A new type of nuclear power technology — small modular reactors that promise lớn produce carbon-neutral energy more safely và efficiently than traditional nuclear power plants — is becoming closer khổng lồ a reality as a handful of companies push to lớn overcome key regulatory hurdles.

Bạn đang xem: Safety of nuclear power reactors

The U.S. Government has not yet approved the reactors, which require significantly less space than a typical nuclear plant & produce nuclear energy on a comparatively smaller scale, for construction, but it is signaling a willingness to vị so in the future.

In December, the Nuclear Regulatory Commission, the independent government agency tasked with ensuring the safety of nuclear power nguồn plants, granted the Tennessee Valley Authority the first-ever early site permit for a small modular reactor project. The Tennessee utility currently has no plans to lớn build & operate SMRs, but the permit gives it the option if it chooses to pursue that giải pháp công nghệ later on.

If it did, the NRC’s decision would mark the first step in a long approval process, but industry watchers still see the move as an important indicator of where the công nghệ is headed.

Last week, the Department of Energy invited companies that specialize in advanced nuclear giải pháp công nghệ to pitch their designs as part of a government effort to keep the U.S. Competitive globally when it comes to lớn nuclear technology.

SMRs have been hailed as one way the U.S. Could combat climate change. The new technology could help boost the nation’s production of nuclear power, which emits no carbon dioxide.

Still, it will likely be several years before any one of the current SMR designs is in operation as businesses, federal agencies & local communities try khổng lồ navigate the best path forward for the new technology.

What is a small modular reactor?

As the name suggests, small modular reactors produce smaller amounts of energy than typical nuclear reactors. Lớn be considered an SMR, the reactor cannot generate more than 300 megawatts per module, compared lớn current nuclear reactors which can produce anywhere from 500 megawatts to more than 1,000 megawatts. One SMR thiết kế from the Portland, Oregon-based company Nu
Scale would produce 60 megawatts, enough energy lớn power 45,000 homes.


Left, a cross section visualization of Nu
Scale Power, LLC. All rights reserved. The works owned by Nu
Scale Power, LLC may not be copied or used to lớn create derivative works without Nu
Scale’s express permission.

But several SMR units could be combined into a network và built khổng lồ scale based on the needs of the communities they serve. Their power đầu ra could also be adjusted after they are operational based on consumer demand or the availability of electricity produced by other sources at a given time of day or year.

“Small modular reactors can be designed lớn ramp up & down with the demand in a very flexible way, in a more cost effective manner, in order lớn fit more neatly with this emerging new electricity system that will include renewables as well as other sources of supply,” said William Magwood IV, director-general of the Nuclear Energy Agency, an intergovernmental agency that promotes global cooperation on nuclear technology.

SMR companies say their reactors would also require far less land than existing nuclear plants. Nu
Scale has designed a 720 megawatt project that would be comprised of 12 reactors — enough lớn power 540,000 homes — và sit on 35 acres. At that size, it would be 17 times smaller than a traditional nuclear plant producing the same amount of electricity, according to lớn the company.

SMRs can also be built in a factory and shipped khổng lồ the location where they’ll eventually operate, cutting down construction costs. Magwood compared that process to lớn manufacturing a commercial airliner, whereas constructing a traditional large nuclear power plant is more analogous to building an entire thành phố block.

How are SMRs designed khổng lồ improve safety?

Major nuclear accidents are fairly uncommon and account for far fewer deaths than accidents in other energy sectors. But the ones that have occurred — Chernobyl, Three Mile Island and Fukushima — have led to widespread concern about the safety of nuclear energy production. The 1979 Three Mile Island accident remains the most serious nuclear accident in U.S. History, where “a small amount of radioactive material” was released but no injuries occurred.

The companies that are developing small modular reactors hope to lớn address those concerns by building new safety features into their designs.

Scale uses light-water technology, where water is used khổng lồ keep their cores from overheating, similar khổng lồ what is used in today’s nuclear power nguồn plants. But there are key differences.

Existing reactors use pumps to maintain a constant flow of water to lớn cool their cores & are equipped with backup diesel generators to keep that process going in the sự kiện of a power nguồn outage. When these complex systems fail, as they did in the Fukushima Daiichi nuclear power plant in nhật bản in 2011, the vi xử lý core can overheat và risk catastrophic failure. Nu
Scale’s SMR relies on natural forces of heating và cooling that combine with gravity to circulate water through its system, eliminating the need for pumps.

Marc Nichol, the senior director of new reactors at the Nuclear Energy Institute, an industry lobbying group that promotes nuclear power, said that “greatly reduces” the potential for an accident.

“As you simplify & make these machines smaller, you actually increase the safety of these such that you can design out potential accidents and eliminate backup equipment that would have been required,” Nichol said.

Power, a nuclear innovation company founded by Bill Gates and headquartered in Bellevue, Washington, has designed two models that use liquid sodium and molten salt rather than water as a coolant. The boiling point of liquid sodium is higher than the temperature produced by the nuclear reaction itself, so the company says the reactor will not overheat.

Power’s molten salt reactor, meanwhile, mixes that heated, liquid salt with its fuel. This action creates a loop that circulates through the system naturally as it heats & cools, eliminating the need for an outside force to keep the process going.

Proponents of SMRs herald the improvements built into this next generation of nuclear technology, but these facilities have not yet been built and tested.

Edwin Lyman, director of nuclear power nguồn safety at the nonprofit Union of Concerned Scientists, said he’s concerned that the companies that kiến thiết SMRs are “putting too much stock” in what they claim to be inherent safety features.

It is easier to prevent the cores of SMRs from overheating and potentially melting down given their smaller kích thước and power output, Lyman said, but he argued that backup safety measures are still important.

Reactors are complex systems, và Lyman said computer-simulated accident scenarios may miss potential shortcomings of proposed designs. Unexpected consequences can arise, he argued, once a facility is actually up & running.

Lyman argues that the new reactors will need multiple layers of safety features, “so that if you’ve guessed wrong or your analysis has uncertainties & you’ve missed something, there’s a backup.”

How close are SMRs to lớn becoming a reality?

The Nuclear Regulatory Commission is currently in the process of reviewing Nu
Scale’s SMR design. If the process goes smoothly, Nu
Scale could be approved as early as September, making it the first lớn be given the green light to lớn license out its kiến thiết for construction.

Power initially had an agreement lớn construct both a demonstration plant and, later, a phối of commercial liquid sodium reactors in China. But those plans were derailed as a result of the Trump administration’s trade dispute with that country, which prohibited the export of advanced nuclear technology. Now, the company is looking to move forward in the U.S. Instead.

Scale & Terra
Power appear to be two industry leaders in the U.S., but several other companies are also in the process of designing SMRs and because the regulatory process takes years, it’s unclear which company, if any, will kết thúc up being the first to lớn build & operate an SMR plant in the U.S.

As seen with a Nevada solar plant, fast-developing giải pháp công nghệ also provides companies the opportunity to lớn leap-frog one another & make competitors’ projects that have been in the works for years obsolete.

Could nuclear energy help mitigate the effects of climate change?

In addition to renewable energy sources, the United Nations Intergovernmental Panel on Climate Change has said nuclear energy could play an important role in mitigating the effects of climate change.

But before the công nghệ can expand, the panel pointed out that concerns regarding nuclear power, such as safety, economic efficiency & waste management, should be addressed.

Since 1990, nuclear power has accounted for nearly trăng tròn percent of the United States’ total electricity production. The federal government intends to lớn continue relying on existing reactors as long as it can, & many facilities have been permitted to lớn extend their licenses for several decades.

Information Library
Country Profiles
Countries T-ZNuclear Fuel Cycle
Safety & Security
Non-power Nuclear Applications
Our Association
trang chủ / Information Library / Safety và Security / Safety of Plants / Safety of Nuclear power nguồn Reactors
Safety of Nuclear power Reactors

(Updated March 2022)

From the outset, there has been a strong awareness of the potential hazard of both nuclear criticality and release of radioactive materials from generating electricity with nuclear power.As in other industries, the design và operation of nuclear nguồn plants aims to minimise the likelihood of accidents, & avoid major human consequences when they occur.These are the only major accidents to have occurred in over 18,500 cumulative reactor-years of commercial nuclear power nguồn operation in 36 countries.The evidence over six decades shows that nuclear power nguồn is a safe means of generating electricity. The risk of accidents in nuclear nguồn plants is low and declining. The consequences of an accident or terrorist attack are minimal compared with other commonly accepted risks. Radiological effects on people of any radioactive releases can be avoided.


In relation khổng lồ nuclear power, safety is closely linked with security, và in the nuclear field also with safeguards. Some distinctions apply:

Safety focuses on unintended conditions or events leading lớn radiological releases from authorised activities. It relates mainly lớn intrinsic problems or hazards.

No industry is immune from accidents, but all industries learn from them. In civil aviation, there are accidents every year và each is meticulously analysed. The lessons from nearly one hundred years’ experience mean that reputable airlines are extremely safe. In the chemical industry & oil-gas industry, major accidents also lead to lớn improved safety. There is wide public acceptance that the risks associated with these industries are an acceptable trade-off for our dependence on their products and services. With nuclear power, the high energy density makes the potential hazard obvious, và this has always been factored into the design of nuclear nguồn plants. The few accidents have been spectacular & newsworthy, but of little consequence in terms of human fatalities. The novelty value and hence newsworthiness of nuclear nguồn accidents remains high in contrast with other industrial accidents, which receive comparatively little news coverage.

Harnessing the world's most concentrated energy source

In the 1950s attention turned to harnessing the power of the atom in a controlled way, as demonstrated at Chicago in 1942 and subsequently for military research, và applying the steady heat yield lớn generate electricity. This naturally gave rise khổng lồ concerns about accidents and their possible effects.However, with nuclear power, safety depends on much the same factors as in any comparable industry: intelligent planning, proper design with conservative margins & back-up systems, high-quality components and a well-developed safety culture in operations.The operating lives of reactors depend on maintaining their safety margin.

A particular nuclear scenario was loss of cooling which resulted in melting of the nuclear reactor core, & this motivated studies on both the physical & chemical possibilities as well as the biological effects of any dispersed radioactivity. Those responsible for nuclear power technology in the West devoted extraordinary effort lớn ensuring that a meltdown of the reactor core would not take place, since it was assumed that a meltdown of the bộ vi xử lý core would create a major public hazard, and if uncontained, a tragic accident with likely multiple fatalities.

In avoiding such accidents the industry has been very successful. In the 60-year history of civil nuclear power generation, with over 18,500 cumulative reactor-years across 36 countries, there have been only three significant accidents at nuclear power plants:

Of all the accidents và incidents, only the Chernobyl and Fukushima accidents resulted in radiation doses khổng lồ the public greater than those resulting from the exposure to lớn natural sources. The Fukushima accident resulted in some radiation exposure of workers at the plant, but not such as to lớn threaten their health, unlike Chernobyl. Other incidents (and one 'accident') have been completely confined lớn the plant.

Apart from Chernobyl, no nuclear workers or members of the public have ever died as a result of exposure to radiation due khổng lồ a commercial nuclear reactor incident. Most of the serious radiological injuries & deaths that occur each year (2-4 deaths and many more exposures above regulatory limits) are the result of large uncontrolled radiation sources, such as abandoned medical or industrial equipment. (There have also been a number of accidents in experimental reactors & in one military plutonium-producing pile – at Windscale, UK, in 1957– but none of these resulted in loss of life outside the actual plant, or long-term environmental contamination.) See also Table in Appendix 2: Serious Nuclear Reactor Accidents.


It should be emphasised that a commercial-type power nguồn reactor simply cannot under any circumstances explode like a nuclear bomb – the fuel is not enriched beyond about 5%, và much higher enrichment is needed for explosives.

The International Atomic Energy Agency (IAEA) was set up by the United Nations in 1957. One of its functions was khổng lồ act as an auditor of world nuclear safety, and this role was increased greatly following the Chernobyl accident.It prescribes safety procedures và the reporting of even minor incidents. Its role has been strengthened since 1996 (see later section). Every country which operates nuclear power nguồn plants has a nuclear safety inspectorate and all of these work closely with the IAEA.

While nuclear power nguồn plants are designed lớn be safe in their operation & safe in the event of any malfunction or accident, no industrial activity can be represented as entirely risk-free. Incidents and accidents may happen, and as in other industries, what is learned will lead to lớn a progressive improvement in safety.Those improvements are both in new designs, và in upgrading of existing plants. The long-term operation (LTO) of established plants is achieved by significant investment in such upgrading.

The safety of operating staff is a prime concern in nuclear plants. Radiation exposure is minimised by the use of remote handling equipment for many operations in the chip core of the reactor. Other controls include physical shielding and limiting the time workers spend in areas with significant radiation levels. These are supported by continuous monitoring of individual doses and of the work environment lớn ensure very low radiation exposure compared with other industries.

The use of nuclear energy for electricity generation can be considered extremely safe. Every year several hundred people die in coal mines to lớn provide this widely used fuel for electricity. There are also significant health and environmental effects arising from fossil fuel use. Contrary to lớn popular belief, nuclear nguồn saves lives by displacing fossil fuel from the electricity mix.

Achieving safety: the reactor core

Concerning possible accidents, up to the early 1970s, some extreme assumptions were made about the possible chain of consequences. These gave rise to a genre of dramatic fiction (e.g.The đài loan trung quốc Syndrome) in the public domain & also some solid conservative engineering including containment structures in the industry itself. Licensing regulations were framed accordingly.

It was not until the late 1970s that detailed analyses & large-scale testing, followed by the 1979 meltdown of the Three Mile Island reactor, began lớn make clear that even the worst possible accident in a conventional western nuclear power nguồn plant or its fuel would not be likely to lớn cause dramatic public harm. The industry still works hard to lớn minimize the probability of a meltdown accident, but it is now clear that no-one need fear a potential public health catastrophe simply because a fuel meltdown happens. Fukushima Daiichi has made that clear, with a triple meltdown causing no fatalities or serious radiation doses lớn anyone, while over two hundred people continued working onsite to mitigate the accident's effects.

The decades-long test & analysis programme showed that less radioactivity escapes from molten fuel than initially assumed, và that most of this radioactive material is not readily mobilized beyond the immediate internal structure. Thus, even if the containment structure that surrounds all modern nuclear plants were ruptured, as was the case with one of the Fukushima reactors, it is still very effective in preventing the escape of most radioactivity.

A mandated safety indicator is the calculated probable frequency of degraded bộ vi xử lý core or chip core melt accidents. The US Nuclear Regulatory Commission (NRC) specifies that reactor designs must meet a theoretical 1 in 10,000 year bộ vi xử lý core damage frequency, but modern designs exceed this. US utility requirements are 1 in 100,000 years, the best currently operating plants are about 1 in one million & those likely lớn be built in the next decade are almost 1 in 10 million. While this calculated core damage frequency has been one of the main metrics lớn assess reactor safety, European safety authorities prefer a deterministic approach, focusing on actual provision of back-up hardware, though they also undertake probabilistic safety analysis (PSA) for core damage frequency,and require a 1 in 1 million chip core damage frequency for new designs.

Even months after the Three Mile Island (TMI) accident in 1979 it was assumed that there had been no bộ vi xử lý core melt because there were no indications of severe radioactive release even inside the containment. It turned out that in fact about half the bộ vi xử lý core had melted. Until 2011 this remained the only bộ vi xử lý core melt in a reactor conforming lớn NRC safety criteria, & the effects were contained as designed, without radiological harm to anyone.* Greifswald 5 in East Germany had a partial bộ vi xử lý core melt in November 1989, due to lớn malfunctioning valves (root cause: shoddy manufacture) & was never restarted. At Fukushima in 2011 (a different reactor thiết kế with penetrations in the bottom of the pressure vessel) the three reactor cores evidently largely melted in the first two or three days, but this was not confirmed for about ten weeks. It is still not certain how much of the chip core material was not contained by the pressure vessels & ended up in the bottom of the drywell containments, though certainly there was considerable release of radionuclides lớn the atmosphere early on, & later khổng lồ cooling water**.

* About this time there was alarmist talk of the so-called 'China Syndrome', a scenario where the chip core of such a reactor would melt, and due to continual heat generation, melt its way through the reactor pressure vessel và concrete foundations to keep going, perhaps until it reached đài loan trung quốc on the other side of the globe! The TMI accident proved the extent of truth in the proposition, và the molten chip core material got exactly 15 milimet of the way to đài loan trung quốc as it froze on the bottom of the reactor pressure vessel.

** Ignoring isotopic differences, there are about one hundred different fission products in fuel which has been undergoing fission. A few of these are gases at normal temperatures, more are volatile at higher temperatures, & both will be released from the fuel if the cladding is damaged. The latter include iodine (easily volatilised, at 184°C) and caesium (671°C), which were the main radionuclides released at Fukushima, first into the reactor pressure vessel and then into the containment which in unit 2 apparently ruptured early on day 5. In addition, as cooling water was flushed through the hot core, soluble fission products such as caesium dissolved in it, which created the need for a large water treatment plant to lớn remove them.

Apart from these accidents & the Chernobyl disaster there have been about ten bộ vi xử lý core melt accidents – mostly in military or experimental reactors – Appendix 2 lists most of them. None resulted in any hazard outside the plant from the bộ vi xử lý core melting, though in one case there was significant radiation release due to burning fuel in hot graphite (similar lớn Chernobyl but smaller scale). The Fukushima accident should also be considered in that context, since the fuel was badly damaged & there were significant off-site radiation releases.

Licensing approval for new plants today requires that the effects of any core-melt accident must be confined to lớn the plant itself, without the need lớn evacuate nearby residents.

The main safety concern has always been the possibility of an uncontrolled release of radioactive material, leading khổng lồ contamination & consequent radiation exposure off-site. Earlier assumptions were that this would be likely in the event of a major loss of cooling accident (LOCA) which resulted in a chip core melt. The TMI experience suggested otherwise, but at Fukushima this is exactly what happened. In the light of better understanding of the physics and chemistry of material in a reactor chip core under extreme conditions it became evident that even a severe chip core melt coupled with breach of containment would be unlikely khổng lồ create a major radiological disaster from many Western reactor designs, but the Fukushima accident showed that this did not apply khổng lồ all. Studies of the post-accident situation at TMI (where there was no breach of containment) supported the suggestion, và analysis of Fukushima will be incomplete until the reactors are dismantled.

Certainly the matter was severely tested with three reactors of the Fukushima Daiichi nuclear nguồn plant in nhật bản in March 2011. Cooling was lost about an hour after a shutdown, and it proved impossible khổng lồ restore it sufficiently to prevent severe damage to lớn the fuel. The reactors, dating from 1971-75, were written off. A fourth is also written off due to damage from a hydrogen explosion.

Achieving optimum nuclear safety

A fundamental principle of nuclear power nguồn plant operation worldwide is that the operator is responsible for safety. The national regulator is responsible for ensuring the plants are operated safely by the licensee, & that the thiết kế is approved. A second important concept is that a regulator’s mission is to protect people & the environment.

Design certification of reactors is also the responsibility of national regulators. There is international collaboration among these to varying degrees, andthere are a number of sets of mechanical codes and standards related to unique and safety.

With new reactor designs being established on a more international basis since the 1990s, both the industry & regulators are seeking greater thiết kế standardization and also regulatory harmonization. The role of the World Nuclear Association's Cooperation in Reactor thiết kế Evaluation và Licensing (CORDEL) Working Group & the OECD Nuclear Energy Agency's (NEA's) Multinational thiết kế Evaluation Programme (MDEP) are described in the information page on
Cooperation in Nuclear Power.

An OECD-NEA report in 2010 pointed out that the theoretically-calculated frequency for a large release of radioactivity from a severe nuclear power plant accident has reduced by a factor of 1600 between the early Generation I reactors as originally built và the Generation III/III+ plants being built today. Earlier designs however have been progressively upgraded through their operating lives.

It has long been asserted that nuclear reactor accidents are the epitome of low-probability but high-consequence risks. Understandably, with this in mind, some people were disinclined to lớn accept the risk, however low the probability. However, the physics and chemistry of a reactor core, coupled with but not wholly depending on the engineering, mean that the consequences of an accident are likely in fact be much less severe than those from other industrial & energy sources. Experience, including Fukushima, bears this out.

A 2009 US Department of Energy (DOE) Human Performance Handbook notes: "The aviation industry, medical industry, commercial nuclear power nguồn industry, US Navy, DOE và its contractors, & other high-risk, technologically complex organizations have adopted human performance principles, concepts, & practices to consciously reduce human error và bolster controls in order to reduce accidents and events... About 80% of all events are attributed khổng lồ human error. In some industries, this number is closer to 90%. Roughly 20% of events involve equipment failures. When the 80% human error is broken down further, it reveals that the majority of errors associated with events stem from latent organizational weaknesses (perpetrated by humans in the past that lie dormant in the system), whereas about 30% are caused by the individual worker touching the equipment và systems in the facility. Clearly, focusing efforts on reducing human error will reduce the likelihood of events." Following the Fukushima accident the focus has been on the organizational weaknesses which increase the likelihood of human error.


To achieve optimum safety, nuclear plants in the western world operate using a 'defence-in-depth' approach, with multiple safety systems supplementing the natural features of the reactor core. Key aspects of the approach are:

High-quality design và construction.Equipment which prevents operational disturbances or human failures & errors developing into problems.Comprehensive monitoring & regular testing to detect equipment or operator failures.Redundant và diverse systems khổng lồ control damage to the fuel và prevent significant radioactive releases.Provision to confine the effects of severe fuel damage (or any other problem) to the plant itself.

These can be summed up as: prevention, monitoring, and action (to mitigate consequences of failures).

The safety provisions include a series of physical barriers between the radioactive reactor core và the environment, the provision of multiple safety systems, each with backup và designed to accommodate human error. As well as the physical aspects of safety, there are institutional aspects which are no less important – see following section on International Collaboration.

The barriers in a typical plant are: the fuel is in the form of solid ceramic (UO2) pellets, and radioactive fission products remain largely bound inside these pellets as the fuel is burned. The pellets are packed inside sealed zirconium alloy tubes to khung fuel rods. These are confined inside a large steel pressure vessel with walls up lớn 30 centimet thick – the associated primary water cooling pipework is also substantial. All this, in turn, is enclosed inside a robust reinforced concrete containment structure with walls at least one metre thick. This amounts to three significant barriers around the fuel, which itself is stable up khổng lồ very high temperatures.

These barriers are monitored continually. The fuel cladding is monitored by measuring the amount of radioactivity in the cooling water. The high pressure cooling system is monitored by the leak rate of water, & the containment structure by periodically measuring the leak rate of air at about five times atmospheric pressure.

Looked at functionally, the three basic safety functions in a nuclear reactor are:

To control reactivity.To cool the fuel.To contain radioactive substances.

Xem thêm: Nằm mơ thấy người thân chết nhưng chưa chết nhưng chưa chết đánh con gì?

The main safety features of most reactors are inherent – negative temperature coefficient & negative void coefficient. The first means that beyond an optimal level, as the temperature increases the efficiency of the reaction decreases (this in fact is used lớn control nguồn levels in some new designs). The second means that if any steam has formed in the cooling water there is a decrease in moderating effect so that fewer neutrons are able to cause fission và the reaction slows down automatically.

In the 1950s and 1960s some experimental reactors in Idaho were deliberately tested lớn destruction to verify that large reactivity excursions were self-limiting and would automatically shut down the fission reaction. These tests verified that this was the case.

Beyond the control rods which are inserted to lớn absorb neutrons & regulate the fission process, the main engineered safety provisions are the back-up emergency vi xử lý core cooling system (ECCS) lớn remove excess heat (though it is more to lớn prevent damage to the plant than for public safety) và the containment.

Traditional reactor safety systems are 'active' in the sense that they involve electrical or mechanical operation on command. Some engineered systems operate passively, e.g. Pressure relief valves. Both require parallel redundant systems. Inherent or full passive safety thiết kế depends only on physical phenomena such as convection, gravity or resistance to lớn high temperatures, not on functioning of engineered components. All reactors have some elements of inherent safety as mentioned above, but in some recent designs the passive or inherent features substitute for active systems in cooling etc. Such a thiết kế would have averted the Fukushima accident, where loss of electrical power resulted is loss of cooling function.

The basis of thiết kế assumes a threat where due to accident or malign intent (e.g. Terrorism) there is vi xử lý core melting and a breach of containment. This double possibility has been well studied and provides the basis of exclusion zones and contingency plans. Apparently during the Cold War neither Russia nor the USA targeted the other's nuclear nguồn plants because the likely damage would be modest.

Nuclear power plants are designed with sensors to lớn shut them down automatically in an earthquake, and this is a vital consideration in many parts of the world. (See Nuclear nguồn Plants and Earthquakes paper)

Severe accident management

In addition khổng lồ engineering and procedures which reduce the risk và severity of accidents, all plants have guidelines for severe accident management or mitigation (SAM). These conspicuously came into play after the Fukushima accident, where staff had immense challenges in the absence of power & with disabled cooling systems following damage done by the tsunami. The experience following that accident is being applied not only in kiến thiết but also in such guidelines, and peer đánh giá on nuclear plants are focusing more on these than previously.

In mid-2011 the IAEA Incident và Emergency Centre launched a new secure web-based communications platform to lớn unify and simplify information exchange during nuclear or radiological emergencies. The Unified System for Information Exchange on Incidents và Emergencies (USIE) has been under development since 2009 but was actually launched during the emergency response lớn the accident at Fukushima.

In both the TMI & Fukushima accidents the problems started after the reactors were shut down – immediately at TMI và after an hour at Fukushima, when the tsunami arrived. The need to lớn remove decay heat from the fuel was not met in each case, so core melting started khổng lồ occur within a few hours. Cooling requires water circulation and an external heat sink. If pumps cannot run due khổng lồ lack of power, gravity must be relied upon, but this will not get water into a pressurised system – either reactor pressure vessel or containment. Hence there is provision for relieving pressure, sometimes with a vent system, but this must work & be controlled without power. There is a question of filters or scrubbers in the vent system: these need to be such that they vày not block due to solids being carried. Ideally any vent system should khuyễn mãi giảm giá with any large amounts of hydrogen, as at Fukushima, và have minimum potential khổng lồ spread radioactivity outside the plant.Filtered containment ventilation systems (FCVSs) have been retrofitted to some reactors which did not already have them, or any of sufficient capacity, following the Fukushima accident. The basic premise of a FCVS is that, independent of the state of the reactor itself, the catastrophic failure of the containment structure can be avoided by discharging steam, air and incondensable gases like hydrogen khổng lồ the atmosphere.

The Three Mile Island accident in 1979demonstrated the importance of the inherent safety features. Despite the fact that about half of the reactor chip core melted, radionuclides released from the melted fuel mostly plated out on the inside of the plant or dissolved in condensing steam. The containment building which housed the reactor further prevented any significant release of radioactivity. The accident was attributed to mechanical failure & operator confusion. The reactor's other protection systems also functioned as designed. The emergency core cooling system would have prevented any damage khổng lồ the reactor but for the intervention of the operators.

Investigations following the accident led khổng lồ a new focus on the human factors in nuclear safety. No major thiết kế changes were called for in western reactors, but controls and instrumentation were improved significantly and operator training was overhauled.

At Fukushima Daiichi in March 2011 the three operating reactors shut down automatically, and were being cooled as designed by the normal residual heat removal system using nguồn from the back-up generators, until the tsunami swamped them an hour later. The emergency core cooling systems then failed. Days later, a separate problem emerged as spent fuel ponds lost water. Analysis of the accident showed the need for more intelligent siting criteria than those used in the 1960s, & the need for better back-up power and post-shutdown cooling, as well as provision for venting the containment of that kind of reactor & other emergency management procedures.

Nuclear plants have Severe Accident Mitigation Guidelines (SAMG, or in Japan: SAG), & most of these,including all those in the USA, address what should be done for accidents beyond thiết kế basis, and where several systems may be disabled. See section below.

In 2007 the US NRC launched a research program lớn assess the possible consequences of a serious reactor accident. Its draft report was released nearly a year after the Fukushima accident had partly confirmed its findings. The State-of-the-Art Reactor Consequences Analysis (SOARCA) showed that a severe accident at a US nuclear power nguồn plant (PWR or BWR) would not be likely khổng lồ cause any immediate deaths, & the risks of fatal cancers would be vastly less than the general risks of cancer. SOARCA's main conclusions fall into three areas: how a reactor accident progresses; how existing systems & emergency measures can affect an accident's outcome; & how an accident would affect the public's health. The principal conclusion is that existing resources và procedures can stop an accident, slow it down or reduce its impact before it can affect the public, but even if accidents proceed without such mitigation they take much longer to happen & release much less radioactive material than earlier analyses suggested. This was borne out at Fukushima, where there was ample time for evacuation – three days – before any significant radioactive releases.

In năm ngoái the Canadian Nuclear Safety Commission (CNSC) released its Study of Consequences of a Hypothetical Severe Nuclear Accident and Effectiveness of Mitigation Measures. This was the result of research and analysis undertaken lớn address concerns raised during public hearings in 2012 on the environmental assessment for the refurbishment of Ontario power Generation's (OPG's) Darlington nuclear nguồn plant. The study involved identifying and modelling a large atmospheric release of radionuclides from a hypothetical severe nuclear accident at the four-unit Darlington power plant; estimating the doses to lớn individuals at various distances from the plant, after factoring in protective actions such as evacuation that would be undertaken in response to such an emergency; and, finally, determining human health và environmental consequences due lớn the resulting radiation exposure. It concluded that there would be no detectable health effects or increase in cancer risk. A fuller write-up of it is on the World Nuclear News website.

A different safety philosophy: early Soviet-designed reactors

The April 1986 disaster at the Chernobyl nuclear power plant in Ukraine was the result of major thiết kế deficiencies in the RBMK type of reactor, the violation of operating procedures and the absence of a safety culture. One peculiar feature of the RBMK design was that coolant failure could lead khổng lồ a strong increase in power đầu ra from the fission process (positive void coefficient). However, this was not the prime cause of the Chernobyl accident.It once & for all vindicated the desirability of designing with inherent safety supplemented by robust secondary safety provisions. By way of contrast to lớn western safety engineering, the Chernobyl reactor did not have a containment structure lượt thích those used in the West or in post-1980 Soviet designs.

The accident destroyed the reactor, và its burning contents dispersed radionuclides far và wide. This tragically meant that the results were severe, with 56 people killed, 28 of whom died within weeks from radiation exposure. It also caused radiation sickness in a further 200-300 staff & firefighters, and contaminated large areas of Belarus, Ukraine, Russia & beyond. It is estimated that at least 5% of the total radioactive material in the Chernobyl 4 reactor chip core was released from the plant, due khổng lồ the lack of any containment structure. Most of this was deposited as dust close by. Some was carried by wind over a wide area.

About 130,000 people received significant radiation doses (i.e. above internationally accepted ICRP limits) & continue to lớn be monitored. According to lớn an UNSCEAR report in 2018, about 20,000 cases of thyroid cancer were diagnosed in 1991-2015 in patients who were 18 và under at the time of the accident. The report states that a quarter of the cases in 2001-2008 were "probably" due lớn high doses of radiation, và that this fraction was likely lớn have been higher in earlier years, & lower in later years. However, it also states that the uncertainty around the attributed fraction is very significant – at least 0.07 khổng lồ 0.5– and that the influence of annual screenings & active follow-up make comparisons with the general population problematic. Thyroid cancer is usually not fatal if diagnosed và treated early; the report states that of the diagnoses made between 1991 và 2005 (6,848 cases), 15 proved lớn be fatal. No increase in leukaemia or other cancers have yet shown up, but some is expected. The World Health Organization is closely monitoring most of those affected.

The Chernobyl accident was a unique event and the only time in the history of commercial nuclear power nguồn that radiation-related fatalities occurred.The main positive outcome of this accident for the industry was the formation of the World Association of Nuclear Operators (WANO), building on the US precedent.

The destroyed unit 4 was enclosed in a concrete shelter, which was replaced by a more permanent structure in 2017.

An OECD expert report on the accident concluded: "The Chernobyl accident has not brought to light any new, previously unknown phenomena or safety issues that are not resolved or otherwise covered by current reactor safety programs for commercial power nguồn reactors in OECD thành viên countries." In other words, the concept of 'defence in depth' was conspicuous by its absence, & tragically shown to lớn be vitally important.

Apart from the RBMK reactor design, an early Russian PWR design, the VVER-440/V-230, gave rise lớn concerns in Europe, and a program was initiated to close these down as a condition of EU accession, along with Lithuania’s two RBMK units. See related papers on Early Soviet Reactors và EU Accession, and RBMK Reactors.

However, after the US Atomic Energy Commission published General kiến thiết Criteria for Nuclear power nguồn Plants in 1971, Russian PWR designs conformed, according khổng lồ Rosatom. In particular, the VVER-440/V-213 Loviisa reactors in Finland were designed at that time & modified to lớn conform. The first of these two came on line in 1977.

A broader picture – other past accidents

There have been a number of accidents in experimental reactors & in one military plutonium-producing reactor, including a number of core melts, but none of these has resulted in loss of life outside the actual plant, or long-term environmental contamination. Elsewhere (Safety of Nuclear nguồn Reactors appendix) we tabulate these, along with the most serious commercial plant accidents. The các mục of ten probably corresponds khổng lồ incidents rating cấp độ 4 or higher on today’s International Nuclear event Scale (Table 4). All except Browns Ferry và Vandellos involved damage to lớn or malfunction of the reactor core. At Browns Ferry a fire damaged control cables and resulted in an 18-month shutdown for repairs; at Vandellos a turbine fire made the 17-year old plant uneconomic to lớn repair.

Mention should be made of the accident khổng lồ the US Fermi 1 prototype fast breeder reactor near Detroit in 1966. Due to lớn a blockage in coolant flow, some of the fuel melted. However no radiation was released offsite & no-one was injured. The reactor was repaired & restarted but closed down in 1972.

The well-publicized criticality accident at Tokai Mura, Japan, in 1999 was at a fuel preparation plant for experimental reactors, and killed two workers from radiation exposure. Many other such criticality accidents have occurred, some fatal, và practically all in military facilities prior to lớn 1980. A reviews of these is listed in the References section.

In an uncontained reactor accident such as at Windscale (a military facility) in 1957 and at Chernobyl in 1986 (and to lớn some extent Fukushima Daiichi in 2011), the principal health hazard is from the spread of radioactive materials, notably volatile fission products such as iodine-131 và caesium-137. These are biologically active, so that if consumed in food, they tend to lớn stay in organs of the body. I-131 has a half-life of 8 days, so is a hazard for around the first month, (and apparently gave rise to lớn the thyroid cancers after the Chernobyl accident). Caesium-137 has a half-life of 30 years, & is therefore potentially a long-term contaminant of pastures and crops. In addition to lớn these, there is caesium-134 which has a half-life of about two years. While measures can be taken lớn limit human uptake of I-131, (evacuation of area for several weeks, iodide tablets), high levels of radioactive caesium can preclude food production from affected land for a long time. Other radioactive materials in a reactor bộ vi xử lý core have been shown to be less of a problem because they are either not volatile (strontium, transuranic elements) or not biologically active (tellurium-132, xenon-133).

Accidents in any field of technology provide valuable knowledge enabling incremental improvement in safety beyond the original engineering. Cars and airliners are the most obvious examples of this, but the chemical & oil industries can provide even stronger evidence. Civil nuclear power nguồn has greatly improved its safety in both engineering & operation over its 65 years of experience with very few accidents and major incidents lớn spur that improvement. The Fukushima Daiichi accident was the first since TMI in 1979 which will have significant implications, at least for older plants.

Scrams, seismic shutdowns

A scram is a sudden reactor shutdown. When a reactor is scrammed, automatically due lớn seismic activity, or due lớn some malfunction, or manually for whatever reason, the fission reaction generating the main heat stops. However, considerable heat continues to be generated by the radioactive decay of the fission products in the fuel. Initially, for a few minutes, this is great– about 7% of the pre-scram level. But it drops to lớn about 1% of the normal heat output đầu ra after two hours, to 0.5% after one day, & 0.2% after a week. Even then it must still be cooled, but simply being immersed in a lot of water does most of the job after some time. When the water temperature is below 100°C at atmospheric pressure the reactor is said lớn be in "cold shutdown".

European 'stress tests' & US response following Fukushima accident

Aspects of nuclear plant safety highlighted by the Fukushima accident were assessed in the nuclear reactors in the EU's thành viên states, as well as those in any neighbouring states that decided khổng lồ take part. These comprehensive và transparent nuclear risk và safety assessments, the so-called "stress tests", involved targeted reassessment of each power reactor’s safety margins in the light of extreme natural events, such as earthquakes & flooding, as well as on loss of safety functions & severe accident management following any initiating event. They were conducted from June 2011 to lớn April 2012. They mobilized considerable expertise in different countries (500 man-years) under the responsibility of each national Safety Authority within the framework of the European Nuclear Safety Regulators Group (ENSREG).

The Western European Nuclear Regulators' Association (WENRA) proposed these in response khổng lồ a hotline from the European Council in March 2011, and developed specifications. WENRA is a network of Chief Regulators of EU countries with nuclear power nguồn plants và Switzerland, và has membership from 17 countries. It then negotiated the scope of the tests with the European Nuclear Safety Regulators Group (ENSREG), an independent, authoritative expert toàn thân created in 2007 by the European Commission comprising senior officials from the national nuclear safety, radioactive waste safety or radiation protection regulatory authorities from all EU thành viên states, và representatives of the European Commission.

In June 2011 the governments of seven non-EU countries agreed khổng lồ conduct nuclear reactor căng thẳng tests using the EU model. Armenia, Belarus, Croatia, Russia, Switzerland, Turkey & Ukraine signed a declaration that they would conduct găng tests & agreed lớn peer review of the tests by outside experts. Russia had already undertaken extensive checks. (Croatia is co-owner in the Krsko PWR in Slovenia, và Turkey is building its first nuclear plant.)

The reassessment of safety margins is based on the existing safety studies và engineering judgement khổng lồ evaluate the behaviour of a nuclear power nguồn plant when facing a set of challenging situations. For a given plant, the reassessment reports on the most probable behaviour of the plant for each of the situations considered. The results of the reassessment were peer-reviewed và shared among regulators. WENRA noted that it remains a national responsibility lớn take or order any appropriate measures, such as additional technical or organisational safety provisions,resulting from the reassessment.

The scope of the assessment took into account the issues directly highlighted by the events in Fukushima & the possibility for combination of initiating events. Two 'initiating events' were covered in the scope: earthquake và flooding. The consequences of these –loss of electrical power và station blackout, loss of ultimate heat sink & the combination of both – were analysed, with the conclusions being applicable to lớn other general emergency situations. In accident scenarios, regulators consider power nguồn plants' means khổng lồ protect against và manage loss of chip core cooling as well as cooling of used fuel in storage. They also study means to lớn protect against và manage loss of containment integrity và core melting, including consequential effects such as hydrogen accumulation.

Nuclear plant operators start by documenting each power nguồn plant site. This analysis of 'extreme scenarios' followed what ENSREG called a progressive approach "in which protective measures are sequentially assumed to be defeated" from starting conditions which "represent the most unfavourable operational states." The operators have to explain their means to maintain "the three fundamental safety functions (control of reactivity, fuel cooling confinement of radioactivity)" and support functions for these, "taking into account the probable damage done by the initiating event."

The documents had lớn cover provisions in the plant kiến thiết basis for these events và the strength of the plant beyond its kiến thiết basis. This means the "design margins, diversity, redundancy, structural protection và physical separation of the safety relevant systems, structures and components and the effectiveness of the defence-in-depth concept." This had to focus on 'cliff-edge' effects, e.g. When back-up batteries are exhausted & station blackout is inevitable. For severe accident management scenarios they must identify the time before fuel damage is unavoidable và the time before water begins boiling in used fuel ponds and before fuel damage occurs. Measures lớn prevent hydrogen explosions & fires are lớn be part of this.

Since the licensee has the prime responsibility for safety, they performed the reassessments, và the regulatory bodies then independently reviewed them. The exercise covered 147 nuclear plants in 15 EU countries – including Lithuania with only decommissioned plants – plus 15 reactors in Ukraine and five in Switzerland.

Operators reported to lớn their regulators who then reported progress khổng lồ the European Commission by the end of 2011. Information was shared among regulators throughout this process before the 17 final reports went to lớn peer-review by teams comprising 80 experts appointed by ENSREG & the European Commission. The final documents were published in line with national law and international obligations, subject only lớn not jeopardising security – an area where each country could behave differently. The process was extended lớn June 2012 to lớn allow more plant visits và to địa chỉ more information on the potential effect of aircraft impacts.

The European Commission adopted, with ENSREG, the final căng thẳng tests Report on April 26, 2012 & issued the same day a joint statement underlining the chất lượng of the exercise. The full report & a summary of the 45 recommendations were published on www.ensreg.eu. Drawing on the peer reviews, the EC và ENSREG cited four main areas for improving EU nuclear plant safety:

Guidance from WENRA for assessing natural hazards và margins beyond thiết kế basis.Giving more importance lớn periodic safety reviews and evaluation of natural hazards.Urgent measures lớn protect containment integrity.Measures to lớn prevent và mitigate accidents resulting from extreme natural hazards.

The results of the stress tests pointed out, in particular, that European nuclear power nguồn plants offered a sufficient safety màn chơi to require no shutdown of any of them. At the same time, improvements were needed khổng lồ enhance their robustness khổng lồ extreme situations. In France, for instance, they were imposed by ASN requirements, which took into account exchanges with its European counterparts. A follow-up European kích hoạt plan was established by ENSREG from July 2012.

The EU process was completed at the end of September 2012, with the EU Energy Commissioner announcing that the ức chế tests had showed that the safety of European power reactors was generally satisfactory, but making some other comments & projections which departed from ENSREG. An EC report was presented to the EU Council in October 2012.

In the USA the Nuclear Regulatory Commission (NRC) in March 2012 made orders for immediate post-Fukushima safety enhancements, with a cost of about $100 million across the whole US fleet. The first order required the addition of equipment at all plants to lớn help respond lớn the loss of all electrical power và the loss of the ultimate heat sink for cooling, as well as maintaining containment integrity. Another required improved water level và temperature instrumentation on used fuel ponds. The third order applied only lớn the 33 BWRs with early containment designs, & required 'reliable hardened containment vents' which work under any circumstances. The US industry association, the Nuclear Energy Institute, told the NRC that licensees with these Mark I and Mark II containments “should have the capability lớn use various filtration strategies khổng lồ mitigate radiological releases” during severe events, and that filtration “should be founded on scientific & factual analysis và should be performance-based khổng lồ achieve the desired outcome.” All the measures are supported by the industry association, which also proposed setting up about six regional emergency response centres under NRC oversight with additional portable equipment.

In nhật bản similar găng tests were carried out in 2011 under the previous safety regulator, but then reactor restarts were delayed until the newly constituted Nuclear Regulatory Authority devised và published new safety guidelines, then applied them progressively through the fleet.

Earthquakes & volcanoes

The International Atomic Energy Agency (IAEA) has a Safety Guide on Seismic Risks for Nuclear power nguồn Plants, and the matter is dealt with in the WNA page on Earthquakes và Nuclear power Plants.Volcanic hazards are minimal for practically all nuclear plants, but the IAEA has developed a new Safety Guide on the matter. The Bataan plant in Philippines which has never operated, và the Armenian plant at Metsamor are two known lớn be in proximity lớn potential volcanic activity.

Flooding –storms, tides và tsunamis

Nuclear plants are usually built close to lớn water bodies, for the sake of cooling. The site licence takes trương mục of worst case flooding scenarios as well as other possible natural disasters and, more recently, the possible effects of climate change. As a result, all the buildings with safety-related equipment are situated on high enough platforms so that they stand above submerged areas in case of flooding events. As an example, French Safety Rules criteria for river sites define the safe level as above a flood màn chơi likely to be reached with one chance in one thousand years, plus 15%, và similar regarding tides for coastal sites.

Occasionally in the past some buildings have been sited too low, so that they are vulnerable lớn flood or tidal & storm surge, so engineered countermeasures have been built. EDF's Blayais nuclear plant in western France uses seawater for cooling & the plant itself is protected from storm surge by dykes. However, in 1999 a 2.5 m storm surge in the estuary overtopped the dykes – which were already identified as a weak point and scheduled for a later nâng cấp – and flooded one pumping station. For security reasons it was decided lớn shut down the three reactors then under nguồn (the fourth was already stopped in the course of normal maintenance). This incident was rated 2 on the INES scale.

In 1994 the Kakrapar nuclear power plant near the west coast of India was flooded due to lớn heavy rains together with failure of weir control for an adjoining water pond, inundating turbine building basement equipment. The back-up diesel generators on site enabled bộ vi xử lý core cooling using fire water, a backup lớn process water, since the offsite nguồn supply failed. Following this, multiple flood barriers were provided at all entry points, inlet openings below kiến thiết flood màn chơi were sealed và emergency operating procedures were updated. In December 2004 the Madras NPP & Kalpakkam PFBR site on the east coast of India was flooded by a tsunami surge from Sumatra. Construction of the Kalpakkam plant was just beginning, but the Madras plant shut down safely và maintained cooling. However, recommendations including early warning system for tsunami và provision of additional cooling water sources for longer duration cooling were implemented.

In March 2011 the Fukushima Daiichi nuclear plant was affected seriously by a huge tsunami induced by the Great East nhật bản Earthquake. Three of the six reactors were operating at the time, & had shut down automatically due khổng lồ the earthquake. The back-up diesel generators for those three units were then swamped by the tsunami. This cut power nguồn supply and led to lớn weeks of drama & loss of the reactors. The thiết kế basis tsunami height was 5.7 m for Daiichi (and 5.2 m for adjacent Daini, which was actually mix a bit higher above sea level). Tsunami heights coming ashore were about 14 metres for both plants. Unit 3 of Daini was undamaged và continued to lớn cold shutdown status, but the other units suffered flooding khổng lồ pump rooms where equipment transfers heat from the reactor circuit to lớn the sea – the ultimate heat sink.

The maximum amplitude of this tsunami was 23 metres at point of origin, about 160 km from Fukushima. In the last century there had been eight tsunamis in the japan region with maximum amplitudes above 10 metres (some much more), these having arisen from earthquakes of magnitude 7.7 khổng lồ 8.4, on average one every 12 years. Those in 1983 và in 1993 were the most recent affecting Japan, with maximum heights 14.5 metres & 31 metres respectively, both induced by magnitude 7.7 earthquakes. This 2011 earthquake was magnitude 9.

For low-lying sites, civil engineering và other measures are normally taken to lớn make nuclear plants resistant to flooding. Lessons from Blayais và Fukushima have fed into regulatory criteria. Sea walls have been và are being built or increased at Hamaoka, Shimane, Mihama, Ohi, Takahama, Onagawa, & Higashidori plants. However, few parts of the world have the same tsunami potential as Japan, và for the Atlantic and Mediterranean coasts of Europe the maximum amplitude is much less than Japan.


In any light-water nuclear power nguồn reactor, hydrogen is formed by radiolytic decomposition of water. This needs to be dealt with lớn avoid the potential for explosion with oxygen present, và many reactors have been retrofitted with passive autocatalytic hydrogen recombiners in their containment, replacing external recombiners that needed to lớn be connected và powered, isolated behind radiological barriers. Also in some kinds of reactor, particularly early boiling water types, the containment is rendered inert by injection of nitrogen.

In an accident situation such as at Fukushima where the fuel became very hot, a lot of hydrogen is formed by the oxidation of zirconium fuel cladding in steam at about 1300°C. This is beyond the capability of the normal hydrogen recombiners to khuyễn mãi giảm giá with,and operators must rely on venting to lớn atmosphere or inerting the containment with nitrogen.

International collaboration khổng lồ improve safety

There is a lot of international collaboration, but it has evolved from the bottom, và only in 1990s has there been any real top-down initiative. In the aviation industry the Chicago Convention in the late 1940s initiated an international approach which brought about a high degree of kiến thiết collaboration between countries, & the rapid universal uptake of lessons from accidents. There are cultural & political reasons for this which mean that even the much higher international safety collaboration since the 1990s is still less than in aviation. See also paper on
Cooperation in Nuclear nguồn Industry, especially for fuller description of WANO, focused on operation.

World Association of Nuclear Operators

International cooperation on nuclear safety issues takes place under the auspices of the World Association of Nuclear Operators (WANO) which was mix up in 1989. In practical terms this is the most effective international means of achieving very high levels of safety through its four major programs: peer reviews; operating experience; technical support and exchange; và professional và technical development. WANO peer đánh giá are the main proactive way of sharing experience và expertise, và by the kết thúc of 2009 every one of the world's commercial nuclear power plants had been peer-reviewed at least once. Following the Fukushima accident these have been stepped up khổng lồ one every four years at each plant, with follow-up visits in between, and the scope extended from operational safety lớn include plant thiết kế upgrades. Pre-startup reviews of new plants are being increased.

IAEA Convention on Nuclear Safety

The IAEA Convention on Nuclear Safety (CNS) was drawn up during a series of expert màn chơi meetings from 1992 to lớn 1994 & was the result of considerable work by Governments, national