Edited from World Nuclear Association information paper "Safety of Nuclear Power Reactors," March 2012. www.world-nuclear.org/info/inf06.html. Copyright © 2012 by the World Nuclear Association. All rights reserved. Reproduced by permission.
"It is now clear that no one need fear a potential public health catastrophe simply because a fuel meltdown happens."
In the following viewpoint the Nuclear World Association argues that the nuclear industry has been successful in avoiding major accidents. Only three accidents—at Chernobyl, Three Mile Island and Fukushima—have occurred in more than fifty years, resulting in less severe consequences than feared, according to the association. Accidents such as the meltdown at Fukushima, the association maintains, will lead to continuous safety improvements in reactor design, operational safety, and accident management. The association also claims that current reactor design can withstand most natural disasters and terrorist attacks. Strict oversight and international collaboration make nuclear power safer than any other form of energy production, the association concludes. The World Nuclear Association promotes nuclear power and supports companies in the industry.
As you read, consider the following questions:
- What were the three major nuclear accidents, according to the viewpoint?
- What are the key aspects of the "defence-in-depth" approach, as explained in the viewpoint?
- What changes does the viewpoint state were made to the Nuclear Regulatory Commissions' security requirements after the 9/11 terrorist attacks?
In the 1950s attention turned to harnessing the power of the atom in a controlled way, as demonstrated [by the] Chicago [Pile 1 (CP-1), the world's first man-made nuclear reactor] in 1942 and subsequently for military research, and applying the steady heat yield to generate electricity. This naturally gave rise to 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 and back-up systems, high-quality components and a well-developed safety culture in operations.
The Nuclear Industry Has Successfully Avoided Accidents
A particular nuclear scenario was loss of cooling which resulted in melting of the nuclear reactor core, and this motivated studies on both the physical and chemical possibilities as well as the biological effects of any dispersed radioactivity. Those responsible for nuclear power technology in the West devoted extraordinary effort to ensuring that a meltdown of the reactor core would not take place, since it was assumed that a meltdown of the 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 over 14,500 cumulative reactor-years of commercial operation in 32 countries, there have been only three major accidents to nuclear power plants—Three Mile Island [on March 28, 1979, in Pennsylvania], Chernobyl, [on April 26, 1986, in the Ukraine] and Fukushima [on March 11, 2011, in Japan]—the second being of little relevance to reactor design outside the old Soviet bloc.
An Accident Is Unlikely to Cause Dramatic Harm
It was not until the late 1970s that detailed analyses and large-scale testing, followed by the 1979 meltdown of the Three Mile Island reactor, began to make clear that even the worst possible accident in a conventional western nuclear power plant or its fuel would not be likely to cause dramatic public harm. The industry still works hard to 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 has made that clear, with a triple meltdown causing no fatalities or serious radiation doses to anyone, while over two hundred people continued working on the site to mitigate the accident's effects.
The decades-long test and analysis program showed that less radioactivity escapes from molten fuel than initially assumed, and 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 it has been with at least one of the Fukushima reactors, it is still very effective in preventing escape of most radioactivity....
Of all the accidents and incidents, only the Chernobyl and Fukushima accidents resulted in radiation doses to the public greater than those resulting from the exposure to natural sources. The Fukushima accident resulted in some radiation exposure of workers at the plant, but not such as to threaten their health, unlike Chernobyl. Other incidents (and one "accident") have been completely confined to the plant.
Apart from Chernobyl, no nuclear workers or members of the public have ever died as a result of exposure to radiation due to a commercial nuclear reactor incident. Most of the serious radiological injuries and 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....
It should be emphasised that a commercial-type power reactor simply cannot under any circumstances explode like a nuclear bomb—the fuel is not enriched beyond about 5%.
We Can Learn from Accidents
The International Atomic Energy Agency (IAEA) was set up by the United Nations in 1957. One of its functions was to act as an auditor of world nuclear safety, and this role was increased greatly following the Chernobyl accident. It prescribes safety procedures and the reporting of even minor incidents. Its role has been strengthened since 1996. Every country which operates nuclear power plants has a nuclear safety inspectorate and all of these work closely with the IAEA.
While nuclear power plants are designed to be safe in their operation and 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, will lead to progressive improvement in safety....
The main safety concern has always been the possibility of an uncontrolled release of radioactive material, leading to contamination and 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 core melt. The TMI [Three Mile Island] 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 core under extreme conditions it became evident that even a severe core melt coupled with breach of containment would be unlikely to create a major radiological disaster from many Western reactor designs, but the Fukushima accident showed that this did not apply to all. Studies of the post-accident situation at Three Mile Island (where there was no breach of containment) supported the suggestion, and analysis of Fukushima is pending.
Certainly the matter was severely tested with three reactors of the Fukushima Daiichi nuclear power plant in Japan in March 2011. Cooling was lost after a shutdown, and it proved impossible to restore it sufficiently to prevent severe damage to the fuel. The reactors, dating from 1971-75, were written off. A fourth is also written off due to damage from a hydrogen explosion....
Nuclear Accidents Are Less Severe than Other Industrial Accidents
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 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 to be much less severe than those from other industrial and energy sources. Experience, including Fukushima, bears this out....
The use of nuclear energy for electricity generation can be considered extremely safe. Every year several thousand people die in coal mines to provide this widely used fuel for electricity. There are also significant health and environmental effects arising from fossil fuel use. To date, even the Fukushima accident has caused no deaths, and the IAEA reported on 1 June 2011: "to date, no health effects have been reported in any person as a result of radiation exposure."...
Defence-in-Depth Achieves Maximum Security
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 and construction,
- equipment which prevents operational disturbances or human failures and errors developing into problems,
- comprehensive monitoring and regular testing to detect equipment or operator failures,
- redundant and diverse systems to control damage to the fuel and 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)....
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 design depends only on physical phenomena such as convection, gravity or resistance to 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 design would have averted the Fukushima accident, where loss of electrical power resulted in loss of cooling function.
The basis of design assumes a threat where due to accident or malign intent (e.g. terrorism) there is 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 power plants because the likely damage would be modest.
Nuclear power plants are designed with sensors to shut them down automatically in an earthquake, and this is a vital consideration in many parts of the world....
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 power 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. Full analysis of the accident is pending, but it is likely that the results will include more attention being given to siting criteria and the design of back-up power and cooling, as well as emergency management procedures.
Nuclear plants have Severe Accident Mitigation Guidelines (SAMG, or in Japan: SAG), and most of these, including all those in the USA, address what should be done for accidents beyond design basis, and where several systems may be disabled....
European "Stress Tests" Following the Fukushima Accident
Assessment of the aspects of nuclear plant safety highlighted by the Fukushima accident is being applied to the 143 nuclear reactors in the EU's 27 member states, as well as those in any neighbouring states that have decided to take part. These so-called "stress tests" involved targeted reassessment of each power reactor's safety margins in the light of extreme natural events such as earthquakes and flooding ... as well as on loss of safety functions and severe accident management following any initiating event....
The results of the reassessment will be peer-reviewed and shared among regulators. They may indicate a need for additional technical or organisational safety provisions. WENRA [Western European Nuclear Regulators Association] noted that it remains a national responsibility to take any appropriate measures resulting from the reassessment....
Severe Accident Management
In addition to engineering and procedures which reduce the risk and 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 and with disabled cooling systems following damage done by the tsunami. The experience following that accident is being applied not only in design but also in such guidelines, and peer reviews on nuclear plants will focus more on these than previously.
In mid 2011 the IAEA Incident and Emergency Centre launched a new secure web-based communications platform to unify and simplify information exchange during nuclear or radiological emergencies. The Unified System for Information Exchange on Incidents and Emergencies (USIE) has been under development since 2009 but was actually launched during the emergency response to the accident at Fukushima....
International Collaboration to Improve Safety
There is a great deal of international cooperation on nuclear safety issues, in particular the exchange of operating experience under the auspices of the World Association of Nuclear Operators (WANO) which was set 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; and professional and technical development. WANO peer reviews are the main proactive way of sharing experience and expertise, and by the end 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 to one every four years at each plant, with follow-up visits in between, and the scope extended from operational safety to include plant design upgrades. Pre-startup reviews of new plants are being increased.
The IAEA Convention on Nuclear Safety (CNS) was drawn up during a series of expert level meetings from 1992 to 1994 and was the result of considerable work by Governments, national nuclear safety authorities and the IAEA Secretariat. Its aim is to legally commit participating States operating land-based nuclear power plants to maintain a high level of safety by setting international benchmarks to which States would subscribe....
The Convention entered into force in October 1996. As of September 2009, there were 79 signatories to the Convention, 66 of which are contracting parties, including all countries with operating nuclear power plants....
Problems with Ageing Nuclear Plants
Several issues arise in prolonging the lives of nuclear plants which were originally designed for 30- or 40-year operating lives. Systems, structures and components (SSC) whose characteristics change gradually with time or use are the subject of attention.
Some components simply wear out, corrode or degrade to a low level of efficiency. These need to be replaced. Steam generators are the most prominent and expensive of these, and many have been replaced after about 30 years where the reactor otherwise has the prospect of running for 60 years. This is essentially an economic decision. Lesser components are more straightforward to replace as they age, and some may be safety-related as well as economic. In Candu [Canadian pressurized heavy water] reactors, pressure tube replacement has been undertaken on some older plants, after some 30 years of operation.
A second issue is that of obsolescence. For instance, older reactors have analogue instrument and control systems, and a question must be faced regarding whether these are replaced with digital in a major mid-life overhaul, or simply maintained.
Thirdly, the properties of materials may degrade with age, particularly with heat and neutron irradiation. In some early Russian pressurized water reactors, the pressure vessel is relatively narrow and is thus subject to greater neutron bombardment than a wider one. This raises questions of embrittlement, and has had to be checked carefully before extending licences.
In respect to all these aspects, periodic safety reviews are undertaken on older plants in line with the IAEA safety convention and WANO's [World Association of Nuclear Operators] safety culture principles to ensure that safety margins are maintained....
In the USA most of the more than one hundred reactors are expected to be granted licence extensions from 40 to 60 years. This justifies significant capital expenditure in upgrading systems and components, including building in extra performance margins. There is widespread agreement that further extensions may be justified, and this prospect is driving research on ageing to ensure both safety and reliability in older plants....
Nuclear Plants Can Withstand Acts of Terrorism
Since the World Trade Centre attacks in New York in 2001 there has been concern about the consequences of a large aircraft being used to attack a nuclear facility with the purpose of releasing radioactive materials. Various studies have looked at similar attacks on nuclear power plants. They show that nuclear reactors would be more resistant to such attacks than virtually any other civil installations. A thorough study was undertaken by the US Electric Power Research Institute (EPRI) using specialist consultants and paid for by the US Dept. of Energy. It concludes that US reactor structures "are robust and (would) protect the fuel from impacts of large commercial aircraft"....
Looking at spent fuel storage pools, similar analyses showed no breach. Dry storage and transport casks retained their integrity. "There would be no release of radionuclides to the environment".
Similarly, the massive structures mean that any terrorist attack even inside a plant (which are well defended) and causing loss of cooling, core melting and breach of containment would not result in any significant radioactive releases.
However, while the main structures are robust, the 2001 attacks did lead to increased security requirements and plants were required by NRC [Nuclear Regulatory Commission] to install barriers, bulletproof security stations and other physical modifications which in the USA are estimated by the industry association to have cost some $2 billion across the country....
Nuclear Reactor Operation Is Safer than Generating Coal-Based Power
The designs for nuclear plants being developed for implementation in coming decades contain numerous safety improvements based on operational experience. The first two of these advanced reactors began operating in Japan in 1996.
One major feature they have in common (beyond safety engineering already standard in Western reactors) is passive safety systems, requiring no operator intervention in the event of a major malfunction....
Many occupational accident statistics have been generated over the last 40 years of nuclear reactor operations in the US and UK. These can be compared with those from coal-fired power generation. All show that nuclear is a distinctly safer way to produce electricity.