The safety of nuclear power
Author : Amy Hollamby
28 July 2010
Nuclear power has gained support as the need to meet climate-change commitments by lowering greenhouse gas emissions, and gaining energy independence become increasingly significant political issues. Nuclear power plants can provide low-carbon electricity for as long as 60 years.
The safety of nuclear power
However, the use nuclear power is limited by political and environmental opposition, in addition to widespread public concern regarding safety. In Europe concerns continue to grow regarding the dangers of an excessive reliance on Russian gas supplies so is nuclear power is the answer?
In the history of civil nuclear power there have only been two major reactor accidents - Three Mile Island and Chernobyl. The Three Mile Island, USA, nuclear power plant accident on March 28, 1979 demonstrated the importance of inherent safety features. The accident was attributed to mechanical failure and operator confusion. The reactor’s other protection systems operated as designed. The incident was caused by a cooling system malfunction resulting in a partial meltdown of the reactor core. Some radioactive gas was released a couple of days after the accident, but not enough to cause any dose above background levels to local residents. There were no injuries or adverse health effects. The accident happened at 4am, when the reactor was operating at 97% power. It involved a relatively minor malfunction in the secondary cooling circuit which caused the temperature in the primary coolant to rise. This in turn caused the reactor to shut down automatically. Shut down took about one second. At this point a relief valve failed to close, but instrumentation did not reveal the fact, and so much of the primary coolant drained away so that the residual decay heat in the reactor core was not removed. The core suffered extensive damage as a result. The containment building which housed the reactor further prevented any significant release of radioactivity. The design of the reactor did not require any major design changes but the operators were unable to diagnose or respond properly to the unplanned automatic shutdown. Deficient control room instrumentation and inadequate emergency response training proved to be root causes of the accident and led to a new focus on the human factors in nuclear safety. Public confidence in nuclear energy, particularly in USA, declined sharply following the Three Mile Island accident. It was a major cause of the decline in nuclear construction through the 1980s and 1990s.
In contrast, the Chernobyl reactor did not have a containment structure like those used in the west or post-1980 Soviet designs. The incident at the Chernobyl nuclear power plant in Ukraine, occurred on 26 April 1986 and was due to a flawed reactor design coupled with serious mistakes made by the plant operator. Many believe it was a direct consequence of Cold War isolation and resulting absence of any safety culture. The resulting steam explosion and fires released at least 5% of the radioactive reactor core into the atmosphere and downwind. The plume drifted over large parts of the former Western Soviet Union, Eastern Europe, Western Europe, and Northern Europe. Large areas in Ukraine, Belarus, and Russia had to be evacuated with over 336,000 people resettled.
Two Chernobyl plant workers died on the night of the accident, and 28 further people died within weeks as a result of acute radiation poisoning. However, many more died much later due to cancer or other complications.
Since then, Chernobyl has been considered the worst tragedy in human history and is classified as the seventh level incident in the "International Nuclear Event Scale". Three Mile Island rated five.
Both Chernobyl and the Three Mile Island incident highlight what happens when there is an accident at a nuclear power plant and explains the unsurprisingly large amount of opposition to the use of nuclear power, especially considering the number of other renewable forms of energy on offer which are not linked to such extreme hazards. However, are these fears founded? It is important to remember that Chernobyl and Three Mile Island are the only major accidents to have occurred in some 14,000 cumulative reactor-years of commercial operation in 32 countries. Of all these accidents and incidents, only Chernobyl resulted in radiation doses to the public greater that those resulting from the exposure to natural sources. Other incidents have been completely confined to the plant. Furthermore, 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 reactor incident.
Hazards associated with nuclear power:
Hazards associated with nuclear power can be either due to human and/or mechanical factors.
Nuclear material may be hazardous if not properly handled or disposed of. Even when properly contained, fission byproducts which are no longer useful generate radioactive waste, which must be properly disposed of. In addition, material exposed to neutron radiation—present in nuclear reactors—may become radioactive in its own right, or become contaminated with nuclear waste. Furthermore, toxic or dangerous chemicals may be used as part of the plant's operation, which must be properly handled and disposed of.
Nuclear material can be unstable and possibly generate unexpected behavior, resulting in an uncontrolled power excursion 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. Experience has proved otherwise in any circumstances relevant to Western reactor designs. In the light of better understanding of the material in a reactor core under extreme conditions it became evident that even a severe core melt coupled with breach of containment could not in fact create a major radiological disaster from any Western reactor design. Studies of the post-accident situation at Three Mile Island supported this.
The amount of heat generated in a nuclear reactor can be huge causing immense pressure to build up in the vessel, resulting in a steam explosion, which happened at Chernobyl. However, the reactor design used at Chernobyl was peculiar in many ways. It utilised a positive void coefficient, meaning a coolant failure could cause a strong increase in power output from the fission process. All reactors built outside the former Soviet Union have had negative void coefficients, a passively safe design. More importantly though, the Chernobyl plant lacked a containment structure. Western reactors have this structure, which acts to contain radiation in the event of a failure.
Since the World Trade Centre attacks in New York in 2001 there has also been concern about the consequences of a large aircraft being used to attack a nuclear facility with the purpose of releasing radioactive materials. However, 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.
Those responsible for nuclear power technology in the West have devoted extraordinary effort to ensure 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 fatalities. In avoiding such accidents the industry has been outstandingly successful.
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, highlighted that even the worst possible accident in a conventional western nuclear power plant or its fuel could not cause serious harm to the public. The industry still works hard to minimise the probability of a meltdown accident, but it is now clear that no-one need fear a potential public health catastrophe.
Tests have shown that less radioactivity escapes from molten fuel than initially assumed, and that this radioactive material is not readily mobilised beyond the immediate internal structure. Thus, even if the containment structure that surrounds all modern nuclear plants were ruptured, it would still be highly effective in preventing escape of radioactivity.
Operational safety is a prime concern for those working in nuclear plants. Radiation doses are controlled by the use of remote handling equipment for many operations in the 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 to ensure very low radiation exposure compared with other industries.
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, such as ‘The China Syndrome’, in the public domain and also some solid conservative engineering including containment structures (at least in Western reactor designs) in the industry itself. Licensing regulations were framed accordingly.
The safety of nuclear power
One mandated safety indicator is the calculated probable frequency of degraded core or core melt accidents. The US Nuclear Regulatory Commission (NRC) specifies that reactor designs must meet a 1 in 10,000 year core damage frequency, but modern designs exceed this. The best currently operating plants are about 1 in 1 million and those likely to be built in the next decade are almost 1 in 10 million.
Regulatory requirements today are that the effects of any core-melt accident must be confined to the plant itself, without the need to evacuate nearby residents.
Achieving nuclear safety
To achieve optimum safety, nuclear plants in the West 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).
The safety provisions include a series of physical barriers between the radioactive reactor core and the environment, the provision of multiple safety systems, each with backup and designed to accommodate human error.
The barriers in a typical plant are: the fuel is in the form of solid ceramic 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 form fuel rods. These are confined inside a large steel pressure vessel with walls up to 30 cm 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.
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, and the containment structure by periodically measuring the leak rate of air at about five times atmospheric pressure.
The three basic safety functions in a nuclear reactor are: to control reactivity, to cool the fuel and to contain radioactive substances.
The main safety features of most reactors are inherent - negative temperature coefficient (as the temperature increases the efficiency of the reaction decreases) and negative void coefficient. Beyond the control rods which are inserted to absorb neutrons and regulate the fission process, the main engineered safety provisions are the back-up emergency core cooling system (ECCS) to remove excess heat (though it is more to prevent damage to the plant than for public safety) and 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, eg 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.
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. 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.
The main 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.
These designs are one or two orders of magnitude safer than older ones in respect to the likelihood of core melt accidents, but the significance of that is more for the owner than the neighbours, who - as Three Mile Island showed - are safe also with older types.
The safety of nuclear power
All of this evidence shows that although the safety of nuclear power is often under question, accident statistics are extremely low, especially compared to the use of other fossil fuels for energy. Many safety systems are in place to reduce the risk of a meltdown accident, and even if one should occur, regulatory requirements state that the effects must be confined to the plant itself, without the need to evacuate nearby residents. The option of nuclear power is currently being discussed in the UK.
The use of nuclear power in the UK is likely to undergo some changes following the formation of the new coalition government and the surprising appointment of Chris Huhne as Britain’s energy and climate minister by David Cameron. Nuclear energy companies are likely to be unnerved by this choice given fervent Liberal Democrat opposition to a cornerstone of Conservative energy policy: ten new nuclear power stations in the next couple of decades to ensure security of supply and a zero-carbon source of electricity. Huhne is sceptical about nuclear power but is interested in supporting the development of renewable energy.
The energy companies planning nuclear investments have put on a brave face, determined to press ahead with tens of billions of pounds of investment in new nuclear power stations, believing that combined Tory and Labour support will be enough to see legislation on new nuclear power stations pushed through parliament. Plus, both Nick Clegg and Chris Huhne have confirmed that their opposition to nuclear is not theological, but based on the excessive costs of the new build, hinting that they could be willing to compromise.
Both parties agree not to award the industry any direct subsidies. This could still leave the door open for the Tories’plan to boost nuclear investment by artificially raising the price of carbon allowances traded on the EU emissions market.
The full coalition agreement said the Liberal Democrats would continue to argue against more nuclear power stations from within the coalition, but would agree to abstain on votes over a new national energy planning statement. The agreement specifically sets out that Lib Dem opposition to nuclear power would not be taken as vote of no confidence in the coalition.
The agreement says that a Lib Dem spokesperson will argue against the policy statement when debated in the House of Commons. Lib Dem MPs will abstain, which is likely to let the policy through with the support of Labour MPs.
However, the agreement is careful to make it clear that the new build programme should only be extended to the replacement of the existing, ageing fleet of power stations, and that each one will be subject to the normal planning process. The document also commits the coalition to developing a “smart” electricity grid, funding carbon capture and storage projects, and developing offshore wind generation
However, there remains a sizeable threat that the Liberals could force a time-consuming and costly public inquiry that delays the new build. The idea that Chris Huhne will have to formulate regulatory policy and set out a timetable for nuclear is likely to be a considerable worry. Most destabilising is the fact that policy will probably not be clear for some time, for Huhne is going to have to square his Department’s theoretical support for nuclear with his own views (in a previous speech on energy) that:
“No private sector investor has built a nuclear power station anywhere in the world without lashings of government subsidy since Three Mile Island and Chernobyl. The World Bank refuses to lend on nuclear projects because of the long history of overruns. Our message is clear, No to nuclear, as it is not a short cut, but a dead end.”
To conclude, Increasing energy needs, fluctuating oil and gas prices, increased competition for dwindling supplies, and rising concerns about global warming are all encouraging countries to reconsider nuclear energy. Accidents related to coal and oil production such as Britain’s Piper Alpha disaster in 1988, which resulted in 167 lives being lost, and more recently the BP Deepwater Horizon oil spill continue to cause significant detrimental health and environmental effects, and perhaps highlight that nuclear power is the safer option.