Safety of Nuclear Power Reactors

Safety of Nuclear Power Reactors Nuclear Issues Briefing Paper 14 November 2003

  • From the outset, there has been a strong awareness of the potential hazard of both nuclear criticality and release of radioactive materials.
  • There have been two major reactor accidents in the history of civil nuclear power - Three Mile Island and Chernobyl. One was contained and the other had no provision for containment.
  • These are the only major accidents to have occurred in over 10 000 cumulative reactor-years of commercial operation in 32 countries.
  • The risks from western nuclear power plants, in terms of the likelihood and consequences of an accident or terrorist attack, are minimal compared with other commonly accepted risks.
  • The operation of many nuclear reactors in the former Eastern Bloc is of international concern, and a program of international assistance is helping to improve their safety.
  • There have been two major accidents in the history of civil nuclear power generation;

  • Three Mile Island (USA 1979) where the reactor was severely damaged but radiation was contained and there were no adverse health or environmental consequences
  • Chernobyl (Ukraine 1986) where the destruction of the reactor by explosion and fire killed 31 people and had significant health and environmental consequences. A table showing all reactor accidents, and a table listing some energy-related accidents with multiple fatalities are appended.

    These two significant accidents occurred during more than 10,000 reactor-years of civil operation. Only the Chernobyl accident resulted in loss of life or radiation doses to the public greater than those resulting from the exposure to natural sources. Other incidents (and one 'accident') have been completely confined to the plant. (There have also been a number of accidents in experimental reactors and 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.)

    It should be emphasised that a commercial-type power reactor simply cannot under any circumstances explode like a nuclear bomb.

    The International Atomic Energy Agency (IAEA) was set up by the United Nations in 1957. One function was to act as an auditor of world nuclear safety. It prescribes safety procedures and the reporting of even minor incidents. Its role has been strengthened in the last decade. Every country which operates nuclear power plants has a nuclear safety inspectorate and all of these work closely with the IAEA.

    Safety is also 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.

    One mandated safety indicator is the calculated 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. US utility requirements are 1 in 100,000, 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. The Three Mile Island accident in 1979 was the only one in a reactor conforming to NRC safety criteria, and this was contained as designed, without radiological harm to anyone. ]

    Regulatory requirements today are that any core-melt accident must be confined to the plant itself, without the need to evacuate nearby residents.

    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. At Chernobyl this tragically happened and the results were severe, once and for all vindicating the extra expense involved in designing to high safety standards.

    The use of nuclear energy for electricity generation can be considered extremely safe. Every year over one 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.

    Achieving optimum nuclear safety: Western reactors

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

  • high-quality design & construction
  • equipment which prevents operational disturbances developing into problems
  • redundant and diverse systems to detect problems, control damage to the fuel and prevent significant radioactive releases
  • provision to confine the effects of severe fuel damage to the plant itself. The safety systems 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. Safety systems account for about one quarter of the capital cost of such reactors.
  • These include control rods which are inserted to absorb neutrons and regulate the fission process, and the back-up cooling systems to remove excess heat. In addition, most reactors are designed with an inherent feature called a negative void coefficient. This means that beyond an optimal level, as the temperature increases the efficiency of the reaction decreases (especially if any steam has formed in the cooling water). This is due to a decrease in moderating effect so that fewer neutrons are able to cause fission and the reaction slows down automatically. Other physical features also enhance safety. For instance, in a typical reactor the fuel is in the form of solid ceramic (UO2) pellets, and radioactive fission products remain bound inside these pellets as the fuel is burned. The pellets are packed inside zirconium alloy tubes to form fuel rods. These are confined inside a large steel pressure vessel with walls about 20 cm thick, which, in turn, is enclosed inside a robust concrete containment structure with walls at least one metre thick.

    Nuclear power plants are designed to shut down automatically in an earthquake, and this is a vital consideration in many parts of the world. (see paper on Earthquakes) The Three Mile Island accident in 1979 demonstrated the importance of such systems. The containment building which housed the reactor prevented any significant release of radioactivity, despite the fact that about half of the reactor core melted. The accident was attributed to mechanical failure and operator confusion. The reactor's other protection systems also functioned as designed. The emergency core cooling system would have prevented the accident but for the intervention of the operators.

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

    By way of contrast, the Chernobyl reactor did not have a containment structure like those used in the West or in post-1980 Soviet designs.

    Since the World Trade Centre attacks in New York 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 - see Appendix 3.. The latest and most thorough study was undertaken by the Electric Power Research Institute using specialist consultants and partly funded 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".

    The analyses used a fully-fuelled Boeing 767-400 of over 200 tonnes as the basis, at 560 km/h - the maximum speed for precision flying near the ground. The wingspan is greater than the diameter of reactor containment buildings and the 4.3 tonne engines are 15 metres apart. Hence analyses focused on single engine direct impact on the centreline and on the impact of the entire aircraft if the fuselage hit the centreline (in which case the engines would ricochet off the sides). In each case no part of the aircraft or its fuel would penetrate the containment. 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".

    Switzerland's Nuclear Safety Inspectorate studied a similar scenario and reported in 2003 that the danger of any radiation release from such a crash would be low for the older plants and extremely low for the newer ones.

    A different safety philosophy: Early Soviet-designed reactors The April 1986 disaster at the Chernobyl nuclear power plant in the Ukraine was the result of major design 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 to a strong increase in power output from the fission process ( positive void coefficient). However, this was not the prime cause of the Chernobyl accident. The accident destroyed the reactor, killed 31 people, 28 of whom died within weeks from radiation exposure. It also caused radiation sickness in a further 200-300 staff and firefighters, and contaminated large areas of Belarus, Ukraine, Russia and beyond. It is estimated that at least 5% of the total radioactive material in the Chernobyl-4 reactor core was released from the plant, due to 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) and are being closely monitored. About 800 cases of thyroid cancer in children have been linked to the accident. Most of these were curable, though about ten have been fatal. No increase in leukaemia or other cancers have yet shown up, but some is expected. The World Health Organisation 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 that radiation-related fatalities occurred. The destroyed unit 4 was enclosed in a concrete shed ("sarcophagus"), which now requires remedial work.

    An OECD expert report on it concluded that "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 reactors in OECD Member countries." International efforts to improve safety

    The IAEA has given a high priority to addressing the safety of nuclear power plants in eastern Europe, where deficiencies remain. However, energy demand in these countries is such that there is little flexibility for closing even those plants which are of most concern, though the European Union is bringing pressure to bear, particularly in countries which aspire to EU membership.

    A major international program of assistance has been carried out by the OECD, IAEA and Commission of the European Communities to bring early Soviet-designed reactors up to near western safety standards, or at least to effect significant improvements to the plants and their operation.

    Modifications have been made to overcome deficiencies in the 13 RBMK reactors still operating in Russia and Lithuania. Among other things, these have removed the danger of a positive void coefficient response. Automated inspection equipment has also been installed in these reactors. cf briefing paper # 56 supplement.

    The other class of reactors which has been the focus of international attention for safety upgrades is the first-generation of pressurised water VVER-440/230 reactors. These were designed before formal safety standards were issued in the Soviet Union and they lack many basic safety features. Eleven are operating in Bulgaria, Russia, Slovakia and Armenia, under close inspection.

    Later Soviet-designed reactors are very much safer and the most recent ones have Western control systems or the equivalent, along with containment structures. There is a great deal of international cooperation on nuclear safety issues, in particular the exchange of operating experience.

    In 1996 the Nuclear Safety Convention came into force. It is the first international legal instrument on the safety of nuclear power plants worldwide. It commits participating countries to maintain a high level of safety by setting international benchmarks to which they subscribe and against which they report. It has 65 signatories and has been ratified by 41 states. Reporting nuclear incidents The International Nuclear Event Scale (INES) was developed by the IAEA and OECD in 1990 to communicate and standardise the reporting of nuclear incidents or accidents to the public. The scale runs from a zero event with no safety significance to 7 for a "major accident" such as Chernobyl. Three Mile Island rated 5, as an "accident with off-site risks" though no harm to anyone, and a level 4 "accident mainly in installation" occurred in France in 1980, with little drama. Another accident rated at level 4 occurred in a fuel processing plant in Japan in September 1999. See INES table. Other accidents have been in military plants. Advanced reactor designs

    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.

    Safety relative to other energy sources

    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. Two simple sets of figures are quoted in the Table below and that in the appendix. A major reason for coal's unfavourable showing is the huge amount which must be mined and transported to supply even a single large power station. Mining and multiple handling of so much material of any kind involves hazards, and these are reflected in the statistics.

    Comparison of accident statistics in primary energy production. (Electricity generation accounts for about 40% of total primary energy). Fuel Immediate fatalities 1970-92 Who? Normalised to deaths per TWy* electricity Coal 6400 workers 342 Natural gas 1200 workers & public 85 Hydro 4000 public 883 Nuclear 31 workers 8 *Basis: per million MWe operating for one year, not including plant construction, based on historic data which is unlikely to represent current safety levels in any of the industries concerned.

    Source: Ball, Roberts & Simpson, Research Report #20, Centre for Environmental & Risk Management, University of East Anglia, 1994; Hirschberg et al, Paul Scherrer Institut, 1996; in: IAEA, Sustainable Development and Nuclear Power, 1997; Severe Accidents in the Energy Sector, Paul Scherrer Institut, 2001). SOURCES IAEA, 1993, IAEA Yearbook 1993 ANSTO, 1994, The Safety of Nuclear Power Reactors, Nuclear Services Section Background Paper Nuclear Energy Institute, Source Book, 1995 OECD NEA, 1995, Chernobyl Ten Years On. Nuclear Engineering International, August 1999 Twilley R C 2002, Framatome ANP's SWR1000 reactor design, Nuclear News, Sept. EPRI Dec 2002 report on NEI web site - For further information Return to Index Uranium Information Centre Ltd A.C.N. 005 503 828 GPO Box 1649N, Melbourne 3001, Australia phone (03) 9629 7744 fax (03) 9629 7207