Safety of Nuclear Power Reactors - Nuclear Issues Briefing Paper 14

Published December 5th, 2000 - 02:00 GMT

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 more than 9000 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, are negligible 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 greatly improving the safety of these reactors.  


There have been two major accidents in the history of civil nuclear power generation;  


Three Mile Island, United States in 1979, where the reactor was severely damaged but radiation was contained and there were no adverse health or environmental consequences.  


Chernobyl, Ukraine in 1986, where the destruction of the reactor by a steam explosion and fire killed 31 people and had significant health and environmental consequences.  


These two accidents have occurred during more than 9000 reactor-years of civil operation. Only the Chernobyl accident has resulted in radiation doses to the public greater than those resulting from the exposure to natural sources, or in loss of life.  


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 has resulted in loss of life outside the actual plant, or long-term environmental contamination.)  


It should be emphasized 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 and one function was as an auditor of world nuclear safety. 


It prescribes safety procedures and the reporting of even minor incidents, and its role has been strengthened in the last decade. Each country has a nuclear safety inspectorate which works in collaboration with the IAEA.  


By any comparison, nuclear energy applied to electricity generation is extremely safe. There are over one thousand deaths each year in coal mines, mostly providing fuel for the main alternative to nuclear energy. There are also health effects from the actual use of fossil fuels.  


Achieving optimum nuclear safety: Western Reactors:  

To achieve optimum safety, nuclear plants in the western world operate with a 'safety-in-depth' approach, with multiple safety systems. These 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.  


As well as the control rods which are inserted to absorb neutrons and the back-up cooling systems to remove excess heat, 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 less neutrons cause fission, hence 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. The radioactive fission products remain bound inside the fuel pellets. The pellets are inside zirconium alloy tubes, forming fuel rods.  


The rods are inside a large steel pressure vessel with walls about 20 cm thick. This in turn is enclosed inside a robust concrete containment structure with walls a meter or more thick.  


The Three Mile Island accident in 1979 demonstrated the importance of such systems. The containment building which housed the reactor prevented 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, and the emergency core cooling system would have prevented the accident but for the intervention of the operators.  


Investigations following the accident resulted in 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 adopted in the West and in post-1980 Soviet designs.  


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 is such that coolant failure leads to a strong increase in power output from the fission process, though that 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), 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 about 3 to 4 percent 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 leukemia 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 that radiation-related fatalities occurred. The destroyed unit 4 was enclosed in a concrete shed ("sarcophagus"), which now requires remedial work.  


Safety deficiencies remain in the other reactors at Chernobyl, though two have since shut down. However, energy shortages in the Ukraine necessitate the continued operation of the remaining one (along with 12 others, which supply one third of the country's electricity).  


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 the former Eastern Bloc. Current 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.  


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, Ukraine and Lithuania. Automated inspection equipment has also been installed in these reactors. 


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


Later Soviet-designed reactors are very much safer than the RBMK and early VVER types mentioned above, and the most recent ones have Western control systems.  


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.  


It should be noted that a 1996 study was done on the likely effect on Australia of the hypothetical catastrophic failure of a Chernobyl-type nuclear reactor in Indonesia. The study appears to assume massive release of radioactive contaminants, unprecedented for the kinds of reactors envisaged.  


Reporting nuclear incidents:  

The International Nuclear Event Scale (INES) was developed by the IAEA and OECD in 1990 to communicate and standardize 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", and a level 4 "accident mainly in installation" occurred in France in 1980, with little drama. Another accident provisionally rated at level 4 occurred in a fuel processing plant in Japan in September 1999.  


Advanced reactor designs:  

The designs for nuclear plants being developed for implementation next century 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 major malfunctions.  

Note: originally published on October 1999 


© 2000 Mena Report (

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