Table of Contents
The Energy Challenge
Nuclear Power Facts
Nuclear Power Advantages
The Salient Points
Annex I: The DECADES Project
Annex II: Nuclear Power Case Studies
The safety concept
Nuclear power plants are generally built to high safety standards. Nevertheless, there have been two serious accidents. The first occurred in 1979 at Three Mile Island (TMI) in a widely used reactor type, resulting in serious reactor core damage, but inconsequential environmental releases. The second occurred seven years later in 1986 at Chernobyl in a unique reactor type used only in the former Soviet Union, resulting in serious environmental consequences.
Many lessons have been learned from these two events. The Chernobyl accident brought out a failing in the graphite reactor design which permitted a rapid power escalation under an abnormal operational condition. The loss of coolant water flow that occurred did not lead to an immediate automatic shutdown, which is a requirement for other reactor types. Most importantly, the environmental consequences of the Chernobyl accident compared with the negligible consequences of the TMI accident confirmed the importance of the principal reactor safety concept that incorporates three protection barriers to prevent environmental radioactive releases [Fig.: Protective Barriers].
The first protection barrier, the ceramic fuel and its cladding, retains the radioactive products of the nuclear fission process. The second, the strong metallic primary circuit consisting of the reactor vessel and connecting pipes, retains radioactive material released in the event of fuel damage. The final and ultimate barrier, typified by a large cylindrical containment structure of pre-stressed concrete enclosing the reactor primary system - many with inside steel liners and some with a double walled concept as used in the large number of French standardized plants - retains radioactive material that could be released from a primary circuit failure. It was the lack of a sufficient containment barrier in the Chernobyl design which led to the serious environmental consequences.
Containment designs also consider external events. Experiments to simulate direct hits from jet aircraft, carried out with high speed projectiles fired into walls of concrete and steel, demonstrate little damage. Containment damage from all postulated severe accidents would be in the form of structural cracks allowing only minimal environmental releases.
The impact of Chernobyl
The consequences of the disastrous Chernobyl accident remain a focus of concern. Some 6% of the radioactive contents of the reactor core was released into the atmosphere, with radioactive iodine and caesium of greatest relevance to human health.
The accident resulted in 31 short term deaths, with 28 due to extremely high radiation exposures. An additional 106 people experienced serious radiation effects. Some 200 000 workers, known as liquidators, involved in cleanup activities during 1986 and 1987, received average exposures of twice the yearly permitted occupational exposure and similar to the exposure received every year by individuals in high radon areas of Europe. A few thousand received more than ten times the permitted occupational exposure and several dozen workers received exposures considerably higher. The total number of listed liquidators eventually rose to more than 600 000, with most of the additional individuals receiving limited exposures.
Of the some 116 000 inhabitants evacuated from the 30 km exclusion zone around the Chernobyl site, 95% received less than the average for the initial group of liquidators. The more than 400 000 residents living in areas that were classified as strict control zones received significantly less than that, their exposure occurring principally during the early months following the accident.
For the 1 116 000 total individuals in the three major groups (600 000 liquidators, 116 000 exclusion zone evacuees and 400 000 residents of strict control zones) who received by far the highest exposure from the Chernobyl releases, the predicted long term radiation induced cancer deaths and normally non-fatal thyroid cancers are reported in the proceedings of a 1996 international conference co-sponsored by the IAEA, WHO and the European Union. The report projects some 3 500 radiation induced cancer deaths, mainly late in life, in addition to some 200 000 anticipated cancer deaths from other sources - somewhat more than a 0.3% increase in the cancer death rate. The estimate is consistent with the atomic bomb survivor studies, which project a 0.7% increase for the survivors who received a larger as well as a more harmful rapid radiation exposure.
The single radiation related health impact that has been observed to date is a sharp increase in thyroid cancers among children exposed to short lived radioactive iodine. Some 800 cases in children under 15, three of which were fatal, were documented by 1996, with the total incidence of this treatable illness projected to rise to several thousand. There is no evidence to date of an increased incidence of other malignancies including leukaemia, the most sensitive indicator of radiation induced effects (Table I).
LONG TERM HEALTH EFFECTS OF CHERNOBYL
Number of people Cancer deaths from other causes Cancer deaths from radiation (increase) Emergency workers 1 000 180 20 (2.0%) Liquidators (nationwide) 650 000 90 000 2 000 (0.3%) Evacuated 1986 115 000 17 000 400 (0.3%) Infants 1986 (up to 4 years) 1 000 000 a b
a - Fifty thyroid cancers (treatable).
b - Several thousand thyroid cancers (treatable).
Numerous reports of cancer deaths of people living in the contaminated zones of Belarus, the Russian Federation and Ukraine and among liquidators have not been substantiated. Significant mental health disorders could be a consequence of the accident's broad and severe psychological, economic and social impact. The effects of measures intended to limit radiation exposure, causing lifestyle changes through resettlements, changes in food supplies and restrictions in activities, were compounded by a generally deteriorating larger economic and social environment.
There were short term environmental impacts, including lethal exposures to coniferous trees and some small mammals within a 10 km zone around the reactor site. The natural environment had begun to recover visibly by 1989 and sustained impacts on ecosystems have not been observed. An evaluation of long term hereditary effects in plants or animals will take many more years. No statistically meaningful hereditary effects have ever been observed as a result of human exposure to significant levels of radiation.
Some nuclear power facts
How likely is another serious environmental release? There are currently 15 Chernobyl type nuclear power plants that have operated on average for about 17 years each. Although some may be shut down early, others may operate at least through their 30 year design life. More plants of this type are not expected to be built. With the exception of some of the early Soviet designed units, the remaining 420 nuclear power plants in the world have structural containments around the principal reactor primary system components. There has been a large ongoing global co-operative effort to improve the safety of all operating Soviet designed plants that has included significant modernization of instrumentation and equipment.
There are already more than 8 000 reactor-years of accumulated operational experience worldwide, equivalent to an average of 20 years of operation for each nuclear power unit. Building on this large experience base, today's reactors incorporate improved safety measures and are designed to rule out an environmental release in the case of a severe accident. Designers believe the newest plants would suffer no more than one severe core damage accident in 100 000 reactor-years of operation and this without a subsequent environmental release.
Advanced nuclear power plants with an even smaller severe accident possibility are under development. The full spectrum of advanced designs ranges from evolutionary types with enhanced safety features to entirely new designs which introduce innovative safety concepts. The new concepts include passive - sometimes referred to as inherent - safety features based on natural convection coolant flow, making safety less dependent on active components such as pumps and valves and on human performance. A high temperature helium gas cooled reactor with a unique fuel design has been developed and operated. It employs spherical fuel particles coated with layers of ceramic that remain intact and retain virtually all fission products at temperatures as high as 1600 °C. Any fuel failure during severe accident conditions would be gradual and a rapid release of fission products would not occur (Box 5).
SOME ADVANCED DESIGN CHARACTERISTICS
- Evolutionary - large size [1 400 MW(e)], improved reliability, enhanced safety features
- Smaller and simpler - medium size [600 MW(e)], simplified systems, passive safety features
- Modular gas cooled - variable size [200-400 MW(e)], helium coolant, inherent fuel safety
- High temperature gas cooled - large size [1 000 MW(e)], high efficiency, helium coolant, inherent fuel safety
- Innovative - new concepts, passive safety features
Global nuclear safety culture
Over the years a global nuclear safety culture has evolved through international collaborative efforts to strengthen safety worldwide. Binding international agreements, codes of practice, non-binding safety standards and guides along with international review and advisory services now exist.
A Convention on Nuclear Safety entered into force in October 1996. At the first Meeting of the Parties to take place in April 1999, national safety reports covering civil nuclear power operations will be examined and a summary report of findings made available. The recent updating of the international regime for civil liability for nuclear damage that includes a Convention on Supplementary Funding along with the new Joint Convention on the Safety of Spent Fuel Management and on the Safety of Radioactive Waste Management is further evidence of the growing infrastructure of legal and other commitments that bind countries in nuclear safety matters (Box 6).
Nevertheless, the most convincing demonstration of the adequacy of the global nuclear safety culture will be the performance of existing plants and the avoidance of any major safety event in the future. Through the activities of national regulatory bodies, the World Association of Nuclear Operators (WANO) and the numerous national and international utility organizations, the nuclear industry's efforts to promote safety are closely scrutinized. The IAEA has developed a broad range of well used safety services that allow international experts to review and advise on safety matters.
BINDING INTERNATIONAL AGREEMENTS
- Civil liability for nuclear damage (1963) as amended (1997)
- Physical protection of nuclear materials (1980)
- Early notification of a nuclear accident (1986)
- Assistance in the case of a nuclear accident or radiological emergency (1986)
- Nuclear safety (1996)
- Safety of spent fuel management and safety of radioactive waste management (1997)
Beyond doubt the Chernobyl accident was a severe accident in all its dimensions. For comparative purposes a review of other large energy related as well as other industrial accidents is needed. While the perception of nuclear accidents may not change, such a review provides some perspective. In the industrial sector, the well known 1984 Bhopal accident at a chemical plant in India caused some 3000 early deaths and several hundred thousand severe health effects.
In the energy sector, dam failures and overtopping have caused thousands of deaths and massive disruption in social and economic activities with the displacement of entire towns - the Varont dam overtopping in Italy and dam failures in Gujarat and Orissa in India are three such examples, each with several thousand fatalities. Severe coal mine accidents causing several hundred deaths are not rare. Explosions and major fires in the oil and gas industry have involved both occupational and public fatalities and injuries. A pipeline gas leak explosion in the Urals involved 500 fatalities. Energy sector accidents have also led to severe environmental damage, such as the 1989 Exxon Valdez oil tanker accident in Alaska.
If risk assessments considered only short term severe accident fatalities (Table 2), the reported data would indicate that hydroelectric and gas fuel cycles have led to the largest single event fatality numbers. However, to draw conclusions about the relative safety of the various energy systems, fatalities and morbidity - occupational as well as public - over the longer term must be considered. This is being discussed. Equally important are the maturity of the technology, the quality and maintenance of equipment and the safety and environmental controls.
SHORT TERM FATALITIES (1970 - 1992)
Fatalities Average fatalities Events Range Total per GW(e).a Coala 133 5-434 6 418 0.32 Oil 295 5-500 10 273 0.36 Natural gas 88 5-425 1 200 0.09 Liquid propane gas 77 5-100 2 292 3.1 Hydro 13 10-2500 4 015 0.8 Nuclear 1 31 31 0.01
a - The total is some 10 times higher if accidents with less than 5 fatalities are included.