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Nuclear safety and radioactive waste management
How safe is nuclear energy?
M. A. Quaiyum
The use of nuclear fission i.e. splitting the atom for the peaceful purposes, notably for power generation was started in the 1950s. Today, the world produces as much electricity from nuclear energy as it did from all sources combined in 1960. Civil nuclear power can now boast over 13,000 reactor years of operation experience and supplies almost 16 per cent of global electricity needs, in 30 countries. Contributions of different primary sources of energy are shown at “World Electricity”. Safety has always been an important consideration from the very beginning of the development of nuclear reactors. With such cautious approach to safety, it is no wonder that the operation of reactors to date has an impressive track record. It is also true that this glamour of high profile story of nuclear industry has been tarnished to a great extent due to the accidents at Three Mile Island(TMI-2) in 1979 and Chernobyl in 1986. Against this background, the question of radiation and nuclear safety have been brought before a wider audience in the international community. The Chernobyl accident has, in fact, added an entirely new dimension to the issue of safety. On the other hand, the said accidents have opened up new eyes to the scientists, engineers and technocrats who have been facing the challenges of a totally new and powerful technology. As a result, they had ample opportunity to have a deeper insight into the problem as a whole and the world nuclear community at large felt the need of establishing greater international co-operation on safety. Hence, there were major developments in the worldwide harmonization in nuclear and radiation safety. The United Nations prepared new International Basic Safety Standards for Protection Against Ionizing Radiation and for the Safety of Radiation Sources. It is reported that this trend towards greater harmonization of safety criteria seems likely to continue. The present paper discusses, in some detail, the current practices in connection with safety and radioactive waste management in normal operation of nuclear reactors and also the different ways of mitigating accidental conditions, and touches upon safety research for future reactors.
2. EFFECTS OF RADIATION ON HUMAN BODY: The human body has developed an instinctive reaction to heat, light and to some extent to ultraviolet radiation. Unfortunately the same instinct does not work against ionizing radiation. As a matter of fact, the nature of ionizing radiation is such that one cannot see, feel, sense or identify the same without the aid of any scientific instruments. It causes harm silently without any notice. Consequently, severe radiation damage may be suffered without any realization at the time on the part of the exposed subject. The nature and the extent of the symptoms that develop vary with the individual. They depend on the type of radiation, on the depth to which the radiation has penetrated, on the extent of the body exposed, on the amount of radiation absorbed, and also upon whether the exposure was chronic, i.e, repeated or prolonged so as to lead to a cumulative effect, or acute, i.e., received in one large dose. The changes produced by radiation on the body are of two types, namely somatic and genetic effects. Somatic effects are those experienced directly by the exposed individual; on the other hand, the genetic effects are not evident in the irradiated person but become apparent in subsequent generations. The consequences of somatic effects may be further divided into two categories. There are first, the early effects that start to be felt within a short time, e.g., from a few minutes to several hours after exposure. Then there may be late or delayed effects of radiation which do not appear until months or even years later. There is no threshold of radiation dose for such effects. The risk is, however, age dependent. The children are more prone to risk than the adult. The fetus in particular is very sensitive to radiation.
3.SAFETY PRECAUTIONS: All work with radioactive substances, from the use of isotopes as tracers, nuclear reactor operation to the large- scale processing of spent reactor fuel involves some danger and hence requires that adequate safety precautions be taken. In order to make a power reactor safe enough to be relied upon as a source of energy special considerations are given to, among others, its design, operation, site selection, implementation and manpower training. Radioactive fission products have to pass four barriers, namely the fuel, fuel can, pressure vessel/pressure tube and finally the containment before they become harmful to the operating personnel and the population in the vicinity of the power plant. After the fuel has been in the reactor for a certain period, much of the uranium has already fissioned and as such considerable amount of fission products have built up in the fuel. These spent fuels are highly radioactive. They have to be cooled by putting in water and then they may be stored or reprocessed. In the latter case, any left over uranium and the plutonium that has been formed are removed. If every system works as it is designed to do, the operation of a reactor is very boring mundane job. Problem arises only when something goes wrong and system behaves in an unwarranted and unpredicted way. It may also be appreciated that a nuclear power plant is indeed a complex industrial facility, the majority of the tasks of which are performed by machines. Human operators are, however, involved to a great extent in their design, installation, testing, maintenance and operation. The performance of a person working within a complex mechanical system depends on that person’s capabilities, limitations and attitudes, as well as on the quality of instructions and training provided to him. The interface between a machine and its operators in any industrial project is usually known as human factor. Several human factors that contribute to the unwarranted situations mentioned above have now been identified. Changes have, accordingly, been made in providing training so that such situation or complexity that is popularly known as accident can be managed by human factor in a better way.
4.TMI ACCIDENT: It is well known that the nuclear industry owes much of its design, operation and maintenance philosophy to the fossil fuel industry. An event like TMI-2 reminds, however, everyone that a nuclear power plant is much more complex machine than any other power plant. Immediately after TMI-2 accident, early human factors work has, in fact, focused on ways to manage this complexity. The following changes are incorporated:
4.1. Training now accompanies all regular and emergency preparedness tasks. Through the use of computer simulators, training is made to be as realistic as possible.
4.2. Procedures for both normal and emergency operations were updated to be more technically accurate, better defined, and entirely comprehensible. For tasks performed infrequently and for emergencies, procedures were expanded in detail and clarity.
4.3. Control room design and layout modifications have been carried out in existing plants to reduce the probability of design-induced error. Such improvements can lead to the prevention of accidents or better management of accidents if they occur, although care must also be taken to avoid causing human errors by changing layouts or designs with which the operator is familiar.
4.4. Reports are now required for every event which occurs out of the ordinary, a form is completed describing the event, its probable cause and other pertinent information. This information is compiled and evaluated statistically for assessing the likelihood of accidents via methods such as Probabilistic Risk Assessment. If human error information is accurately reported, such statistics are useful for future human factors improvement.
4.5. Equipment design, maintenance and testing now focus on improving the identification of equipment and access to it, providing better technical manuals and written procedures, and by designing ergonomically better tools and instruments. Improving the work environment—for example, by avoiding extreme temperatures, noise and inadequate lighting can reduce maintenance errors further
Most of these human factors issues are interrelated. As mentioned, training accompanies all regular and emergency preparedness task; however, it also must be used to validate the effectiveness of changes made in the name of human factors. For example, human error during training activities has also been used to explicitly determine how a system or component may fail and what safeguards can be incorporated by the designer to prevent or mitigate such failures. One of the major human factors contributions that has been made since the TMI-2 accident has been the introduction of Safety Parameter Display System(SPDS). As required by the USNRC, every nuclear power plant control room must include a SPDS. NUREG-700 specifically provides information on SPDS requirements based on lessons learned from the TMI-2 accident, recommendations for general emergency preparedness, and human factors criteria, respectively. By the end of the 1980s, the USNRC was fairly satisfied with the ability of computer codes to simulate nuclear plant response under adverse conditions. In 1987 they published NUREG-1230, “Compendium of ECCS research for Realistic LOCA Analysis.” This was followed by a Rule change published in regulatory Guide 157 presenting a best-estimate approach to licensing nuclear power plants. This approach removed much of the conservatism of 10 CFR 50 Appendix K; however, it requires application of statistics to assure system integrity during an accident.
5. RESEARCH FALLOUT FROM CHERNOBYL: The fallout of Chernobyl accident has been written and analyzed from just every point of view imaginable. The immediate cause of the Chernobyl accident was an ill conceived and mismanaged electro-mechanical engineering experiment. Ultimately, in order to conduct this experiment, reactor staff had to bypass multiple layers of safety systems that could have prevented the reactor instability that the Chernobyl had actually experienced. While this unprecedented safety culture had been the primary cause of the accident, a number of other factors as mentioned at the following paragraph had contributed to the extensiveness of the fallout.
The design of the Chernobyl reactor presents a number of problems that can only exasperate a severe accident. Unlike the western-styled Light Water Reactors, Chernobyl’s RBMK reactor design contains the following recognized design flaws:
Inherently unstable under conditions of loss-of-coolant.
Inherently unstable under conditions of increased temperature.
Lacks sufficient containment.
The absence of a containment structure was specially important. Post-accident analyses indicated that if there had been a U.S.-style containment, none of the radioactivity would have escaped, and there would have been no injuries or deaths.
The social fallout from Chernobyl put pressure on the nuclear industry to take a harder look at safety in general and the safety mechanisms that prevent severe accidents in particular. Following the Chernobyl accident, governments and private industry sponsored research in severe accidents research, advanced reactors designs, new engineered safety systems, and refined regulation. As a matter of fact, much of this work only extended the path of research begun with the accident at the Three Mile Island. Ultimately, attention focused on raising the safety standard of the nuclear industry internationally. The International Nuclear Safety Programme(INSP) was established for this purpose. The INSP offers training and technology transfer to under-developed nuclear countries. They have also been actively involved in offering direct services to address specific needs of operating reactors worldwide. As Three Mile Island and Chernobyl demonstrated, severe accidents- those accidents that deform or melt fuel-can happen. For western-designed reactors which are designed with a containment, it was generally believed that the containment integrity would be maintained for most of the severest accidents imaginable. This accidents would result in Direct Containment Heating (DCH) situation in which molted core material breaches the reactor pressure vessel and enters the containment.
The SURTSEY Direct Heating Test facility was built to answer questions about DCH for severe accidents. The SURTSEY facility was sponsored by the USNRC and built at Sandia National Laboratory in Albuquerque, New Mexico. Ultimately, much of the result from the SURTSEY were captured in new phenomenological models that could be incorporated in to computer codes. The USNRC sponsored the development of the CONTAIN computer code at Sandia and much of the results at SURTSEY were incorporated into CONTAIN. Another development has been the coordinated effort of US Nuclear industry, through the Electric Power Research Institute (EPRI) and the US Department of Energy (DOE) to document the industry’s requirements for advanced light water reactor (ALWR) plants. The most studied part of the new design features of the ALWR are the safety systems. Designers have taken two different approaches to address safety concerns:1) redesign of conventional systems for improved reliability and safety and 2) develop passive safety systems. Suggestions for the conventional system redesign included:
Increasing the number of ECCS trains from two to four.
Maintain a common ECCS and containment spray water to eliminate the need to switch from an external source and provide a semi-closed system.
Inject coolant directly into the reactor pressure vessel.
Rapid system depressurization to bring pressure below the shut-off head of the ECCS pumps.
Suggestions for passive safety systems included:
Gravity-driven emergency coolant systems
System designs that facilitate natural circulation
Automatic depressurization systems to bring pressure below ECCS coolant source pressures.
Integrated containment cooling systems to mitigate severe accidents.
The above-mentioned choices were included in advanced passive designs developed by GE(SBWR) and Westinghouse(AP600.)
6. POST 9-11 ACTIONS: Since September 11, 2001, NRC has strengthened security at nuclear facilities by working experts using state-of-the-art structural and fire analyses to realistically predict the consequences of terrorist acts. These studies confirm that, given robust plant designs and the additional engineered safety, security, and emergency preparedness and response, it is unlikely that significant ill consequences would result from a wide range of terrorist attacks, including one from a landing aircraft.
7. RADIOACTIVE WASTES: Like all other industrial ventures, generation of electricity also produces wastes. Unlike other wastes the latter are radioactive. Hence, these wastes have to be stored and disposed of in ways that protect human health and the environment. Although wastes are generated from every stage of the nuclear fuel cycle, the energy content of uranium is exceptionally high and as such waste per unit of electricity produced is small. It has always been recognized that radioactive wastes are potentially hazardous, so both industry and government ensure that they are managed properly. In order to achieve the required standards of radioactive waste management, the nuclear industries throughout the world classify radioactive wastes into a number of categories. A large portion of radioactive waste produced from the nuclear fuel cycle has radiation levels similar to, or not much higher than, the natural background level. Such waste is relatively easy to deal with. On the other hand a small proportion is highly radioactive and requires particularly careful management.
Radioactive wastes are classified on the following general considerations:
how long the waste will remain radioactive;
the concentration of the radioactive material in the waste; whether the waste is heat-generating.
The above-mentioned considerations will determine the suitable disposal methods. For this purpose, the internationally accepted categories are- Very Low Level Waste(VLLW), Low Level Waste(LLW), Intermediate Level Waste(ILW) and High Level Waste(VHW).
VLLW contains negligible radioactivity and is considered suitable for disposal with domestic refuse. LLW contains only small amounts of radioactivity with negligible amounts of long-lived activity. Low level waste does not require shielding during its handling and transport and is suitable for shallow land burial. This type of waste accounts for the great bulk of radioactive waste from the nuclear fuel cycle and typically comprises paper, rags, tools, clothing, filters and other such materials. These wastes are often compacted or incinerated in order to reduce the volume and disposed of according to local regulations.
ILW contains higher amounts of radioactivity and requires shielding. This waste typically comprises resins, chemical sludges and metal fuel claddings, as well as contaminated equipment and waste from decommissioning. Treatment and disposal of ILW varies depending on the waste form and whether it is short-or long-lived. Many types are solidified or immobilized in a solid, non reactive material such as cement or bitumen. In general, short-lived ILW can be disposed of in a shallow land burial, but long-lived ILW must be disposed of in a manner similar to that which is used for high level waste.
HLW are of two types. Spent fuel which is not reprocessed and the fission products released from spent fuel by reprocessing. HLW is highly radioactive and contains long-lived radioactivity. It generates a considerable amount of heat and requires cooling for many years before disposal. Most of the radioactivity from nuclear power is in the HLW, but the actual quantity of it is small, eg about 25 kg of spent fuel each year to provide electricity for a thousand people. Both types HLW must be treated, prior to disposal. HLW from reprocessing is vitrified into solid blocks of a special type of borosilicate glass. For direct disposal, spent fuel requires encapsulation in containers made, for example, of steel or concrete. In either case, there is a cooling period of 20 to 50 years between removal from the reactor and disposal, with the conditioned HLW being retained in interim storage. HLW may be disposed of in geological repositories that will lie 500 to 1000 meters deep.
Radioactive wastes resulting from the nuclear fuel cycle are responsible for only a small fraction of the radiation that is received from all sources. The world has evolved with radiation and radioactivity being natural occurrences. Natural background radiation accounts for about 90 per cent of the total amount of radiation received by average person in the course of a year, and medical treatments much of the rest.
8. CONCLUSION: The nuclear industry, the governments and the nuclear community as a whole are committed to manage the operation, maintenance, handling & transport of nuclear materials, and managing the waste that arise from the nuclear fuel cycle. All countries which use nuclear power or nuclear materials in any form have created agencies or specialist departments for managing activities mentioned above. These organizations are responsible for establishing rules & regulations for activities connected with nuclear materials including the operation of nuclear power plants. They are also responsible for issuing licenses and for establishing programmes for research and development, for treatment and conditioning of the waste, for interim storage and for eventual geological disposal.
The Presidential Orders-15 of 1973 mandated all peaceful uses of nuclear energy including nuclear power in Bangladesh. For a long time, there were, however, no legal instruments to control all radiological and other nuclear activities that naturally include harmful ionizing radiation. Such a state of affairs can never be desirable. To the relief of the people of the country and also to the international arena, Nuclear Safety and Radiation Control (NSRC) Act-1993 (Act No. 21 of 1993) was passed by the National Assembly in 1993. The Nuclear Safety and Radiation Control (NSRC) Rules-1997 ( SRO No. 205-Law/97) were notified in the Bangladesh Gazette on September 18, 1997.
The writer is Professor of Mechanical Engineering, and former Chairman of Atomic Energy Commission, Bangladesh.
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