David Okrent

ON INTERGENERATIONAL EQUITY AND ITS CLASH WITH INTRAGENERATIONAL EQUITY AND ON THE NEED FOR POLICIES TO GUIDE THE REGULATION OF DISPOSAL OF WASTES AND OTHER ACTIVITIES POSING VERY LONG-TERM RISKS

This article begins with some history of the derivation of 40 CFR Part 191, the U.S. Environmental Protection Agency (EPA) standard that governs the geologic disposal of spent nuclear fuel and high-level and transuranic radioactive wastes. This is followed by criticisms of the standard that were made by a Sub-Committee of the EPA Science Advisory Board, by the staff of the U.S. Nuclear Regulatory Commission, and by a panel of the National Academies of Science and Engineering. The large disparity in the EPA approaches to regulation of disposal of radioactive wastes and disposal of hazardous, long-lived, nonradioactive chemical waste is illustrated. An examination of the intertwined matters of intergenerational equity and the discounting of future health effects follows, together with a discussion of the conflict between intergenerational equity and intragenerational equity. Finally, issues related to assumptions in the regulations concerning the future state of society and the biosphere are treated, as is the absence of any national philosophy or guiding policy for how to deal with societal activities that pose very long-term risks.

Risk perception and risk management: on knowledge, resource allocation and equity

Abstract This article is an outgrowth of the opening article given at a pair of invited sessions on “Risk Perception versus Risk Analysis” at the PSAM 3 Conference held on Crete in June 1996. The first introductory section provides some review of the relevant issues and raises some general questions about possible changes in the emphasis and directions of research on risk perception and related social science studies. The second section looks in a little more detail at issues related to public participation in the deliberations and decision-making concerning significant societal ventures involving risk. Section 3 examines the role and importance of knowledge in risk perception. the relatively brief fourth section raises questions about the publics perception of geologic disposal of high-level radioactive wastes as something catastrophic, to be “dreaded”. Section 5 looks at the bases for allocation of resources for improving the public health and safety. And the sixth section examines intragenerational equity and the conflict which can arise between it and intergenerational equity.

The use of neural networks and a prototype expert system in BWR ATWS accidents diagnosis

Abstract Neural networks and a symbolic expert system are employed to form a prototype system in BWR anticipated transients without scram (ATWS) accidents diagnosis. Unsupervised learning based on discovery of cluster structures (Paos approach) and back propagation (BP) neural networks are used to group and memorize different ATWS patterns. Multiple training data sets derived from output files of the SABRE computer code are used in training the BP networks to cope with oscillations of reactor power and other parameters. Tests of neural networks recall correctness are performed, given the presence of time shift of sample data, random noise and incomplete information. The expert system can simulate the ATWS strategy developed by Pennsylvannia Power & Light company. It undertakes diagnosis, using an event tree structure, and can execute the trained BP networks. The expert system is also able to deal with temporary loss of reactor water level information.

Modeling of Equiaxed Grain Growth in Uranium Dioxide Fuels

Grain size is one of the most important parameters affecting fission gas release and swelling of nuclear fuels. On the other hand, fission gas bubbles in previously irradiated fuel interact with moving grain boundaries and affect subsequent grain growth. Equations are developed that describe equiaxed grain growth in oxide fuel that has undergone previous irradiation (burnup) at temperatures too low to promote grain growth. The resulting expressions relate the grain growth rate at the new high temperature to the percent burnup (or fission gas content) experienced during the prior low-temperature period.

Use of an influence diagram and fuzzy probability for evaluating accident management in a boiling water reactor

A new approach is presented for evaluating the uncertainties inherent in severe accident management strategies. At first, this analysis considers accident management as a decision problem (i.e., applying a strategy compared with do nothing) and uses an influence diagram. To evaluate imprecise node probabilities in the influence diagram, the analysis introduces the concept of a fuzzy probability. When fuzzy logic is applied, fuzzy probabilities are easily propagated to obtain results. In addition, the results obtained provide not only information similar to the classical approach, which uses point-estimate values, but also additional information regarding the impact of using imprecise input data. As an illustrative example, the proposed methodology is applied to the evaluation of the drywell flooding strategy for a long-term station blackout sequence at the Peach Bottom nuclear power plant. The results show that the drywell flooding strategy is beneficial for preventing reactor vessel breach. It is also effective for reducing the probability of containment failure for both liner melt-through and late overpressurization. Even though uncertainty exists in the results, flooding is preferred to do nothing when evaluated in terms of two risk measures: early and late fatalities.

A Historical Perspective on Seismic Risk Assessment and Bayesian Updating

Drawing on an evaluation of past and current risks to light water power reactors arising from seismic design and construction errors, as well as the results of recent probabilistic risk assessments, a historical comparison is made between expert estimates of seismic risk and the implications of experience with past seismic deficiencies. A few methods of annual Bayesian updating of expert opinion on seismic risk as a function of new information are compared. The implications of the results include the suggestions that care be exercised in using a self-estimate of uncertainty in performing weighting and that the use of common information may lead consciously or unconsciously to a disadvantageous excessive weight placed on prevalent rather than innovative opinion.

Alternative Risk Management Policies for State and Local Governments

The last decade has seen a very rapid expansion of activity by the federal government with regard to societal hazards and risks. Although federal regulatory decisions have tended to avoid quantifying the question “How safe is safe enough?” there is growing attention, on the federal level, to efforts to quantify risks and to examine various policies concerning “acceptable” or “assumable” risk. However, matters pertaining to many hazards and risks fall under the direct control of state, county, or municipal government. While there exists much current effort on risk quantification and risk management at the federal level, little information is available on the extent and magnitude of risks subject to local control. In addition, little has been done to examine the ramifications of various possible risk management policies at the state or municipal level.

The effect of accident conditions on the molten core material relocation into the lower head of a PWR vessel with application to TMI-2

Abstract The damage progression of the reactor core and the slumping mechanism of molten material to the lower head of the reactor vessel were examined through simulation of severe accident scenarios that lead to large-scale core damage. The calculations were carried out on a Three Mile Island Unit 2 configuration using the computer code SCDAP/RELAP5/MOD3.2. Different accident scenarios were simulated. The high-pressure injection and makeup flow rates were changed. The extreme case with no water being added during the accident was examined. Reflood by restart of coolant pump 2B was also studied. Finally, the size of the power-operated relief valve opening was also changed. The effects of these accident scenarios on the accident progression and the core damage process were studied. It is concluded that, according to code MOD3.2, the molten material slumped to the lower head of the reactor vessel when the junction of the top and side crusts failed after the molten pool had reached the periphery of the core. When the effective stress caused by pressure imbalance inside and outside of the crust became larger than the ultimate strength of the crust, the crust failed mechanically.

An expert system to estimate time, rate, and magnitude of release of important radioactive isotopes given a loss of containment integrity

A prototype expert system is developed to provide information to emergency planners on the time, rate, and magnitude of release of important radioactive isotopes given a loss of containment integrity. Basic to the method is the anticipated availability of instruments to measure concentration of significant individual radioisotopes in containment {approximately} 1/min. The escape of radioisotopes is assumed to be proportional to that of noncondensable gases, which are monitored and/or predicted in part.

A look at safety goals and safety design trends for advanced light water power reactors

Analyse de la politique de la Nuclear Regulatory Commission (Etats-Unis) en matiere de surete des reacteurs nucleaires. Presentation des modifications de conception des reacteurs a eau legere ayant entraine une amelioration de la securite