Table 4.24 Summary of Potentially Major but Unquantified Issues impact probability cannot be given. For this reason potentially major, but unquantified, risks should be given prominence comparable to the quantified risks discussed above. Table 4.24 is a listing of potentially major (Category A) but unquantified issues identified for the six technologies considered. Estimates of expected health and safety impact levels have been developed for certain catastrophic events (i.e., events with low probability of occurrence but high impact per event), in particular, for fission reactor systems.However, these impacts were not included as quantified issues in this study because of inherently high uncertainties associated with predicting occurrence rate and impact per occurrence. Furthermore, averaging expected catastrophic impacts over plant lifetime does not indicate the full significance of these potential events. The issues of potential diversion of fission fuel for weapons use and the potential for a crash of a SPS spacetransport vehicle into an urban area are also included in the potential catastrophic event category. Through engineered safeguards, the probability of occurrence of these events can be reduced to very low levels, but essentially zero probability is very difficult if not impossible to achieve at reasonable cost. A further important distinction concerning unquantified issues is whether the potentially affected persons are part of the general public or are workers producing or operating the system. Issues in the latter category (e.g., emissions from solar cell production, emissions of toxic materials from the fission system fuel cycle, liquid-metal fire hazards in the LMFBR and fusion systems) affect a well-defined group, i.e., occupational workers, and those impacts can be more easily monitored and mitigating actions implemented. In contrast, impacts from low-level microwave radiation, if they exist, may be difficult to identify because of their potentially small and subtle effects on a large exposed group.
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