Engineers often face the general question of which approximations are appropriate for a given analytical task. In particular, when is a simpler model useful if a more complex model also exists? This paper explores this question in the domain of radiotoxicity relative to geologic disposal performance dose assessments.
Criterion 1 requires that the simpler approach, radiotoxicity, must be calculated correctly. The concept of ingestion radiotoxic inventories is analogous to the inventory of toxic chemicals in other industries. From a decision analysis perspective, it is also somewhat analogous to the nuclear reactor safety concept of “passive safety.” This paper explains some of the issues in calculating radiotoxicity, motivated by the author's observations of errors in the literature: not accounting for radioactive progeny, misunderstanding natural “ore,” and focusing on transuranic (TRU) isotopes without adequate attention to actinide decay products. For example, Th/233U fuel cycles do have lower amounts of TRU isotopes than U/239Pu fuel cycles, but that does not necessarily mean lower long-term hazard.
Criterion 2 requires that the uncertainties in the more complex approach, performance dose assessments, must raise issues for the assessments' intended purposes - in which case, radiotoxic inventory may be of assistance until those issues are resolved. Performance dose assessments were developed for, and are legally the way to show, compliance with regulations, but the uncertainties are large. Less obvious is the degree to which dose assessments are applicable to other purposes - comparing fuel cycle options prior to site selection and showing the safety of a fuel cycle and waste management approach to the public. In the last sense especially, performance dose assessments are analogous to probabilistic risk assessments for nuclear reactor safety. The United States lacks a selected consensus site, selected fuel cycle approach (direct disposal versus recycling), and selected waste form. Thus, the paper does not intend to discuss all the issues with performance dose assessments but rather intends to focus on only those performance dose uncertainties that raise issues when comparing fuel cycles. Uncertainties associated with whether a generic geological environment is attractive or a specific location meets requirements are beyond the scope of this paper.
Ingestion radiotoxicity correlates with heat, gamma, and inhalation radiotoxicity. Thus, options that are relatively high in ingestion radiotoxicity tend to be high in other parameters. Therefore, reduction in ingestion radiotoxicity means both that the potential source term for release is lower but also that one driving force for release (heat) is also lower. However, the most important time frames differ as heat is mainly an issue in decades and centuries after reactor discharge, but ingestion radiotoxicity is mainly an issue during longer time periods. Ingestion radiotoxicity points to the importance of actinides in long-term waste management, followed by specific fission products such as 99Tc, 129I, 93Zr, 135Cs, and 79Se. TRU isotopes were important in four of five generic geologic environments recently studied independently with used fuel disposal experts as were the same fission products as noted from radiotoxicity - in part because waste assessments must include multiple exposure pathways including human intrusion and drilling into the waste. Dose calculations that were made for used fuel disposal may be misleading if extrapolated to disposal of only the remnants of used fuel separation because the chemistry of waste will differ. Radiotoxicity can be misleading if used to strictly rank order among individual isotopes if a specific disposal option is well known. Hybrid approaches that incorporate radiotoxicity and features of full performance assessments may have value.