DISPOSAL CRITICALITY IN THE VERY LONG TERM

This paper addresses the deep geologic disposal of spent nuclear fuel (SNF), and how the considerable knowledge as to The evaluation of disposal criticality safety and the related the origins and characteristics of the world’s uranium design process for criticality control is divided into four orebodies can be used in the design of SNF disposal methods overlapping, but generally sequential time and spatial regimes that assure the criticality safety of such disposal in the very as follows: long term. The very long term refers to the extended period following postulated degradation of the waste package barrier (1) Potential criticality in the original configuration within the by groundwater, the postulated gradual dissolution of the intact waste package uranium in the SNF, its subsequent transport by the groundwater, and its postulated precipitation to form a uranium (2) Potential criticality in various possible degraded deposit in the far field geologic environment of the general configurations within the failed waste package envelope repository locale. The possibility that such a sequence of events could lead to a nuclear criticality in the uranium (3) Potentially critical accumulations within the near field deposit, and the consequences of such a criticality need to be (emplacement drift), outside of the original waste addressed as a part of the overall criticality evaluation of SNF package(s) disposal. The only extensive data on the aggregate behavior of uranium in natural environments is the data on the origins and (4) Potential critical accumulations in the very long term in characteristics of the world’s uranium deposits. Furthermore, the far field. because U-235 decays less slowly than U-238, the natural enrichments were considerably higher when some of these Each of the foregoing regimes must be addressed uranium deposits were formed: two billion years ago, the individually, and integrated with the other regimes as part of natural enrichment was about 3.6%, compared to the current the criticality control design of the waste disposal package, the 0.711%. Thus, depending upon their times of formation, the engineered barrier and the repository emplacement world’s uranium orebodies provide data as a function of configuration. It is noted that as long as a nuclear criticality enrichment. The high-grade portions of the single orebody in designer can postulate a critical configuration for the quantity which natural criticalities have occurred, at Oklo in Gabon, of uranium and associated materials being evaluated, the Africa, provide one unique source of relevant data. However, probability of a criticality is no less than the probability of that the high grade portions of the other orebodies formed during postulated configuration, which may be very small in a natural and since that same general time frame did not go critical, and setting, but not zero. Thus disposal criticality design requires may provide equally useful information regarding the a probabilistic approach. Further, except where the probability conditions required for natural criticalities. Finally, if there are is zero, an analysis of the safety consequences of the differences in the natural environment when the early high postulated criticality, and the resulting safety risk (probability grade uranium deposits were formed, as compared to the times consequences) is also required. present, such differences need to be reflected in the evaluation of SNF disposal criticality