Significant Period of the Hazard

In analyzing the safety of a waste repository, it is crucial to know the time period under consideration. A number of geologic processes and events are relevant for the safety analysis only if the time frame exceeds a certain range. As it is obvious that the hazard of a waste repository due to the decrease of its radioactive inventory will eventually approach a level that is no longer significant, it will be feasible to estimate a time frame for the safety analysis. This time frame will be called the significant period of the waste repository hazard. The estimation of

Figure 11.29 Radioactivity of in­dividual radionuclides in HLW from the LWR uranium fuel cycle. Re­processing, 150 days after reactor dis­charge; enrichment, 3% 23SU; burnup,

30,0 MWd/MT heavy metal; resi­dence time, 1100 days; 0.5% uranium and 0.5% plutonium remaining in HLW.

such a significant period of hazard may be considered the first step in an iteration that may need refinement before the safety analysis is completed.

Definition of a significant level. To define a level of significance for the geologic waste repository hazard, a point of reference is required. The hazard of naturally occurring uranium in equilibrium with its daughters is frequently used as such reference. This choice implies the reasonable assumption that an artificial hazard equal to that of naturally occurring uranium is not considered significant because the natural uranium hazard is inevitable and people have been living with it all the time.

Such a comparison of hazards is meaningful only for similar chemical species and if the barriers protecting people from the hazards are at least qualitatively similar. This is true for a geologic waste repository as compared to a uranium deposit. The locations are similar, that of waste is even likely to be more favorable, and the key radionuclides involved, particularly 226 Ra and its parents, behave similarly.

As for the location, many uranium deposits occur considerably closer to the surface than a waste repository is supposed to be located. Therefore, radionuclides from uranium deposits may have to travel a shorter distance than those from waste repositories. Moreover, groundwater at greater depth is usually less mobile. The geologic containment of the waste repository is not taken into account as a barrier because the significant period of the hazard is supposed to be the period for which the integrity of the geologic containment is to be analyzed. Even disregarding this barrier, it is reasonable to consider the remaining barriers of a waste repository similar to those of a natural uranium deposit.

The specific radionuclides reponsible for the waste hazard are important because of their different mobilities when migrating with groundwater. Figures 11.29 and 11.30 show the long-term radioactivities and ingestion hazard indices of the most significant radionuclides in LWR uranium waste versus time. Beyond 500 years, the ingestion hazard is controlled by americium, plutonium, and eventually by radium as a uranium daughter. The neptunium itself contributes to the ingestion hazard, but less than 10 percent. The ingestion hazard of natural uranium is that of its daughter radium, and consequently over a long period of time is identical

Figure 11.30 Ingestion hazard index (defined in Sec. 2.1) of individual radio­nuclides in HLW from the LWR uranium fuel cycle. Reprocessing, 150 days after reactor discharge; enrichment, 3% 235U; bumup, 30,000 MWd/MT heavy metal; residence time, 1100 days; 0.5% uranium and 0.5% plutonium remaining in HLW.

Figure 11.31 Range of ingestion hazard index of HLW and range of reference in­gestion hazard index of naturally occur­ring uranium.

with the ingestion hazard of waste. Plutonium and americium have essentially the same mobility as uranium. The mobility of radium is correlated with that of its parent uranium. Only the not very abundant neptunium is faster by a factor of 100 [B8].

The conclusion is that a comparison of ingestion hazard indices of waste in a geologic repository and of naturally occurring uranium is a reasonable basis for the definition of a significant level of the waste hazard.

Estimation of the significant period of the waste hazard. Figure 11.31 shows a band of long-term ingestion hazard indices of HLW from various fuel cycles and a line corresponding to unreprocessed LWR fuel versus time. It shows also a horizontal band representing various reference levels [L4, L5].

Reference level means the quantity of natural uranium whose ingestion hazard index is used as a reference to which that of the waste from 1 MT of heavy metal reprocessed is compared. These quantities according to different approaches are as follows:

The quantity of natural uranium to be mined for the production of the heavy metal reprocessed. This type of reference has already been used in Chap. 8 because it is the most general one with no special assumption about the form of the natural uranium involved. Its disadvantage is the strong dependence on fuel-cycle type. With an equilibrium LMFBR fuel cycle, for instance, the quantity of uranium to be mined becomes close to zero and, consequently, the period of significance of the waste hazard becomes extremely long. To maintain its applicability, the uranium equivalent must always be calculated on the virtual basis that all power has been generated from freshly mined uranium.

The volume of natural U308 equal to the volume of solidified waste from reprocessing 1 MT of heavy metal. This volume is assumed to be 80 liters as an average. For unreprocessed fuel 120 liters have been used. U308 has been chosen as the standard uranium species because this is the radioactive concentrate in a uranium ore just as solidified waste is the radioactive concentrate in a waste repository. Moreover, it is a sufficiently generalized uranium species. This reference leads to a dependence of the significant period on the waste oxide concentration in the waste form.

The waste from 1 MT of heavy metal is assumed to be evenly distributed in that volume of rock which is required to accommodate the boreholes for the corresponding number of waste blocks, disregarding rock above and beneath the boreholes. The waste blocks are assumed to have 20 w/o waste oxides and to be arranged in a hexagonal array with 10-m distances. The ingestion hazard index of a unit volume of this homogenized disposal field is compared to the ingestion hazard index of the same volume of 0.2 percent uranium ore. This approach leads to a dependence of the significant hazard period on the density of waste in the host rock of the geologic repository.

The range of intersection between the ingestion hazard index band and the horizontal band indicates the range of significant periods of the hazard. These significant periods vary in a relatively narrow range, namely, between 500 and 10,000 years for the whole variety of waste from different fuel cycles except for unreprocessed fuel.

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