2.7.2.1. Radiological data

Individual radionuclide and total activities A summary of some of the methods used to determine radionuclide inventories is given in Table 2.5.

This information is required not only as input for a repository safety assessment, but is also used for dose rate, heat output and radiotoxicity calculations.

The number of radionuclides presented in a radionuclide inventory is programmespecific, e.g., in the Swiss programme, for model inventories all radionuclides of halflife greater than 60 days plus important short-lived daughters are initially considered (i.e., a total of 147 radionuclides). Following a simplified safety analysis (screening exercise), this list is reduced to those radionuclides that are safety-relevant and only for this inventory is the full safety assessment study performed.

As an example, the nuclides selected after the simplified safety assessment, those selected after the analysis of the near-field and, finally, those that dominated releases from the far-field in the case of the studies performed for a potential Swiss L/ILW repository (Nagra, 1993) are given in Table 2.6.

Heat output Heat outputs are determined using WBq 1 conversion factors based on the determined radionuclide inventory. For L/ILW, heat output is not normally of great interest (apart from ensuring that it is not too high for ILW-LL). However, for HLW wastes, this parameter determines the size of the repository, in that disposal canister heat output limits are required to ensure that the engineered barrier system (EBS) functions as required. These limits are a compromise between the spacing between waste packages and the dimensions and spacing of the tunnels (see also chapters 3 and 5).

Dose rates If no production data exist, these can be calculated with varying degrees of sophistication. A simple, user-friendly code is Microshield (Grove, 1998) but, like all tools, the limitations must be understood if it is to be used for more than simple modelling. If more accurate values are required, and especially if complicated

Table 2.5

Overview of methods used to determine radionuclide inventories