Decay in radioactivity of high-level waste from reprocessing one tonne of spent PWR fuel

Radioactivity (GBq)

107

total

fission products

actinides

106

105

104

Original Ore

103

102

Gbq = 109 becquerel

The straight line shows the radioactivity of the corresponding amount of uranium ore.

NB both scales are logarithmic.

Source: OECD NEA 1996, Radioactive Waste Management in Perspective.

Fig. 2.18. Radioactive waste in perspective: time taken for reprocessed waste to attain the same activity as the original uranium ore from which the spent fuel was produced.

produced is shown in Fig. 2.18. However, it must be pointed out that the specific activities are still significantly different, i.e., the activity is distributed over different volumes (0.11 m3 of vitrified HLW is approximately equivalent to 700 m3 of original uranium ore).

2.7.5. Simplifying the number of waste types

In any national radioactive waste programme, a bewildering range of waste types can arise, based on the waste immobilisation matrix (e.g., type of cement used) or container type (e.g., steel drum, concrete box). To be in the position to perform an efficient (i.e., cost effective) safety assessment, it is therefore necessary to group wastes and summarise their properties into representative waste types. Generally, this can be viewed as a strict application of common sense, e.g., combining HLW waste types with L/ILWSL waste types will not result in a sensible representative waste type. Further, organiccontaining wastes should not be diluted with inorganic-containing wastes due to the differing behaviour of the organics in a post-closure repository. Nor, for example, is it

a The values inserted into this table are illustrative. The position of priority 1 is clear but 2 or 3 will depend on whether more weight is put on the inventory or the material content. Therefore, the positioning of the priority tends to become more subjective the higher the number. However, using such a priority key table, it is possible to identify whether it is wise to group waste types together (to reduce the number of waste types to be considered) and also to prioritise any work that is required in obtaining additional information, etc.

recommended that -wastes be combined with pure -wastes, even if they contain the same type of materials, as the long-lived -wastes require longer periods of immobilisation in a repository than the -wastes.

Although there are many options available for waste combination, one simple way of assessing what should or should not be combined, and to determine where the maximum amount of effort should be invested, is to use a waste priority key table (Table 2.7) where the following material and inventory priorities may be established.

2.7.5.1. Radionuclide inventory priorities

For a L/ILW-SL repository, definitions normally exist which give either average or maximum total or individual radionuclide activity values. Therefore, for the first round of prioritisation, categories can be defined on this basis. For the next inventory when, e.g., total inventories of individual nuclides have been defined, this process can be repeated to consider these additional criteria.

Category 1: >10% of limit

Category 2: 0.1% < inventory values < 10% of limit

Category 3: <0.1% of limit

For L/ILW-LL and HLW, this system is less applicable as the importance of each waste type must be (as far as activity content is concerned) uniformly high and, correspondingly, the level of knowledge should also be as high as possible.

2.7.5.2. Material priorities

Based on the amount (wt%) of organic material in the waste relative to the cement immobilisation matrix, the following type of table (Table 2.8) can be created (please note, however, that these values are merely indicative of an approach, but are not necessarily applicable to a particular disposal programme where such data need to be produced with the input of safety analysis experts).

The classification applies if any one of the criteria is reached within a specific waste type. Again, as for the radionuclide inventories, this sort of approach is more suitable to L/ILW-SL inventories, but can be used for L/ILW-LL as well, i.e., for wastes of similar activity levels,