Many properties need to be considered in waste form development. Mechanical properties are important from the point of view of material integrity in a storage or disposal environment. Of particular importance is the long-term chemical durability of the waste form as this will influence the release of toxic elements due to leaching under disposal conditions. Chemical durability can also change over very long timescales depending on the radiation stability of the waste form, particularly as this may adversely affect the dissolution rates through, for example, the generation of corrosive radiolysis products in the leaching solution.

Durability studies performed using a modified MCC-1 procedure on AWE monolithic material which had undergone accelerated ageing trials through the substitution of 238Pu for 239Pu demonstrate this adverse effect. 238Pu-doped material which had been aged for 1820 days (total a radiation fluence of 4 x 1018g-1) was leached in water at 40°C for 28 days and com­pared with 239Pu-doped material (unaged) and the results are summarized in Table 25.8. It can be seen that the release of elements from the aged 238Pu

samples is considerably greater than that from the unaged 238 or 239 samples.