inventory of a repository for long-lived ILW. Degradation of cellulose under alkaline conditions forms various hydroxy-carboxylic acids that have the capacity to form complex molecules with actinide ions. Under these circumstances, the solubility of the actinide in question can be increased by orders of magnitude and sorption similarly decreased. The effect of these changes is to significantly increase the calculated postclosure doses from the actinide elements (e.g., Hooper et al., 2000). The effect depends strongly on the concentration of degradation products in the repository porewater and it diminishes as these degradation products are removed from the repository by groundwater flow and by their further degradation by microbes.

8.2.3.5. Gas generation

Several gases can be generated by radioactive waste. The most significant in terms of volume is hydrogen, which is produced by the corrosion of metals present either in the waste itself or as containers (see Chapter 5 for details). The hydrogen produced by these corrosion reactions is non-radioactive, but it is possible that it could be produced at a sufficient rate and at sufficient volume to perturb the EBS or even, in a very low permeability claystone, the host rock itself (see discussions in Nagra, 2002a).

Some radioactive gases are produced, albeit in much lower volumes. These include 3H, radon and 14C-containing CO2 and CH4. Due to its short half-life, 3H is not usually significant in a repository. Radon also has a short half-life, but, because it is a decay (or daughter) product of the 238U chain, it will continue to be produced as long as U remains in the repository. Even so, the consequences are rarely significant as the U is solubilitylimited in both the EBS and the host rock.

14C-containing CO2 and CH4 arise through processes such as the microbial degradation of 14C-containing organic wastes in L/ILW. The relatively long half-life of 14C means that, if these gases form in significant amounts, they can impact on post-closure safety. This possibility means that R&D has been carried out to calculate the rate of 14CO2 and 14CH4 production, measure the rate at which 14CO2 reacts with cement porewater and estimate the uptake of 14CO2 and 14CH4 by plant roots1. More recently, it has been shown that 14C can be released in an organic form during the corrosion of radioactive steel (e.g., from a reactor core) and new R&D is now being focused on this area (see, e.g., JAEA, 2007)

8.3. Conventional R&D

Most of the R&D that is done to support radioactive waste disposal does not require hot cells, glove-boxes or other remote handling equipment. This work may be subdivided into three main categories: engineered barriers, radionuclide migration in the geosphere and biosphere-related work. Studies are mostly carried out in an open laboratory or else in the field.

1 Uptake by plants that are ingested by humans is one exposure pathway that produces significant radiological impact (e.g., Nirex, 2001); another is drinking water extracted from rivers (e.g., JNC, 2000).