Waste form behaviour

SCK•CEN has more than 25 years of experience in research concerning the compatibility of waste forms with the final disposal environment. Waste forms that are relevant for Belgium include intermediate level (bituminized waste, cementitious waste) and high level waste (spent nuclear fuel, and high level vitrified waste).

Our expertise lies especially in the study of the chemical interactions between the waste forms and the disposal environment, and the ageing of the waste forms. The main objectives of our research are to determine values for the lifetime of the waste forms, and to investigate the possible influence of waste form degradation on the retention properties of the disposal environment. Our studies consist of laboratory and in situ experiments supported by modelling.

High-level vitrified waste

Coupled leaching/diffusion tests are used to verify the long-term predictions on the performance of SON68 waste glass. SON68 (COGEMA R7T7) is the borosilicate glass used by COGEMA (France) to reprocess and vitrify spent fuel from nuclear power plants.

The tests:

  • Provide realistic data on the corrosion of SON68, at a high backfill swelling pressure, representative temperature, and in the presence of γ- and α-radiation.
  • Estimate the thermal and radiolytic gas generation in backfill materials.

Figure 1. Experimental set-up for the investigation of coupled processes.

CORALUS is an integrated in situ corrosion test used to assess and demonstrate the performance of the SON68 glass in conditions that are representative for those expected in a disposal system relying on the use of a clay-base backfill material. CORALUS includes:

  • Highly α-doped SON68 glass samples
  • Clay-based backfill materials (with high swelling pressure)
  • Contact with the Boom clay host formation
  • Controlled temperature
  • Presence of γ-radiation

Figure 2. Three-dimensional view of a CORALUS test tube with 60Co sources.

Spent nuclear fuel

Our test programmes aim at studying the UO2 dissolution behaviour over very long time scales (simulated fuel ages up to 90000 years), both in clayey and cementitious environments. Specific items that we investigate are:

  • The influence of α-activity on the dissolution rate
  • The influence of H2 on the dissolution rate

The dissolution rate of UO2 is determined with static and dynamic leach tests.

Bituminized waste

Our bitumen studies focus on the impact of bituminized waste (Eurobitum, produced by the Eurochemic/Belgoprocess reprocessing facility) on the long-term safety functions of the Boom clay host formation. Two types of disturbances can be distinguished:

  • A mechanical disturbance, caused by the build-up of pressure in and around the waste, (a) due to the uptake of water by the dehydrated salts embedded in the waste, and (b) due to gas generation by anaerobic corrosion of the steel drums, radiolysis, and microbial activity
  • A chemical disturbance by the release of large amounts of (a) NaNO 3 and (b) water-soluble, organic, potentially complexing molecules due to radiolytic and chemical degradation of the bitumen.

Cellulose-containing waste

Cellulose-containing waste, mainly paper filters that incorporate α-emitters (a.o. plutonium and americium) cannot be incinerated and is cemented for deep geological disposal. After emplacement of the waste containers, the disposal galleries will probably be backfilled with concrete.

In the highly alkaline and anaerobic environment of the disposal site, the decomposition of cellulose will yield water-soluble degradation products that may increase the solubility of radionuclides in the clay formation through complexation. Therefore we investigated the effect of cellulose degradation products on the solubility and sorption behaviour of Pu(IV) and Am(III).

From our experimental work and associated safety calculations we concluded that the presence of cemented cellulose-containing waste forms in a geological disposal site in the Boom Clay formation will not cause problems concerning the influence of degradation products on the radionuclide behaviour.

Contact: Druyts Frank