دانلود رایگان مقاله انگلیسی مدل سازی انتقال واکنش پذیر سیمان پر برای خرد کردن زباله های رادیواکتیو - الزویر 2018

ABSTRACT

Cementitious materials are included in many geological disposal facilities for radioactive wastes, including the UK candidate backfill material Nirex Reference Vault Backfill (NRVB). As part of an ongoing programme of work to assess NRVB performance, 1-D reactive-transport models have been constructed to simulate reaction with different illustrative groundwater compositions. Variant cases were also produced to explore the effect of model assumptions on the predicted behaviour of the backfill. Depending on groundwater composition, cement alteration pathways included: carbonation; external sulphate attack; chloride attack; the formation of magnesiumrich solids; and the precipitation of secondary aluminosilicate minerals. In general, the models suggest that the pore space in backfill associated with radioactive waste disposal systems is likely to become clogged (to some degree) over time close to backfill-rock interfaces resulting in a reduction in capacity for solute transport. However, the models do not include all relevant process couplings which is potentially, an area for further work.

6. Conclusions

1-D reactive-transport models were constructed to simulate a block of cementitious NRVB (1 m thick) that is in contact with two illustrative groundwater compositions with different sulphate and chloride concentrations. The models suggest that different groundwater compositions will result in different cement alteration pathways that have different implications for pH evolution (and therefore the potential behaviour of contaminants released from wastes). The alteration of hydrogarnet to Friedel's salt in the presence of water with higher dissolved chloride concentration resulted in an increase in cement porewater pH up to a value of ~13, which would also potentially increase the potential for alteration of the host rock by hyperalkaline fluids. In contrast, the formation of products associated with reaction with Mgrich waters would tend to lead to a reduction in pH to ~10 and in a system dominated by calcium carbonate precipitation, pH may fall to ~8 depending on pCO2. The models also show that thaumasite could potentially form as a product of sulphate attack on NRVB, something that has not been considered in previous work.