Accelerated Leaching Methods For Cementitious Nuclear Wasteforms and The Impact of Superplasticisers on the Leaching Behaviour

The Intermediate Level Wastes (ILW), produced during the reprocessing of spent nuclear fuel, are commonly encapsulated in a blast furnace slag (BFS) and Portland cement (PC) composite and stored in repositories. These cements are required to safely immobilise these wastes for long durations of time. Additives are used in the cementitious wastes to further enhance them for example superplasticisers are used in general cement industry to increase fluidity, resulting in large reductions of water required for mixing, giving high early strength development, a decrease in permeability and ultimately significant reduction in costs. However superplasticisers contain molecules that can complex insoluble radioelements such as plutonium and uranium increasing their solubility and in turn mobility. This can be problematic while the cementitious wastes are stored. The standard storage conditions in the UK are that a cement is stored in a steel canister, then a backfill cement such as NRVB (Nirex Reference Vault Backfill cement) is poured over this cement, allowed to set after which the container is sealed. These conditions expose the waste forms to potential contact with ground water in the event of damage to the container. It is therefore important to study the leaching behaviour of these cementitious waste forms and simulate any resultant degradation that would occur over the very long durations they are expected to be stored. The impact of superplasticisers on these leaching behaviours is also investigated to understand the impact these additives have. The project investigated the leaching behaviour of caesium (Cs) and strontium (Sr) within a 90wt% blast furnace slag (BFS) and 10wt% Portland cement (PC) system (9:1 BFS:PC). A static leaching method was used to monitor the behaviour of Cs and Sr in solution without changing it. Three accelerated leaching methods were then used, semi-static wherein the solution (deionised water) was changed at regular intervals, acidic using ammonium nitrate (NH4NO3) solution and electrical using a current of 5A/m2 and 25 A/m2. If successful these methods can help characterise the suitability of cement formulations for distant future use. The semi-static method showed small increases in leaching, replacing the solution reset the concentration gradient and kept the pH near neutral. The acidic leaching method increased leaching rates significantly however caused the formation of phases that would not occur naturally and also suppressed Al leaching. The electrical methods caused the largest increase in leaching rates, with the closest similarity to the non-accelerated leaching conditions, the crystal structure was similar to that in static leaching and the hydrate phases formed were similar. A current of 25 A/m2 over a 1-3 day period would leach the same amount of Cs and Sr elements as static leaching for 2 years. The leaching behaviour of Cs and Sr is dependent on how they are retained within the cement. The project found that Cs remains as CsNO3. Sr(NO3)2, however, was not present in the hardened cement and retained within the hydrogarnet and a nitrated AFm phase. Secondly the impact of superplasticisers on the leaching behaviour of the cement matrix containing cerium was also explored using ADVA cast 551. Cerium was chosen as opposed to caesium or strontium as the main concerns in industry due to superplasticisers is regarding the complexion of uranium and plutonium, and Ce is a good simulant for plutonium. It was found that the introduction of ADVA cast 551 increased the fluidity and workability of the cement, did not alter the cement crystal structure however did increase the leached amounts of Ce by approximately 30%. In storage conditions this complexed Ce would come into contact with the cement backfill. A flow through leaching test was used on an NRVB cement sample and synthesised solutions of the leached Ce including the ADVA cast 551 to monitor how much Ce would pass through. It was found that similar amounts of Ce had passed through the NRVB whether the solution contained superplasticiser or not. This project found that the leaching behaviour of ions was dependent on how the ion is present and in which phase. Due to Cs and Sr being incorporated differently the Cs leaches much faster than the Sr, as it is not incorporated into any phase. The Sr is associated with several phases, however is mainly found to be associated with AFm type phases. The most efficient leaching accelerant method is the electrical method; therefore with further investigation this could potentially be used to explore the optimal cement compositions for cementitious waste immobilisation. This could also be used in cement samples containing superplasticiser, given that this increases mobility over short durations, the next step would be to investigate the long-term impacts, via simulation using electrical leaching.