Legacy Ponds and Silo Wastes

Hydrogen gas retention in nuclear waste sludge is of interest on many nuclear licensed sites. The risk of hydrogen hold-up in either man-made voids or within fracture zones inside a sludge bed and a sudden release of gas following bed disturbance is clear.

Despite this, much remains to be understood about the likelihood of large gas pockets forming within sludge and the features of nuclear waste sludge that may increase or decrease the risk of this happening.

Gas hold-up within nuclear waste sludge is a complex issue. The amount of hold-up depends on the physical properties of the sludge, with strong evidence relating gas retention to sludge strength. The exact amount of hold-up depends on a wider range of factors such as rheology, layering, composition, particle size, pressure and depth of sludge. It is assumed in the available industry literature that the sludge is homogeneous with no stratification. This is unlikely given the history of waste deposition. The presence of a layered sediment bed with each layer having different gas generation rates and yield strengths may be important. The development of yield strength over time is also important. Sludge ages slowly with an observed increase in the yield strength. If gas production is rapid, and gas is retained on the particles as they form the sludge bed, then a considerable volume of gas may be trapped during ageing. What effect this has on the development of the yield stress is as yet unknown. It is feasible that this leads to a significant decrease in the aged yield strength. This could have significant impact on the nature of the bed and routes for gas release. The retention and trapping of gas in a sludge bed as it forms may also lead to a low-density bubble/particle mix that is inherently unstable as the density will always be close to that of the fluid. Such a system may be susceptible to ‘turn-over’ and catastrophic gas release.

Our focus is to develop an improved understanding of how gas is retained and released from nuclear waste sludge. We will explore how gas influences the properties of realistic simulant sludge materials and how ageing of sludge is influenced by the simultaneous production of gas. We expect this project will be closely integrated with known issues at a number of nuclear sites, ensuring interest in the work and relevant outputs for licensed nuclear environments.