Engineering news
Westinghouse and Sellafield have tested what they claim is an effective method for the removal of hydrogen from shielded boxes containing nuclear waste.
Nuclear waste that contains metallic spent nuclear fuel pieces or sludge generates hydrogen through radiolysis and chemical reactions. These waste materials can be stored in shielded boxes with filtered vents for removal of the hydrogen to prevent formation of flammable gas mixtures. Commercially available filters are designed for applications without shielding, such as for fitting the opening of a 200 litre drum.
Drilling through shielding can allow unwanted hydrogen accumulation, Westinghouse subsidiary Fauske & Associates said. Working with Sellafield, the firm said it had conceived and tested “an effective method for hydrogen removal from shielded boxes with significant hydrogen rates”. This hydrogen removal method minimises the number of filters required for passive storage of spent metallic nuclear fuel pieces and zeolites, Fauske said.
Some manufacturers supply filters that are suitable for removal of hydrogen from unshielded nuclear waste containers such 200 litre drums. The rate of hydrogen removal through a filter varies with filter size and materials. The key filter specification provided by the manufacturer is known as the filter coefficient, Fauske explained, expressed in units of moles hydrogen per second per mole fraction difference across the filter.
However, shielded containers are made of much thicker materials than conventional containers. In order for the hydrogen to escape from the container, it must first pass through a channel drilled into the shielding material, known as the flow path, then through the filter and out into the surrounding atmosphere. The rate at which hydrogen escapes from the container depends upon the difference in hydrogen concentration between the two sides of the filter.
Because shielding keeps the fuel, and the bulk of the hydrogen, away from the filter, the hydrogen flow rate through the filter is reduced. So, removal of hydrogen through any filter is less effective in a shielded container than it would be for the same filter on an unshielded container. This can mean that many more filters are required, Fauske said.
Hydrogen removal from shielded containers poses a greater problem in some cases than others. For systems where the hydrogen source is radiolysis, this is usually not an issue. However, for systems where the hydrogen source is chemical reactions, the source rate is typically much larger than from radiolysis, and this might make hydrogen removal impractical.
Fauske said: “We have performed modelling for [a] design which demonstrates that the efficiency of the double-bore system can be in the range of 80% to 90%, depending upon the geometry and the hydrogen concentration in the container. Thus, the number of filters required for a given application may be, for practical purposes, unaffected by the extra resistance offered by the shielding, and at worst it is only weakly affected.
“We have confirmed the expected performance of the dual-bore design through experiments for different filter types and variation in hydrogen concentration.”