By James P. Burlebach, PhD, Manager, Waste Technology & Post-Fukushima Services, Fauske & Associates
Nuclear waste that contains metallic spent nuclear fuel pieces or sludge generates hydrogen as a product of chemical reactions and from radiolysis of water. These waste materials may be stored in shielded boxes with filtered vents for removal of the hydrogen to prevent formation of flammable gas mixtures. Bore holes drilled through shielding add resistance to hydrogen removal that can allow unwanted accumulation of hydrogen.
The innovative work described here was performed by Fauske & Associates, (FAI) a wholly owned subsidiary of Westinghouse Electric Company, LLC, in partnership with Sellafield, Ltd. In partnership, we have conceived, modeled, and experimentally verified an effective method for hydrogen removal from shielded boxes with significant hydrogen generation rates. This innovation minimizes the number of filters required for passive storage of spent metallic nuclear fuel pieces and other hydrogen-generating waste streams.
Many commercially available filters are suitable for removal of hydrogen from unshielded nuclear waste containers such as 200 L drums. The rate of hydrogen removal through a filter varies with filter size and materials. The key filter specification provided by the manufacturer is the filter coefficient, expressed in units of moles hydrogen per second per mole fraction difference across the filter. The size of filter for a given application is chosen based on the hydrogen source rate and the required upper limit for hydrogen concentration.
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 (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. 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.
For example, a shielded container with a bore hole of 20 mm diameter and 300 mm length drilled through the shielding would allow hydrogen to escape at only one-tenth the rate that it would in an unshielded container (in engineering terms, the system efficiency is 10%). As a consequence, the number of filters required would increase tenfold.
The key to hydrogen removal from a shielded container is to reduce the flow resistance such that the filter is the main resistance. We have developed an innovative arrangement to promote hydrogen flow to the filter via a pair of bore holes in the shielded container lid, Figure 1. This design takes advantage of buoyancy induced natural circulation. Gas from the container flows up one of the bore holes into the plenum beneath the filter and then down the other bore hole, returning to the container. We have performed modeling for this design which demonstrates that the efficiency of the double bore system can be in the range of 80% to 90%. This high efficiency minimizes the number of vents required.
Figure 1. Double bore hole arrangement with single filter
For additional information, be sure to stop by Dr. Burelbach's poster session titled Innovative Hydrogen Removal Design for Self-Shielded Containers at the WM2015 Conference this Wednesday, March 18 from 1:30 - 5:00 in the first floor foyer of the Phoenix Convention Center, or contact Dr. Burelbach at 630-887-5221 or burelbach@fauske.com
Nuclear plant, nuclear safety
