Conceptual design of a cold methane moderator system for the European Spallation Source (ESS)

As part of the work for the target station of the planned European spallation source (ESS) the Central Department of Technology at the Forschungszentrum Juelich GmbH is also concerned with the moderators, particular attention being given to the development of cold methane moderators. This report discusses the technical feasibility of solid methane moderators. Methods to tailor the neutron output by adding absorption materials (decouplers or poisons) are not considered here, neither are composite moderators. Based on the given target-moderator-reflector assembly of the ESS project a concept for the ESS cold methane moderators has been developed and is being examined at the Forschungszentrum Juelich. According to this moderator concept the moderator is a fixed bed of small spheres, which makes moderator container filling homogeneous and reproducible. Since spheres form a defined packed bed, cooling of the moderator bed by H2 is reliable. The process of filling the moderator container and of removing the pellets is batchwise to ensure complete removal of the pellets, so that no spent methane pellets accumulate in the system. For removal of the moderator spheres the fixed bed in the moderator container is fluidized with subsequent hydraulic transport of the pellets. The spent methane pellets are separated from the transport fluid and the methane is released over the stack or purified and reused. Depending on the kind and amount of the radioactive isotopes present these may have to be separated and stored. (orig.)