A case study to bound the search space of the optimization problem for the PSBR beam tube

We present a simple methodology for reducing the extent of the search space of the modular optimization code package developed for the size and shape optimization of the beam tube assembly at the Penn State Breazeale Reactor (PSBR). In this method, we express the origin of the neutron output at the beam tube exit in two components depending on the location of their last scattering collision in (i) the Bi gamma shield; or (ii) the moderator (e.g. H₂O or D₂O) illuminating the beam tube. We compute the contribution of these two components to the neutron flux at the beam tube exit by performing numerical experiments using the three-dimensional particle transport code TORT on a model configuration. We illustrate the results of this approach with various moderator materials, comparing the strength and spectrum of the outgoing neutron beam, and indicating how those affect the search space size. Results demonstrate that the neutrons originating at the beam tube base contribute more output neutrons at the beam tube exit than neutrons from all other origination locations. Hence, this result enables defining a small search space, thus reducing the optimization procedure's computational time.