Radiation transport analysis in support of the Spallation Neutron Source target station neutron beam line shutters

The Department of Energy has initiated the design and construction of the Spallation Neutron Source (SNS) to be built at the Oak Ridge National Laboratory, The SNS consists of an accelerator system that will provide protons, which will strike a liquid mercury target inside the shielding monolith, and through the spallation nuclear reaction process produce neutrons. The neutrons that are produced are slowed down via four moderators that surround the mercury target. These moderated neutrons will then exit the shielding monolith via 24 possible different neutron beam lines, through 18 independent primary shutters, to the neutron experiment area. Each neutron beam line can be closed by raising that beam line's shutter, which will place a shutter neutron beam stop in the path of neutrons traveling towards the neutron experiment area. The neutron experiment area is where scientists and engineers will be able to perform neutron scattering experiments. This work involved the design of the materials and configuration of the shutters to meet radiation limits on the outside surface of the shielding monolith and in the neutron experiment area, while trying to reduce construction cost. This analysis resulted in a new shutter design so that the peak dose rate on the outside surface of the shielding monolith is about 0.1 mrem/hr, which satisfies the project limit of 0.25 mrem/hr. A neutron beam line plug was also designed, so that the peak dose rate on the outside of the shielding monolith is still 0.1 mrem/hr in the case of uninstrumented neutron beam lines. Lastly, the design of the shutter neutron beam stop inside of each shutter was optimized to allow a peak dose rate of 1.0 mrem/hr in the neutron experiment area when the shutter is in the closed position. This meets the project limit of 2.0 mrem/hr. The entire analysis amounts to a total cost savings of about 958 thousand dollars.