Egress Shielding Studies for the SNS Accelerator System

The radiation transport and dose levels at four egresses located in the Spallation Neutron Source (SNS) accelerator system were analyzed based on controlled and uncontrolled beam losses in the accelerator system. A six-step hybrid Monte Carlo/Discrete Ordinates approach was employed to solve these problems using the Monte Carlo code MCNPX and the discrete ordinates code DORT along with the coupling tools MTD and DTD. MCNPX served to characterize the generation and leakage of secondary radiation from the accelerator and beam line structures, whereas DORT performed the analyses of the radiation fields (neutrons and gammas) in the accelerator tunnels and walkways of the egress. The coupling tools facilitated generation of the boundary sources from one transport step to the next. In this effort, large detailed accelerator models were built for MCNPX to properly describe the different types of linac structures, the beam transport and focusing elements (dipoles, quadrupoles, sextupoles), and the beam collimators. The studies confirmed that the present egress designs were adequate to attenuate the dose in the linac tunnel of up to 100 rem/hr to a level of about 0.5 mrem/hr at the egress exit during normal operation. The egress in the accumulator ring is located at the entrance of the collimator section, a section with a high beam loss rate. For this reason, a dose level in the tunnel of 400 rem/hr was estimated along with a dose level at the exit of the egress of ∼3 mrem/hr, which makes it a limited occupancy radiation area. (authors)