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Description
The Second Target Station (STS) Project at Oak Ridge National Laboratory (ORNL) is currently completing the final design of components related to Target Systems and Instrument Systems. These components include the rotating water-cooled tungsten target, two liquid-hydrogen moderators, instrument bunker shielding, and many others. Following completion of the Proton Power Upgrade (PPU) at the Spallation Neutron Source (SNS), the proton accelerator now produces 1.3 GeV protons at a frequency of 60 Hz. One quarter of these pulses will deliver 700 kW of beam power to the STS target.
This paper discusses analyses of the prompt dose rates associated with the STS standard heavy neutron beamline shutter when the standard heavy shutter attenuator intercepts the neutron beam inside the instrument bunker. The shutter is closed when the attenuator is rotated into the beam to shut off neutron transmission and allow users to change samples. The standard heavy neutron beamline shutter refers to the full assembly, including the shutter attenuator, drive motor, and associated components. The primary objective of these analyses is to evaluate the performance of the shutter attenuator downstream of the shutter, in the direction of the instrument sample location.
There are two major design requirements for the standard heavy neutron beamline shutter:
1) The standard heavy shutter attenuator shall reduce the total effective dose rate to less than 2 mrem/h at 3 m from the downstream face of the attenuator in white beam.
2) The instrument bunker shielding shall limit the total effective dose rate outside the bunker to less than 0.25 mrem/h at 30 cm from surfaces in generally accessible areas, including when the standard heavy shutter is in the closed position.
These analyses were completed using MCNP 6.2. The results show that the standard heavy shutter design meets the specified design requirements with an attenuator consisting of twenty-nine 2.5 cm thick plates. The first twenty plates on the upstream (moderator) side are tungsten, and the remaining nine plates on the downstream (instrument) side are carbon steel.
This paper also describes model creation using GEOUNED, generation of variance reduction parameters with ADVANTG, and the methodology used to ensure adequate sampling of neutrons traveling along the beamline. It also compares these results with those for the primary shutters at the Spallation Neutron Source.
| Topics | T4. Shielding and dosimetry |
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