Study on bulk shielding for a spallation neutron source facility in the high-intensity proton accelerator project

Under the JAERI-KEK High-Intensity Proton Accelerator Project, a spallation neutron source driven by a 3 GeV-1 MW proton beam is planed to be constructed in a main part of the Materials and Life Science Facility. This report describes results of a study on bulk shielding performance of a biological shield for the spallation neutron source by means of a Monte Carlo calculation method, that is important in terms of radiation safety and cost reduction. A shielding configuration was determined as a reference case by considering preliminary studies and interaction with other components, then shielding thickness that was required to achieve a target dose rate of 1 mu Sv/h was derived. Effects of calculation conditions such as shielding materials and dimensions on the shielding performance was investigated by changing those parameters. By taking all the results and design margins into account, a shielding configuration that was identified as the most appropriate was finally determined as follows. An iron shield region extends up to 4.8 m from the center of the source, and it is surrounded by a magnetite concrete shield. For regions at backward angles larger than 105 degrees with respect the proton beam direction, the magnetite concrete shield extends up to 6.4 m from the center. The thickness increases as the angle decreases from 105 degrees to 0 degrees (forward angle), where the magnetite concrete shield reaches to 8.0 m from the center.