Gamma dose measurements and spectroscopy analysis for spallation products in JSNS mercury circulation system

Abstract

Mercury used as a spallation neutron target at JSNS (J-PARC Spallation Neutron Source)� is highly activated by proton beam operation. In order to evaluate the feasibility of hands-on maintenance after nominal beam operation with 1 MW in the where a mercury circulation system was installed, gamma dose measurements and gamma spectroscopy analysis were carried out after low power operation during the commissioning period. By analyzing the data, it was found that radioactive spallation products adhered to the inner surface of piping wall of the circulation systemand those significantly raised the dose rates in the cell. In order to discuss about measures for future hot-cell entry, the gamma dose rates after the nominal operation were evaluated based on the measured data obtained in this work. 1) is a multipurpose research facility with muon and neutron generation target. The operation of the J-PARC spallation neutron source (JSNS) in MLF started in May 2008, and an effort is being made now to increase the proton beam intensity up to 1 MW together with many kinds of researches on materials and life science. 2. Mercury Target and Circulation System. In JSNS, mercury was selected as a material of neutron generation target in consideration of the neutron generation efficiency and the cooling. A drawing of the mercury target vessel and a mercury circulation system on a trolley is shown in Fig 1. Neutrons are generated by bombarding mercury in the target vessel with a proton beam, and the mercury heated by incident protons and accompanying nuclear reactions is circulated in the loop between the target vessel and the mercury circulation system for cooling. The target vessel is inserted to the core of the neutron station consisted of moderators, reflector, neutron beam shutters and shields. For the maintenance, the trolley is moved to the hot cell prepared for handling of activated devices. Mercury in the circulation system is highly activated due to radioactive isotopes produced via spallation reactions. Therefore most of the maintenance works for the mercury target and the circulation system were designed to be done by using remote handling system equipped in the cell. However a few works, which is expected to happen once or twice during the facility lifetime of about 30 years, will need to be done by hands-on; this will necessitate entrance of workers to the cell. Mercury in the circulation system can be transferred to the drain tanks under the circulation system on the bottom of the