Evaluation of Neutron Flux Distribution for Radioactivity Analysis of Biological Shield Concrete Required for Study on Decommissioning of Tokai Power Station

INTRODUCTION The Tokai Power Station (Tokai 1), a 166MWe gas-cooled graphite-moderated reactor of advanced Calder-Hall type, ceased commercial operation in March 1998 after 32 years of service, and spent fuels are being discharged from the core at present for preparation of decommissioning the reactor. Plans for the decommissioning that include future dismantling work, have been studied since the beginning of nineties. Measurement and prediction by analysis of neutron flux distribution and radioactivity of the biological shield concrete had been performed for several years before the permanent cessation of the reactor, and also are being continued. The obtained data have been used to evaluate the area affected by neutron irradiation and the corresponding rank of burial disposal of radioactive wastes. These evaluations are essential to prepare specific decommissioning plans and estimate decommissioning cost and so need precise estimation of radioactive concentration. The biological shields formed from ordinary concrete consist of primary and secondary ones. On the north and south of the reactor room surrounded by the primary shield, there exist areas where gas ducts of the primary cooling system are located between the primary and secondary shields. These areas are so called duct enclosures. In the primary shield composing the duct enclosure, there exist several penetrations with large openings for primary cooling gas ducts, air blast ducts called as shield cooling ducts that lead cooling air to the inner surface of the primary shield facing the reactor vessel and rupture doors. The building structure of the duct enclosure and the locations of the penetrations are shown in Figure 1. These penetrations cause neutron streaming through the primary shield from the reactor room, and enhance neutron flux level of the duct enclosure and radioactive concentration of the surrounding shield concrete and also steel components during the reactor operation. Existence of these penetrations forms complex distribution of neutron flux in the duct enclosure together with two cylindrical openings for vertical piping of primary cooling gas ducts and a large square central opening in the boundary floor between neighboring stories. This paper presents evaluation of neutron flux distribution and radioactivity of concrete cores by measurement and analysis for estimation of the radioactive concentration at the dismantling work of the biological concrete shield composing the duct enclosure area.