Evaluation of neutron flux distribution for radioactivity analysis of biological shield concrete required for study on decommissioning of Tokai 1 Power Station

The Tokai Power Station, a 166 MWe gas cooled reactor, ceased commercial operation in March 31, 1998 and at present spent fuels are being discharged. Meanwhile, a specific plan for the decommissioning to be performed after the fuel discharging is being studied. The reactor is surrounded by thick primary biological shield, but large penetrations such as primary cooling gas ducts filled with CO2 and shield cooling ducts and also the large opening covered with thin steel doors called rapture doors exist in the primary biological shield. They cause big neutron leakages into the outer region, called duct enclosure area, surrounded by secondary biological shield, and then enhance the radioactive nuclide concentration in the surrounding shield wall concrete. The radioactive nuclide concentration in this area distributes widely from the low level radioactive waste to below the clearance level. Its precise evaluation, which depends on data of neutron flux distribution, is essential for estimation of dismantling work, dismantling cost, disposal cost of the dismantled waste etc. of the decommission planing. In order to obtain the neutron flux distribution data, a few kinds of activation metal foils were placed at many positions in the space and on the wall surface in the duct enclosure area during the reactor operations. Also neutron transport calculation covering several floors was performed with use of three-dimensional discrete ordinates code, which is suitable for complicated geometry with some neutron streaming paths from the reactor room through the primary shield. By comparing results of calculation and measurement, good correlation was confirmed between them. (author)