A Parametric survey on advanced shielding subassemblies for reducing power generation in the IVST in the JOYO upgrade program

Kawashima, Masatoshi*; not registered; not registered

The JOYO-upgrade program has been planned for enhancing the present irradiation capacity with increase of the maximum power level and with improving its load factor. To achieve a higher load factor by reducing refueling intervals, the preseut work discussed a possibility of disusing the exclusive IVST pots in a invreased reactor power level. It is reqiured that power generation in the pot has to be suppressed to a level less than that can be coolable by natural convection, by replacing the present stainless steel reflectors to advanced B4C-shielding subassemblies. A parametric survey was made on some of a basic specification of the advanced shielding subassemblies as a concrete approach. The principal results obtained by this survey are as follows. (1)Use of the advanced shielding subassemblies can offer disuse of the exclusive IVST pots even with the upgraded power level up to 1.5 times larger than that of the present power level. (2)To achieve this target, two outermost layers of the reflectors are replaced by the advanced B4C-shielding subassemblies. This modification can be done within a slight alteration of the present flow distribution from lower pressure plenum as is done in the present version of the MK-2 core. (3)A 19-pin structure with 100cm height of the shielding materials in the axial length are proposed as a basie specification. Natural boron will be used for the 9-th row position and condensed boron-10 pellets will be installed for the 10th outermost row. These specifications are determined by effects of shielding ability, pellet maximun temperatures, PCMI and life of the subassembly. (4)The arrangement of the advanced shielding subassemblies makes the power generation of a fuel assembly in the IVST pots reduced considerably to the level which can be cooled by natural convection within a pot. But the power level is almost the same as the allowable upper limits of the power generation in a pot. Further discussion will be needed as for ...