To suppress the increase of core diameter by making void coefficient negative without drastically lowering the height of core.
In a core of reactor constituted by arranging long size fuel assemblies 2 and short size fuel assemblies 3 in a square grid, the long size fuel assemblies comprise axial blankets 4 at the upper and the lower parts of the long size fuel parts and the short size fuel assemblies 3 comprise water excluding vessels 5 at the upper part of the short size fuel parts and axial blankets 4 at the lower part. The water excluding vessel 5 is provided with a gas purge hole at the upper part and a coolant inlet hole at the lower part. As steam stagnates in the water excluding vessel 5 in normal operation and excludes cooling water, neutron leak from the water excluding vessel 5 is enhanced during increasing of void coefficient in the region outside the water excluding vessel 5, which maintains the pressure in the water excluding vessel 5 equal to the reactor pressure, and reactivity variation during power rising is suppressed.
WATANABE SHOICHI
UEDA KIYOSHI
TSUBOI YASUSHI
