JP2835104B

PURPOSE:To prevent the contact of control rods and a moderator by segmenting a control rod channel to two regions; an upper region and lower region, and setting the width of the coolant flow passage in the lower region wider than the width of the coolant flow passage in the upper region. CONSTITUTION:The control rod channel 10 inserted with the control rods 12 is segmented to the two regions; the upper region and the lower region. The width of the coolant flow passage in the lower region is set wider than the width of the coolant flow passage in the upper region. Since gaseous helium as a coolant flows down from above to below in the control rod channel 10, the temp. of the gaseous helium and the moderators 6a, 6b increase successively toward the lower part. Since the width of the coolant flow passage in the lower region of the control rod channel 10 is set wider than the width of the coolant flow passage in the upper region in this case, the contact of the control rods 12 with the control rod channel 10 occurs in the upper region part and the heating up of the control rods 12 is prevented in the event of such contact.