A nuclear reactor having a remotely removeable and replaceable thermal and/or radiation insulating shield for the head. This shield is formed of interlocked integrated shielding units of thermal insulating material. Each unit has a vertical member and a horizontal member. Each unit has a lifting-and-lowering rod on its top and it may be raised vertically by a remotely operable clamp engaging the head. The interlocking surfaces of adjacent units are so formed that when any unit is raised or lowered vertically its interlocking surface readily slides along the interlocking surfaces of the units adjacent to it with which it is interlocked.