Patent Number: 
Section: claims

1. A nuclear reactor including: a nuclear reactor vessel containing a body of coolant liquid defining a coolant liquid surface therein,  a guard vessel spaced from said nuclear reactor vessel to form an annulus space therebetween, and  a thermal load reducing system for controlling thermal stresses in said nuclear reactor vessel comprising:  partition members disposed in said annulus space at a position above the surface of the coolant liquid body in said reactor vessel,  means for circulating a low temperature gas via the annulus space and above the partition members to cool down the adjacent reactor vessel wall disposed above said coolant liquid surface, and  means for circulating a higher temperature gas via the annulus space from a level lower than the surface of the coolant liquid body to the partition members for raising the temperature of the reactor vessel wall disposed below the surface of the coolant liquid body within the reactor vessel. 2. The thermal load reducing system in a nuclear reactor vessel according to  claim 1 , wherein said low-temperature gas is circulated at a constant flow velocity during normal operation of said nuclear reactor, and said high-temperature gas is circulated only during starting operation of said nuclear reactor. claim 1