Investigation of the thermal inertia and thermal stresses of heat-transferring elements in a solid-coolant nuclear reactor

Computational studies of specific questions arising when solids are used in nuclear reactors as the cooling medium are described. The concept of using approximately 1 mm in diameter spherical heat-carrying elements made of pyrolytic carbon coated graphite as a medium for transmitting heat from a fuel element to a steam generator is examined. A computational analysis of the internal stresses arising in 1–10 mm in diameter spherical elements transferring heat and the temperature lag of the heat-carrying elements relative to the temperature of the medium under cyclic heating and cooling is performed. The results of experimental studies were used to determine the boundary conditions of the problem. Supplying heat uniformly over the surface and through a finite number of contact points, which is characteristic for a fill consisting of spherical particles, was modeled. The transmission of heat through a finite number of contacts results in a complicated stress state of the heat-transmitting elements and a higher thermal inertia. It is shown that the internal stresses are weak in small heat-carrying particles, but when the diameter is increased to 10 mm the stresses from thermal cycling become comparable to the ultimate strength.