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

Abstract

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.This article presents the results of a computational study of specific particulars of using a solid substance as a coolant in nuclear reactors. The internal stresses in heat-transmitting elements of the coolant during heat transfer and, correspondingly, thermal cycling and thermal inertia during the propagation of heat inside the elements fundamentally distinguish a solid coolant from liquids and gases. To ensure that heat is transferred from the core to a secondary heat exchanger various designs were proposed for large-size heat-transferring elements which are caused to move in special directions by driving mechanisms.The concept of using small heat-carrying particles moving through the core and heat exchangers without orientation and only under gravity with direct contact with the surface of the fuel elements was recently proposed and validated. To realize this variant, the optimal nuclear and thermophysical properties of the coolant must be combined, and properties securing the ability to move under gravity through a core with constant porosity must be imparted to the particles. In [1] it is proposed that the technology for fabricating microfuel elements be used to make heat-carrying particles similar in shape and size to microfuel. The heat-carrying particles do not contain nuclear fuel and consist of a graphite kernel and a pyrolytic carbon coating. These particles combine a small neutron-absorption cross section, high heat capacity, and the ability to withstand high