Operating conditions of fuel elements in NRE reactor (and test conditions of NRE fuel elements in IVG-1 reactor) are associated with essential presence of thermal stresses in them, because all the fuel elements heat from the internal energy is selected by the hydrogen flow pumped through the fuel assembly. The most intense heat removal from the surface of the fuel element (creating a radial temperature drop AT over its cross-section) takes place in the inlet HS, operating in the temperature range of brittle state that means the first and second HS in the 6-cell FA or the first, second, and third HS in the 8-cell FA.

In the course of the reactor steady-state operation, the temperature drop in each fuel element increases from zero to maximum and remains at that value during the

Fig. 7.4 The radial distrib­ution of temperature T, the axial thermal stresses Ot and axial residual radiation stress Or resulting in NRE fuel ele­ments at various stages of its operation in flowing techno­logical channels

start-up (the parabolic form of the temperature distribution in cross-section of fuel element is shown in Fig.7.4). Temperature drop value is determined by the heat flux qs from the surface of a fuel element, thermal conductivity X of the fuel element material and the radius r. This temperature drop AT creates dangerous thermal macro stresses ot with tensile component at the fuel element surface (see the diagram of axial ot in Fig.7.4). The values of the axial and tangential tensile stresses ot can be found [8] from the formula:

ot = aEAT/[2(1 — v)]= a Eqsr/[4X(1 — v)], (7.1)

where a—coefficient of thermal expansion; E—Young modulus; v—Poisson’s con­stant.

Thermal stresses ot pose a real danger for fuel elements operating only at tem­peratures up to 1,600K. Because in these conditions, the brittle refractory carbides will not have relaxation capability of these time elastic macro stresses. If the stress ot exceeds the tensile strength of fuel element material there will be cracks on its surface. Cracks in themselves do not affect the operation of the fuel elements, but their appearance leads to very negative consequences, as in the future failure of fuel rods weakened by cracks by bending, bandage and vibration loads (bending loads occur in the fuel elements in the event of the longitudinal beams twisting in HS, and bandage—under the influence of radiation thickening of fuel elements).

Along with the negative aspect of hazardous thermal stresses presented in the fuel elements there is a positive sign consisting of fuel elements hardening due to the appearance of compressive residual radiation macro stresses on their surfaces or (see diagram of axial or in Fig.7.4). Residual stresses or arise again because of the presence of radial temperature drop AT in the irradiated fuel element, which provides radiation swelling heterogeneity of the fuel element material. Due to different rates

of annealing of resulting radiation defects volume of peripheral (cooler) regions of the fuel element will increase stronger than the internal volume of (hotter) regions. This will ensure the appearance of compressive residual stresses on the fuel ele­ment surface and tensile stresses inside, as the internal regions preventing excessive expansion of the surface regions, will create an elastic compression in them.