Stored energy in the graphite of power-producing reactors

Abstract

The effects of the atomic displacements produced in graphite by the collisions of fast neutrons are of great importance in the technology of graphite-moderated nuclear power reactors. This paper describes experimental and theoretical work on the stored energy associated with such displacements, the rate of release of this stored energy as a function of temperature, and the thermal conductivity of graphite, over a range of irradiation and annealing conditions. Some large effects are observed, particularly in the rate of accumulation of radiation damage at various temperatures of irradiation, as well as many other effects of fundamental interest. After high doses at low irradiation temperatures the rate of release of stored energy with increasing temperature can approach the specific heat of unirradiated graphite, but a relatively small increase in the irradiation temperature brings about a drastic reduction in the amount of energy stored. The conditions under which this radiation damage is produced are very complicated, for they involve not only sequences of spontaneous rearrangements amongst the displaced atoms during and after the irradiation treatment but also the total intensity and the energy spectrum of the neutrons, and it is impossible to separate all the variables in a strict sense. Various experiments have however been made, and theoretical and semi-empirical procedures developed, which enable results to be correlated satisfactorily over a range of conditions including those in both graphite-moderated and water-moderated reactors. The paper shows how the rate of stored energy release may be estimated for practical values of temperature and irradiation dose.