Microfuel Behavior Under the Action of Neutron Pulses in the BIGR Reactor

Abstract

The results of experimental and computational investigations of microfuel behavior under the action of neutron pulses in the BIGR reactor are presented. The methods used in the present work made it possible to determine the change in the structure of the irradiated samples, specifically, to record interlayer gap formation in the microfuel and the actual fracture of the microfuel. The results of this work could be helpful for evaluating the service life of microfuel and the consequences of emergency situations in HTGR as well as for developing and perfecting the corresponding computational software. The foundation for developing the HTGR was created back in mid-1990s by advances made in our and other countries in the technology of gas turbines and high-efficiency heat exchangers. It was assumed that an energy conversion system based on the high-efficiency Brayton gas-turbine cycle with efficiency ~50% will be implemented in this reactor [1]. The fuel considered for HTGR was comprised of microfuel: fuel particles with a multilayer protective coating which are homogeneously distributed in a graphite matrix, forming a fuel compact with a prescribed shape [2]. As a rule, microfuel consists of spherical uranium-dioxide particles (kernels) confined in a strong, multilayer, high-temperature shell consisting of layers of pyrocarbon and silicon carbide capable of effectively containing the fission products. The use of microfuel opens up prospects for substantially increasing the fuel temperature in reactors and thereby the efficiency with which atomic energy is converted into electricity. The aim of the present work is to investigate the serviceability of microfuel under the conditions of pulsed nuclear heating. The results of experimental and computational studies of the behavior of samples of microfuel under the action of neutron pulses in the BIGR reactor are presented below. In the experiments, the surface temperature of the microfuel and the energy released in the kernel were determined and possible fracture of the microfuel was recorded. The experiments with fracture of the microfuel approximately reproduced the processes occurring in design-base and unanticipated emergency regimes under conditions characteristic for HTGR. The experiments were accompanied by careful temperature and thermomechanical calculations. Two reasons were proposed for the appearance and development of possible accidents in HTGR: 1) sharp increase in the reactivity, one consequence of which is an increase in the neutron flux density, energy release and temperature of the fuel elements; 2) reduction in or cessation of the coolant flow in the core, directly resulting in an increase in the fuel temperature. An increase in the temperature of the kernels can result in their melting, undesirable chemical reactions and local thermal stresses, i.e., damage to and even destruction of the fuel elements. In addition, this can cause a reduction in the tem