Possibility of using mixed dioxide fuel in the VK-300 reactor

Abstract

The problem of utilizing the plutonium accumulated in spent fuel from power reactors has become more acute in the last few years for reasons including the existence of excess weapons plutonium which appeared as a result of nuclear disarmament. Any plutonium is a valuable energy resource, which should be burned with the highest efficiency in fast reactors, but the relatively high construction and operation costs is postponing the commercial utlization of such reactors indefinitely. At the same time, the quite wide prevalence of thermal reactors in power production that are capable of utilizing plutonium in a mixed oxide fuel makes it possible to tackle the solution of this problem now.Substantial practical progress in this respect has been achieved in some countries in western Europe [1]. They include France, Germany, Switzerland, and Belgium, where together with investigations on closing the fuel cycle on fast and thermal reactors [2], recycling of mixed oxide fuel in light-water reactors is being successfully implemented. At the end of 1997 the number of such reactors reaches 25, while in France the number reached 14. The total load of mixed oxide fuel in these reactors is about 700 tons with respect to the heavy metal (h.m.).In our country practical experience in utilizing mixed oxide fuel has been gained only for fast reactors [3], but the possible use of this fuel in thermal BBER and RBMK reactors is under investigation [4, 5]. The problems of recycling fuel are not being considered for the time being. Primarily very simple schemes for adding 'plutonium to the fuel cycle of these reactors are being examined. These schemes essentially consistof replacing some of the uranium fuel with uranium-plutonium fuel without any large changes in the construction of the fuel assemblies and the core. The investigations show that the use of plutonium in water-moderated water-cooled reactors, such as the VK-300 reactor, changes the neutron-physical characteristics of the core:The efficiency of absorbers (control rods and boric acid) decreases because of hardening of the neutron spectrum. The energy of the thermal neutrons approximately doubles when 235U is replaced by plutonium.The temperature coefficient of the reactivity with respect to the moderator becomes more negative (approximately by 40%), while the Doppler coefficient remains almost unchanged.The delayed-neutron fraction decreases by a factor of 1.5-2. The coefficient of nonuniformity of energy release increases. These changes can complicate the reactivity compensation system and the transient and accident operating regimes and they can also decrease the thermotechnical reliability of the reactor plant. At the same time, analysis of variants of BBER-1000 and RBMK cores with up to 1/3 of the fuel assemblies containing mixed oxide fuel showed that the reactivity coefficients and the nonuniformity of energy release in such reactors remain within admissible limits. Similar conclusions can also be drawn with respect to foreign PWRs (see, for example, the investigations i...