PRODUCTION OF ANNULAR AND COMPACT TYPE BURNABLE ABSORBER NUCLEAR FUEL PELLETS BY POWDER METALLURGY AND SOL GEL ROUTE

Year 2020, Volume: 32 Issue: 1, 19 - 42, 23.09.2020

Ahmet Yaylı

Abstract

One of the major challenges of the nuclear industry is to improve the performance, safety and lifetime of reactors. In this area there are many efforts focused on the research and development of new materials in order to extend the fuel lifetime, increase the burn-up and optimize the power density distribution. With this aim a neutron absorber material is usually incorporated into the UO2 nuclear fuel. Gadolinium is an excellent burnable poison because it presents a large cross section for neutron absorption and allows the compensation of the excess reactivity of the fuel in the beginning of its life. The need to improve reactor performance through longer cycle lengths or improved fuel utilization has been apparent since the beginning of commercial nuclear power generation. Among several modifications introduced as a consequence, the fuel initial enrichment has been increased, which means that the additional amount of fissile material (235U) in the reactor core has to be compensated by the introduction of additional neutron absorber material in the reactor core. This compensation was initially achieved only by using neutron absorber materials assembled in control rods or/and by addition of soluble absorber (boric acid) in the reactor coolant. In Boiling Water Reactors (BWR), the use of soluble absorber in the coolant/moderator was prohibited for technological reasons. In Pressurized Water Reactors (PWR), boric acid as a soluble absorber added to the coolant/moderator has been routinely used, but the increase in initial fuel enrichment cannot be indefinitely compensated by increasing the boric acid concentration. Beyond a certain concentration, thermal expansion of water at start-up reduces the quantity of boron in the core, resulting ultimately in a positive moderator reactivity coefficient, which is an unacceptable situation regarding to the safe reactor operation. This is the reason why the introduction of solid burnable absorbers (or burnable poison) within the fuel rods was considered. The use of a burnable poison in nuclear reactors provides the necessary negative moderator reactivity coefficient at the beginning of core life and help to shape the core power distributions (Yayli,1995, Böhm, 1987 ) The poison material should have a high neutron absorption cross section and form daughter products with low absorption cross sections. Then, as soon as the irradiation proceeds, the burnable poison burns up and the macroscopic absorption cross section decreases. From a nuclear viewpoint, gadolinia is an excellent burnable poison, having a high neutron absorption cross section coupled to a burn up rate that, if properly designed, can match approximately the 235U depletion, minimizing the reactivity penalty at the end-of-cycle (EOC) (Stogen, 1982, Brandberg 1973)

Keywords

Burnable Absorber, nuclear fuel, gadolinia, annular type pellets, , solgel

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