Use of erbium as a burnable absorber for the VVER reactor core life extension
The paper presents the results of a computational and theoretical analysis dealing with the use of erbium as a burnable absorber in VVER-type reactors. Partial refueling options for the reactor life extension (to a year and a half to two years) are considered, the refueling ratio being equal to three for the former and to two for the latter. Erbium is expected to be present in all fuel elements in the FA with the same weight content.
The influence of the erbium weight content on such reactor and fuel neutronic characteristics as burn-up, reactivity coefficients, residual volume of «liquid control», and volumes of the liquid radioactive waste formed has been estimated. Calculations were performed based on a simplified refueling model without FA reshuffling. An infinite mesh of polycells, consisting of FAs with different in-core times, was considered. The neutron escape from the core was taken into account through the selection of the critical value KҐ at the end of life.
Erbium does not burn up altogether for the core life which affects the fuel burn-up reduction as compared with the liquid system for excessive reactivity compensation. The reduction is 0.7% per 0.1% of the erbium weight load in the fuel element. This, however, also reduces the maximum content of the boron absorber in the coolant and the accumulation of LRW in the ratio of 5% per 0.1% of the erbium weight load.
Erbium influences the spectral component of the coolant temperature reactivity coefficient which turns out to be negative even with a minor weight fraction in fuel and the decrease in the boron absorber fraction leads to a positive value of the density reactivity coefficient. As the result, the total coolant temperature reactivity coefficient has a negative value throughout the life.
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