In this work, the fuel assembly of type TVS-2006, which is used in Russian reactor VVER-1200, is considered. A proposed fuel of Thorium and Uranium-233 ((Th232+U233)O2 has been investigated to replace the fuel currently used (U238 as a fertile and U235 as fissile). This has been done in order to compare the; conversion ratio, reproduction factor (η), burnup, effective delayed neutrons fraction (βeff), Minor Actinides (MA) inventory, and Temperature coefficients of reactivity. The calculations performed using the Monte Carlo code (SERPENT-2.31) to analyze and compare the two fuels. Results showed that using U-233 in the Thorium base will considerably increase the reactivity at the Beginning Of Cycle (BOC), increase the conversion ratio, and burn up. Besides, the safety parameters for the proposed Th-based fuel, in general, lower than that for Low Enriched Uranium (LEU) but still acceptable.
The paper presents the results of a computational and theoretical analysis concerned with the use of erbium as a burnable absorber in VVER-type reactors. Partial refueling options for the reactor life extension to 18 and 24 months is considered, the refueling ratio being equal to three for the 18-month life and to two for the 24-month life. 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 neutronic characteristics of the reactor and fuel as burn-up, reactivity coefficients, residual volume of “liquid" control, and amounts of the liquid radioactive waste (LRW) formed was assessed.
The calculations were performed using a simplified model of refueling without FA reshuffling. An infinite array of polycells consisting of FAs with different in-core times was considered. The escape of neutrons from the core was taken into account by selecting the critical value K∞ at the end of life.
Erbium does not burn up in full for the lifetime which affects the fuel burn-up as compared with the liquid excessive reactivity compensation system. The reduction is 0.7% per 0.1% of the erbium weight load in the fuel elements. This, however, also reduces the maximum content of the boron absorber in the coolant and the LRW accumulation 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 its minor weight fraction in fuel elements, and a reduction in the boron absorber fraction leads to a positive value of the density reactivity coefficient. As a result, the overall coolant temperature reactivity coefficient has a negative value throughout the lifetime.
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