Effect of the Burnable Absorber Arrangement on the VVER(1200 Fuel Assembly Neutronic Performance
Optimizing the use of fuel in a power reactor is a task of current concern. However, little attention has been given to investigating the dependences among the enrichment used, the content of gadolinium oxide in fuel rods, and the life time in combination with evaluating the efficiency of using Gd loaded fuel elements with different Gd2O3 contents. The paper considers fuel assembly versions for VVER-1200 reactors having different enrichments for fuel elements, including those with Gd, and different contents of gadolinium oxide in fuel. A comparative analysis is presented for assemblies with homogeneous Gd2O3 arrangements in each fuel element and with profiled Gd2O3 arrangements. In the latter case, profiling depends on the neutron flux density in the layer which includes fuel elements with Gd. This suggests that the arrangement of gadolinium oxide proportionally to the neutron flux density will improve the FA neutronic performance. The results were obtained using SERPENT (a continuous-energy multi-purpose three-dimensional Monte Carlo particle transport code). The assemblies with the used parameters for a 12-month fuel cycle have shown the method under consideration to be inefficient for a period of over 300 eff. days. With an increased enrichment of the gadolinium oxide contained, the use of profiled versions has turned out to be more rational for longer periods (up to 900 eff. days). Therefore, this phenomenon is relevant for the reactor life whereas it proves to be insignificant for the fuel life. It is emphasized that the complex relationship between the content of gadolinium and uranium in the assembly and the effective multiplication factor for the profiled assembly and the initial assembly should be considered in detail.
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