# Nuclear Science and Techniques

《核技术》(英文版) ISSN 1001-8042 CN 31-1559/TL     2019 Impact factor 1.556

Nuclear Science and Techniques ›› 2019, Vol. 30 ›› Issue (8): 129

• NUCLEAR ENERGY SCIENCE AND ENGINEERING •

### Recomended strategy and limitations of burnable absorbers use in VVER fuel assemblies

Lenka Frybortova 1,2

1. 1 UJV Rez a.s., Hlavni 130, 250 68 Husinec - Rez, Czech Republic
2 Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Brehova 7, 115 19 Praha 1, Czech Republic
• Received:2018-12-13 Revised:2019-04-16 Accepted:2019-04-17
• Contact: Lenka Frybortova E-mail:hle@ujv.cz
• Supported by:
This work was supported by the Technology Agency of the Czech Republic (No. TE01020455).
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Lenka Frybortova. Recomended strategy and limitations of burnable absorbers use in VVER fuel assemblies.Nuclear Science and Techniques, 2019, 30(8): 129
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Abstract: There is an obvious effort to increase the burn up of used fuel assemblies in order to improve fuel utilization. A more effective operation can be realized by extending the fuel cycles or by increasing the number of reloadings. This change is nevertheless connected with increasing the uranium enrichment even above 5% of 235U. Burnable absorbers are widely used to compensate for the positive reactivity of fresh fuel. With proper optimization, burnable absorbers decrease the reactivity excess at the beginning of the cycle, and they can help with stabilization of power distribution. This paper describes properties of several materials that can be used as burnable absorbers. The change in concentration or position of the pin with a burnable absorber in a fuel assembly was analyzed by the HELIOS transport lattice code. The multiplication factor and power peaking factor dependence on fuel burn up were used to evaluate the neutronic properties of burnable absorbers. The following four different materials are discussed in this paper: Gd2O3, IFBA, Er2O3, and Dy2O3. Gadolinium had the greatest influence on fuel characteristics. The number of pins with a burnable absorber was limited in the VVER-440 fuel assembly to six. In the VVER-1000 fuel assembly, 36 pins with a burnable absorber can be used as the assembly is larger. The erbium depletion rate was comparable with uranium burn up. Dysprosium had the largest parasitic absorption after depletion.