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Conference Article: Irradiation performances of the SUPERPHENIX type absorber element

Kryger, B.; Gosset, D. (CEA Centre d'Etudes de Saclay, DRN/DMT/SEMI/LEMA, Cedex (France)); Escleine, J.M. (CEA Centre d'Etudes de Cadarache, DRN/DEC/SDC.LEMC, Cedex (France))

Abstract

Several aspects of the irradiation behaviour of the SUPERPHENIX type absorber element are presented in this paper. A large programme of irradiation tests was performed in PHENIX to assess and to improve the absorber pin design whose main characteristics for the first load are: a sodium bonded and vented pin with high density (96 % TD) and highly enriched (up to 90 at % of boron 10) B4C pellets. We present and discuss the main post-irradiation results obtained by this programme with concerns the behaviour of both B, C pellets (fragmentation, swelling, helium release, thermal conductivity evolution) and stainless steel clad (embrittlement by carburization, mechanical interaction). It appears that the residence time of the first load of SUPERPHENIX control rods is clearly limited by mechanical interaction between B& and the clad, and particularly by relocating of small fragments of B,C at beginning of life in the initial gap. The irradiation performed in PHENIX led to fix the residence time of the first load of control rods to 240 e.f.p.d. The analyses of the effects limiting the residence time have enabled us to propose an extension of this time by two measures. The first one is reduction of the capture rate in boron carbide. This measure was brought into operation by mean of lowering at 48 at % the boron 10 enrichment of the B4C pellets in the lower part of the pin. The second measure is preventing the fragment relocation by adoption of a thin stainless steel shroud enclosing the pellet stack. The efficiency of these measures was proved in several irradiation tests (ANTIMAG experiments) in PHENIX. A burn-up of 220 x 10’20 capt/cm3 was achieved without any dimensional change of the pin diameter. The shroud failed but could nevertheless prevent any pellet cladding deformation. Thus, these results have enabled us to fix a residence time of 640 e.f.p.d for the third load of the SUPERPHENIX control rods. The achievement in the future of lifetime up to 1 000 e.f.p.d. will require the development of both advanced absorbing materials and pin designs.

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key words: control elements; design; fbr type reactors; irradiation; neutron absorbers; optimization; performance; post-irradiation examination; reliability; safety

Reference:: Technical committee meeting on absorber materials, control rods and designs of backup reactivity shutdown systems for breakeven cores and burner cores for reducing plutonium stockpiles. Obninsk (Russian Federation). 3-7 Jul 1995; International Atomic Energy Agency, Vienna (Austria); IAEA-TECDOC--884, pp:124-137