Advances in Control Assembly Mtls for Water Reactors (IAEA

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Microscopic (cm'1) (barn) 755 ±2 3813 104 descendants absorption resonances (bam) abs. 3 ± 1 55 hafnium as cladding material only as far as those properties be improved : the elaboration of a 'Hafnaloy', which would be improved in the same extent than zirconium into 'Zircaloy', is required. 3. AN ADVANCED ABSORBER : HfB? We present here a brief review of the main properties of this compound. Hafnium diboride is a very refractory material, its melting temperature is about 3380 °C (table 2) [7]. It is a ceramic which has then a brittle mechanic behaviour.

On an efficiency and cost point of view, boron carbide is very interesting but it undergoes an important swelling and an extended fracturation [1]. A significant part of those damages come from important capture gradients throughout the pellets radius, due to the high thermal neutron absorption of 10B, leading to self-shielding effects. This could be reduced by the use of other cladding materials, with a high neutron absorption cross-section. Hafnium is such a material and different studies are in progress, aiming first to the elaboration of hafnium tubes [2], [3] and second to the estimation of the efficiency of control rods consisting of a hafnium 49 cladding containing B4C pellets, which is related in section 4.

A combination of thermomechanical and irradiation-induced phenomena are involved : Slump and creep of the Ag-In-Cd absorber under the effect of compression by the rodlet spring; the effect of slump is to bridge the gap between the absorber and the cladding. The higher the absorber temperature is, the greater is the slump. Irradiation-induced swelling: neutron flux induces changes of absorber phase and transmutation of Indium into Tin, Silver into Cadmium, and creates microstructural defects. 62 CARDS CONTINUOUS GUIDE FUEL ASSEMBLY FIGURE 1 TYPICAL RECORDED WEAR FOR A LOW MOBILITY RCCA The two phenomena act together to close the gap between absorber and cladding.

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