ICONE11-36554 AXIAL OFFSET ANOMALY : EFFECT OF USING ENRICHED BORIC ACID AS CHEMICAL SHIM ON B^<10> HOLDUP IN CRUD
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概要
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A significant number of current pressurized water reactors (PWRs) are experiencing axial offset anomaly (AOA), a condition that continues to elude prediction and thus creates an operational difficulty of being unable to accurately estimate safety margin. The cause of AOA is complex, involving both thermal-hydraulics and chemistry of core operation and design. Corrosion products deposit preferentially as a porous layer in the upper portion of the core and this porous deposit is termed "crud". The axial shift in neutron flux takes place because of the uptake of neutron absorbing B^<10> into the crud from the chemical shim in the coolant. The present chemical shim used in PWRs in the US is Natural Boric Acid (Nat. BA) with 19.8%B^<10>. The use of Enriched Boric Acid (EBA) in PWRs in the US is envisaged because of the conversion of fuel to MOX within a span of 5 to 10 years. The presence of increased amount of Pu^<239> in MOX fuel increases reactivity and causes higher energy levels of neutrons. The absorption cross-section of B^<10> for higher energy neutrons is lower and hence the need for more B^<10>. The required B^<10> can be introduced in two ways : 1) Using EBA, with the same concentration of Boric Acid. 2) Using Natural Boric A cid (Nat. BA), but with higher concentration of Boric Acid. This paper compares the amount of B^<10> holdup in the crud for these two cases. It is found that the B^<10> adsorbed at any given depth in the crud will be lower when EBA is used if the enrichment level is higher than a threshold value. This threshold value is governed by the affinity of the crud to the components in solution and the local concentrations of the components. The local concentrations across the depth of the crud vary exponentially and hence the threshold values are a function of the depth of the crud. The total B^<10> adsorbed across the crud will be lesser when the Enrichment level is above an effective threshold value, which can be determined from the accumulated effect over the depth of the crud. A simplified one-dimensional concentration model is used to show that the B^<10> holdup due to formation of Ni, Fe, Zr compounds will be the same for both cases. From this analysis we can conclude that, the B^<10> holdup in the crud will be lower when EBA is used, if the enrichment level used is above the effective threshold value, which is governed by the adsorption parameters.
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著者
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Radhakrishnan S
Department Of Nuclear Plasma And Radiological Engineering University Of Illinois At Urbana-champaign
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Jones Barclay
Department of Nuclear, Plasma and Radiological Engineering University of Illinois at Urbana-Champaig
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Jones Barclay
Department Of Nuclear Plasma And Radiological Engineering University Of Illinois At Urbana-champaign