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Enhancing Nuclear Safety


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Low-moderated 4.738-wt.%-enriched uranium dioxide fuel rod arrays


Congress title :ICSBEP
Congress location :CHICAGO

Summary

A low-enriched and low-water-moderated array composed of UO2 (4.738 wt. % 235U) fuel rods at a 1.075-cm or 1.1-cm square pitch is considered in this experimental programme. These experiments were subcritical approaches extrapolated to critical, with the multiplication factor reached being very close to 1.000 (within 0.1 %). Four experiments are evaluated of which all are acceptable for use as benchmark experiments. These experiments were carried out in the testing equipment called “Apparatus B” in the experimental criticality facility of CEA research centre at Valduc in 1998. The experimental programme is described in the basic report (see Reference 1). Fuel rods used in these experiments are Zircaloy “reclad rods”. Further data about these rods will be gathered in an additional report. The main concern of nuclear manufacturers being to store the highest quantity of fuel assemblies in limited areas, a solution consists in decreasing the pitch between fuel rods in order to decrease the reactivity of individual assemblies. However, a strict validation of calculation routes is needed to ensure that calculations for such pitches are correct. That is the purpose of this benchmark. The total uncertainty in terms of reactivity is around 0.170%, which is may be overestimated. It should be noticed that the keff is very sensitive to the external rod diameter and the pitch between rods. The approach consisting in dividing the propagated keff by square root of 9 to account for the compensations due to fluctuations of the population is questionable. However, a division by square root of total number of rods would underestimate the result. The right uncertainty value is surely between these two values.

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