Choosing an upper bound medium among U-Pu mixtures for criticality studies, when the maximum uranium enrichment and plutonium content is not the most penalizing hypothesis
Congress title :ICNC 2007
Congress location :Saint-Petersbourg
Congress date :28/05/2007
In the framework of the use of mixed nuclear fuels, different uranium-plutonium mixtures can be processed with varying parameters like uranium enrichment, plutonium content and isotopic composition. In criticality safety evaluation of U-Pu configurations, it is often assumed that the maximum uranium enrichment together with the maximum plutonium content is the most penalizing hypothesis.
However, this paper describes two types of configurations where such an assumption is not valid and may lead to a wrong choice of the upper bound medium:
- in the case of mass control expressed in fissile material mass (235U and all the isotopes of Pu) combined with a limitation on the maximum plutonium content, the mixture with the maximum uranium enrichment may not be the upper bound medium;
- in the case of mass control combined with a moderation control expressed in water volume, the mixture with the maximum plutonium content may not be the upper bound medium.
The first one of these two problems comes from incoherence between the two control variables, raising the possibility that an increase in uranium enrichment cause a decrease in plutonium mass. The second one comes from 235U, which is more reactive than 239Pu for intermediate moderation ratios.
Three plutonium isotopic compositions (100/0/0/0, 95/5/0/0, 71/17/11/1) in homogeneous U-Pu-H2O mixtures are studied, with uranium enrichment varying between 0.72 and 100%, plutonium content between 0 and 100%, and moderation ratio between 0 and 1000.
The main results are presented and, to conclude, this paper determines, for both configurations, keys to accurately choose an upper bound medium:
in the case of mass control expressed in fissile material mass, domains are expressed in terms of uranium enrichment and plutonium content, in which the minimum critical mass either increases or decreases with uranium enrichment,
in the case of mass control combined with a moderation control expressed in water volume, limits are given on uranium enrichment, for which considering as an upper bound medium the one with either the maximum or 100% plutonium content is valid (outside these limits, mixtures with various Pu content must be studied).
It must be noticed that all the limits and domains given in this paper are only valid for the three plutonium isotopic compositions studied and for homogeneous mixtures.