Optimization of Moderator to UO₂ Ratio for 16×16 and 17×17 Fuel Assemblies with Varied Enrichment and Boron Concentration

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Introduction

The moderator-to-fuel ratio is the ratio of the number of moderator nuclei within the volume of a reactor core to the number of fuel nuclei. As the core temperature increases, the density of moderator decreases with thermal expansion which causes a hardening of neutron spectrum in the reactor core resulting in higher resonance absorption. In practice, water-moderated reactors are designed with a moderator – fuel ratio so that the reactor is operated in an under-moderated condition as an increase in temperature results in the addition of negative reactivity, and the reactor becomes more selfregulating. With this, composite effects of moderator density, fuel temperature, and other phenomena must be balanced to ensure system stability under all operating conditions.

Methodology

The simulations in this report were done using CASMO3, and the configuration was set for no fuel zoning. For both 16×16 and 17×17 assemblies, the fuel enrichment was varied to be 3.0% and 4.5%, as well as the boron concentration to be 1000 ppm and 10 ppm. The reference state condition was set to constant where T_f (fuel temperature) = 960.95 K, T_m (moderator temperature) =586 K. For each case, k-inf (at zero burnup) vs moderator-fuel ratio plots were generated from the simulation results. The highest point of the plot represents the optimum moderator – fuel ratio.

Results and Discussion

It is found that as fuel enrichment was increased from 3.0% to 4.5%, the optimum moderator – fuel ratio also increases for both assemblies and boron concentration. Figure 1 shows a super-imposed graph of k-inf vs moderator – fuel ratio for all cases. An increase in the moderator-to-fuel ratio decreases k-inf due to the dominance of the decreasing thermal utilization factor. Below this point, a decrease in the moderator-to-fuel ratio decreases k-inf due to the dominance of the increased resonance absorption in the fuel. When boron concentration is increased, the optimum moderator – fuel ratio decreases for the same enrichment and fuel assembly. This is obvious because of the presence of boron in the coolant that aids moderation as boron is a neutron poison due to its neutron absorbing properties. For the comparison between two assemblies for same values of boron concentration and fuel enrichment, 17×17 assembly has higher moderator to fuel – ratio compared to its corresponding configuration for 16×16 assembly. It is also observed that curves for both assemblies are almost identical and has close values for k-inf.

Conclusions

In this report, the optimum moderator – fuel ratio is determined when k-inf reaches its maximum value. The effect on the moderator – fuel ratio by varying boron concentration, enrichment, and fuel assembly configuration is presented. When fuel enrichment is increased, optimum moderator – fuel ratio increases, while it decreases with increasing boron concentration. Moreover, there is no significant difference on the k-inf and moderator – fuel ratio for both assemblies.