2.3. Analysis of Heterogeneous Fuel Geometries

Heterogeneous Pu-MA-Th fuel assembly geometries offer some potential advantages over the homogeneous Pu-MA-Th fuels. Assuming that Pu and MA come as separate streams from the separation process, the relative amounts of Pu and MA in the fuel can be varied so that most of the Pu can be concentrated in one type of fuel assemblies while all of the MA can be concentrated in another type of assemblies. Therefore, for each assembly type, the fuel lattice can be optimized to a certain degree for preferential destruction of either Pu or MA. Additionally, different fuel management schemes can be applied to the fuel assemblies containing different relative amounts of Pu and MA. As a result, assembly types that required to reach higher burnup (and, therefore, higher TRU destruction efficiency) can reside in the core for larger number of cycles than other types of fuel assemblies.

An assessment of these potential advantages for an equilibrium core containing heterogeneous fuel configurations were performed with the CASMO-4 computer code that allows 2D transport and burnup calculations of 2x2 segment (“colorset”) of fuel assemblies of different types. A schematic diagram of such 2x2 colorset of fuel assemblies is shown in Figure 2.3.1.

Th–Pu assembly Th–Pu-MA assembly

Th–Pu-MA assembly

Th–Pu assembly

Figure 2.3.1. Example of CASMO 2x2 colorset layout.

The analysis of heterogeneous fuel configurations was performed through variation of fuel composition (Pu:MA:Th ratio) and variation of lattice H/HM ratio for each assembly type and the destruction efficiencies and destruction rates for Pu and MA averaged over the entire colorset

were examined. At the same time, a number of constraints were imposed in order to ensure feasibility of realistic designs:

- Pu content in Pu-MA-Th assembly is large enough to provide colorset pin-power peak of less than 1.2

- Variation of fuel pin diameter in H/HM optimization is in the range of ± 20 % of the reference one

- Colorset average Pu to MA ratio is the same for all calculated cases and the TRU isotopic vector used is as presented in Table 2.2.IV. - Colorset average TRU loading is selected to provide average colorset discharge burnup corresponding to fuel cycle length of about 18 months assuming 3 batch reloading scheme for both types of fuel assemblies comprising the colorset.

The results of this preliminary analysis show that no significant improvements in TRU destruction efficiencies or destruction rate can be achieved with heterogeneous fuel configurations via lattice optimization under the imposed constraints. The residual fraction of MA in the heterogeneous colorset never exceeded the corresponding value of homogeneous fuel of the same composition if simultaneous discharge of the entire colorset is assumed. This is due to relatively low power fraction generated by the Pu-MA-Th assemblies. However, above 50% of MA can be potentially destroyed (not including U233 and other Th chain nuclides) if Pu-MA-Th assemblies are driven to 100 MWd/kg burnup due to less frequent than Pu-Th assemblies refueling.

Although the Pu-MA-Th assemblies exhibit very flat reactivity behavior during irradiation, the heterogeneous cores have higher power peaking factors than homogeneous cores due to the fact that Pu-MA-Th assemblies are sub-critical during the entire irradiation period.

The calculated colorset average reactivity coefficients are comparable to those of the homogeneous Th-Pu-MA assembly. However, small local βeff in MA containing assemblies in the colorset may challenge the fuel performance in control rod ejection accident. The effect of small βeff will be offset to some extent by slightly more negative Doppler coefficient than in the reference UO2 case.