Abstract

In the case of a closed fuel cycle, minor actinides transmutation can lead to a strong reduction in spent fuel radiotoxicity and decay heat. In the heterogeneous approach, minor actinides are loaded in dedicated targets located at the core periphery so that long-lived minor actinides undergo fission and are turned in shorter-lived fission products. However, such targets require a specific design process due to high helium production in the fuel, high flux gradient at the core periphery and low power production. Additionally, the targets are generally manufactured with a high content in minor actinides in order to compensate for the low flux level at the core periphery. This leads to negative impacts on the fuel cycle in terms of neutron source and decay heat of the irradiated targets, which penalize their handling and reprocessing. In this paper, a simplified methodology for the design of targets is coupled with a method for the optimization of transmutation which takes into account both transmutation performances and fuel cycle impacts. The uncertainties and performances of this methodology are evaluated and shown to be sufficient to carry out scoping studies. An illustration is then made by considering the use of moderating material in the targets, which has a positive impact on the minor actinides consumption but a negative impact both on fuel cycle constraints (higher decay heat and neutron) and on assembly design (higher helium production and lower fuel volume fraction). It is shown that the use of moderating material is an optimal solution of the transmutation problem with regards to consumption and fuel cycle impacts, even when taking geometrical design considerations into account.

Highlights

  • Minor actinides transmutation is the process of removing selected nuclides (Am, Cm and Np) from the waste and submitting them to a neutron flux in order to turn them into fission products

  • The optimization methodology of the heterogeneous transmutation strategy developed in [11] was coupled here with the assembly pre-design algorithm described above. This methodology is based on the characterization of the entire transmutation process in the blankets based on four parameters, namely: – the r-factor, which is an estimator of the neutron spectrum in the blankets

  • It was shown that consumption in excess of 8.8 kg per TWhe could be obtained with a fully optimized geometrical assembly design characterized with wide pins, thick cladding and high gases expansion plenum, this within expected constraints linked to fuel reprocessing

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Summary

Introduction

Minor actinides transmutation is the process of removing selected nuclides (Am, Cm and Np) from the waste and submitting them to a neutron flux in order to turn them into fission products. They experience a high level of neutron flux which increases the performances of the process This has several drawbacks, the main one being that minor actinides loading leads to a hardening of the neutron spectrum, which has potentially negative impacts on the core feedback coefficients [3]. This leads to a pollution of the entire fuel cycle with minor actinides, which are strong alpha and neutrons emitters and limits the flexibility of the transmutation process as it becomes dependent on the fuel management. The same methodology will be reviewed and illustrated using the example of neutron spectrum modification in the blankets

Design of minor actinides bearing blankets
Description of the optimization process
Uncertainty analysis of the meta-model approach
Analysis of an example: the case of the moderated approach
Qualitative analysis
Impact of spectrum softening on the inventory
Geometrical optimization of the assembly
Findings
Complete optimization with a genetic algorithm
Full Text
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