Abstracts and Available Papers Presented at the
2000 International RERTR Meeting

A Sophisticated Computational Method for HEU-LEU Conversion
of the German FRJ-2 Research Reactor Using MCNP

R. NABBI, G. Thamm, J. WOLTERS

Research Center Jülich
Leo-Brandt-Straße
52425 Jülich, Germany

Abstract

Due to a more than four-year shutdown of FRJ-2, for comprehensive inspection and refurbishment work, the HEU-LEU conversion plan was extended beyond the original time schedule. In the meantime a sophisticated computational method has been developed as a requirement of the licensing authority– for reactor physics calculations, core conversion studies and fuel element performance analysis. For this purpose, an MCNP model in very high fidelity was developed and coupled with a depletion code. The fuel elements, absorber arms, and the moderator zones were highly segmented, and all core and reflector components were included.

The results of the calculation shows that the reactivity value of each core at any burnup step is predicted with a high accuracy (within the 2s confidence interval). The flux and burnup distribution in all core configurations with HEU and individual LEU fuel elements, respectively, are also in very good agreement with the experimental results. In the case of a single LEU fuel element, the neutron flux is approx. 5 % lower than in an HEU fuel element resulting in an extended irradiation time in the reactor core.

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