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Conference Article: PBMR Fuel Sphere Source Terms

C.C. Stoker, F. Reitsma, PBMR (Pty) Ltd, Centurion, SOUTH AFRICA

Abstract

A thorough evaluation of the fuel sphere source terms, that is, the determination of the nuclide inventories, photon sources and decay heat, is fundamental to any reactor design. The accurate calculation of the fuel sphere source terms depends to a great extent on the one-energy-group neutron spectrum averaged cross-sections used for the fuel depletion analyses. The continuous reloading scheme of the Pebble Bed Modular Reactor (PBMR) necessitates special considerations, since the fuel is not stationary. As a fuel sphere moves through the reactor, the neutron flux spectra changes continuously due to, amongst other reasons, temperature, neutron leakage, spatial position, material composition and reactor power. In this paper the behaviour of the one-group microscopic cross-sections, as the fuel circulates through the core, is evaluated and the effect on the fuel sphere source terms quantified. VSOP-99 and the ORIGEN-JÜl-99 code system, which uses time-dependent multi-pass reactor data generated by VSOP-99, are used. From the results it is clear that the different rates at which the fuel spheres move through the reactor core, and the different flux values and flux spectra in the different areas in the core, cause the fuel depletion to become largely dependent on the space time history of the fuel sphere. A simplified one-dimensional reactor model in SCALE was used to generate for the average equilibrium core conditions, a single ORIGEN-S cross-section library. The ORIGEN-S fuel depletion with this cross-section library is probably acceptable for scoping evaluations, but should be used with caution for specific reactor design applications.

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key words: PBMR; Decay Heat; Depletion; ORIGEN-JÜl; VSOP-99; ORIGEN-S
Reference:
Proceedings of the Conference on High Temperature Reactors, Beijing, China, September, 22-24, 2004
International Atomic Energy Agency, Vienna (Austria)
HTR-2004, pp:1-15