ADVANCES IN THE DEVELOPMENT OF A SUBGROUP METHOD FOR THE SELF-SHIELDING OF RESONANT ISOTOPES IN ARBITRARY GEOMETRIES

The subgroup method is used to compute self-shielded cross sections de fined over coarse energy groups in the resolved energy domain. The val idity of the subgroup approach was extended beyond the unresolved ener gy domain by partially faking into account correlation effects between the slowing-down source with the collision probability terms of the t ransport equation. This approach enables us to obtain a pure subgroup solution of the self-shielding problem without relying on any form of equivalence in dilution. Specific improvements are presented on existi ng subgroup methods: an N-term rational approximation for the fuel-to- fuel collision probability, a new Pade deflation technique for computi ng probability tables, and the introduction of a superhomogenization c orrection. The absorption rates obtained after self-shielding are comp ared with exact values obtained using an elastic slowing-down calculat ion where each resonance is modeled individually in the resolved energ y domain.