MC{sup 2}-3/DIF3D Analysis of the ZPPR-15 Axial Expansion Experiments
The Zero Power Physics Reactor (ZPPR) fast critical facility at the Argonne National Laboratory-West (ANLW) site in Idaho was built in 1969 to obtain neutron physics information necessary for the design of fast breeder reactors. The ZPPR-15 phases A through D were part of a series of cores intended to model a 330 MWt Integral Fast Reactor (IFR) concept. The ZPPR-15 assembly consisted of an inner core zone, an outer core zone of higher fissile content, a thin radial blanket and a thick steel reflector. The inner and outer core zones were of approximately equal volumes. The core zones simulated a sodium-cooled, metal-fueled fast reactor with approximately 10 wt.% zirconium in the target fuel composition. Phases A and B simulated an equilibrium cycle UPu-Zr fuel core where plutonium created in the blankets was sufficient to negate the need for enriched {sup 235}U feedstock. Phase D simulated a beginning of life U-Zr fuel core with enriched uranium fuel although approximately 10% of the fuel in the outer zone of the Phase D core was plutonium because {sup 240}Pu was required to provide a neutron source for the modified source multiplication technique used for many reactivity worth measurements such as sodium void worth in ZPPR. Phase C simulated the intermediate state between beginning of life and equilibrium cycle conditions with 50% enriched uranium fuel and 50% plutonium fuel in the inner and outer core zones. These configurations represent the various stages of reactor operating conditions. Phase A did not contain zirconium (metal fuel is typically alloyed with Zr) while Phases B, C and D did contain zirconium although the limited plate inventory restricted the zirconium zone to the inner core zone. This set of experiments was performed between April 1985 and July 1986 and focused on validation of both the concept and modeling. In each phase, there were multiple loadings for sub-critical, critical, and with regard to this manuscript, axial expansion worth measurements. Combined, these cores can be used to assess the accuracy of physics codes and evaluated cross section data for IFR concept reactors. This paper presents a recent effort that was made on retrieving validation data from the ZPPR-15 experiments and on deterministic modeling and analysis using MC{sup 2}-3/DIF3D for code validation. Axial expansion experiments and ZPPR-15D loading 206 were selected for this analysis. An explicit modeling of the axial expansion (introduction of gaps between sections of drawer) of ZPPR-15D loading 206 yielded a 32% over-prediction in the worth of the axial expansion with the MC2-3/DIF3D models. This indicates a consistent bias between ZPPR-15B and ZPPR-15D with regard to the deterministic modeling of the experiments. As part of future work, we intend on investigating whether the cross section processing or homogenization model is to blame, but we note with such small changes in k-effective in a large model, it is rather difficult to obtain zero errors as is typical for other worth measurements. (authors)