Safety Calculations and Benchmarking of Babcock and Wilcox Designed Close Spaced Fuel Storage Racks

Responding to the existing industry concern over spent fuel storage capacity, fuel storage racks have been redesigned to give closer spaced storage capabilities. In this redesign, the Babcock and Wilcox (B&W) objective was to offer an optimum balance between maximum storage and minimum cost, with an adequate safety margin. Design conditions were:

1. B&W Mark “B” and Mark “C” pressurized water reactor fuel assemblies
2. Uo₂ at 3.5 wt% enriched or MO₂ at 4.5 wt% enriched
3. k_eff ≤ 0.95.

To achieve good geometric representations of the fuel assemblies and storage racks, KENO Monte Carlo computer codes were used. (Parametric studies were made of k_eff versus can spacing, thickness, and boron concentration for an infinite storage pool) The UO₂ studies used KENO II with a 16-energy-group neutron structure and the standard Hansen-Roach library. The MO₂ studies used KENO IV with a 12 3-energy-group structure. The KENO calculations were then bench-marked against critical experiment data. Systematic errors caused by biases in the calculational techniques were identified by comparing (benchmarking) calculated values with experimental values. Random errors caused by statistical errors in Monte Carlo calculations, and material and fabrication tolerances were combined before being added to applicable systematic errors. Final results indicate that with a lattice spacing of 33.02 cm (13 in.) and a 0.340-cm (0.134-in. )-thick unborated stainless-steel can, all nuclear safety, thermal-hydraulic, and seismic design requirements are met. This basic design is a conservative approach to the fuel storage problem that utilizes current technology, and offers an optimum balance of the design variables at the minimum cost with an adequate safety margin. For some plants, where variables such as fuel enrichment are different, closer spaced racks using nonpoisoned stainless-steel cans can be utilized. The same basic design can also be used to retro-fit operating plants and increase storage capability from the conventional cores (1-yr discharge) to over cores (8-yr discharge).