Storage of burned PWR and BWR fuel

In the last few years, credit for fuel burnup has been allowed in the design and criticality safety analysis of high-density spent-fuel storage racks. Design and operating philosophies, however, differ significantly between pressurized water reactor (PWR)- and boiling water reactor (BWR)-type plants because: (1) PWR storage pools generally use soluble boron, which provides backup criticality control under accident conditions; and (2) BWR fuel generally contains gadolinium burnable poison, which results in a characteristically peaked burnup-dependent reactivity variation. In PWR systems, the reactivity decreases monotonically with burnup in a nearly linear fashion (excluding xenon effects), and a two-region concept is feasible. In BWR systems, the reactivity is initially low, increases as fuel burnup progresses, and reaches a maximum at a burnup where the gadolinium is nearly depleted. In any spent-fuel storage rack design, uncertainties due to manufacturing tolerances and in calculational methods must be included to assure that the highest reactivity (k/sub eff/) is less than the 0.95 US Nuclear Regulatory Commission limit. In the absence of definitive critical experiment data with spent fuel, the uncertainty due to depletion calculations must be assumed on the basis of judgment. High-density spent-fuel storage racks may be designed for both PWR and BWR plants with credit for burnup. However, the design must be tailored to each plant with appropriate consideration of the preferences/specifications of the utility operating staff