The assumption of 151 damaged fuel rods is more conservative than the existing design and licensing basis value of 125 fuel rods. Consistent with DAEC refueling procedures, a post-shutdown period of 60 hours is credited for radioactive decay in determining the release activity inventory, which is greater than the existing design and licensing basis of 24 hours. All the activity in the gap between the fuel pellets and the cladding of the damaged fuel rods is assumed to be released instantaneously into the pool. A pool water iodine decontamination factor of 200 is used, which is higher than the value of 100 used in the existing licensing basis analysis. NMC assumed no decontamination for noble gases released in the pool and full retention of all aerosol and particulate fission products by the pool water.
Any activity leaving the pool enters the reactor building.
All of the FHA activity is assumed to be released within 2 hours from the reactor building as a ground release, with no credit for holdup or dilution by the reactor building, and no credit for operation of the standby gas treatment system. Not crediting any dilution, holdup, or cleanup by the standby gas treatment system of the activity released from the pool represents a more conservative basis than that used in the existing licensing basis FHA analysis. Therefore, the proposed action to change the TS and the licensing and design bases regarding the design-basis FHA does not represent a significant offsite radiological impact to the human environment. Using the above AST and the updated atmospheric dispersion values, NMC evaluated the dose to operators in the control room assuming that operators manually actuate control room isolation within 10 minutes. The analyses also assumed 1000 cubic feet per minute of unfiltered inleakage into the control room and TSC, even though both areas are designed to be pressurized to preclude such inleakage after an accident.