Document: NRC Regulatory Guide
Document ID: 4d46a966-d280-43da-9b03-8b0abe7b29ce
Document Type: regulatory_guide
Title: Alternative Radiological Source Terms for Evaluating Design Basis Accidents at Nuclear Power Reactors (Rev. 1)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2120/ML21204A065.pdf
Revision Date: 2023-05
Chapter: 
Section ID: RG-1.183
CFR Part: 
CFR Title: 

Content:
ved isotopes, the 2011 ANS-5.4 release model standard recommends multiplying the best estimate predictions by a factor of 5.0 to obtain upper tolerance release fractions. DG-1389, Appendix J, Page J-6 J-3.2 For long-lived isotopes, established model uncertainties associated with the NRC-approved fuel rod thermal-mechanical code should be applied, either deterministically or sampled within a statistical application methodology, to obtain high-confidence upper tolerance release fractions. J-4. Nominal fuel design specifications (excluding tolerances) may be used. J-5. Actual in-reactor fuel rod power histories may diverge from reload core depletion calculations because of unplanned shutdowns or power maneuvering. As a result, the rod power history or histories used to predict gap inventories should bound anticipated operation. Rod power histories used in the fuel rod design analysis based on core operating limits report thermal-mechanical operating limits or radial falloff curves should be used. The fuel rod power history used to calculate gap inventories should be verifiable. J-5.1 The calculation supporting the bounding gap inventories in Table J-1 used a segmented power history (SPH) for both the boiling-water reactor and pressurized-water reactor limiting designs. Seven different power histories were considered, with each running at 90 percent of the bounding rod average power, with the exception of running at the linear heat generation rate limit for approximately 9 to 10 gigawatt-days per metric ton of uranium (GWd/MTU) burnup (rod average) at seven different burnup intervals. Given that no single fuel rod will dominate the bounding power envelope, a SPH approach is an acceptable alternative to assigning fuel rod power at the maximum, burnup-dependent power level over the fuel rod lifetime. J-6. Higher local power density (Fq) promotes more local FGR. Higher rod average power (Fr), along with a flatter axial power distribution (Fz), promotes more FGR along the