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:
rradiated fuel to infer characteristics of aerosols under light-water reactor severe accident conditions. The SOAR recommends a log-normal distribution for aerosols in the reactor coolant system (AMMD 1.0 ߤ݉ with a geometric standard deviation, ߪ௚, of 2.0), and provides PHÉBUS-FP aerosol measurements in containment (AMMD of 3.0 ߤ݉ and ߪ௚ of 2.0). Considering the MHA LOCA modeling approach which considers no pipe break and where the deposition properties after reflood are based on the characteristics of the containment aerosol (i.e., considers the effects of an active emergency core cooling system) the methods in Appendix A, Regulatory Positions 5.6.2 and 5.6.3 should assume a log-normal aerosol diameter distribution with an AMMD of 2.0 ߤ݉ and ߪ௚ of 2.0. Assume as fixed values, a ܥ௦(݀௔) of 1 and viscosity of 1.93x10-5 Pa-sec. At least 10,000 trials are necessary to develop a settling velocity distribution dataset. Note that while methods discussed in Regulatory Positions A- 5.6.2 and A-5.6.3 are, in part, discussed in Reference A-19, Reference A-19 does not establish regulatory positions. For example, regarding Reference A-19, input parameters such as the AMMD, assumed in example calculations and statements regarding the validity of the existing 20-group method are not endorsed in this regulatory guidance. The multi-group method should include the following assumptions and steps to estimate removal coefficients: a. Discretize the settling velocity dataset into at least 2,000 equal-width groups. Assign relative probabilities for each group by dividing the number of data points within each group by the sample size (e.g., 10,000 trials) to determine the group probabilities. Identify the mid-point of each group to represent the settling velocity for that group. b. Compute each group, aerosol filter efficiency using the following method. By rearranging Equations 2 and 3 from Reference A-16, the filter efficiency, ߟ௙௜௟௧, is computed by