Document: NRC Regulatory Guide
Document ID: ceaeb6d7-d85a-46d3-a1b6-70059a498965
Document Type: regulatory_guide
Title: 12/2001 (Rev. 1)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0131/ML013100014.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.78
CFR Part: 
CFR Title: 

Content:
The weights presented in the table are directly proportional to the toxicity limit. If a particular chemical has a toxicity limit of 500 mg/m3, the weights from the table (based on 50 mg/m3) should be increased by a factor of 10. Air Exchange Rate The weights in the table are inversely proportional to the air exchange rate. If a control room has an exchange rate of 2.4 per hour, the weights from the table (based on 1.2 per hour) are decreased by a factor of two. In other words, the weights are appropriately adjusted for the actual fresh air exchange rate. The current crop of control rooms with automatic isolation capabilities may have leakage characteristics different from those listed in the Table. Again, appropriate adjustments of weight should be made based on the actual air exchange rate. It should be noted that the use of an air exchange rate of less than 0.06 per hour for an isolated control room requires that the control room leakage rate be verified by periodic field testing. Pasquill Stability Category If it is determined that the worst 5th-percentile meteorology is better (Condition E) or worse (Condition G) than Condition F (the condition used in the table) at a given site, the following adjustments should be made to the table: Pasquill Stability Category Weight Multiplication Factor E 2.5 F 1.0 G 0.4 The Pasquill Stability Category F represents the worst 5th-percentile meteorology observed at the majority of the nuclear power plant sites. REGULATORY ANALYSIS A draft regulatory analysis was published with the draft of this guide when it was published for public comment (DG-1087, February 2001). No changes were necessary, so a separate value/impact statement for Revision 1 of Regulatory Guide 1.78 has not been prepared. A copy of the draft value/impact statement is available for inspection or copying for a fee in the NRC’s Public Document Room at 11555 Rockville Pike, Rockville, MD, under DG-1087. An electronic version of DG-1087 is available in the