Document: NUREG-0800
Document ID: e3249d77-0411-430a-9793-06aea8669075
Document Type: srp
Title: FISSION PRODUCT CONTROL SYSTEMS AND STRUCTURES
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0520/ML052070474.pdf
Revision Date: 2023-06
Chapter: 6
Section ID: 6.5.3
CFR Part: 
CFR Title: 

Content:
y Guide 1.52. The reviewer 92 should consult with ETSBPERB concerning filter system efficiencies. 93 The exhaust systems may be one of several designs. Common designs are: (1) Straight exhaust through charcoal and high-efficiency particulate air (HEPA) filters. Primary containment leakage to these regions 94 is assumed to go directly to the filter with no mixing or holdup in the region being filtered. (2) Recirculation system with split inflow (some exhausted through filters and some recirculated to the region being treated). Primary containment leakage to the region being treated is assumed to be directly to the intake of the recirculation fan. There, a fraction of it (the ratio of exhaust to total flow) is exhausted through the filters; the balance is then assumed to return to the region being treated. The placement of the system intake and return headers is examined to determine that return flow from the fans does not have a direct path to the intake again. Credit for mixing in 50% of the region is given for fission products returned by the DRAFT Rev. 3 - April 1996 6.5.3-10 recirculation system to the secondary volume if the header placement is satisfactory. (3) Other variations on the recirculation system are (a) filters in the recirculation line, (b) filters in both the recirculation line and the exhaust line, and (c) high exhaust flow to reduce the negative pressure to several centimeters (inches) water gauge, and then no 95 exhaust with recirculation only for some time period. The sizing of the system fans for the volumes they are maintaining at a negative pressure may be critical in determining the ratio of exhaust flow to recirculation flow. Past history shows secondary containment structures are considerably more leaky than applicants anticipated (2 to 5 times as great as anticipated), and fan exhaust flows have been increased after testing to account for this. (When identical flow rates are predicted for two volumes which differ by a factor of 10 or