Resolution of Generic Safety Issues: Issue 25: Automatic Air Header Dump on BWR Scram System ( NUREG-0933, Main Report with Supplements 1–34 )
This issue concerns the slow loss of control air pressure in the scram system of BWRs.31 Air pressure dropping at a certain rate will first allow some of the CRD scram outlet valves to open slightly, thus filling the scram discharge volume with water but allowing little or no control rod movement. Eventually, the rods will try to scram but the scram will be impaired in a manner similar to what happened at Browns Ferry Unit 3 on June 28, 1980.74 Meanwhile, the dropping air pressure can cause a transient (e.g., via feedwater controller lockup) which would normally call for a scram.
Immediate action taken by the staff following the Browns Ferry event resulted in a technical resolution of the issue. The generic safety evaluation report30 identified new requirements for scram discharge volume designs. These requirements have been approved and implemented in the forms of orders125 from the Division of Licensing, dated January 9, 1981 to BWR licensees. Thus, this issue has been RESOLVED.