United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 79-12: Short Period Scrams at BWR Facilities

                               UNITED STATES 
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C.  20555

                                May 31, 1979

                                                      IE Bulletin No. 79-12 

SHORT PERIOD SCRAMS AT BWR FACILITIES 

Summary: 

Reactor scrams, resulting from periods of less than 5 seconds, have occurred
recently at three BWR facilities.  In each case the scram was caused by high
flux detected by the IRM neutron monitors during an approach to critical. 
These events are similar in most respects to events which were previously 
described by IE Circular 7707 (copy enclosed). 

Description of Circumstances: 

The following is a brief account of each event. 

1.   Oyster Creek - On December 14, 1978, the reactor experienced a scram as
     control rods were being withdrawn for approach to critical, following 
     a scram from full power which had occurred about 15 hours earlier.  The
     moderator temperature was 380 degrees F and the reactor pressure was 
     190 psig.  Because of the high xenon concentration the operators had 
     not made an accurate estimate of the critical rod pattern.  The 
     operator at the controls was using the SRM count rate, which had 
     changed only slightly, (425 to 450 cps) to guide the approach.  Control 
     rod 10-43 (first rod in Group 9) was being withdrawn in "notch 
     override" to notch position 10, when the reactor became critical on an 
     estimated 2.8 second period.  The operator was attempting to reinsert 
     the rod when the scram occurred.  Failure of the "emergency rod in" 
     switch to maintain contact due to a bent switch stop, apparently 
     contributed to the problem. 

2.   Browns Ferry Unit 1 - On January 81, 1979, the reactor experienced a 
     scram during the initial approach to critical following refueling.  The
     operator was continuously withdrawing in notch override" the first 
     control rod in Group 3 (a high worth rod) because the SRM count rate 
     had led him to believe that the reactor was very subcritical.  A short 
     reactor period, estimated at 5 seconds, was experienced.  The operator 
     was attempting to reinsert control rods when the scram occurred. 
.

IE Bulletin No. 79-12                                       May 31, 1979 
                                                            Page 2 of 3 

3.   Hatch Unit 1 - On January 31, 1979, the reactor experienced a scram 
     during an approach to critical. Control rod 42-15 (fifth rod in Group 
     3) was being continuously withdrawn in "notch override" when the scram 
     occurred, with a period of less then 5 seconds. The temperature was 
     about 200 degrees F with effectively zero xenon. 

As indicated above, these short period trips occurred under a wide variety 
of circumstances. They did have several things in common, however. In none 
of these cases was an accurate estimate of the critical position made prior 
to the approach to critical. In each case a rod was being pulled in a high 
worth region. Finally, in each case the operator, believing that the reactor
was very subcritical, was pulling a rod on continuous withdrawal. 

Action to be Taken by Licensees: 

For all GE BWR power reactor facilities with an operating license: 

1.   Review and revise, as necessary, your operating procedures to ensure 
     that an estimate of the critical rod pattern be made prior to each 
     approach to critical. The method of estimating critical rod patterns 
     should take into account all important reactivity variables (e.g., core
     xenon, moderator temperature, etc.). 

2.   Where inaccuracies in critical rod pattern estimates are anticipated 
     due to unusual conditions, such as high xenon, procedures should 
     require that notch-step withdrawal be used well before the estimated 
     critical position is reached and all SRM channel indicators are 
     monitored so as to permit selection of the most significant data. 

3.   Review and evaluate your control rod withdrawal sequences to assure 
     that they minimize the notch worth of individual control rods, 
     especially those withdrawn immediately at the point of criticality. 
     Your review should ensure that the following related criteria are also 
     satisfied: 
     
     a.   Special rod sequences should be considered for peak xenon 
          conditions. 

     b.   Provide cautions to the operators on situations which can result 
          in high notch worth (e,g. first rod in a new group will usually 
          exhibit high rod worth). 

4.   Review and evaluate the operability of your "emergency rod in" switch 
     to perform its function under prolonged severe use. 
.

IE Bulletin No. 79-12                                       May 31, 1979 
                                                            Page 3 of 3 

5.   Provide a description of how your reactor operator training program 
     covers the considerations above (i.e., items 1 thru 3). 

6.   Within 60 calendar days of the date of issue of this Bulletin, report 
     in writing to the Director of the appropriate NRC Regional Office, 
     describing your action(s) taken, or to be taken, in response to each of
     the above items. A.copy of your report should be sent to the United 
     States Nuclear Regulatory Commission, Office of Inspection and 
     Enforcement, Division of Reactor Operations Inspection, Washington, 
     D.C. 20555. 

For all BWR facilities with a construction permit and all other power 
reactor facilities with an operating license or construction permit, this 
Bulletin is for information only and no written response is required. 

Approved by GAO B180225 (R0072); clearance expires 7/31/80. Approval was 
given under a blanket clearance specifically for identified generic 
problems. 

Enclosures: 
1.   IE Circular No. 77-07 
2.   List of IE Bulletins Issued 
          in Last Twelve Months  
Page Last Reviewed/Updated Tuesday, July 23, 2013