IE Circular 77-07, Short Period During Reactor Startup


                                APR 12 1977 

J. P. O'Reilly, Director, Region I
N. C. Moseley, Director, Region II
J. G. Keppler, Director, Region III
E. M. Howard, Director, Region IV
R. H. Engelken, Director, Region V 


The subject document is transmitted for issuance by close of business three 
days after date of this letter. The Circular should be issued to only 
holders of BWR Operating Licenses. Also enclosed is a draft copy of the 
transmittal letter. 

                                        Dudley Thompson, Acting Director 
                                        Division of Field Operations 
                                        Office of Inspection and Enforcement

1.   Circular 77-07
2.   Draft transmittal letter

Transmittal letter for Circular 77-07 to each holder of a BWR Operating 


The enclosed circular 77-07 is forwarded to you for information. If there 
are any questions related to your understanding of the actions required, 
please contact this office. 

                                        (Regional Director) 

IE Circular 77-07 

Circular 77-07                                          Date:              
Page 1 of 2 



Recent events of concern to the NRC occurred at the Monticello causing 
Dresden BWRs involving inadvertent high reactivity in Short periods during 
reactor startup. 

At Dresden Unit No. 2 on December 28, 1976 during a reactor startup 
following a scram from unrelated causes about 9 hours earlier, a rod 
withdrawal of one notch resulted in a rapid power rise associated with a 
reactor period of about one second and caused an Intermediate Range Monitor 
(IRM) Hi-Hi flux scram. The IRM was on its most sensitive scale. The 
moderator was essentially without voids and the reactor water temperature 
was 338F. A similar event occurred at this facility on August 17, 1972. 

At Monticello on February 23, 1977, following a reactor scram about 10 hours
earlier from unrelated causes, a reactor period of about one second was 
experienced during startup before the reactor tripped on IRM Hi-Hi flux. The
IRM was on its most sensitive scale and the short period resulted from the 
withdrawal of a control rod one notch. The reactor moderator had few voids 
and the water temperature was 480F. 

The two most recent events were similar in the following respects: 

1.   Prior to the earlier, unrelated scram, both plants had been operating 
     at or near full power with axial flux peaking in the bottom portion of 
     the core. 

2.   The time form the earlier scrams to the subsequent startups maximized 
     the xenon concentrations in the core. 

3.   High worth rod locations were similar and both plants were using the 
     same generic control rod pattern (identified as B1) 

4.   Prior to the IRM scram at both facilities, dramatic indications of high
     notch worth had been seen with rod withdrawals resulting in periods 
     ranging from 10 to 30 seconds, which were terminated by reinsertion of 
     the rod. 

Circular 77-07                                          Date:              
Page 2 of 2 

Review of the events showed that all of the systems including the reactor 
protection system functioned as required. Analyses indicate that the 
combination of essentially no voids in the moderator and high xenon 
concentration accounted for the conditions that resulted in the control rod 
notch acquiring an unusually high differential reactivity worth which 
approximated one-half percent delta K/K at Monticello. This excessive worth 
of rod notch was the result of essentially no voids in the moderator and 
peak xenon conditions which necessitated the withdrawal of significantly 
more control rods than is normally required to reach criticality. The 
resultant flux distribution at criticality magnified the normal axial 
peaking at the top of the core due to the heavy xenon concentrations at the 
bottom. Additionally, the radial contribution to flux peaking was enhanced 
due to the withdrawal of peripheral rods. 

A review of NRC records showed that after the earlier event at Dresden Unit 
No. 2 on August 17, 1972, corrective measures were taken for the subsequent 
startup consisting of notchwise withdrawal of the group of rods. This 
corrective action was taken only for that operating cycle. 

Evaluation of these events indicates that essentially trouble-free startups 
can be accomplished by avoiding the peak xenon with no moderator voids 
condition or possibly by the use of a rod pattern developed for these 
particular conditions. 

These events indicate a need for all licensees of operating BWRs to review 
their startup procedures and practices to assure that their operating staff 
has adequate information to perform reactor startups avoiding such short 
periods in the event that the above described conditions of peak xenon with 
no moderator voids exist at the time of startup. Operators should be made 
aware that extremely high rod notch worths can be encountered under these 
conditions. The procedures should include requirements for a thorough 
assessment following the occurrence of a short period before any further rod
withdrawals are made. These considerations should be included in the 
operator training and requalification training programs. 

No written response to this Circular is required. If you need additional 
information regarding this matter contact the Director of the cognizant NRC 
Regional Office. 


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