IE Circular 77-07, Short Period During Reactor Startup
CR77007
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
IE CIRCULAR 77-07, SHORT PERIOD DURING REACTOR STARTUP
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
Enclosures:
1. Circular 77-07
2. Draft transmittal letter
.
Transmittal letter for Circular 77-07 to each holder of a BWR Operating
License.
Addressee:
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.
Signature
(Regional Director)
Enclosure:
IE Circular 77-07
.
Circular 77-07 Date:
Page 1 of 2
SHORT PERIOD DURING REACTOR STARTUP
DESCRIPTION OF CIRCUMSTANCES:
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.
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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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