ML20084G288
| ML20084G288 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 12/14/1972 |
| From: | Worden W COMMONWEALTH EDISON CO. |
| To: | Anthony Giambusso US ATOMIC ENERGY COMMISSION (AEC) |
| References | |
| 6957, NUDOCS 8304210553 | |
| Download: ML20084G288 (2) | |
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50437 Commonwealth Edison Company 73 WEST ADAMS STREET
- CHICAGO. ILLINOIS Ake e teoly rez POST OPPICE SOX 747
- CHICAOO, IL LIN OIS 4049 0 Dresden Nuclear Power Station
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"I';r Morris, Illinois 60450 December 14, 1972 A-
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Mr. A. Giambusso V.. /
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Deputy Director for Reactor Projects
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$pA;p,'7 U. S. Atomic Energy Co= mission
s Washington, D.C.
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Subject:
License DPR-19, Dresden Nuclear Power Station, Section 6.6.C.2 of the Technical Specification
Dear Hr. Giambusso:
Upon re-evaluation of an event which occurred on August 17, 1972; i.e., a Dresden Unit #2 scram occurred during startup from a IRM HiHi flux trip, we wish to inform you of the event that occurred on that date.
_ PROBLEM AND INVESTICATION On August 17, 1972, Unit #2 was in the process of being brought critical following a reactor scram at 1845 the same day.
The reactor had a moderator temperature of approximately 4300F and had a Xenon inventory in excess of the full power equilibrium condition.
At 2330, a check of the Startup Range Monitors indicated only suberitical multiplication cnd the Intermediate Range Monitors vere downscale. At this time the operator began withdrawing continuously from notch 2 to notch 10, the second control rod (B-07) in group #8 of sequence B-E, Rev. 1 (6/19/72), the neutron flux increased rapidly.
' Die Intermediate Range Monitors responded and exceeded 1207., thus initiating a reactor scram on "IRM HiHi flux".
A review of the startup and intermediate range recorders indicated that a short period had resulted from the withdrawal of control rod B-07.
f' Prior to a subsequent startup, investigation of the control rod pattern at the time of critical indicated that control rod B-07 was one of j
the strongest control rods in the reactor.
The conditions of the reactor, high temperature and maximum Xenon, made it necessary to withdraw more
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control rods than usual for criticality.
This results in withdrawing control rods that are no longer in a checker board pattern, a situation not planned for in the sequence.
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n Mr. A. Giambusso December 14, 1972 CORRECTIVE ACTION For the subsequent startup it was decided to " bank" withdraw group #8 in a notchwise fashion in order to minimize the amount of reactivity inserted in any one withdrawal.
The reactor was brought critical at nearly the same state conditions in an orderly manner.
Analysis of the worth of control rod B-07 at the time of the scram, and analysis of the sequence itself was requested from Produc' tion Nuclear Reactor Analysis Department (Commonwealth Edison) and General Electric Company. A temporary revision to the sequence was provided by G.E. which consisted of taking the same action that was employed by the station when pulling the second critical.
General Electric also recommended bypassing one group in the sequence until higher power has been reached.
Computer analysis of the situation indicated that control rod B-07 had a worth of approximately 1.50 to 1.70%Ak/k and notches 0 to 10 accounted for 707. of the total vorth. No reactivity anomaly existed and there were no safety related problems during this incident or subsequent startups.
The worth of the rod is less than 2.5% o k as specified in the Technical Specifications.
A permanent revised sequence has been issued for Unit 2 and Unit 3 to provide a broader range of geometrically low worth rods so that elevated temperatures and maximum Xenon conditions will not force the critical to be attained at a point where red worths are enhanced by the lack of voids and distortion of the rod worth due to axial Xenon transients. As additional protection against high notch worths during maximum Xenun conditions and elevated temperatures (scram recovery)3 an operating order has been issued that requires bank withdrawal, in a notch wise fashion, of rod Group #5 and subsequent Groups till criticality is achieved.
3.m[ j Ihm-fx.W.P.Worden Superintendent WPW:sdb l
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