ML19276G800
| ML19276G800 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 07/23/1979 |
| From: | Counsil W NORTHEAST UTILITIES |
| To: | Grier B NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| References | |
| NUDOCS 7908270525 | |
| Download: ML19276G800 (3) | |
Text
74 IMHrrHI!AST IFFII.FFIES
,,%..,n PO BOX 270
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HARTFoAD, CONNECTICUT 06101 (203) 666-6911
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, ll July 23, 1979 Docket No. 50-245 Mr. Boyce H. Grier, Director NRC Regional Office U. S. Nuclear Regulatory Commission Region I 631 Park Avenue King of Prussia, PA 19406
References:
(1)
I&E Bulletin #79-12, "Short Period Serrms at BWR Facilities", dated May 31, 1979.
(2) Letter, R. E. Engle (GE) to D. F. Eisenhut (NRC), "NRC Concerns Regarding Worths of Dropped Control Rods During Reactor Startup with High Xenon", dated April 11, 1978.
Gentlemen:
Millstone Nuclear Power Station, Unit No.1 Response to I&E Bulletin #79-12 Reference (1) states instances of BWR scrams due to high control rod notch worths during startups and specifies five actions to be taken by the licensees, to reduce the frequency of these events.
It is the purpose of this letter to reply to Reference (1) by describing the actions taken at Northeast Nuclear Energy Company (NNECO) to reduce the probability of these incidents occurring at Millstone Unit No. 1.
NNECO believes these incidents have no effect on the safe operation of the unit, in that no fuel damage is believed to have
. lted from any of these incidents.
However, NNECO is taking the following actions to minimize future occurrences.
Item 1 Millstone Unit No.1 has modified the rod withdrawal seque..ce in a manner that should virtually eliminate the probability of occurrences. By applying the sequences to all startups, the need for a critical prediction is eliminated.
NNECO does not believe that an accurate critical prediction is possible without an unrealistic expenditure of resources and that modifying the withdrawal sequence on the basis of an approximate prediction could actually increase the potential for high notch worth scrams.
7908270 M F g
. 4 With any rod withdrawal sequences, the worth of each rod and notch varies with local conditions such as moderator temperature, nodal exposure, and number of notches withdrawn. These conditions greatly vary the number of notches required to provide a quantity of ri ctivity.
In addition, complex xenon conditions in the reactor core make it extremely difficult to predict the notch worths of each of the control rods. Therefore, the solution to the problem of eliminating or reducing the short period /high notch worth scrams is contained in our response to Items 2 and 3, which do not rely on estimated critical prediction.
Items 2 and 3 The rod withdrawal sequence utilized at Millstone Unit No.1 originates from the General Electric Company's Reduced Notch Worth Procedure (RNWP) (Reference (2)).
The RNWP makes two basic modifications to the classic BWR withdrawal sequence:
(1) Prior to 50% control red density (Group 1 - 4 withdrawn), a Banked Position Withdrawal Sequence is used. This bant.ng of all the rods within a group flattens the flux distribution and prec *udes pulling of high worth rods from high flux regions.
In addition to General Electric's Banked Position Withdrawal Sequence, the rod withdrawal sequence has been modified to further reduce the rod notch worths by single notching rods in groups 3 and 4 from notch 04 to 12 for each rod prior to advancing to the next group (i.e., all rods in group 3 notched from 04 to 06 prior to notching the group from 06 to 08).
Prior to this modification, rod groups 3 and 4 were notched from 00 to 04, 04 to 08, 08 to 12, and 12 to 48.
(2) Beyond 50% rod density, successive control rods are selected for withdrawal from the region of lowest flux. This assures that the lowest worth available rod is always selected for withdrawal.
During all reactor startups, reactor operators are provided a warning in the station procedure " Approach to Criticality, OP-201" which states that under certain xenon conditions, extremely high rod notch worths can be experienced and directs that when criticality is approached, as determined by instrumentation response, the operator should withdraw each rod in a group one notch and repeat this sequence until the group is at its designated position. This process should be followed for at least one group af ter criticality is achieved.
In the event that single notch rod withdrawal causes an unanticipated short period, no further rod withdrawal should be made until the Reactor Engineer / Designee has assessed the situation and recommended a course of action.
The rod withdrawal sequences for all reactor startups utilize the Reduced Notch W' rth Procedure and the Banked Position Withdrawal Sequence, and are available o
for your inspection.
In addition to these modified rod sequences, all SRM's are monitored during startup to assure optimum use of the most significant data.
The SRM records are used to record the highest SRM channels until the IRM systen can be utilized to monitor the startup.
Item 4 A review of the operating history of the Emergency Rod In Switch indicates there have been no operating problems associated with this switch since the start of 9
e a commercial operation in December,1970. This switch is used infrequently with normal rod motion controlled by the rod in/out switch. Therefore, there is no reason to doubt its capability to perform its function over long periods of time, due to our past operating history. The rod-in function for any number of notches can be provided by the rod in/out swl.tch or the Emergency Rod In Switch. The operator, therefore, has two means available for rod insertion, in addition to the rod scrams.
Item 5 The Training Department provides adequate training to all Reactor and Senior Reactor Operators during the initial training and during the annual requalifica-tion training for Items 1 through 3 in the following ways:
(1) Training personnel provide a lecture pertaining to this material.
(2) Reactor Engineering personnel provide instruction on the avoidance of short periods and reduced rod notch withdrawal sequence.
(3) All LER's and NRC Bulletins pertaining to these items, and other applicable information, are read and acknowledged by licensed personnel.
(4) During the BWR Simulator Training, Items 1 through 3 are again addressed to all licensees.
NNECO believes the action described herein will reduce the frequence of these scrams to the minimtm possible. We feel that these actions adequately address the concerns expressed in Reference (1).
Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY
/h. l W. G. Counsil Vice President