ML20084Q679

Ao:On 730921,rod Withdrawal Error Resulted in Average Planar Linear Heat Generation Rate in Excess of Tech Specs.Cause Not Stated.Rod Cooling Technique Discontinued.Operator Cautioned Re Importance of Care During Rod Selection

ML20084Q679
Person / Time
Site:	Quad Cities
Issue date:	10/01/1973
From:	Stephenson B COMMONWEALTH EDISON CO.
To:	Oleary J US ATOMIC ENERGY COMMISSION (AEC)
References
BBS-73-201, NUDOCS 8306130534
Download: ML20084Q679 (4)

Text

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Commo[ ;cith Edison Quad-CitiesWuclear Power Station O  !

1 Post Offica Box 216 l Cordova, Illinois 61242 i

. Telephone 309/654-2241 4

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Q 8,I BBS-73-201

'.N QM J. F. O' Leary, Director Directorate of Licensing Regulation U. S. Atomic Energy Commission Washington, D.C. 20545

Subject:

Quad-Cities Nuclear Power Station, Unit 2 Docket No. 50-265; DPR-30, Appendix A Sections 3 5.J and 6.6.B

Dear Mr. O' Le ary:

On September 21, 1973 a rod withdrawal error on Unit 2 resulted in an average planar linear heat generation rate in excess of that specified in Figure 3.5.1 of Section 3 5.J of the Tech-nical Specifications.

This abnormal occurrence was previously

• reported to you by telegram on September 21, 1973 PROBLEM AND INVESTIGATION On the evening of Septenber 20, 1973 Unit 2 CRD H-6 was ex-periencing high temperature alarms. An accepted technique was being used temperature drives. to cool a few fully withdrawn rods with high rod This consists of giving the withdrawn a brief notch override, continuous withdrawal signal.

This particular drive had risen in temperature and had been cooled at least 3 or 4 times during the night.

At 0327 on September 21, 1973 the unit operator inadvertantly selected H-6. At that an adjacent rod, H-7, instead of the intended hot rod, time control rod H-7 was located at position

32. The operator, believing the selected rod to be at position 48, applied the continuous withdrawal signal.  :

RBM rod blocks were received at several points in the withdrawal but did not deter the eventual _ positioning of the rod at 48, full out. i

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Mr. F. J. O' Leary -

2- October 1, 1973 1

At 0830 a Technical Staff nuclear engineer made his morning inspection. He noticed an LPRM high alarm on the A level of string 32-25. On closer inspection he discovered the mis-positioned control rod and immediately obtained a TIP trace of the adjacent LPRM location. The control rod was then re-turned to its proper position, 32.

Subsequent evaluation of the TIP trace revealed that, while the average planar LHGR limit had been exceeded, the peak heat flux and MCHFR limits had not been violated. The calculated values were:

APLHGR = 13 72 kw/ft ,

PHP =

127.22 w/cm2 LHGR =

17.29 kw/ft MCHFR = 2.48 All of the above values occurred at node 5 of fuel assembly 31-26. The nodal exposure was 5066 MWD /T which corresponds to an APLHCR limit of 12.35 kw/ft.

EVALUATIO:1 AND CORRECTIVE ACTIO:1 SAFETY IMPLICATIONS At higher power and rated flow conditions, any time a non-pheripheral control rod is selected, intermittent Rod Block Monitor trips are received without any control rod movement as the reactor gross power drifts up and down slightly. Con-sequently RBM rod blocks often occur while center rods are selected during the rod cooling operation. The RBM alarms ,

which the process computer logged during- the withdrawal of H-7, were evidently interpreted by the operator as being associated with normal power fluctuations. The RBM did, however, perform its intended function of preventing continuous withdrawal.

As a result the rod was withdrawn over a 3 minute period in steps of several notches. This undoubtedly reduced the severity of the transient.

The local xenon transient resulting from the error-had not yet turnod around at the time the error was discovered. Conditions recordr J immediately prior to rod insertion - to position 32 thcevrore represent lowest local xenon inventory. In addition the thermal power as indicated by the computer was approximately 1% greater at the time of discovery than at the time of the error. It may therefore be concluded that the calculations

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O O Mr. J . F. O ' Leary October 1,1973 represent the worst case steady state conditions which occurred in the interval the rod was out of position.

Since MCHFR was well above the design value of 19, the only safety implications of this occurrence are those related to the fuel densification LCO. These are considered minor in view of the length of time that the limit was exceeded and the fact that only one core location was involved.

DETERMINATION OF CAUSE AND CORRECTIVE ACTION Two measures have been taken to prevent recurrence of this type of error. First, the rod cooling technique has been discontinued. By doing so the source of this error has been removed. Although the technique has been successful in pre-venting excessive drive temperatures for short periods of time, the benefits have not yet been proven essential to proper drive maintenance.

All rod movements at high power, such as xenon compensation or rod exercising, are made on a single notch basis. With the absence of the rod cooling procedure, there are now no circumstances when a rod is given a continuous withdraw signal during normal high power operation.

Secondly, all operators have been cautioned as to the importance of extreme care during rod selection and movement of any kind.

The occurrence itrelf has also greatly impressed the operators on this aspect of safe reactor operations.

CUMULATIVE EXPERIENCE Prior to establishment of APLHGR limits the unit was operated frequently at heat fluxes of similar magnitude in the bottom of the core. This was done intentionally to achieve an exposure distribution consistent with the Haling principle of core management. At the time no operational or safety hazards were assumed to exist and no problems were experienced. The pre-vious power distributions of this magnitude were approached, however, over a much longer period of time and with great care for remaining within the original design heat flux limits.

For the case of a withdrawal error of a deeper rod (positions 6 or 22 in this case), it is safe to assume that the withdrawal would be terminated sooner than for shallow rods (positions 30 to 46). Withdrawing shallow rods has little, if any, effect on gross thermal power, primarily redistributing the local' flux. Consequently one RBM channel sees a large increase from the A level LPRMs somewhat balanced by the decreasing C level

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u o o Mr. J . F. O ' Leary October 1, 1973 LPRMs. The other channel sees an initial increase in the B level LPRMs soon followed by decreases in both B and D levels.

Withdrawal of a deep rod, on the other hand, has a large effect on gross power and increases are seen on all levels of the LPRM strings . The RBM uppersetting is therefore reached much sooner, blocking further withdrawal. In addition the APRM rod block, which is presently set N3% conservative, may also block withdrawal of an exceptionally powerful rod.

Very truly yours, COMM0IlWEALTH EDISO!! COMPNIY QUAD CITIES NUCLEAR POWER STATIO!!

A fy=tI7-~

.B. tephenson Super tendent '

BBS/lk cc: Directorate of Regulatory Operations, Region III -

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