ML20217M930

Safety Evaluation Accepting Continued Operation W/O High Startup Rate Trip by Nene for Millstone,Unit 2

ML20217M930
Person / Time
Site:	Millstone
Issue date:	08/19/1997
From:	NRC (Affiliation Not Assigned)
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ML20217M923	List: ML20217M921 ML20217M930
References
NUDOCS 9708250365
Download: ML20217M930 (2)

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SAFETY EVALUATION BY_J R OFFICE OF NUClfAR REACTOR REGULATION RELATING TO CONTINUED OPERATION WITHOUT A HIGH STARTUP RATLTfif NORTHEAST NUCLEAR ENERGY COMPANY MILLSTONE NUCLEAR DOWER STATION. UNIT 2 DOCKET NO. 50-336 1.0 INTRODUCllDA A startup rate (SUR) trtp was originally provided in the older Combustion Engineering (CE) plants with analog protection systems (Palisades, Fort Calhoun, Maine Yanke, Calvert Cliffs, and St. Lucie) to provide protection against the classic startup accident in which control roos are withdrawn at a maximum rate from a suberitical condition. With the SUR trip operational, events initiated from subcritical conditions were assured of having much less consequences than events initiated from criticci conditions. Thus, the original Millstone, Unit 2, Final Safety Analysis Report (FSAR), as well as current updated FSARs, discuss the more severe events that do not rely on crediting this trip for protection. This situation evidently led the licensee for Millstone, Unit 2, and CE personnel to incorrectly assume that the SUR trip was not credited in the plant's safety analyses.

Based on this incorrect assumption, the licensee requested removal of the SUR trip from the Millstone Unit 2 Technical Specifications (TS) in 1978. The NRC allowed elimination of trip from TSs and also required physical removal from the reactor protection system (RPS) to eliminate any possible degradation that could affect other safety-related equipment. All other CE analog plants still incorporate this trip in their RPS, although not necessarily in their TSs. .

Subsequently, CE has informed the licensees of CE plants of the original design intent of this trip and the inadvisability of removing it from the RPS without due consideration of the events against which it provides protection.

CE recommended that utilities considering removal of this trip function from the RPS should assure themselves that the power excursion events initiated from suberitical conditions, which are currently protected by the SUR trip, will be afforded similar protection by other trips remaining in the RPS.

Therefore, Siemens Power Corporation (SPC), the current fuel vendor for Millstone, Unit 2, reanalyzed the uncontrolled control element assembly (CEA) bank withdraul event from a subcritical or low-power condition and credited the variable overpower (V0P) trip rather than the SUR trip.

2.0 EVALUATI6B The conscquences of an uncontrolled CEA bank withdrawal event from subcritical or low-power conditions are most limiting when the event is initiated during 9708250365 970819 DR ADOCK 050003 6

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the lowest possible power level (which maximizes the power overshoot), the most limiting CEA insertion configuration (which mrximizes the axial and radial power peaking and the CEA bank reactivity worths), and the lowest possible reactor coolant system (RCS) pressure (which maximizes the margin to departure-frsm-nucleate-boiling [DNB:). Based on this, initiation during Mode 3 is more limiting than initiat'on during Mode 2. Therefore, the event was initiated during Mode 3 for the analysis. The analysis also assumed a conservatively small negative Doppler coefficient and the most positive moderator coefficient allowed by the Millstone, Unit 2, TSs.

The event initiator wes assumed to be a single failure, which causes the l

uncontrolled withdrawal of al,1 regulating CEA banks, in the normal withdrawal sequoce and overlap from 10' of rated power at a conservatively fast i withdrawal rate of 50 in./ min. This is acceptable because the CEAs are wired together into preselected bank configurations, which prevent them from being l withdrawn in other than their respective banks. Power is supplied to the banks-in such a way that no more than two banks can be withdrawn at the same time and _in-their proper withdrawal sequence. The analysis further assumed that the event is preceded by an extended shutdown, which inaximizes the power overshoot, and also occurs at a beginning-of-cycle condition, which minimizes the negative Doppiar feedback. Both of these assumptions are conservative and, therefore, acceptable.

The transient is terminated by the V0P trip with a minimum DNB ratio of 1.50, well above the 95/95 safety limit of 1.17 for the XNB DNB correlation. The peak fuel centerline temperature reaches 2521*F, well below the fuel melting temperature of U0 3 . Therefore, the requirements of General Design Criterion (GDC) 20, that protection be automatically initiated, and GDC 25, that a single failure of the protection system does not result in violation of specified acceptable fuel design limits, have been satisfied.

3.0-[9NCLUSION The NRC staff has reviewed the revised analysis for Millstone Unit 2 during Cycles.10 through 13 and has concluded that the V0P trip provides acceptable fuel protection against exceeding DNBR or fuel centerline melt limits.

Therefore, the NRC staff agrees with NNECO's conclusion that continued operation of. Millstone, Unit 2, without the SUR trip is acceptable for future

-cycles that are bounded by the conditions of the SPC reanalysis as documented in Siemens Report EMF-97-014, Rev 0, " Millstone Unit 2 Uncontrolled CEA Bank Withdrawal from Subcritical/startup Analysis," dated February 1997.

Principal Contributor: L. Kopp Date: August 19, 1997 i

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