ML19329D935
| ML19329D935 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 12/15/1978 |
| From: | Stello V Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| NUDOCS 8004080944 | |
| Download: ML19329D935 (13) | |
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. UNITED STATES GF AMERICA NUCLEAR REGULAT:s f C0!?'.ISSION In1the i'.aer ci
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SACRAMENTO MUNICIPAL UTILITY DISTRICT
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Docket No. 50-312
' Rancho Seco Nuclear Ger.erating Station EXEMPTIOri I.
The Sacrt ento ";r.icir:1 l'tility District (the licensee), is the holder of Facility Operating License No. D?R-54 which authorizes the operation of the nuclear power reactor knowr, as Rancho Seco Nuclear Generating Station (the facility) at steady state reactor power levels not in excess of 2772 megawatts thermal (rated power).
The facility consists of a
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Babcock and:Wilcox Company (ELW) designed pressurized water reactor (PWR) located at the _ licensee's site in Sacramento' County, California.
II.
In accordance with the-requirements of the Commission's Emergency Core Cooling System (ECCS) Acceptance Criteria,10 CFR 50.46, the licensee submitted on July 8,1975,- an ECCS evaluation for the _ facility.
The ECCS performance submitted by the licensee was based upon an ECCS Evaluation Model developed by B&W, the desicner of the Nuclear St'eam Supply System for this facility.
The B&W ECCS Evaluation Model had been previously 4
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fcund to conform to the r2;uirements of the Ccrmissien's ECCS *.cc: :.n:e Criteria,10 CFR Part 50.45 and Appendix 1:.
The evaica:icn :.dicc;;;
that with the -limits set forth in the facility's Technical Specificaticas, the ECCS cooling performance for the facility v:ould conform with the criteria contained in-10 CFR 50.45(b) which govert, calculated peak clad temperature, maximum cladding oxidation, maximum hydrogen generation, cooiable geometry and long-term cooiing.
Ca April 12, 1978, B&W informed the Cca-ission that it had determined that in the event of a small break Loss of Coolant Accident (LOCA) on the discharge side of a reactor coolant pump, high pressure injection l
(HPI) flow to the core could be reduced somewhat.
Subsequent calcu-l lations indicated that in'such a case the calculated peak clad temperature l
might exceed 2200F.
Previous small break analyses for _B&W 177 fuel assembly (FA) lowered loop plants had identified the limiting small break to be in the suction line of the reactor coolant pump.
Recent analyses have shown that the dis-charge line break is more limiting than the suction line break.
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The Rancho Seco Nuclear Generating Station has an ECCS configuration l
whichiconsists of.two HPI trains.
Each train has a HPI pump and the 1
- train injects into two of.the four' reactor coolant system (RCS) cold t
legs oN~ the discharge. side of the RCS pump.
(There is also a third HPI pump installed.). The two parallel HPI trains are connected but are kept isolated by manual valves _(known as the crossover valves) that are i
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Up:n receiving a safety injection sicnal the HPI pumps are started and valves in the four injection lines are opened.
Assuri.; '.::: cf :ff:':a pcder and the worst single failure (failure of diesel to start) 001y one H?! pump would be available and two of the fcur injectic, va'.es would fail to open.
If a small break is postulated to occur in the RCS piping between the RCS pump discharge and the reactor vessel, the high pressure injection flow injected into this line (7 bout half of the output of one high pressure pump) could flow cut the break.
Therefore, for the worst combination i
I of break location and single failure, only one-half of the flow rate of a single high pressure ECCS pump would contribute to maintaining the coolant inventory in the reactor vessel.
This situation had not been
. previously analyzed and B&W had indicated that the limits specified in
-10 CFR 50.45 may be exceeded.
Following discovery of this proble.5, B&W stated that they had analyzed
. a sppctrum of small breaks in the pump discharge line and had determined that to meet the limits of 10 CFR 50.a5, operator action was recuired to open the two manually operated crossover valves and to manually align theLtwo motor driven ' isolation valves which had failed to open.
This
-would allow the flow from-the one HPI pump to feed all four reactor d
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coolantIlegs.
8&W assumed that 30% of the flor! would be lost thret,9 the" break and:70% would refill the core.. Et,' letters dated I.;ril' 14
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and 21.-1978, supplemented by discussions with the staff, the licensee committed to provide.fsr the necessary cpSrator actions within the
-required time frace.
That is, in the event' of a small break and a liditing single failure, ranual. action.would be taken to begin cpenia; these trives within.five ninutes and have them fully opened and cn
-adequate flow split obtained-within 10 minutes after initiation of the event.
To ~i;ctiitate this operation, the licensee committed to maintain one of the serias-connected, manually operated crossover
-valves'normally open. The anu,yres performed by B&W-assumed that the flow split was established at-660 seconds by operator action.
?We therefore concluded that the modeling of operator action used in the' analyses was.a reasonable approximation of the operator action that' actually will be 'taken. provided specific procedures were(prepared and followed to assure such action.
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- By letter dated July lE,197E, E&W submitted a sumary of analyses -
of this event.
This sumary described the methods used and the I
results obtained for snail breaks in the pump discharge piping for I
.a reactor pcner of 2772 ikt, which is the rated power level of i
. Rancho:Seco.
The results provided in this sumary were obtained
- using the BLU ECCS Evaluation Model with two rodificaticns.
These mcdifications, which involve use of a two nede inner vessel simulation and phase distributional rultipliers for bubble rise in all control volumes within the reactor vessel, were described in a B&W letter to the staff, dated May 26, 1978, and have been reviewed and approved by the staff.
By letter dated July 18, 1978, the licensee stated that he had reviewed
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the B&W submittal' of July 18, 1978, and had found the conclusions L
acceptable and applicable to Rancho Seco.
Based on this review, the licensee requested authorization to operate Rancho Seco at 10C"; full i
power (2772-tht).*
In a submittal dated July 7,1978, the licensee also confirmed that procedures for operator action consistent with the: assumptions of the B&W analyses had been implemented, that drill'
.had been. conducted which verified that the required operations could readily ibe. completed in less time than -assumed in the B&W analyses and that L
all.five operating shifts had been trained in the procedures.
Repre-
/sentatives of the Comnission's regional office state that they have verified the licensee's implementation of the procedures and have
- *- In a previcusilicensing action related to this matter the p
maximum-power level ' of-Rancho Seco had.been. limited to 2080 Mwt
. by the Comnission'.s Order for. Modification of License dated b
Aprili26, 1978.
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inspected 'the licensee's training records to verify that training in the
. procedures was conducted. ' Eased on the above, we concluded that.
the procedures implerented by"the licensee relative to. operator action-in,the ever.: cf a s Ell break are acceptable.
In addition, in his l'etter dated 4pril 21, 1978, the licensee committed to submit by July 21,-1978, a proposal for any long-term modification (to-elimincte the need for prompt operator action) considered-appropriate.
Regarding the licensee's recuest for authorization to operate the facility-at full power (2772 Mwt), we. reviewed the B&W submittal
. of July 18,'1978, which-presented the results of analyses performed for reactor coolant. pump discharge line break sizes 0.15, 0.10, 2
0.085,'0.07, 0.055'and 0.04 ft at a reactor power level of 2772 Mwt.
Based on these results, B&W stated that with operator action con-2 sistent with that.modeled in the analyses, a 0.07 ft discharge line break is the most limiting case.
In this case, core 'uncovery occurs for a' out'410 seconds and the conservatively calculated b
peak clad ~ temperature is :approximately 1092*F.
This temperature is well below the limit 1specified in 10 CFR 50.46(b).
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Based on our, review of these analyses, we found that the calculations 2
-supported the conclusion that 0.07 ft discharge line break was the mostlimiti$gcase. The analyses submitted used a sicplified input tc the FDA" code for the distribution of steam sources which the licentee described as conservative.
However, these analyses did not previde adequate justification that this approach -
Was clearly conservative.
Accordir.;1y, we cculd not conclude that operation of Rancho Seco at 2772 Mwt would be fully in conformance with 10 CFR 50.46. On tne other hand, for operation of this facility at power le<els up to 2772 Mwt, the ECCS performance calculations for limiting small break did indicate that this break had a very substantial cargin on peak c',ad-temperature below the limits of 10 CFR 50.46(b) if operator action censistent with that assumed in the analyses was properly-taken. Therefore, because of this very-
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s0bstantial margin on peak clad temperature, the NRC staff concluded I
that operation of Rancho _ Seco at power levels of-up to 2772 Mwt in accordance with the operating procedures previously identified would not endanger life or property or the common defense and security.
The license conditions previously imposed by Order of April 26, 1978,
-were therefore modified to:
(1) require submission of a reevaluation of ECCS cooling performance wholly in conformance with 10 CFR 50.46.,
except for the. credit for completion of operator action within 10 -
minutes after initiation.of the event; (2) limit the maximum steadf e
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t state reactor core power level to 2772 P;:t; (3) recuire operation
- in accordance with procedures described in the licensee's letters of April 14, 1978, as supplemented by letters dated April 21 and July 7,1978 (except that the maximum time for completion of operator
' action was 10 minutes); and (4) require submission as soon as possible of a description and safety evaluation of a proposed plant modification which would eliminate reliance on prompt operator action.
These conditions were added to the Rancho Seco license by
. Order for modification of License dated July 21, 1978.
Since that.
- time, B&W 'has provided in.their letter of August 11, 1978 additional information concerning the simplified input used in the F0AM code portion of the ECCS performance analyses submitted July 18, 1978.
The staff has reviewed this additional information and on the basis 4
of its review has concluded that the small break LOCA analyses which used this simplified F0AM code input method are acceptably conservative
. and in conformance with the performa1ce criteria of 10 CFR 50.46 and Appendix K to Part 50. As noted previously, however, these
- analyses assume completion of the local operator action as described in L the licensee's ' letters of April 14, 21 and July 7,1978, within
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' ten minutes following the initiation of the event.
The original concern. in this' matter c rived from an unexpected but nevertheless. inadequate assessment of a spectrum of breaks.
This
- deviation'from 10 CFR 50.46.has been ameliorated on a temporary basis-
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by the actionsjdiscusred herein.
However, continued reliance on -
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- prompt cperator. action to perform the required steps to as:ure
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plant safety over a period of years into the future is undesirable
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= and should be remedied as promptly as possible.
To this extent, still remains until modifications are made the~origirial -
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- to - elimina te Tr:e on pr:mpt cperator acticns.
To remedy this defect the itte..see, in accordance with license Condition 4 of the present Order,, submitted on July 20, 1978, a description and safety evaluation of a proposed plant modification which would eliminate reliance'on the prompt operator action noted above.
Additional information concerning the proposed modification har been supplied by the licensee's letters of October 9, November 22 and December 4,-1978.
In addition, in his letters of November 22 and December'4,1978, the licensee comnitted to complete implementation of this proposed modification prior to Rancho Seco operation in Cycle 4.
Justification for this implementation schedule was provided in the licensee's letter of December 4, 1978.
.-The-licensee, in his letter of November 22,.1978, also. requested
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an exemption ~from the provisions of 10 CFR 50.46 untii such modifications wereLigplemented.
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9 With respect to this request for an exemption, we note trat the conclusions drawn in our Order of July 21, 1978 remain valid and have been further supported by our subsequent conclusions regarding the acceptability of the simplified input used in the FOAM code.
A::crdingly, we conclude that' operation of Rancho Seco at power levels up to 2772 Mat in accordance with the referenced procedures for operator action until n:difications are completed 'to achieve full compliance witn 10 CFR 50.46 will not endanger life or property or the common defense and security.
We have reviewed the modification proposed by the licensee to eliminate reliance on prompt operator action.
This modification consists of replacement of the AC motors present.ly installed on the valve operators for the four high preseure injection valves and the reactor coolant normal makeup valve, with similarly qualified DC motors. Because Rancho Seco has four Class i battery banks, the proposed ~ change allows the' injection and makeup valve operators to be powered from reliable power sources different from those used for 1
'the high pressure injection pumps. Accordingly, a single failure wouldLnotidisable both a pump and its associated injection valves.
LThe licensee' has presented a single failure analysis which demonstrates that in the presence of _a small break LOCA, no single failure combined withLa' loss of offsite power would prevent supply of an adequate total
- flowo{coolingwater.
The' licensee will'also provide modificatic,s u.=
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which will limit the degree of opening-of the injection valves upon receipt.f a high pressure injection signal.
This is being done to
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assure a proper flow split between the four i jection lines and to prevent
- pump runout. The-licensei. has committed to perform tests to verify that these criteria are r.ec.
The licensee has also provided a battery loading analysis which adequately demonstrates the ability of the battery banks to accc-Mate this additional load.
Therefore, based en cur revie, of tr.e -licensee's submittal we conclude that upon installatio-of the modificaticn, as proposed, and upon completion of testing to verifv attainment of the flow split' assumed in the S&W analysis of July 18, lW8, the emergency ccre cooling system will fully conform to the requirements
' of 10-CFR 50.46.
F Thus, while the ECCS for Rancho Seco does not fully comply witn our requirements, appropriate actions have been taken to remedy the defect in a timely manner, and to mitigate the consequences of a small bnaakLOCA, should such an accident occur prior to implementation of acceptable modifications. As a condition of granting an exemption,-
adherence.to prescribed operator actions and implementation of the 1
proposed modifications prior to operation in Cycle 4 are being
- made conditions of the facility operating license.
These conditions will remain in force only for the interval of time about one' year until the proposed modifications of the ECCS ere -
completed. - The jublic interest is served by issuing this exemption for Rancho Seco~ in that in the absencecf an exemption, shutdown of the facility would be required.
Loss of this large block of
- generating capacity-could adversely affect electric system b
reliability and.thus possibly adversely affect the-public.
. III.
Copies of the following docunents are available for inspection at the Commission's Public Document Room at 1717 H Street, Washington, D.C.
20555, and are being placed in the Conmission's local public document room at the Sacramento City-County Library, Sacratento, California.
(1) Letters from J. J. flattiros to R. W. Reid, Chief, Operating Reactors ' Branch #4, dated April 17 and 21,1978.
(2) Order for Modification of License, Docket No. 50-312, dated July 21,1978.
(3) Letters from J. H. Taylor to S. A. "arga, Chief, Light Water Reactors Branch #4, dated May 26, July 18 and August 11, 1978.
(4) Letters from J. J. Mattimoe to R. W. Reid, Chief, Operating Reactors Branch #4, dated suly 7 and 18, October 7, November 22 and December 4,1978.
IV.
WHEREFORE, in accordance with the Commission's regulations as set forth in 10 CFR 50.12, the licensee is hereby granted an exemption from the provisions of 10 CFR Part 50, Paragraph 50.45(a).
With respect to Rancho Seco this exemption supersedes the Order for Modification of License dated July 21, 1978, and the license is hereby conditioned as follows:
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, (1) ~ Until implenentation of the modifications defined in ~ (2) below, the facility shall be operated in accordance with the procedures for operator action described in the licensee's letter dated
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April'14, 1978~ as-supplemented by letters dated April 21 and July 7,-
1978, except.that the maximum time for completion of operator actian_ shall be 10 minutes after initiation of the event, 'and
' (2) Authorizati:n to operate the facility in the absence of implementation of the moolfications to eliminate reliance on prompt operator. action, as described in the licensee's letters of July 20, October 9,and November 22:,1978 is limited to Cycle 3.
FOR THE NUCLEAR REGULATORY COMMISSION Victor Stello, Jr., Director Di ision of Operating Reactors Of ice of Nuclear Reactor Regulation Dated at.Bethesda, Maryland, this 15th day of December 1978.
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