ML19351F093
| ML19351F093 | |
| Person / Time | |
|---|---|
| Site: | La Crosse File:Dairyland Power Cooperative icon.png |
| Issue date: | 12/05/1980 |
| From: | Crutchfield D Office of Nuclear Reactor Regulation |
| To: | Linder F DAIRYLAND POWER COOPERATIVE |
| References | |
| LSO5-80-12-010, LSO5-80-12-10, NUDOCS 8012290475 | |
| Download: ML19351F093 (6) | |
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WASHINGTON, D. C. 20555
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Docket No. 50-409 LS05-80-12-010 Mr. Frank Linder General Manager Dairyland Power Cooperative 2615 East Avenue South La Crosse, Wisconsin 54601
Dear Mr. Linder:
We have completed our review of your Report NES-81 A0033 entitled "LACBWR Rod Drop Probability Study," submitted by letter dated March 29, 1979, which was submitted to assist us in determining whether a system to monitor rod patterns should be backfitted tc the La Crosse Boiling Water Reactor. We have found, as documented in the enclosed Evaluation, that such backfitting is not necessary.
Sincerely, M:'N:
Dennis M. Crutchfield,
,ief Operating Reactors Branch #5 Division of Licensing
Enclosure:
1 As stated l
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4 Mr. Frank Linder December 5, 1990 cc Fritz Schubert, Esquire Director, Standards and Criteria Staff Attorney Division Dairyland Power Cooperative Office of Radiation Programs 2615 East Avenue South (ANR-460)
La Crosse, Wisconsin 54601 U. S. Environmental Protection Agency
- 0. S. Heistand, Jr., Esquire Washington, D. C.
20460 Morgan, Lewis & Bockius 1800 M Street, N. W.
Washington, D. C.
20036 U. S. Environmental Protection Agency Mr. R. E. Shimshak Federal Activities Branch La Crosse Boiling Water Reactor Region V Office Dairyland Power Cooperative ATTN: EIS COORDINATOR P. O. Box 135 230 South Dearborn Street Genoa, Wisconsin 54632 Chicago, Illinois 60604 Coulee Region Energy Coalition Charles Bechhoefer, Esq., Chairman ATTN: George R. Nygaard Atomic Safety and Licensing Board P. O. : ox 1583 U. S. Nuclear Regulatory Comission La Crosse, Wisconsin 54601
'ii ngt on, D. C.
20555 La Crosse Public Library Dr. t 'orge C. Anderson 800 Main Street Department of Oceanography La Crosse, Wisconsin 54601 University of Washington Seattle, Washington 98195 U. S. Nuclear Regulatory Comission Resident Inspectors Office Mr. Ralph S. Decker Rural Route #1, Box 225 Route 4, Box 190D Genoa, Wisconsin 54632 Cantridge, Maryland 21613 Town Chairman Dr. Lawrence R. Quarles Town of Genoa Kendal at Longwood, Apt. 51 Route 1 Kenneth Square, Pennsylvania 19348 Genoa, Wisconsin 54632 Thomas S. Moore Chairman, Public Service Comission Atomic Safety and Licensing Appeal Board of Wisconsin U. S. Nuclear Regulatory Comission Hill Farms State Office Building Washington, D. C.
20555 Madison, Wisconsin 53702 Ms. Anne K. Morse Alan S. Rosenthal, Esq., Chairman Coulee Region Energy Coalition Atomic Safety and Licensing Appeal Board Post Office Box 1583 U. S. Nuclear Regulatory Comission Lacrosse, Wisconsin 54601 Washington, D. C.
20555 U. S. Nuclear Regulatory Comission Mr. Frederick Milton Olsen, III Resident Inspectors Office 609 North lith Street Rural Route #1, Box 225 Lacrosse, Wisconsin Genoa, Wisconsin 54632
N NRC STAFF EVALUATION OF LA CROSSE R0D DROP PROBABILITY STUDY MARCH 21, 1979 DAIRYLAND POWER COOPERATIVE DOCKET NO. 50-409 Introduction The Dairyland Power Cooperative has submitted a document, NES-81 A0033, entitled "LACBWR Rod Drop Probability Study" for our review.
The report was prepared by Nuclear Energy Services and was submitted as part of the documentation to be used to determine whether the Lacrosse Boiling Water Reactor (LACBWR) should be required to install a system to enforce a particular rod withdrawal sequence in order to prevent the occurrence of a rod drop accident producing a peak fuel enthalpy greater than 280 cal /gm.
Description of Report The document reports on a study of the probability of occurrence of a rod drop accident resulting in a peak fuel enthalpy of greater than 280 calories / gram in the LACSWR reactor. The series of events which must occur to produce a 4
dropped rod are identified and event probabilities are defined.
The core conditions required to produce a high rod worth are investigated and the rod worth required to produce a peak enthalpy of 280 calories per gram is calculated.
t An occurrence probability for the event of lea than 10 per reactor year has been adopted as the acceptance criterion for the study results. Both best estimate values and conservative (upper bound) values for the component probabilities have been defined,
.The control rod pattern and control rod sequence in LACBWR are described.
The conditions necessary to produce a 280 cal /gm peak enthalpy are given.
These include a reactor power greater than 45 percent of full power, a drop distance of greater than 35 inches and the.tip of the stuck rod cast be in the top half of the core.
The following events are a necessary prelude to the rod drop accident: the rod must become disconnected from the drive, it must stick in the top ha',f of the core, j
the disconnect must go undetected, the stuck rod must have a high worth, and the rod must drop when the reactor power is greater than 45% of full power.
2 s
Evaluation of Report The calculations of the consequences of the rod drop accident were performed with the BUKIt1 code.
This code is the Babcock and Wilcox version of the MEKIf? code developed for EPRI by MIT.
It is a three-dimensional, two group reactor kinetics code and is capable of treating the three-dimensional aspects of the rod drop accident correctly.
An adiabatic calculation is performed - that is, no credit is taken for heat transfer to the noderator during the transient.
The initial power rise is assumed to be terminated by Doppler feedback in the fuel.
Final shutdc<in is assumed to be due to control rod insertion by scram caused by a high flux trip.
Initial reactor conditions are assumed to be those appropriate to hot zero power.
The result of the calculation - that a dropped rod reactivity worth of greater than 1.4 percent reactivity change is required to produce a peak enthalpy of 280 calories per gram - is consistent with that obtained for large BWRs and is acceptable.
The analysis to detennine core conditions for which a potential dropped rod worth greater than 1.4% reactivity change exists shows that such worths exist only in the latter portion of the withdrawal sequence - above about 45 percent of full This is contrary to experience in large BWRs, where the large worths occur power.
only at low power.
Several features of LACBWR and its operation contribute to this difference.
During early portions of the rod withdrawal sequence all the rods are withdrawn, essentially as a bank.
This is made possible by the fact i
that small individual rod motions are permitted in LACBWR as opposed to 6-inch notch increments in large BWRs. Banking of the rods in this manner leads to very low dropped rod worths.
Dairyland has considered withdrawal errors in this range l
and concluded that, even with such errors, potential rod worths are smaller than the worths at high power.
The drop speed in LACBWR is considerably greater than that in later BWRs since l
the control rods in LACBWR are not provided with a velocity limiter. Thus the rod: drops for essentially the whole core length before the void distribution changes and relatively large net reactivity changes are possible.
We have reviewed the assessment of core conditions and events necessary for the
-n occurrence of a rod drop accident with unacceptable consequences.
These are 1
similar in nature to those discussed in the staf f response to the ACRS on*
generic item IIA-2
" Control Rod Drop Accidents (BWR)" and are acceptable.
i The values of the various probabilities and the basis for their choice are given.
A maximum value of 1.2 x 10'4 for the rod disconnect probability is based on the fact that no disconnects have been observed in 25,000 complete withdrawals in
-5 LACE!!R. An " expected" value of 4x10 for this probability is based on the GE ex erience which is expected to be conservative for LACEUR.
An expected
-3 value of 10 for the probability of sticking is used based on a similar value in the NRC study referred to above.
Similarly a value of 10-2 (corresponding to 5 stuck rods per year) is used as an upper bound value.
The probability of non-detection of the stuck rod is based on the number of separate rod motions required to move the rod drive 35 inches from the stuck rod.
Since rods are moved 1/2 - to 1-inch increments a total of at least 35 separate rod withdrawals are required.
Each of these motions has an observable effect on reactor power.
The bounding value of the probability is detemined by assuming that only eight detection opportunities exist and that the non-detection probability is 0.25 at each opportunity.
The total bounding value of the non-detection probability is then (0.25)8: 1.5x10 The expected
-5
-8 individual probability is taken to be 0.1 and the total value is 1.0 x 10 Not all rods in the LACBWR have the potential for having a worth of greater than 1.4% reactivity change.
For cycle 6, for example, only 16 of the 29 rods
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could have such worth.
In order to allow for possible different rod patterns or loading patterns in future cycles it is assumed that 21 of the 29 rods might have such worth.
Also, the reactor must be operating above 45% full power in l
order to achieve the large worths.
A conservative estimate is that such operations occur for ten months out of the year. Finally it is assumed that the probability of the rod's actually dropping is unity.
The result.of combining all these prob-
-I4 abilities is an expected probability of 8.2 x 10 per year for a rod drop resulting in a peak fuel enthalpy of more than 280 cal /gm and a " worst-case" probability of 4.8x10 Both these values are less than the criterion of 10-7 The various
~9 prebabilities assumed in the analysis are consistent with those employed in the staff study referenced above and are acceptable.
- X ' Statistical Examination of the RDA in Some BWRs" presented to the ACRS subcoitr:ittee on reactor safety on March 23, 1976.
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As described above the results of the rod drop accident are calculated usino the adiabatic assumption.
Recent studies by a staff consultant, Brookhaven National Laboratories
- show that, in the operating range in which the moderator is at saturation conditions, including thermal-hydraulic feedback in the analysis greatly reduces the peak fuel enthalpy.
As a result the calculated rod worth required to exceed 2e0 cal /gm peak fuel enthalpy is greatly increased (by a factor of 2 or more). Thus the analysis presented here is conservative since zero power conditions were assumed to detemine the consequences of the accident while the large rod worths necessary to exceed the peak fuel enthalpy limits occur at high power.
Evaluation Procedure Ue have reviewed the subject document under the guidelines presented in the
/
Standard Review Plan, Sections 4.3 and 15.4.9.
Sufficient infomation is presented in the document to permit the conclusion that state-of-the-art analysis methods are used, judicious choices of important parameters are made and ccnservative results are obtained.
The choice of 10 events /per reactor year for the acceptance criterion on probability of occurrence is consistent with the stcff's previous deliberations on this matter and has been accepted by the Advisory Committee on Reactor Safeguards in the resolution of Generic Item IIA-2. L'e find this criterion to be acceptable.
The calculation of the probability of occurrence of the rod drop accident is also con:istent with the calculation performed in the staff study.
Because of this fact and the fact that conservatisms exist in the evaluation of the effects of a rod drop accident coupled with the large margin to the acceptance criterion in the result, we conclude that the analysis of the probability of a rod drop accident resulting in a peak fuel enthalpy greater than 280 cal /gm is acceptable.
Regulatory Position Based on our review we conclude that the backfitting of the Lacrosse Boiling Water Reactor to install a device to monitor the rod withdrawal sequence is not necessary.
"Themal-Hydraulic Effects pn Center Rod Drop Accidents in a Boiling Uater Reactor, BNL-NUREG-28109, July 1980.