ML20138C513

Topical Rept Evaluation of WCAP-10395, Statistical Evaluation of LOCA Heat Source Uncertainties. Rept Unacceptable for Ref in License Applications Due to Failure to Evaluate Accuracy of Computer Codes

ML20138C513
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Issue date:	12/03/1985
From:	Office of Nuclear Reactor Regulation
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ML20138C496	List: ML20138C492 ML20138C513
References
NUDOCS 8512130035
Download: ML20138C513 (8)

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l -TOPICAL REPORT EVALUATION BY THE REACTOR SYSTEMS BRANCH I 0F WCAP-10395 l

" Statistical Evaluation of LOCA Heat Source Uncertainty" l I

r Summary of Topical Report The topical report provides a statistical evaluation of some of the

. uncertainties contained in the Westinghouse large break LOCA evaluation model. This is done by comparing five of the parameters calculated by the evaluation model with best estimate predictions for these parameters. The sensitivity of peak cladding temperature which would result from a LOCA is then investigated, using varying degrees of conservatism. Westinghouse r concludes that the evaluation model provides a safety margin beyond a 95%

probability level even when some of the conservatism is omitted.

Specifically, Westinghouse evaluated the conservatism in: 1) decay heat,

2) metal-water reaction, 3) initial power level uncertainty, 4) initial pellet

,. temperature, and 5) core peaking factor. Westinghouse does not propose to remove all the conservatism. Requirements for the first three parameters are specified in the NRC Regulations (Appendix K) and could not be changed without a change in the regulations. The conservatism in all five parameters is evaluated to gauge the overall conservatism in the model predictions. This is

! done by comparison to models which Westinghouse considers to be best estimate, including the associated uncertainties, e

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The evaluation model utilizes the 1971 ANS standard for decay heat generation ,

predictions with a 1.2 multiplier to account for uncertainties. This model is l

specified by Appendix K. Westinghouse utilizes the 1979 ANS standard for best I estimate predictions.

t l The evaluation model utilizes the Baker-Just equation for metal water reaction r predictions as required by Appendix K. Westinghouse uses the Cathcart-Powell i

equation based on Oak Ridge test data to calculate metal water reaction rates ,

in their best estimate calculations. The metal water reaction model is not ,

significant in best estimate calculations, since the predicted peak cladding

! temperatures are below the threshold for the reaction.

The Appendix K evaluation model requires that the initial core power level be assumed to be 27.' above the licensed power level. Westinghouse has analyzed the calormetric uncertainty in reactor thermal power measurements, and believes that the uncertainty is closer to 1% for a four-loop plant.

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l The present evaluation model utilizes initial fuel pellet temperatures (stored i

! energy) at the worst time in life. Uncertainties are included to account for il jj hydraulic and nuclear effects on enthalpy rise as well as local variations in

t ii power, fuel rod and pellet fabrication. Westinghouse contends that the added

!i conservatisas in temperature calculation and core power peaking are not strictly required by Appendix K, and are therefore candidates for possible

'i e reduction in conservatism.

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! The peak cladding temperature results for a postulated !nrge break LOCA were ,

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+ predicted using various degrees of conservatism. This was done by modifying t l the evaluation model to calculate the best estimate case, and by determining g

the sensitivity of peak cladding temperature to changes in the five parameters  !

as each was varied independently with the other parameters at the best estimate l

! values. The peak cladding temperature was calculated to increase as L

conservatism was increased in the following order:

1. Best estimate assumptions at the most adverse time in plant life.

2. Parameters set at the conservative 955 probability level.

3. Best estimate at the most adverse time in plant life with Appendix K models used only when required by the regulations. l

4. Current evaluation model.

Westinghouse concludes that, since the results from the 955 probability level case are less severe than the case with only the required Appendix K assumptions, the power and fuel temperature uncertainties any be removed from the evaluation model. The removal would be implemented by modifications to methodology for calculation of plant operational peaking factors. The

.j calculational results from the evaluation model reflected in the LOCA limit curve of 2he technical specifications would remain unchanged. Therefore, plant

[: specific peak cladding temperature analyses for postulated LOCAs would not need to be repeated. What would change is the removal of peaking factor ,

i uncertainties, including those from measurement, engineering tolerances, and I  !

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L 4-i fuel rod bow which are added to the moveable incore detector measurements.

i Westinghouse contends that, by inclusion of uncertainties in the Appendix K l LOCA calculation for fuel temperature and local power, there is no requirement t

$ to add the uncertainties to the incore measurements. This amounts to removal of an 8.15% penalty in allowable plant peaking factors. Proposed standard technical specifications are provided which describe revised methodology for

calculation of power distribution limits for plants with and without an Axial Power Distribution Monitoring System (APDMS).  ;

i Staff Position and Conclusions Recent analyses by the NRC staff and its contractors indicate that the most probable peak cladding temperatures that would be experienced during the limiting large break LOCA would be in the 1000*F to 1200*F range. These results have been obtained from advanced computer codes (TRAC and RELAPS),

developed independently at two separate national laboratories. Industry

,. calculations with realistic LOCA computer codes reach the same conclusion; namely, that there is approximately a 1000*F to 1200*F margin between the peak cladding temperature expected during the limiting large break LOCA and the

-10 CFR 50.46 limit of 2200*F. These analytical estimates are now well verified a by LOFT, semiscale, 20/3D, and other experiments. The staff has concluded that i

! this margin can be reduced without adverse effects on plant safety, and that i acceptable reduction in this margin may be warranted to avoid unnecessary l  ! restrictions in operation as a result of excessive conservatism imposed in ECCS l evaluations.

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ii The staff has established an approach that would permit a reduced margin in il '

large break LOCA ECCS analyses without changing the required features of i,

l} Appendix K Reference 1. The approach is outlined as follows:  ;

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!I 1. Employ "best estimate" models to calculate the realistic or most l llL probable peak cladding temperature. '

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2. Using the "best estimate" models, calculate the peak cladding I -

temperature at the conservative 95% probability level. The 95%

probability analysis should include the precision with which the j L code can calculate actual behavior, input or plant parameter uncertainties (such as power level, initial temperatures, pressures l

4 and nuclear parameters).

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l- 3. Perform a conventional ECCS analysis using the realistic model l

l augmented only with the required features of Appendix K. This would i

,., constitute a new evaluation model which confoms to Appendix K.  !

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i The above approach is similar to that followed by Westinghouse.

lI The WCAP-10395 methodology is deficient, however, in several areas. The ii i! principal deficiencies are listed below

1, II (h l 3; 1. The accuracy of the computer codes was not evaluated. The staff

! requires that both random and systematic errors be considered, based 1

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I t on comparison to experimental data from separate effects as well as integral system data.

, 2. Interdependence of variables was not accounted for in determining the peak cladding temperature at the 95% probability level. For example, j sensitivities determined at best estimate conditions may nc,6 ue

! ,' appropriate at higher peak cladding temperatures.

3. Westinghouse should demonstrate that the uncertainty from all j significant parameters used in the analyses has been considered.

Only five parameters are considered in WCAP-10395, and no j determination was made about the significance of other parameters. l

4. The sensitivity to single failure assumption was not evaluated. The

, limiting single failure for the evaluation model may not be limiting .

j if the conservatism in the model were relaxed.

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5. The evaluations in WCAP-10395 are limited to a single typical plant.  ;

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Before any plant can obtain model relaxation, the staff requires that plant specific calculations be performed or that justification for use of generic calculations be provided.

i j 6. The sensitivity of peak cladding temperature to the uncertainties in r

. initial fuel temperature and power peaking was calculated in WCAP-10395.

t i Westinghouseconcludedthattheseuncertaintiescouldberemoved i

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r I with adequate conservatism remaining. The topical report, however, f

( requested removal of the uncertainty provisions which are added to F

the nuclear power measurements. The equivalency between the

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! calculational uncertainties and the measurement uncertainties should be demonstrated.

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(- Because of the above considerations, the staff cannot approve WCAP-10395 in its present fom. We understand, however, that Westinghouse is planning to submit

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p additional evaluations of LOCA model conservatisms which will address the deficiencies in the WCAP-10395 evaluations.

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- 1 REFERENCE 1

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,' I'nformation Report, William Dircks for the Commissioners, " Emergency Core a

ji Cooling System Analysis Methods," SECY-83-472, November 17, 1983.

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