ML20205M009
| ML20205M009 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe, Maine Yankee, 05000000 |
| Issue date: | 10/25/1988 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20205M007 | List: |
| References | |
| NUDOCS 8811020279 | |
| Download: ML20205M009 (4) | |
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ENCLOSURE 1 SAFETY EVAltlATION BY TPE OFFICE OF NUCLEAR REALTOR REGULATION RELATING TO THE RELAP5YA CODE FOR USE IN pWR SMALL EREAK LOCA ANALYSES FOR MAINE YANKEE AND YANKEE R0WE PLA G To satisfy the requirements of NUREG-0737. TM! Action Item II.K.3.30, Yankte Atomic Electric Company (YAEC) submitted RELAP5YA described in the topical report YAEC-1300P, "RELAP5YA: A Computer Program for Light Water Reactor System Thermal-Hydraulic Analysis," for NRC review and approval as a licensing method for a small break loss-of-coolant-accident (LOCA) analysis. NRC has used technical assistance by the Idaho National Engineering Laboratory (INEL) under contract FIN-86039 in the review of the topical report. The NRC has reviewed the Technical Evaluation Report (TER) on the evaluation which is in Appendix A of this Safety Evaluation (SE). The staff concurs in the reconnendations made by INEL and finds that the RELAP5YA is acceptable as a licensing method for a SBLOCA analysis under the conditions stipulated. The conditions are identified in Section 6 of tite attached TER and are restated as follows:
1.
RELAP5YA is applicable for modeling LOCA events with the break sizes up i
2 to 0.7 ft. If YAEC uses the code for SBLOCA with break sizes larger Y
than 0.7 ft, VAEC must ensure that the code predicts important phenomena j,*
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l which may occur as discussed in Appendix A.
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2.
Use of RELAP5YA is restricted to analyzing SBLOCAs where the accumulators f
do not empty and the hydrogen concentration remains above 5 cc H /kg of p
l water 50 that radiolytic decomposition of water is not a source of noncondensible gas. Otherwise, additional justification must be provided i
in the plant specific applications.
3.
RELAP5YA overpredicts tha condensation rate during a pressurizer insurge. YAEC stated condensation of vapor in system high points and the pressurizer refill portion of a SBLOCA are beyond the point of interest
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2 because the core is covered and there is no longer a question of plant safety. Therefore, RELAP5YA cannot be used to analyze SBLOCAs during periods of condensation of vapor in system high points and pressurizer insurge. Otherwise, further justification nust be provideo in the plant specific applications.
4 Due to the large interphase mass transfer coefficients used in RELAPSYA, the code is unable to calculate the presence of superheated vapor unless the volune void fraction is 1.0.
Therefore, for any analysis where it appears the presence of vapor superheat would have a significant effect on the system response or the peak clad temperature, VAEC should justify that the model noda11 ration was detailed enough to account for vapor superheat. This would include any SBLOCA analysis where the two-phase mixture level dropped below the top of the core due to long term core boiloff.
5.
RELAP5YA also overpredicts the condensation rate when subcooled ECC is injected. To mitigate the effects of injecting subcooled ECC, YAEC developed ECC modeling guidelines. These guidelines specify that the ECC water be modeled with a tenperature of 200'F instead of the nominal plant temperature of approximately 100'F. This guideline is acceptable and any deviation from it must be justified.
In addition, the analysis used to justify that the cold leg nodalization at the injection point is f
sufficient to represent. condensation induced by the ECC used 200*F i
water.
If colder ECC is used in a licensing analysis, the effect of cold leg nodalization on condensation must be reassessed and the results c
provided for NRC review, 6.
The nine modifications reviewed in Section 2.2 that apply to PWR SBLOCA analyses are acceptable for use in licensing analyses with the following restrictions:
a.
The Moody critical flow model is correctly implemented. However, plant licensing analyses must use the model options recomended by
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YAEC, i.e., the donor cell static pressure and enthalpy as input to the Moody critical flow table (see Section 2.P.2 of Appendix A),
b.
The multiple surface radiation model is acceptable for licensing applications provided justification of the view factor matrix used is given or, if a code is used to generate the view factor matrix, the code should be r,ubmitted for NRC review and approval (see Section 2.2.7 of Appendix A).
c.
The fuel behavior models are acceptable as implemented provided the rod internal pressure temperature offset is greater than 2*F (see Section 2.2.9 of Appendix A).
7.
There are a large number of options available M the user in RELAPSYA.
0 To ensure that the code is used within its capability and with proper input, the licensee should provide future plant specific licensing submittals justifying all selected options and input dsta, including defaults.
8.
YAEC noted that the following items will be performed on a plant specific basis and documented in the plant submittal:
time step sensitivity r
studies; break size sensitivity studies; a list of metal heat slabs used
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at various nodes including justification of any metal structures not j
represented in the analysis; clarification of how the multiple surface i
radiation model is used in EM analyses; justification that clad geometry changes during a SBLOCA do not change the radiation model view factors, justification of any nonzero value for the direct moderator heating fraction; and justification of the pressurizer nodalization used in SBLOCA licensing analyses.
i 9.
YAEC will also provide justification of the steam cenerator nodalization in SBLOCA licensing analyses on a plant specific basis with the plant submittal. The steam generator nodalization will be determined based on
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4 sensitivity studies to show convergence of the PCT, In order to calculate low flow and reflux cooling conditions any nodalization proposed by YAEC should have at least eight nodes to represent the U-tubes (see Section 5 of Appendix A).
- 10. The analyses that assessed RELAP5YA's ability to calculate the core thermal hydraulic response during core uncovery and recovery had nodalizations that used at least six volumes in the core. Therefore, YAEC is required to use at least six volumes in the core for its plant licensing analyses. More detail may be needed if calculating vapor superheat is important in the analysis. The remplete nodalization used in a plant licensing analysis should be consistent with current modeling guidelines or the nodal 12ations used in the assessment calculations. Any differences should be justified.
- 11. Certain models, as discussed in Section 4 of Appendix A, are not acceptable for use in licensing applications. These models include known errors and YAEC chose not to incorporate available update corrections.
12.
In several of the integral assessment calculations, the system pressure was underpredicted and, as a result, accumulator injection began earlier than in the experiment. The system depressurization rate wes calculated f
better when a discharge coefficient less than 1.0 was used.
The effect of discharge coefficients on the calculated results is the same as 6
varying the break area, Therefore. YAEC is required to perform a break size study to include the worst SBLOCA case for the plant specific
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h licensing applications, Based on its review of the YAEC's submittals and INEL's TER, the staff concludes that REi.AP5YA is acceptable for use in SBLOCA analyses and licensing applications when above conditions are satisfied.
Principle Contributor: S. Sun