ML19011A006

2019/01/11 Nuscale SMR TR RAIs - Request for Additional Information Letter No. 9646 (Erai No. 9646) Topical Report, Thermal Hydraulics 1.05, SPRA

ML19011A006
Person / Time
Site:	PROJ0769
Issue date:	01/11/2019
From:	NRC
To:	NRC/NRO/DLSE/LB1
References
Download: ML19011A006 (4)

Text

Regulatory basis 10 CFR 52.47(a)(2) requires, in part, a description and analysis of engineered safety features and barriers that must be breached before a release of radioactive material to the environment can occur. In performing this assessment, an applicant shall assume a fission product release from the core into the containment and use the expected demonstrable containment leak rate. 10 CFR 52.47(a)(27) requires a description of the design-specific probabilistic risk assessment and its results. 10 CFR 51.55 requires an environmental report addressing the costs and benefits of severe accident mitigation design alternatives (SAMDA).

Request for additional information The NuScale Final Safety Analysis Report (FSAR) Revision 2 uses the methodology in the NuScale Accident Source Term Methodology topical report TR-0915-17565-P, Revision 2, to calculate radiological consequences. Section 3.3.7 of the topical report states that the containment is assumed to leak at the design basis limit leak rate for 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> and then at half of the design basis limit leak rate thereafter. Table 12.2-28 of the FSAR implements this assumption as 0.2% per day for the first 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> of the accident and 0.1% per day after 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. Section 6.2.6 of the FSAR states that the specified maximum allowable containment leak rate, L a, is 0.20 weight percent of the containment air mass per day at the calculated peak accident pressure, P a, identified in Section 6.2.1.

In discussions with NRC staff, NuScale stated that the topical report and the FSAR implement the containment leak rate assumptions in Regulatory Guide 1.183. However, the containment leak rate assumptions in Regulatory Guide 1.183 are based on containment designs which have a larger containment air mass compared to NuScale's evacuated containment design. This difference is illustrated by a staff independent MELCOR confirmatory calculation for a NuScale severe accident scenario using a containment hole sized to give a containment leak rate of 0.2% per day when the containment is filled with air at 1000 psia and 72 F. The staff's calculation predicted a leak rate of 0.7% per day following core damage and that the 0.7% percent per day leak rate would continue beyond 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. The higher leak rate is due to the higher severe accident mole fractions of hydrogen and steam (which are less dense than air) in containment for the NuScale design. The leak rate is scenario-dependent because the amount of hydrogen generated is scenario-dependent. The leak rate also could depend on the amount of xenon and krypton released.

Question The staff has determined that a containment leak rate of 0.7% per day could result in a larger release of radioactive material to the environment and higher radiological consequences. As such, NuScale is requested to provide technical justification in the topical report for the containment leak rate assumed in the MHA radiological consequence assessment, including the reduction in the leak rate at 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />; or to revise the topical report to use a containment leak rate applicable to NuScale accident scenarios. The technical justification should address how the basis for technical specification containment leakage rate requirements is reflected in the assumed containment leakage rate during an accident. If the containment leak rate is changed in the topical report, NuScale should provide revisions to documents that are affected by this change, including the assumptions and results in FSAR 15.0.3, "Design Basis Accident Radiological Consequence Analysis for Advanced Light Water Reactors," in FSAR section 19.1.4.2.1.4, "Release Categories," in the Environmental Report, and in the EPZ Topical Report.