ML17055A500

Enclosure 4 to Amendment Nos. 212 and 198, Resolution of Licensee Comments on Safety Evaluation - Risk-Informed Approach to Resolve Generic Safety Issue 191

ML17055A500
Person / Time
Site:	South Texas
Issue date:	07/11/2017
From:	Lisa Regner Plant Licensing Branch IV
To:	Gerry Powell South Texas
Regner L, DORL/LPLIV, 415-1906
References
CAC MF2400, CAC MF2401
Download: ML17055A500 (9)

Text

NRC Staff Resolution of STP Comments on Draft Safety Evaluation NRC Staff Draft Safety Evaluation (SE) Editorial Comments from STP Nuclear Operating Company (STPNOC)

SE or Attachment STPNOC Reason for No. Section and Page STPNOC Proposed Change Proposed Change NRC Staff Resolution 1 Section 3.1, pg. 6 The sumps are located at the Elev. Correction for the proper floor Change accepted.

(- minus) 11-foot 3-inch level of the elevation.

reactor containment building.

2 Section 4.4.1, However, the licensee stated that Although we dont disagree and did Change accepted.

pg. 19 piping in the containment is describe ASME requirements, fabricated, designed, constructed, quality requirements, and safety and examined (preservice factors, we could not identify where inspections) with rigorous we specifically made this statement.

engineering requirements including safety factors.

However, the staff noted that the licensee described ASME code requirements for design, fabrication, construction, and examination of containment piping, and addressed associated safety factors.

3 Section 4.4.3, The licensee used guidance in STPs UFSAR licensing basis is Change accepted. Added new pg. 21 RG 1.82 (remove note 51 re 2012 identified in the LAR as RG 1.82 endnote 58.

RG). draft Rev. 1, 1983.

SE or Attachment STPNOC Reason for No. Section and Page STPNOC Proposed Change Proposed Change NRC Staff Resolution 4 Section 4.5.2.2, In its letter dated October 20, 2016, The SE phrasing suggests that Partially accepted; the edits to the pg. 28, 29 the licensee provided estimates of STPNOC used geometric mean for second sentence are accepted. The the risk attributable to debris. The one configuration and arithmetic addition of the new sentence is not licensee presented risk results using mean for a different configuration accepted because the licensees the arithmetic mean in one case and and might apply the arithmetic position is not significant to the NRC the geometric mean to allow aggregation for some conditions. staffs review.

comparison in another. The licensee Table 9 in Section 4.5.1 of Att. 1-3 to also stated its licensing position that LAR Supplement 3 (10/20/2016) the geometric mean is the most includes a head to head appropriate method and provided its comparison of delta-CDF results for basis in its response to APLAB, geometric and arithmetic means.

Results Interpretation - Uncertainty STPNOC stated its licensing position Analysis: RAI 2. Per NUREG-1829, in the paragraph below the table that providing analysis results under the geometric mean is the most differing assumptions helps identify appropriate method and referenced the sensitivity of the results to those its basis in a RAI response.

assumptions. The NRC staff reviewed the licensees information and concludes that the sensitivity analysis of the risk results to the choice of aggregation method is an acceptable way to address this source of uncertainty because it is consistent with the recommendation in NUREG-1829.

5 Section 4.5.2.6.2, For DEGBs, D is equal to the inner Agree - contradicts the square root No change, since the current SE pg. 34 diameter of the pipe and a spherical of 2 discussion mentioned in No. 6 version in ADAMS correctly states this jet is assumed. below (but this - just D - is the sentence.

correct interpretation).

SE or Attachment STPNOC Reason for No. Section and Page STPNOC Proposed Change Proposed Change NRC Staff Resolution 6 Section 4.5.2.6.2, In case of a full pipe break, the See Comment 5. Change accepted.

pg. 35 licensee defined an equivalent break size based on 2 times the inner pipe diameter (i.e., the DEGB size is the diameter of a circular opening twice the area of the inner cross-section of the pipe). used a spherical ZOI based on the material L/D using the applicable pipe ID.

7 Section 4.5.2.6.2, In case of a full pipe break, the Is correctly stated in No change, since the current SE pg. 36 licensee uses a spherical ZOI based Section 4.5.2.6.2, page 34. version in ADAMS correctly states this on the material L/D using the See Comment 5. sentence.

applicable pipe ID defined an equivalent break size based on 2 times the inner pipe diameter (i.e.,

the DEGB size is the diameter of a circular opening twice the area of the inner cross-section of the pipe).

8 Section 4.5.2.7, Debris settling is not credited for fine LAR (August 20): The majority of Change accepted.

pg. 46 debris in the debris transport fiber fines (98.5%) destroyed from analyses98.5% of fine debris is insulation in the ZOI are transported transported to the RCB recirculation to the containment pool. The other pool. 1.5% of debris not transported to the RCB sump is trapped in inactive cavities during pool fill. The transport modes and their contributing fractions to the containment pool for ZOI-generated fiber fines are described below.

SE Attachment 2, Long-Term Core Cooling Methodology and Evaluation Results Assessment Comments from STPNOC SE or Attachment STPNOC Reason for No. Section and Page STPNOC Proposed Change Proposed Change NRC Staff Resolution 1 LTCC The licensee provided justification Reference should be to Question 36 Change accepted.

Methodology, the licensee referenced (See draft SE reference 27, pg. 74 Section A.4.1, responses to previous questions 31 of 77).

pg. 9 and 35 36 2 LTCC The licensee provided a description Could not find that we specifically Change accepted.

Methodology, of the structured process used to made this statement, although the Section A.4.2.1.1, identify and define the accident staff might have concluded from the pg. 11 scenario in response to SNPB-3-2 review of the responses that STP (Reference 24 and 25) and put appropriate focus on these SNPB-3-4 (Reference 22). areas.

STPNOC stated that the process for accident scenario identification focused on three areas: The staff review of the STPNOC process determined that it addressed the areas below.

3 LTCC However, the licensee cautioned We cannot find this cautionary Partially accepted; modified as:

Methodology, that the figure of merit for statement. The collapsed liquid Section A.4.2.1.4, determining the limiting break level is important, but PCT is the However, the licensee recognized pg. 19 should not be PCT, but the core accepted regulatory figure of merit. that the figure of merit for collapsed liquid level. determining the limiting break should not be PCT, but the core collapsed liquid level.

SE or Attachment STPNOC Reason for No. Section and Page STPNOC Proposed Change Proposed Change NRC Staff Resolution 4 LTCC Because the LTCC EM is only used Check for understanding: If STP Disagree. The licensee is limited in its Methodology, to perform the simulations described needs to rerun in the future (for the use of this methodology as specified in Section A.4.2.2.5, in the RAI responses provided by same cases but for, say a different the A.4.3 Conclusions.

pg. 23 the licensee, the NRC staff has block limit, more or less, different determined that consideration of blockage timing, and so forth, we future licensing calculations was not should be able to do so provided we needed. The NRC staff has use the same methodology that was concluded that this criterion does reviewed.

not apply.

(No change suggested) 5 LTCC While the licensee provided We would understand this to still Disagree. The licensee is limited in its Methodology, justification for the use of the LTCC allow STP to apply the STP EM for use of this methodology as specified in Section A.4.2.2.7, EM for the simulation of the 16-inch similar sensitivity studies using the the A.4.3 Conclusions.

pg. 24 hot-leg breaks (and various same methodology reviewed by sensitivity studies), complete NRC.

accident-specific guidelines were not provided as the approval was limited to only those simulations already submitted to the NRC.

Therefore, future use of the LTCC EM beyond the methodology and application reviewed by the staff requires prior review and approval by the NRC staff. Thus, the NRC staff concludes that this criterion does not apply.

6 LTCC The CCFL model is applied at the Upper nozzles are where the CCFL Partially accepted. Modified to state Methodology, upper nozzles core plate, a plate is of concern rather than the upper top of the core versus core plate.

Section A.4.2.3.4, containing numerous holes which core plate, per se. The NRC staff notes that CCFL is pg. 27 separates the fuel from the upper typically checked for both the top of plenum. the fuel (i.e., upper nozzles) and the upper core plate, and is applied at which ever one has the least flow area.

SE or Attachment STPNOC Reason for No. Section and Page STPNOC Proposed Change Proposed Change NRC Staff Resolution 7 LTCC Wallis with smooth edges Comment: In our understanding, No changes made.

Methodology, over-predicted the superficial CCFL is correlated by the superficial Section A.4.2.3.4, velocities in data with a few number velocity of the steam (which is The NRC staff notes that CCFL has to pg. 27 of holes, but under-predicted the assumed positive), and superficial do with the velocities of the steam and superficial velocities in data with a velocity of the liquid. When the the liquid, but it also has to do with the moderate number of holes and liquid velocity equals the steam liquid/vapor interface and the friction greatly under-predicted the velocity, it is stopped (flow begins to created in each.

superficial velocities of data with a be counter-current).

large number of holes. This was the correlation chosen by the licensee for the LTCC EM.

(No change suggested) 8 LTCC Therefore, a likely SI flow path is This same argument should apply Accepted and modified:

Methodology, down through the core support plate for cores that are not designed as Section A.4.2.3.4, on the periphery, down the low leakage since power sharing Original:

pg. 28 periphery fuel bundles, and then into will always be an artifact of In reality, while CCFL may occur in the center of the core to make up for multi-region cores. For example, channels which have significant any loss due to boil off. Given the even with low leakage design, we amounts of steam generated, such as open lattice nature of the core, this showed that internal low power center regions of the core, it would flow path would in reality be almost sharing regions had downflow. likely not occur near the core unaffected by CCFL. However, due periphery. Therefore, a likely SI flow to the manner in which the core is path is down through the core support meshed and the CCFL model is plate on the periphery, down the applied, this flow path is not periphery fuel bundles, and then into possible in the simulation. The NRC the center of the core to make up for staff finds that this is likely to be a any loss due to boil off. Given the conservatism in the licensees open lattice nature of the core, this analysis. flow path seems almost unaffected by CCFL, but due to the manner in which (No change suggested) the core is meshed and the CCFL model is applied, this flow path is not possible in the simulation. The NRC staff finds that this is likely to be a very large conservatism in the licensee analysis.

Modified:

In reality, while CCFL may occur in hotter channels which generate significant amounts of steam (e.g.,

those commonly found in central regions of the core), it would not be likely to occur in lower power channels (e.g., those commonly found in the core periphery). Therefore, a likely SI flow path is down through the core support plate on the periphery, down the periphery fuel bundles, and then into the center of the core to make up for any loss due to boil off. Given the open lattice nature of the core, this flow path would in reality be almost SE or Attachment STPNOC Reason for No. Section and Page STPNOC Proposed Change Proposed Change NRC Staff Resolution unaffected by CFL. However, due to the manner in which the core is meshed and the CCFL model is applied, this flow path is not possible in the simulation. The NRC staff finds that this is likely to be a conservatism in the licensees analysis.

9 LTCC While CCFL was found to restrict CCFL will occur at the top nozzles, Partially accepted; modified as:

Methodology, the flow from the upper plenum into which were modeled in STPs LTCC Section A.4.2.4.2, the core in the analysis, it is EM. While CCFL was found to restrict pg. 32 possible that this results from an the flow from the upper plenum into oversensitivity to CCFL caused by the core in the analysis, it is modeling the core and thus the possible that this results from an upper core plate top nozzles where oversensitivity to CCFL caused by CCFL occurs, as a single radial modeling the core - specifically, node. the top of the core where CCFL occurs - as a single radial node.

10 LTCC The staff believes this is likely Should clarify that this is the staffs Change accepted.

Methodology because in the two channel model, opinion. It is not included in our Section A.4.2.4.7, liquid can flow into the average assessment and we have not pg. 35 channel and steam and liquid exit performed analyses to support it.

through the hot channel, but in the base case all liquid flowing into the core and all steam and liquid exiting the core must go through the same channel (i.e., the same node).

SE or Attachment STPNOC Reason for No. Section and Page STPNOC Proposed Change Proposed Change NRC Staff Resolution 11 LTCC Safety parameters are those Confirming the intent of the staffs While STPNOCs comment did not Methodology, parameters which have limits to statements: The uncertainties we request any change to the SE, they did Section A.4.2.5, ensure plant safety, such as the addressed were pertaining to the request confirmation of the staffs pg. 37 specified acceptable fuel design debris issue in LTCC. It appears intent. They clarified that the limits (SAFDLs) required by General from the following paragraph that uncertainties addressed were Design Criterion 10 from 10 CFR 50 those were reviewed and found to pertaining to the debris issues in the Appendix A. Examples of safety be acceptable. LTCC. While there were some parameters are PCT, cladding uncertainties associated with the oxidation thickness, departure from debris itself in the LTCC simulations, nuclear boiling ratio (DNBR), and these were very limited. For example, critical power ratio (CPR). the only direct uncertainty associated with debris was on the blockage time No explicit uncertainty analysis was of the core inlet. This uncertainty was prescribed or performed for the addressed by assuming a LTCC EM. However, the NRC staff conservatively short blockage reviewed specific aspects of the time. There were other uncertainties LTCC EM to confirm that specific associated with the LTCC simulation uncertainties would be accounted which the staff considered to be the for in the analysis. dominating uncertainties and which were addressed in the SE.

(No change suggested) 12 LTCC The NRC staffs conclusions herein We understand this means other Disagree. The licensee is limited in its Methodology, are specific to the South Texas plants cannot use this without prior use of this methodology as specified in Section A.4.3, Project and future uses of this LTCC NRC approval; however, STP can the A.4.3 Conclusions.

pg. 46 EM require prior review and continue to use it in accordance with approval by the NRC staff for those the methodology we submitted.

specific details and plant design.

(No changes suggested)