ML24346A257
| ML24346A257 | |
| Person / Time | |
|---|---|
| Site: | 05200050 |
| Issue date: | 12/11/2024 |
| From: | NuScale |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24346A130 | List:
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| References | |
| LO-175762 | |
| Download: ML24346A257 (1) | |
Text
Response to SDAA Audit Question Question Number: A-15.0.4-1 Receipt Date: 06/17/2024 Question:
Neither FSAR Section 15.0.4, nor other FSAR locations, such as Chapter 4, discuss the potential need for post-event recovery actions to account for fluid boron concentration and boron distribution in the module when exiting extended passive cooling modes to ensure shutdown margin limits are appropriately preserved. Further, as illustrated in Figure 1-1 of TR-124587, Extended Passive Cooling and Reactivity Control Methodology, the LTR excludes post-event return to service design capability from its scope. Provide justification that all post-event recovery scenarios following the 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> extended passive cooling event timeframe do not necessitate consideration of fluid boron concentration and boron distribution in the module by an operator, or propose markups to FSAR Section 15.0.4 or another suitable location to include language that this is an important consideration. Language similar to that provided in Revision 5 of the DCA FSAR Section 15.0.4 should be considered.
Response
The US460 Standard Design Approval Application (SDAA) Final Safety Analysis Report (FSAR)
Section 15.0.4 is updated to include discussion of important considerations for post-event recovery actions similar to FSAR Section 15.0.4 in the NRC-approved Design Certification Application (DCA) for the US600.
Markups of the affected changes, as described in the response, are provided below:
NuScale Nonproprietary NuScale Nonproprietary
NuScale Final Safety Analysis Report Transient and Accident Analyses NuScale US460 SDAA 15.0-31 Draft Revision 2 15.0.3.7.6 Radiological Analysis of the Iodine Spike Design-Basis Source Term Section 15.6.5 presents the LOCA analysis, which shows no fuel failures occur. The design has DBEs that result in primary coolant entering an intact containment and the iodine spike DBST is used to bound the radiological consequences of these events. The design-basis iodine spike DBST and the beyond-design-basis CDST described in Section 15.10 are each assessed against the radiological criteria of 10 CFR 52.137(a)(2)(iv). If both analyses show acceptable dose results, then 10 CFR 52.137(a)(2)(iv) is met.
Section 3.2.6 of Reference 15.0-6 provides the methodology for the radiological consequences of the iodine spike DBST, with design-specific details listed below.
- 1) A generic failure is assumed to occur inside the CNV, resulting in the release of all 100,000 lbm of primary coolant from the RCS to the CNV.
- 2) The iodine and noble gas coolant activity is calculated based on the maximum concentrations allowed by design-basis limits for each of the iodine spiking scenarios. The primary coolant contains an assumed concentration of 5.8E-02 Ci/gm DE I-131 for the coincident iodine spike scenario and 3.5 Ci/gm DE I-131 for the pre-incident iodine spike scenario. For both iodine spiking scenarios, the primary coolant is assumed to contain 16 Ci/gm DE Xe-133.
- 3) At 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />, it is assumed the reactor is shut down and depressurized and releases through the containment to the environment stop.
There are no single failures for this event that could result in more severe radiological consequences.
Doses are determined at the EAB, the LPZ, and for personnel in the MCR and the TSC. The MCR model is described in Section 15.0.3.6.1. The potential radiological consequences of the iodine spike DBST are presented in Table 15.0-10.
15.0.4 Safe, Stabilized Condition Safety analyses of DBEs are performed from event initiation until a safe, stabilized condition is reached. A safe, stabilized condition is reached when the initiating event is mitigated, the acceptance criteria are met, and system parameters (for example inventory levels, temperatures, and pressures) are trending in the favorable direction.
For events that involve a reactor trip, system parameters continue changing slowly as decay and residual heat are removed and the RCS continues to cool down. No operator action is required to reach or maintain a safe, stabilized condition.
Audit Question A-15.0.4-1 Additional considerations are discussed to show Chapter 15 acceptance criteria are not challenged beyond the safe, stabilized condition. Extended passive cooling for decay and residual heat removal is discussed in Section 15.0.5. Boron concentrations
NuScale Final Safety Analysis Report Transient and Accident Analyses NuScale US460 SDAA 15.0-32 Draft Revision 2 and distributions are shown to be acceptable during extended passive cooling for decay and residual heat removal in Section 15.0.5. Boron concentration and distribution in the NPM are important considerations when exiting passive cooling and must be accounted for to ensure subcriticality and coolable geometry are maintained during post-event recovery actions.
15.0.5 Extended Passive Cooling for Decay and Residual Heat Removal There are two systems that perform the safety-related function of decay and residual heat removal from the NPM following a DBE. The DHRS, described in Section 5.4.3, provides decay and residual heat removal while RCS inventory is retained inside the RPV, the containment is maintained in partially evacuated dry conditions, and power is available. The ECCS, described in Section 6.3, provides decay and residual heat removal when RCS inventory is redistributed between the RPV and the CNV after the RVVs and RRVs are opened.
Design-basis events requiring passive cooling from DHRS or ECCS operation progress from initiation of the event to effective DHRS or ECCS operation demonstrating the NPM reaches a safe, stabilized condition, as described in Section 15.0.4. The decay heat removal process continues into the extended passive cooling phase, either with DHRS, natural circulation between the CNV and RPV through the RRVs and RVVs, or a combination of the two.
15.0.5.1 Decay and Residual Heat Removal Scenarios The following are the decay and residual heat removal scenarios:
- 1) DHRS with ECCS actuation blocked by operators after verifying acceptable reactivity conditions
- 2) DHRS with the RVVs and RRVs opening 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> after a loss of normal AC power
- 3) DHRS with the RVVs and RRVs opening 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> after a reactor trip (if ECCS actuation not blocked by operator action)
- 4) ECCS actuation following a LOCA, inadvertent ECCS operation, or DBE with loss of normal AC and EDAS power Significant boron redistribution before ECCS actuation and unacceptable positive reactivity insertion is precluded as shown by analyses performed in accordance with Reference 15.0-8 and discussed below.
Scenario 1 - Decay and Residual Heat Removal using DHRS Non-LOCA events progress from event initiation to the point at which DHRS actuation valves open and MSIVs and FWIVs close to allow DHRS operation. This scenario assumes AC power is available and the post-trip reactivity balance for cold conditions is acceptable. Once reactivity conditions are verified by the operators, the 8-hour ECCS timer is blocked. DHRS cools the NPM and provides