ML24348A012

Response to NuScale Topical Report Audit Question Number A-NonLOCA.LTR-5

ML24348A012
Person / Time
Site:	05200050
Issue date:	12/13/2024
From:	NuScale
To:	Office of Nuclear Reactor Regulation
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Download: ML24348A012 (1)

Text

Response to NuScale Topical Report Audit Question Question Number: A-NonLOCA.LTR-5 Receipt Date: 04/24/2023 Question:

The methodology of screening rod drop cases needs additional description. Section 7.2.15.1 states For the smaller group of cases that do not result in the early reactor trip (( 2(a),(c) However, because power overshoot may occur during the event, these cases are bounded instead by comparison to the withdrawal of a single CRA. (( }} 2(a),(c) Describe what specific parameters in the rod drop and single rod withdrawal cases are being compared, and how the single rod withdrawal analytical limits are derived from the cases analyzed. (( }} 2(a),(c) Please place proposed markups to the LTR in the eRR.

Response

((

}}2(a),(c)

NuScale Nonproprietary NuScale Nonproprietary

((

}}2(a),(c)

NuScale Nonproprietary NuScale Nonproprietary

((

}}2(a),(c)

Section 7.2.15.1 of the non-LOCA topical report, TR-0516-49416, is revised to provide clarifications consistent with the above response. Markups of the affected changes, as described in the response, are provided below: NuScale Nonproprietary NuScale Nonproprietary

Non-Loss-of-Coolant Accident Analysis Methodology TR-0516-49416-NP Draft Revision 5 © Copyright 2023 by NuScale Power, LLC 594 peaking factors for the outer row of fuel assemblies. Sensitivity studies are performed on a variety of parameters, as necessary, to identify the case(s) with a potentially limiting MCHFR. The NRELAP5 MCHFR pre-screening process is employed to identify the cases sent for a detailed subchannel evaluation. The maximum fuel centerline temperature typically occurs at event initiation for those event scenarios with an immediate reactor trip. If the event scenario has a return to power, the maximum fuel centerline temperature is typically bounded by the fuel centerline temperature at HFP because the associated power peak is less than full power. Audit Question A-NonLOCA.LTR-5 As an alternative to performing a system transient analysis, the MCHFR and linear heat generation rate of the dropped rod event can be confirmed to be bounded by other events. Most rod drops result in a reactor trip on high power rate because of the immediate decrease in power from the dropped rod. Figure 7-3 shows the decrease in power from the dropped rod (( }}2(a),(c) as a function of rod worth for a variety of initial power levels. The figure is based on a representative NPM core design and also overlays a representative high power rate trip. (( }}2(a),(c) Sensitivity studies were performed to confirm this behavior using several representative NPM core designs, a range of initial power levels, and a variety of rod worths. (( }}2(a),(c) The alternative bounding method for the rod drop uses this result to screens rod drop cases into two groups: those that result in reactor trip within a short period and those that do not. Audit Question A-NonLOCA.LTR-5 For the larger group of cases that result in the early reactor trip (( }}2(a),(c), the dropped rod, and subsequent reactor trip, cause a decrease in global power. The dropped rod does cause an asymmetry and results in an increase in local power peaking (primarily a radial increase with a small axial increase). The subsequent reactor trip eliminates the asymmetry and the associated local power peaking. Figure 7-4 shows the impact on the local power from the decrease in global power and the increase in local peaking for a representative NPM rod drop case that results in early reactor trip. (( }}2(a),(c) The relevant

Non-Loss-of-Coolant Accident Analysis Methodology TR-0516-49416-NP Draft Revision 5 © Copyright 2023 by NuScale Power, LLC 595 acceptance criteria in Table 7-66 for the first group of the rod drop event can be assessed by the limiting steady-state subchannel analysis. Audit Question A-NonLOCA.LTR-5 For the smaller group of cases that do not result in the early reactor trip (( }}2(a),(c) However, because power overshoot may occur during the event, these cases are bounded instead by comparison to the withdrawal of a single CRA. (( }}2(a),(c) the conditions associated with the dropped rod cases are bounded by the limits used in the analysis of the withdrawal of a single CRA. Therefore, the limiting MCHFR and linear heat generation rate for the dropped rod cases that do not result in early reactor trip are bounded by the results of the single rod withdrawal analysis. This comparison of the rod drop analysis cases to the single rod withdrawal analysis limits is performed on a designcycle-specific basis. A point that falls outside the single rod withdrawal analysis limits, (( }}2(a),(c), would require use of the system transient approach described earlier in this section. When the designcycle-specific comparison is bounding, the relevant acceptance criteria in Table 7-66 for the second group of the rod drop event can be assessed by the limiting single rod withdrawal subchannel analysis. Misalignment of One or More CRAs The misalignment of CRAs occurs as a result of one or more CRAs being left behind when inserting or withdrawing the control bank. These conditions are not evaluated with NRELAP5 as part of the non-LOCA event methodology because this event is not a transient. Instead, the MCHFR is determined as part of a detailed subchannel evaluation.}}