ML24215A229
| ML24215A229 | |
| Person / Time | |
|---|---|
| Site: | 05200050 |
| Issue date: | 08/02/2024 |
| From: | NuScale |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24215A000 | List:
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| References | |
| LO-169995 | |
| Download: ML24215A229 (1) | |
Text
Response to NuScale Topical Report Audit Question Question Number: A-NonLOCA.LTR-4 Receipt Date: 04/24/2023 Question:
The inadvertent decrease in boron concentration calculational method utilizes two boron dilution models: perfect dilution and wave front dilution. Non-LOCA LTR (TR-0516-49416, Revision 4)
Section 7.2.16.1 states: "The wave front model is physically conservative because it assumes the maximum amount of reactivity as the diluted slug of water sweeps through the core. This model does not assume any axial blending to ensure that this reactivity insertion rate is maximized. For all other operating modes where boron dilution is allowed and limited mixing exists, a wave front model is used. These mixing models are generally performed as a hand calculation but may be automated via a spreadsheet or other process.
However, there is not a discussion in SDAA Section 15.4.6, or in the Non-LOCA LTR on the validity of these models and their range of applicability. Given natural circulation depends on the differential coolant density and temperature how have these equations been validated for the SDA design which includes an increase in power and flow?
Response
As described in Section 15.4.6.3.1, two calculational techniques are used to analyze the boron dilution event and provide conservative boron dilution assumptions for the evaluation of both reactivity insertion and loss of shutdown margin. The first technique evaluates the boron dilution by assuming an instantaneous perfect (complete) mixing model and is described further in Section 15.4.6.3.1.1. The second technique evaluates the boron dilution by assuming a slug flow or dilution front (wave front) mixing model and is described further in 15.4.6.3.1.2. Both of these sections provide the mathematical equations (Eqs. 15.4-1 through 15.4-4) and define the input parameters used in the two techniques. These equations are derived from basic first principles. The equations are identical to those used in the design certification application (DCA)
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previously approved by the NRC. The NRC safety evaluation report (SER) for the boron dilution event does not identify any limitations associated with these basic equations. The differences between the DCA design (i.e., the US600) and the SDAA design (i.e., the US460) were reviewed to determine if any would affect the applicability of these basic equations. The only difference identified for potential impact was the additional riser holes present in the US460 design. The additional riser holes allow for additional cross-flow between the riser and downcomer which would promote mixing. The use of the different two calculational techniques, which are bounding of the mixing extremes, ensures that there is no impact from the riser holes.
Therefore, there are no differences between the US600 and US460 designs that affect the applicability of the equations in the two calculational techniques.
The non-LOCA topical report, TR-0516-49416, also provides mathematical equations (Eqs. 7-1 through 7-3) associated with the two calculation techniques. The equations are the same as those presented in the FSAR, although rearranged in some cases and with minor differences in nomenclature. The non-LOCA topical report was previously approved by the NRC and the associated SER does not identify any limitations associated with these basic equations. The equations in the current revision (Rev. 4) are identical to those in the previously approved revision (Rev. 3). As stated before, there are no differences between the US600 and US460 designs that affect the applicability of these basic equations.
The power differences between the US600 and US460 do affect operating temperatures and natural circulation flow as discussed in this audit question. These differences affect the input used when solving Eqs. 15.4-1 through 15.4-4. For the results reported in SDAA Section 15.4.6, the appropriate value for each input parameter has been selected based on the US460 design.
Regarding when each calculational technique should be used, the non-LOCA topical report Section 7.2.16.1 identifies that both techniques are used in Mode 1. The non-LOCA topical report Section 7.2.16.1 further states that the perfect mixing model provides a slower reactivity insertion rate, delaying detection, potentially allowing further loss of shutdown margin, while the wave front model is physically conservative because it assumes the maximum amount of reactivity as the diluted slug of water sweeps through the core. In Mode 1 at 25% power and above, both techniques are used to calculate the reactivity insertion rates, but isolation times and shutdown margin remaining at the time of isolation are calculated with the perfect mixing model. In Mode 1 at hot zero power, both techniques are used to calculate the reactivity insertion rates, but isolation times and shutdown margin remaining at the time of isolation are calculated with the wave front model. In Modes 2 and 3, the wave front model is used to determine isolation times and shutdown margin remaining at the time of isolation. The SER for NuScale Nonproprietary NuScale Nonproprietary
the previously approved non-LOCA topical report revision (Rev. 3) accepted the use of the two techniques for the various mode and power conditions as described.
When comparing the US600 and US460 designs, the temperatures associated with the mode definitions have changed. In addition, the temperature and natural circulation flows at each of the power levels within Mode 1 have changed. However, these differences between the designs are small and do not affect which technique should be used for the various mode and power conditions. The higher flow rates at each power level for 25% power and above for the US460 design compared to the US600 would promote more mixing; therefore, use of the perfect mixing model remains valid. For Mode 1 hot zero power and Modes 2 and 3, the flow rate is comparably low in both designs; therefore, use of the wave front model is valid. For these reasons, the use of the calculational techniques for the various mode and power conditions in the current revision of the non-LOCA topical report (Rev. 4) are identical to those in the previously approved revision (Rev. 3).
The SDAA results in Tables 15.4-13 and 15.4-14 for Mode 1 hot full power and hot zero power, respectively, are reported consistent with the use of two techniques as described in the non-LOCA topical report and summarized above. Similarly, Tables 15.4-15 and 15.4-16 for Modes 2 and 3, respectively, are reported consistent with the use of just the one technique as described in the non-LOCA topical report and summarized above.
In conclusion, the mathematical equations used in the two calculational techniques for the boron dilution analysis are not impacted by design differences between the US600 and the US460.
The inputs used to solve the equations are design specific and the appropriate inputs are used for the US460 design.
No changes to the SDAA are necessary.
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