ML24215A087
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Response to SDAA Audit Question Question Number: A-4.3-4 Receipt Date: 04/17/2023 Question:
In the description of the reactivity coefficients, the flow coefficient of reactivity, which is a measure of the relative reactivity change associated with a core flow change, is missing.
Information similar to that provided in DCA application SAR Section 4.3.2.3.5, Flow Coefficient, would be responsive to this request.
Response
The NuScale power module relies on natural circulation for coolant flow and therefore flow cannot vary independent of power and temperature during normal operation. While the flow coefficient was provided in the DCA, it was not used or needed to demonstrate the safety of the design or ability to meet acceptance criteria. In the interest of optimization, it was removed from the SDA.
Final Safety Analysis Report (FSAR) Section 4.3.1.2 states that the moderator temperature coefficient and Doppler coefficient are the two primary reactivity feedback mechanisms that compensate for rapid reactivity increase, provide inherent reactivity control, and satisfy general design criterion (GDC) 11. Bounding values for the moderator temperature coefficient and Doppler coefficient are applied in the safety analysis. Flow, as a function of power and temperature, is treated conservatively for the events analyzed, so a separate flow coefficient is not used or needed in the SDA.
Markups of the affected changes, as described in the response, are provided below:
NuScale Nonproprietary NuScale Nonproprietary
NuScale Final Safety Analysis Report Nuclear Design NuScale US460 SDAA 4.3-13 Draft Revision 1 in relation to a change in reactor coolant pressure. A change in pressure causes a change in reactivity through a change in the coolant density.
The effects of voiding are accounted for in the density portion of the MTC.
The ITC is the change in reactivity due to the combined change in core average moderator and fuel temperature when the temperature is uniform across the core. The ITC is distinguished from the MTC, which is the change in reactivity due to a change only in moderator temperature. The ITC is important because it is the quantity that can be measured in the plant and is used to develop the NRFs for MTC as described in Reference 4.3-2.
4.3.2.3.3 Power Defect and Power Coefficient The power defect is the sum of the reactivity contributions from a change in moderator and fuel temperatures corresponding to a change in power from full power to zero power. A three dimensional calculation is performed to determine power defect, and therefore axial redistribution is implicitly included.
The maximum and minimum power defect for the equilibrium cycle is shown in Figure 4.3-16.
The power coefficient is the sum of the moderator temperature, fuel temperature, and void coefficient, and is measured over the percent change in power. The minimum and maximum power coefficient is shown in Figure 4.3-17 and is negative at all power levels as shown.
4.3.2.3.4 Differential Boron Worth The differential boron worth is a measurement of the change in reactivity associated with a change in the boron concentration. The differential boron worth coefficient for the equilibrium cycle is provided in Figure 4.3-18.
Audit Question A-4.3-4 4.3.2.3.5 Flow Coefficient Audit Question A-4.3-4 The flow coefficient of reactivity is a measure of the relative reactivity change associated with a core flow change. Due to the natural circulation design of the NPM, moderator temperature differential establishes the flow rate through the NPM. The MTC inherently considers the effects of moderator flow rate on reactivity and a separate flow coefficient is not determined.
4.3.2.3.6 Comparison of Calculated and Experimental Reactivity Coefficients A comparison of calculated and experimental reactivity coefficients is used to derive the NRFs as described in Reference 4.3-2.
NuScale Final Safety Analysis Report Nuclear Design NuScale US460 SDAA 4.3-14 Draft Revision 1 4.3.2.3.7 Reactivity Coefficients in Transient Analysis Audit Question A-4.3-4 The reactivity coefficients, specifically the moderator temperature and Doppler coefficients, are analysis inputs to Chapter 15 transients. Bounding values are calculated using limiting NPM parameters (e.g., moderator temperature, rod position, coolant flow rate, etc.) and are used as design limits in the transient analysis. The exact values of the coefficient used in the analysis depend on whether the transient of interest is examined at the beginning of life or end of life, whether the most negative or the most positive (least negative) coefficients are appropriate to provide conservatism, and whether spatial non-uniformity must be considered in the analysis. Conservative values of coefficients, considering various aspects of analysis, are used in the transient analysis. Details and assumptions for each transient are described in Chapter
- 15. The reactivity coefficients used in transient analyses are confirmed bounding for cycle-specific nuclear designs. Limiting physics parameters, and the direction that is conservative for each Chapter 15 event, is provided in Reference 4.3-2.
4.3.2.4 Control Requirements Core reactivity is controlled by soluble boron in the reactor coolant, CRAs, and burnable poison integral to the fuel pellets, as described in the following sections.
4.3.2.4.1 Soluble Boron The design uses natural boron for soluble boron control. The soluble boron concentration is changed to control relatively slow reactivity changes due to moderator temperature changes from ambient conditions to HZP.
transient xenon and samarium poisoning due to planned power changes.
reactivity effects of fissile inventory depletion and buildup of fission products.
depletion of burnable poison.
Table 4.3-2 shows the boron concentrations for different modes of operation for the equilibrium cycle. The boron concentration variation for the reference equilibrium cycle is shown in Figure 4.3-19.
The addition of boron for reactivity control lowers the pH of the coolant.
Lithium is added to restore coolant chemistry as discussed in Section 5.2.3.
4.3.2.4.2 Control Rod Assemblies The 16 CRAs provide control and shutdown capability for shutdown margin with the highest worth rod stuck out of the core.