ML24346A175
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Response to SDAA Audit Question Question Number: A-4.3-30 Receipt Date: 06/17/2024 Question:
FSAR Section 4.3.2.1 states that regulating bank groups are used during normal plant operation to provide axial power shaping. However, FSAR Section 4.3.2.5 states that CRAs are typically withdrawn from the core while at full power operation, which limits the potential for CRA absorber depletion. Provide FSAR markup that clarifies how CRAs are used for axial power shaping such that assumptions made in CRA absorber depletion analysis remain valid.
Response
Control rods are not used for axial power shaping during normal plant operation. FSAR Section 4.3.2.1 markups are included with this response.
NuScale Nonproprietary NuScale Nonproprietary
NuScale Final Safety Analysis Report Nuclear Design NuScale US460 SDAA 4.3-5 Draft Revision 2 4.3.2 Nuclear Design Description 4.3.2.1 Nuclear Design Description Audit Question A-4.3-30 The core consists of 37 fuel assemblies as described in Section 4.2. Sixteen of the fuel assembly positions contain CRAs. The CRAs are organized into two banks: a regulating bank and a shutdown bank. The regulating bank contains two groups of four CRAs arranged symmetrically in the core. The regulating bank groups are used during normal plant operation to control reactivity and provide axial power shaping. The PDILs restrict the amount by which the two regulating bank groups can be inserted at power as shown in Figure 4.3-1. The shutdown bank contains two groups of four CRAs. The shutdown bank is fully withdrawn during power operation. The shutdown bank is used in the event of a reactor trip and to maintain the reactor shutdown. More information on the fuel and CRAs is provided in Section 4.2 and Section 4.6.
Audit Question A-4.3-11 The fuel cycles are nominally 18 months and equivalent to a minimum 520 effective full power days. Fuel assemblies may incorporate axial and radial zoning to control power distribution and peaking within the core.
The NPM is designed with a heavy reflector to improve neutron economy. The reflector is made of stainless steel, which reflects fast neutrons back into the core and flattens the power distribution to improve fuel performance. The reflector is located between the core periphery and the core barrel; it provides the core envelope and directs flow through the core.
The soluble boron concentration is adjusted throughout the cycle to compensate for reactivity changes due to power level, fuel burnup, fission product poisoning, and burnable poison depletion. The higher concentration at beginning of cycle (BOC) balances the excess reactivity designed into the core to achieve the desired cycle length. The equilibrium cycle has an initial boron concentration of 1052 ppm.
Burnable poison in the form of gadolinia (Gd2O3) may be used within the fuel assemblies when needed to support the core design. The gadolinia is homogeneously mixed with the UO2 in selected fuel rods to provide a favorable radial power distribution, hold down reactivity, and minimize power peaking within an assembly. Although gadolinia is physically compatible with UO2, its addition to the fuel degrades some of the material properties of the UO2. For this reason, fuel containing gadolinia is limited to a lower power generation rate than fuel containing only UO2 based on consideration of centerline melting.
Audit Question A-4.3-11 The equilibrium cycle is the basis for the reference analysis presented in this section. The exact loading patterns, initial and final positions of assemblies, and number of fresh assemblies and their placement depend on the energy