ML24215A088
| ML24215A088 | |
| Person / Time | |
|---|---|
| Site: | 05200050 |
| Issue date: | 08/02/2024 |
| From: | NuScale |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24015A000 | List: |
| References | |
| LO-169995 | |
| Download: ML24215A088 (1) | |
Text
Response to SDAA Audit Question Question Number: A-4.3-11 Receipt Date: 07/05/2023 Question:
Remove the SDAA statement " Each fuel rod may contain reduced enrichment axial blankets at the top and bottom, with a central fully enriched zone in the 4.3.2.1 Nuclear Design Description.
Based on the response to A-4.3-6 and discussions with NuScale, NuScale has confirmed that the fuel with blankets is not used in the US460 design. If blankets are later added in cycle specific core designs, the appropriate Part 52 change process would need to be used.The current safety analyses do not support the use of blanketed fuel.
Response
The requested updates are incorporated as shown in the attached FSAR Section 4.3 markups.
NuScale Nonproprietary NuScale Nonproprietary
NuScale Final Safety Analysis Report Nuclear Design NuScale US460 SDAA 4.3-5 Draft Revision 1 4.3.2 Nuclear Design Description 4.3.2.1 Nuclear Design Description 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 The shutdown bank is fully withdrawn during power operation.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. Each fuel rod may contain reduced enrichment axial blankets at the top and bottom, with a central fully enriched zone. Fuel Aassemblies may also 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
NuScale Final Safety Analysis Report Nuclear Design NuScale US460 SDAA 4.3-6 Draft Revision 1 requirements and the specific power history of an individual cycle. The fuel loading pattern and fuel shuffle pattern for the reference equilibrium cycle are shown in Figure 4.3-2 and Figure 4.3-3, respectively. The equilibrium cycle does not include axial enrichment zoning in the form of axial blankets.
Table 4.3-1 and Table 4.3-2 summarize the reactor core design parameters used in the analysis. The plant operating modes are described in the technical specifications.
4.3.2.2 Power Distribution Power distribution calculations are discussed in the Nuclear Analysis Codes and Methods Qualification topical report (Reference 4.3-2). This report contains a discussion of power distribution uncertainty, including application and a means for updating the uncertainty values. Additional discussion of the power uncertainties used in thermal-hydraulic analysis is provided in Section 4.4.
4.3.2.2.1 Definitions Enthalpy Rise Hot Channel Factor, FH The maximum enthalpy rise hot channel factor, FH, is defined as the ratio of the maximum integrated fuel rod power to the average fuel rod power. The limit on FH is established to ensure the fuel design criteria are not exceeded and the accident analysis assumptions remain valid. This limit ensures the design-basis value for the CHF ratio is met for normal operation, AOOs, and infrequent events. The FH limit is representative of the coolant flow channel with the maximum enthalpy rise. This channel has the highest power input to the coolant and therefore the highest probability for CHF.
Heat Flux Hot Channel Factor, FQ The heat flux hot channel factor (or total peaking factor), FQ, is the ratio of maximum local heat flux on the surface of a fuel rod to the average fuel rod heat flux. The maximum FQ value is used to calculate the peak linear heat generation rate (LHGR). The maximum value of FQ is used to ensure the SAFDLs are not exceeded.
Axial Peaking Factor, Fz The axial peaking factor, Fz, is the maximum relative power at any axial point in a fuel rod, divided by the average power of the fuel rod.