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{{#Wiki_filter:Response to SDAA Audit Question Question Number: A-4.3-31 Receipt Date: 06/17/2024 Question: FSAR 4.3.2.7.2 states that a single CRA is used to induce radial xenon oscillations, however section A.9 of ER-A021-3589 states that the regulating rods in one quadrant are used to induce radial xenon oscillations (2 are shown in Figure A-1). Therefore, it is not clear whether both regulating rods were used to support the results in FSAR 4.3.2.7.2, or whether only one was. If only a single CRA was used in the analysis, provide justification for the generic conclusion that the reactor is inherently stable with respect to radial xenon oscillations. If both regulating rods in the quadrant were used, provide a corrected FSAR markup.
Response
The regulating rods in one quadrant are used to induce radial oscillations for xenon stability evaluations, as described in Section 5.4 of ER-A021-3589, Revision 4, Nuclear Analysis Methodology. FSAR Section 4.3.2.7.2 is corrected in the attached markups. 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 4.3-18 Draft Revision 2 xenon-induced power distribution oscillations require evaluation. There are three potential modes of oscillations possible in PWRs - azimuthal, radial, and axial. Azimuthal oscillations are not likely because of the inherent size and symmetry of the core loading pattern. Radial and axial oscillations are evaluated at BOC and EOC for the equilibrium cycle. The parameter used to characterize the stability of an oscillation is the stability index, which measures the rate of decay of the oscillation over the oscillation period. The rate at which the oscillation decreases is expressed by an exponential function of the form: Eq. 4.3-2 where A(t) is the time dependent oscillation amplitude (1/hr), A0 is the initial value, and t is time. The oscillation stability index,, is obtained using equation: Eq. 4.3-3 where T is the oscillation period (in hours) and n is oscillation number. A positive stability index indicates the oscillations are diverging and therefore, unstable. A negative stability index indicates the oscillations are converging, and therefore, stable. A stability index of zero indicates a neutrally stable oscillation. There are two modes of xenon oscillation that are evaluated, axial and radial oscillations. The stability calculations are performed using the SIMULATE5 code at various times during the equilibrium cycle. 4.3.2.7.1 Axial Oscillations The oscillations are initiated by inserting and then withdrawing the regulating bank from the PDIL after sufficient time elapses for xenon to redistribute in the core. The axial xenon oscillations are then observed. Stability calculations are performed at various times in core life from 25 percent to 100 percent power. 4.3.2.7.2 Radial Oscillations Audit Question A-4.3-31 Radial oscillations are initiated by instantaneously inserting the regulating rods in one quadranta single CRA, which results in both radial and axial oscillations. A t( ) A0et =
1 T---ln An+1 An
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