SEISMIC MARGIN ASSESSMENT (SMA)

SEISMIC MARGIN ASSESSMENT (SMA)

Unique knowledge and application of seismic margin assessment. SMA is one of the widely applied methods of seismic safety assessment. Used for decades globally the approach is comprehensively described in EPRI NP-6041. Previous evaluations suggest that the actual seismic capacity of the NPP structures, systems and components often exceeds the Safe Shutdown Earthquake used for seismic design of the plant. The seismic margin analysis according to EPRI NP-6041 quantifies the margin by which the component capacity exceeds or falls short of the Review level earthquake (RLE) ground motion. The RLE ground motion is typically defined by the uniform hazard spectrum at a return period of 10 000 years (1.0 x10-4 /y). The analysis then develops estimates of high confidence of low probability of failure (HCLPF) capacities of selected SSCs, where in general, failure of a component is defined broadly as the loss of component function. The HCLPF capacities are stated in terms of a selected ground motion parameter (e.g. peak ground acceleration) associated with the selected RLE response spectrum.

The SMA is essentially a deterministic analysis in which the predicted seismic response quantities associated with the RLE are compared with the corresponding seismic capacities.  Because it is deterministic, it does not develop probabilities of failure of components and consequently does not estimate core damage frequencies. Depending on regulatory commitments, the SMA may be performed for selected structures and systems that are required to bring the plant to a safe shutdown. Typically, the plant operations people will select the safe shutdown paths and the safe shutdown equipment list (SSEL). The equipment or components on a success path with the smallest HCLPF capacity defines the HCLPF of that success path. Accordingly, the SMA obtains the ground motion measure such as the peak ground acceleration up to which the plant can be considered to be safe. As SMA analysis is similar to the design methods, this approach is more easily understood by nuclear regulatory authorities.

SMA begins with a screening process which divides the structures, systems and equipment components (SSCs) on the SSEL into two groups. The first group includes those SSCs with HCLPF values a priori judged to be higher than RLE. The second group includes all the remaining SSCs for which HCLPF values have to be explicitly determined. The screening procedure is based on SQUG or EPRI databases and typically compares the site PGA to generic or experience based capacities. The screening is of high significance since the SSCs positively assessed are screened-out as a priori rugged and no further evaluation is needed for them.