personnel: high stress; reduced time for performance of the action; impossibility of the action in-situ due to conditions that prevent from performance of the action; decreased information support in the MCR. X. Analysis of systems and modeling of accident sequences It is recommended to elaborate the accident sequences model of the seismic PSA on the basis of the available accident sequences models of PSA for internal IEs. It is recommended to use event trees and fault trees from models of PSA for internal IEs as the basis for the PSA model being developed including any necessary additional events and/or changes in system models related to dependent failures in systems due to seismic effects. It is recommended to take into account the fact that types and consequences of failures at various seismic force levels can vary, and differ from those considered in PSA for internal IEs for one and the same element. It is recommended to include elements from the final seismic list into the probabilistic model. It is recommended to consider various levels of seismic effect according to the seismic hazard curve that can influence the aggregated probability of severe accidents with fuel melting. Consideration of various levels of seismic effect is recommended by imposing specific boundary conditions. Using AS models elaborated within NPP PSA of level 1 for internal IEs can be possible when developing scenarios of seismic effects to account for specific seismic effect consequences (multiple failures and spurious actuations). In these cases it is recommended to develop new AS models upon condition that the principles of modeling
the seismic damage curve for confidence level Q is determined by the following expression: , (7.2) where Ф(∙) and Ф-1(∙) direct and inverse standard normal distribution functions; Q confidence level. The median curve is obtained from the expression (7.2) by substituting Q=0.5 in (7.2), taking into account that Ф-1(0.5)=0 (or assuming =0, that is without account of the epistemic uncertainty parameter ). The curve for the 95% level of confidence is obtained by substituting Q=0.95 in (7.2). In addition to that, the damage rate can be considered as the mean curve: , (7.3) where determines the total spread. Mean (composite) curve is determined by the following relation: , (7.4) where combined uncertainty. Fig. 7.2 gives an example of the seismic damage rate curves for various fractiles. The representative points of the curves are: median Am corresponding to the conditional probability of 0.5 on the median and mean curves, and the point on the horizontal axis corresponding to the element seismic margin (HCLPF). The 0.05 probability value on the 0.95 confidence level curve or the 0,01 value on the mean (composite) curve corresponds to the HCLPF acceleration.
Uncertainty of the spectrum processing is estimated as the difference of the 2% extended spectrum from the 2% one at frequency of 6.81 Hz. Considering that the extension provides for 99% probability of non-exceedance (+2.33), we will get (at frequency of 6.81 Hz) by the lognormal distribution properties: . Damping The design frequency of the electric motor was 4.23 Hz corresponding to the acceleration 0.81g. It is considered that the 5% damping is median. However, when computing the damping coefficient and its uncertainty () one should first correct the frequency that changed due to the calculation update. Damping was taken into account when calculating the previous coefficient, therefore . Frequency 6.81 Hz is within the zone of response spectra intensification. In this zone, relations of the spectra (accelerations) with various attenuation coefficients (in fractions of the critical one) at the selected frequency are approximately inversely proportional to the square root of the relation of these attenuation coefficients. It is assumed that the 2% spectrum form the boundary corresponding to the standard deviation 2U , while the 5% spectrum is median (U = 0) (such assumption is suggested in the form of the expert evaluation). In this case: Modeling Possible decrease of the natural frequency due to change of damping and propagation of non-linear deformation is considered. are several methods of taking into account the change of loads on the elements due to the mentioned effects. Here the expert approach is used as the basis for such accounting. As mentioned above, the natural frequency 6.81 Hz is within the zone of response spectra intensification. When the natural frequency decreases, the load on the support structure will also decrease in accordance with the response spectrum. It is reasonably to suppose the decrease of the natural frequency by approximately 10% (median value) due to propagation of the non-linear deformations in the pre-failure
Example The mean seismic hazard rate curve approximation has identifiers K1 = 2.32 10-10 and KH = 4.18, acceleration A0 = 0.12g corresponds to frequency H0 = 1.9 1/year. Then the full frequency value HYf = 1.57 10-8 1/year corresponds to the mean seismic hazard rate curve with parameters Am = 0.53g and = 0.4. If, for example, it is expected that the probability of seismic severe accidents will be 20% of the aggregate allowed value of 10-5 1/year for all IEs, then the obtained value HYf is lower by more than order of two and can be excluded. _______________