One of the most complex cases for assessing the nuclear power plant safety is the evaluation of the response of the plant to an earthquake and calculation of the core damage frequency related with this. Plant level fragilities are convolved with the seismic hazard curves to obtain a set of doublets for the plant damage state. The standard methodology of the description of randomness and epistemic uncertainty of the fragility is based on the use of lognormal distribution. In the practice, because of large number and variety of types of components, variety of failure modes, further simplification is needed in spite of simplicity of the mathematic description of the fragility and its uncertainty. Sophisticated modeling and screening methods have to be applied for plant fragility development requiring enormous experience. Several practical assumptions utilized in the seismic PSA showing certain analogy with interval type description of uncertainties. In the paper an attempt is made for outlining some new opti...