A probabilistic method for determining an operability interval for fasteners in a nuclear power plant assembly, the method comprising: receiving, by a processor of a computer, a determined or assumed geometric distribution of a given initial condition of fasteners in the nuclear power plant assembly at an initial time T0, the initial geometric distribution being determined based on inspection of the nuclear power plant or assumed based on past inspection results at other plants; receiving, by said processor of a computer, a determined future fastener failure probability model of the geometric distribution over time, the future fastener probability model providing a probability distribution of future fastener failures as a function of time for all degradation mechanisms experienced by the fasteners in the nuclear plant assembly; generating, by said processor of a computer, a plurality of random future fastener failure patterns by applying the fastener failure model to the geometric distribution at a given time T1 > T0; receiving, by said processor of a computer, postulating fastener spacing rules designed to evaluate the acceptability of fastener failure patterns for the fasteners in the nuclear power plant assembly, said postulating spacing rules including sets of fastener quantity and spacing rules; applying, by said processor of a computer, the postulated fastener spacing rules to the plurality of randomly generated fastener failure patterns for the given time T1 to determine the probability of the randomly generated fastener failure patterns passing the fastener spacing rules at the given time T1, a fastening failure pattern being acceptable if all criteria of the fastener spacing rules are met; iterating, by said processor of a computer, the applying step for a given range of time values T2, T3,..., Tx >T0 and determining a maximum future time Tmax at which a predetermined acceptable probability of the fastener failure patterns passing the fastener spacing rules is met, thereby justifying the acceptability of the fasteners for continued operation of the nuclear power plant assembly; and determining, by said processor of a computer, the operability interval as being the maximum future time Tmax minus the initial time T0. receiving, by a processor of a computer, a determined or assumed geometric distribution of a given initial condition of fasteners in the nuclear power plant assembly at an initial time T0, the initial geometric distribution being determined based on inspection of the nuclear power plant or assumed based on past inspection results at other plants; receiving, by said processor of a computer, a determined future fastener failure probability model of the geometric distribution over time, the future fastener probability model providing a probability distribution of future fastener failures as a function of time for all degradation mechanisms experienced by the fasteners in the nuclear plant assembly; generating, by said processor of a computer, a plurality of random future fastener failure patterns by applying the fastener failure model to the geometric distribution at a given time T1 > T0; receiving, by said processor of a computer, postulating fastener spacing rules designed to evaluate the acceptability of fastener failure patterns for the fasteners in the nuclear power plant assembly, said postulating spacing rules including sets of fastener quantity and spacing rules; applying, by said processor of a computer, the postulated fastener spacing rules to the plurality of randomly generated fastener failure patterns for the given time T1 to determine the probability of the randomly generated fastener failure patterns passing the fastener spacing rules at the given time T1, a fastening failure pattern being acceptable if all criteria of the fastener spacing rules are met; iterating, by said processor of a computer, the applying step for a given range of time values T2, T3,..., Tx >T0 and determining a maximum future time Tmax at which a predetermined acceptable probability of the fastener failure patterns passing the fastener spacing rules is met, thereby justifying the acceptability of the fasteners for continued operation of the nuclear power plant assembly; and determining, by said processor of a computer, the operability interval as being the maximum future time Tmax minus the initial time T0.