A general method has been developed for evaluating the effects of uncertainties in design variables on the ability of a core to meet various transient and steady-state limiting thermal criteria. The performance characteristics of interest are evaluated by digital computer at carefully selected points in the parameter regions of interest, and the results are fitted by least squares to a polynomial approximation. Statistical information and/or limiting values may be readily applied to this approximation to yield a bound on the probability that the thermal design limits will not be exceeded.