[en] Fuel rod performance analyses for design purposes are dependent upon material properties, dimensions, and loads that are statistical in nature. Probabilistic design methods provide a systematic way to reflect the uncertainties in design parameters. PECS-III is a computer code which employs Monte Carlo techniques to generate the probability density and distribution functions for time-to-failure and cumulative damage for sealed plenum LMFBR fuel rods on a single rod or whole core basis. The probability that some measure of rod performance will fall within given limits is estimated by the relative frequency with which the Monte Carlo samples fall within those limits. The PECS-III deterministic model is a one-dimensional structural analysis code. Cladding loading mechanisms considered include fission gas pressure, differential fuel-cladding thermal expansion, and differential fuel-cladding swelling. The model predicts thermal creep strain, irradiation creep strain, irradiation induced swelling, and cumulative damage fractions based on strain and time to rupture. In fuel rod design, it is oftentimes low probability events (e.g. failure) that are of interest. Importance sampling techniques are used in PECS-III to accurately reproduce the output distribution in the area of interest without the high cost of a large number of samples. (Auth.)