Benchmark intercomparison studies are being undertaken to investigate the accuracy of calculation methods. The same nuclear data library, JEF-2.2, is used in all the codes. The objective has been to estimate the uncertainties arising from approximations in the cross-section processing and neutronics methods and to provide guidance on the sources of the differences. The intercomparisons studied in this phase have been simple models, LWR pin cells without leakage and also with leakage treated by means of a buckling.