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Several clean critical experiments were analyzed with ENDF/B-VI data to assess the adequacy of the data for U 2_s, U238 _nd oxygen. These experimentswere 1) a set of homogeneousU23S-H20 assemblies spanning a wide range of hydrogen/ uranium ratio, and 2) TRX-1, a simple, H20-moderatedBettis lattice of slightly-enricheduranium metal rods. The analyses used the Monte Carlo program RCP01, with explicit three-dimensionalgeometry and detailed representation of cross sections.For the homogeneouscriticals,calculatedk values for large, thermal assembliesshow good agreement with experiment._t s supports the evaluatedthermal criticalityparametersfor U2_S. However_ for assemblies with smaller H/U ratios, kcrit values increase significantlywith increasing leakage and flux-spectrumBardness. These trends suggest that leakage is underpredictedand that the resonance eta of the ENDF/B-VI UZ3S is too large. For TRX-1, reasonablygood agreement is found with measured lattice parameters (reaction-rateratios). Of primary interest is rho28, the ratio of above-thermalto thermal U_38 capture. Calculatedrho2B is 2.3 (± 1.7) % above measurement,suggestingthat U238 resonance capture remains slightly overpredictedwith ENDF/B-VI.However, agreement is better than observed with earlier versions of ENDF/B.

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