A pressurized subcritical facility was constructed with its main pressure vessel directly above a 28-in.-diam (71-cm) fission source plate centered on the top face of the General Electric Nuclear Test Reactor (NTR). The main pressure vessel had an inside diameter of 36 in. and inner length (less head) of 48 in. to accommodate light-water-moderated fuel lattices with a keff 0.98, constructed of low-enrichment oxide fuel rods. Maximum operating conditions for the facility were 540°F at 1050 psig. The system was heated electrically at 110 kW to raise the temperature and at 35 kW to maintain it at 540°F. With the NTR at 30 kW, and with lattice keff8 n/(cm2 sec) at lattice center. The facility has been used to carry out a program of measurements of nuclear reaction rates within H2O-moderated subcritical lattices of plutonium-enriched oxide fuel rods.