Instrumented Fuel Assembly 429, an experimental assembly designed to study helium fill gas absorption and fission gas release in pressurized uranium dioxide (UO2) fuel rods used in light water reactors, is operating in the Halden Heavy Boiling Water Reactor in Halden, Norway. Preliminary assessment of results from fuel rod internal pressure measurements and from the postirradiation examination of selected fuel rods indicates that helium fill gas is absorbed into the UO2 during the first several months of operation. The fuel absorbs ∼5.7 × 10−3 cm3 He (STP)/g UO2 at the irradiation conditions of a 5.4-MPa helium pressure and a peak steady-state fuel temperature of 1500 K. The estimated effective helium diffusion coefficient is ∼1 × 10−8 cm2/s. Periodic transients with power increases of up to 50% do not drive the helium from the UO2 matrix.