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Research is in hand on converting RBMK reactors to fuel containing the consumable absorber erbium in order to improve the safety and economy. Our institutes have performed calculations on various ways of reducing the void reactivity coefficient. Erbium is distinguished from other consumable absorbers in that it has a resonance at 0.47 eV in the absorption cross section of t67Er. The mean temperature of the graphite in the RBMK is 200~ higher than the mean temperature of the water, so if the water is lost, the spectrum shifts towards that resonance, which provides an additional component in the void reactivity effect. Adding erbium as Er203 to the fuel reduces the void reactivity coefficient to a level at which it is not necessary to insert additional absorbers in the core. Also, the consumable absorber in a fresh fuel pin substantially reduces the power and reactivity changes on reloading. This greatly simplifies the loading and also monitoring the core power distribution. With 0.41% erbium by mass, all the units retain the same construction, and the annual saving is 4 million dollars per RBMK-1000 unit on account of the increased burnup.To determine the manor characteristics of uranium-erbium fuel in the design of the fuel pins, experiments were done with pins that contained standard fuel loaded with erbium. The gas release from the fuel is a major parameter governing the pin viability. It is necessary to perform experiments with the pins for burnup up to the design level, as was evident from comparative tests on the fresh and spent fuel under the conditions of reactivity surge, and also in thermal tests.We tested cooled pins containing standard and uranium-erbium fuels under identical conditions and measured the yields of fission products in relation to the extent of burnup, temperature, and decay constant, and we also obtained evidence on the states of the material in the fuel and sheaths after working-life irradiation.Apparatus. The tests were done with a swimming pool-type IW-2M research reactor at a power of 15 MW in two ASU-18/2 irradiators in the RISK-SPRINT testers. The channels were loaded in a core cell of diameter 60 mm formed in the second and third rows of the beryllium reflector. An ASU-18/2 channel consisted of a jacket, a sealed ampule, and a mechanism for moving the ampule vertically in the core. The ampul contained an exposed pin in an aluminum radiator, together with inlet and outlet tubes, thermocouples, and a neutron flux monitor. The gap between the radiator and the ampule wall (100 t~m) provided the necessary temperature in the fuel-pin sheath.We prepared the channel and set up a well-defined gas medium in the ampule and installed the carrier gas system for the fission products as part of a gas-handling and pumping system in the RISK-SPRINT testers. A computerized monitoring and control system measured the temperature and thermal neutron flux density continuously.These experimental pins consisted each of a sheath, a column of fuel, and two steel-zirconium junctions. A tungsten-rhenium ther...

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