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This is a report of a study of the effect of alloy additives on the properties of fuel under conditions typical of water cooled reactors. The behavior of uranium oxide fuel with added mixtures of the oxides of aluminum, silicon, niobium, and iron during reactor irradiation of experimental fuel elements is investigated in the MIR research reactor. The feasibility of using aluminum-silicate additives for improving the operating characteristics of fuel pellets under reactor irradiation conditions is demonstrated.One of the approaches for improving the characteristics of fuel elements is to optimize the microstructure of the fuel (grain size, porosity) by means of alloy additives [1].The work reported here was done for the purpose of studying the effect of alloy additives on the properties of fuel under the operating conditions for fuel elements in water cooled reactors. The behavior of uranium oxide fuel with added mixtures of the oxides of aluminum, silicon, niobium, and iron during reactor irradiation of experimental fuel elements was investigated in the MIR research reactor.An experiment with an abrupt change in the power of VVER-1000 fuel elements was conducted in 1991 in the PVK-2 loop of the MIR reactor using a technique developed at the NIIAR [2]. An irradiation device ( Fig. 1) with 72 experimental fuel elements (Fig. 2) was loaded into channel 2-6 in the assemblies for units Nos. 1, 5, 6, and 4 with 18 fuel elements apiece. The characteristics of the fuel elements are listed in Table 1.During the experiments, the device with the test fuel elements is mounted in the channel of a loop assembly in which the parameters of the coolant are maintained at levels corresponding to the VVER. The reactor is brought to a power sufficient to ensure attainment of the required initial conditions of the experiment in the loop channel. The control rods closest to the loop channel lie below it. After all the parameters are stabilized and equilibrium states are reached at this power level, the power is abruptly increased by the required amount. This is done by extracting the nearest control rods with compensation of the resulting positive reactivity by inserting control rods in other sites within the core.Test Procedure. The variation in the parameters during the experiment is illustrated in Fig. 3. The abrupt power increase took place over 13 min. Then the fuel elements were tested at the higher power for another 10.5 h, after which the reactor was stopped to unseal the fuel elements in another loop channel. According to the readings of the shell seal control system, all the fuel elements in this experimental device remained intact. A maximum amplitude of the power increase for the fuel

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