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Tests of prefabricated VVER fuel elements burnup 50-60 MW·days/kg in regimes with cyclic power variation have been performed in a circuit setup of the MIR research reactor. The testing procedures are described, and the designs of the irradiation setups are presented. Some fuel elements are equipped with sensors for performing in-reactor measurements, which yielded the experimental data on the variation of the fuel element parameters during the tests (gas pressure, fuel temperature, length). Some results of post-reactor materials-engineering investigations are presented. All fuel elements remained airtight.It is becoming increasingly necessary to operate nuclear power plants in a load-following regime, specifically, a regime with daily power regulation. The published data show that maneuvering operating regimes of commercial PWR in foreign nuclear power plants have little effect on the number of fuel-element failures [1].Experimental data are needed to substantiate the safety of transferring operating power generating units of VVER reactors to operation with grid-load following. Such data can be obtained in research reactors [2]. In such tests, it is impossible to reproduce the complete operating cycle of the fuel elements of a power reactor, but it is possible to realize critical conditions which determine the serviceability of fuel elements in power maneuvering regimes.The MIR circuit research reactor [3] is best suited for performing such experiments. The structure of the core, making it possible to produce the test conditions for several experimental fuel assemblies simultaneously, the fact that circuit setups satisfying the modern safety requirements are present, and the substantial experience in performing circuit tests make it possible to perform various experiments, including in variable fuel-element operating regimes [4].The interaction between a fuel kernel and cladding is studied as one of the most important factors determining the serviceability of fuel elements under such conditions. Experimental data on the behavior of fuel elements with large burnup, when there is virtually no gap between the fuel and the cladding, in power maneuvering regimes are especially important. Consequently, it is desirable to use for the tests fuel elements from standard power-reactor fuel assemblies which have reached the necessary burnup.The MIR reactor core is 1 m high. This is much smaller than the VVER core. Consequently, in experiments with full-scale fuel elements the required power is reproduced only on an individual section. The fuel elements were remanufactured to expand the experimental possibilities [5]. A fragment with the required dimensions is cut out, in a protective chamber, of a full-scale irradiating fuel element by remote controlled means. The required pressure is produced inside this fragment and the fragment is equipped with new end parts or, if necessary, various sensors, and resealed.

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