VVER-1000 is a physically big reactor having its typical peculiarities, particularly characteristic is the strong manifestation of small deviations in the breeding properties of separate fuel assemblies. If we take three reactors with typical outer dimension of 100, 200 and 300 cm, the same alteration of fuel assembly breeding properties will cause 8, 16 and 25 % power density change, respectively. Physically bigger reactors demonstrate substantial interference of influence of breeding property deviations on the power density field. All the findings mentioned above are important taking into account accidental assembly gap deviations between fuel assemblies during the operation of VVER-1000. The assembly gap deviations are caused by the shape change of the VVER-1000 fuel assemblies during operation, which also resulted in slower drop of control rods and (rarely) their jamming in the lower part of the core. At the moment, thanks to engineering effort, the phenomenon described above is nearly eliminated, and the control rods are timely inserted into the core. At the same time, certain fuel assembly shape alterations are present, and it must be taken into account at power density calculations, particularly in the periphery fuel elements of the assembly. Currently, possible assembly shape change corresponds to half of the estimated uncertainty of the VVER-1000 fuel element power, which adds 2-4 % to the fuel component of the prime cost. Assembly shape change during operation is simulated using thermo-mechanical calculations and is taken into account by means of additional safety factors. The safety factors are determined by the value of maximum single and twofold gaps between fuel assemblies and the factors of sensitivity of the power density burst to the gap size. The rest of the report is concerned with the algorithm for estimating the factors of sensitivity of additional power density burst to the increase in the gap between assemblies.
VVER-type reactor, fuel assemblies, FA breeding properties, inter-assembly gap, safety factor, interference, power density distribution
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