Streaming effects in inter-assembly gaps of fast reactors

In a power LMFBR, the hexagonal assemblies are separated by sodium gaps. Due to the relatively high mean free path of sodium, these gaps introduce a streaming effect which increases the absolute value of the leakage component of the reactivity, compared to the case where sodium is simply smeared with the assembly material. This effect is enhanced in accidental situations where sodium is voided. A theoretical method, based on a reasonable approximation, is proposed for the calculation of these effects. Numerical values of the reactivity changes are given and show that the streaming effects are important. Values of the reactivity change are also given for another accidental case, in which the core is compacted is such a way that the gaps disappear. Some critical facilities contain a 2D array of perpendicular void gaps. A particular method for the calculation of the streaming in such a lattice is proposed, and numerical values are given. This is important for assessing bias factors in the interpretation of integral experiments