Neutron emission from spent fuel assemblies was investigated with BD-100R bubble detectors
stabilised at 200?C in a container using Peltier elements, at 35-450?C external temperature in the vicinity of
spent fuel assemblies. The intensity of fast neutrons was found to fall exponentially with a 0.13 cm-1
attenuation constant, with increasing vertical distance from the assemblies. The fast neutron emission rate
was measured as a function of cooling time and it was seen that only fast neutrons emitted from the top end
of the assemblies were detected. The near-neighbour effect (i.e, the effect of first neighbours) was also
measured. If an assembly is stored in a hexagonal lattice of 160 or 225 mm storage distance, 70-80% or 10-
20% of neutrons, respectively, were found to come from neighbours. The results obtained are discussed for
determining burnup of spent fuel assemblies.
