Measurement of axial neutron flux distribution for PWR spent fuel assembly

Equipment for exponential experiments has been installed in the storage pool of the post-irradiation examination facility, Korea Atomic Energy Research Institute. Functional characteristic tests of the experimental system have been performed. Also prior to induced fission neutron measurements to be performed, axial backgroupd neutron flux distribution has been measured. The spent fuel samples in this study are C15 and J14 assemblies of Kori unit 1 and J44 assembly of Kori unit 2. The declared average burnup of each spent fuel is 32, 37.84 and 35.018 GWd/tU, and cooling time is 19.5, 12.5 and 9.5 years, respectively. As a result of preliminary tests, the average neutron counts obtained for 3 minutes in the central part of C15, J14 and J44 are about 1900, 3800 and 3200, respectively. A dip of the neutron flux density distribution is appeared in the spacer grid position, which is essentially the same pattern with gamma-ray scanning results. Neutron counts seems to be reduced to 30 % at this position in comparison to the fuel position. Also except both bottom and top area of the spent fuel assembly of C15, the time-dependent stability appears to be with in 2 σ error boundary