Measured and predicted variations in fast neutron spectrum in massive shields of water and concrete

The absolute magnitude, and the variations in form, of the fast neutron spectrum during deep penetration (0.8 – 1.1 m) in massive shields of water, ordinary and magnetite concrete have been studied by using threshold detectors (In(n, n′), S(n, p), Al(n, α)). The results have been compared with predictions by two rigorous (NIOBE, Moments method) and two non-rigorous (multigroup removal-diffusion) shielding codes (NRN, RASH D).

The absolute results predicted were in general within 50% of the measured ones, ie. showed as good or better accuracy than thermal and epithermal flux predictions in the same small-reactor configurations. No difference in accuracy was found between the rigorous and non-rigorous methods.

The changes in the relative form of the spectrum (indicated by variations in the (Al/S) and (In/S) reaction rate ratios and amounting to factors up to 3–4 during a one metre penetration in water) were rather accurately (within 10–30%) predicted by all of the methods.

The photonuclear excitation of the 335 keV level used for detecting the In(n, n′) reaction was found to distort completely the In results in water at penetrations > 50 cm.