Measurements of streaming neutrons on nuclear ship 'Mutsu' by a two-detector-method

Streaming neutrons escaping through an air gap located between the pressure vessel and the primary shield of the Nuclear Ship ''Mutsu'' were measured by applying the two-detector-method. The two detectors consisted of a single BF3 counter provided alternatively with different covering arrangements - (a) 3mm thick steel tube + layers of polyethylene sheeting with total thickness of 30mm + 1mm thick Cd plate and (b) same covering as (a) + polyethylene boxing 20mm thick. In order to derive from the count rates obtained with the detectors described above the absolute values of neutron flux and dose equivalent rate, the detectors were calibrated in laboratory by comparison with a reference detector system in neutron field created around a 252Cf source and TCA, a light-water moderated critical assembly. The conversion from measured counts to neutron flux and neutron dose equivalent rate was estimated to incur errors of +-15 and +-40%, respectively. (auth.)