A new type of 14 MeV neutron detector, based on scintillating fibres, was developed for triton burnup measurements in JT-60U deuterium discharges. The detector consists of an array of scintillating fibres embedded in an aluminum matrix, coupled to a magnetic-resistant phototube with a high current output base, enabling count rates up to 100 MHz. The detector characteristics have been investigated using the Fusion Neutron Source (FNS) DT neutron generator. From the time-dependent measurement of 14 MeV neutron emission in neutral beam (NB) heated plasma discharges, the fast triton diffusivity has been evaluated to be 0.05-0.15 m2 s-1. Loss of fast tritons caused by Toroidal Alfvén Eignmode (TAE) instability was observed in the peripheral region of the plasma.