Nuclear reactors

Reference is made to methods for monitoring the flow rate of coolant in a Na-cooled fast reactor. Hitherto this has presented some difficulty and complexity, using flux distortion methods in which a magnetic field is distorted, to an extent dependent on the flow rate, by Na flowing through the field. The object of the method here described is to provide a simpler alternative method based on a different principle. The Na passing through the core of a fast reactor undergoes a nuclear reaction in which a proportion of the 23Na is converted to unstable 24Na. The latter has two modes of decay; the first, by which 99% decays, has a half-life of 15.4 hours, whereas the second has a half-life of 20 msec and is accompanied by the emission of an energetic β-particle with a spectrum whose upper limit is about 6 MeV, whereas the upper limit of the β-particle emission due to the first decay mode is only about 1.4 MeV. The method described comprises absorbing, at a point of the coolant path downstream from the core, but adjacent thereto, the β-particles from the second mode decay of 24Na to 24 Mg, collecting the β-particles as an electric current, amplifying this current to provide an electrical output signal related to the flow rate of the Na through the core, and monitoring the output signal. (U.K.)