Application of the Pulsed Neutron Technique to the Calibration of the Control Rods of Queen Mary College (Double-Core) Reactor

If a short burst of neutrons is injected into a subcritical reactor the neutron flux dies away roughly exponentially with time. Naturally the more sub-critical the reactor is, thje more rapidly the flux dies away. It follows that a measurement of this decay rate gives an indication of the amount by which a reactor is subcritical. This technnique has been applied to the calibration of the control rods of the Queen Mary College reactor. This calibration is compared with the results obtained by the conventional methods of control rod cailbration, i.e. rod-drop and doubling time measurements.

A novel feature of these measurements is that with a double-slab core, reflections of the pulse between the two slabs are observed as subsidiary peaks superimposed upon the exponential decay. A measurement of the time elapse between these subsidiary peaks enables an estimate to be made of the pulse propagation velocity of thermal neutrons in the graphite separating the two slabs of core.