Prediction of power and analysis of results from thermohydraulic and neutronic measurements in a pressurised-water-loop experiment in the DIDO reactor at Harwell

In an experiment, a fissile fuel pin was irradiated simulating typical PWR conditions of pressure, heat flux, water flow, and geometry. The pressurised water loop had a parallel out-of-pile channel with electrically heated test section, which ran under very similar flow and heat-flux conditions, to act as a control on radiation effects on the water chemistry. Thermohydraulic measurements were used at intervals to calculate the pin power, but because the experiment required some bulk boiling of outlet, these did not give a continuous measure of power. Continuous measuring of fissile power was therefore restricted to neutron-flux measurements. The two independent methods of measuring power gives good agreement at the times that thermohydraulic measurements can be taken. This gives confidence in the use of the self-powered neutron-flux detectors to provide a method of measuring fissile power. The burn-up of U-235 and build-up of Pu isotopes is predicted by solution of the differential equations governing the processes, using information obtained from an actinide analysis of a similar fuel pin irradiated in the same experiment hole in DIDO. Absorption cross-sections are fitted to the relevant equations to obtain agreement with the measured results from the actinide analysis. These cross-sections thus include the appropriate resonance integrals for the neutron spectrum in the correct fuel-element position in DIDO. The effective reaction area for fission is predicted to remain constant within 1 percent over 300 days irradiation up to the time when 80 percent of the U-235 has been burnt up. The experiments confirm these interesting results. (Auth.)