Measurements of temperature fluctuations in a gas-cooled power reactor

The results obtained from measurements of thermocouple noise signals from a graphite moderated gas cooled reactor are presented.

An overall survey demonstrated that in the frequency range covered (up to 0·15 Hz), the reactor behaved as a closely coupled system with no significant phase differences between different parts of the core. Furthermore, the temperature noise power spectra could be divided into two distinct frequency regions in this range. In the lower part of the range the observed fluctuations were found to be due to overall reactor power fluctuations, and in the upper part to heat transfer coefficient variations.

A simple model of the noise processes in the reactor has been developed and verified by consideration of the transfer function between coolant and fuel temperatures.

The channel gas outlet thermocouple response, which was required for reactor fault studies, was measured by conventional rod oscillation techniques and noise analysis using both intrinsic fluctuations and refuelling transients. A comparison of the results showed that noise analysis, especially using refuelling transients, was capable of replacing the rod oscillation technique. This gave the advantages of not interfering with the normal operation of the reactor, reducing the experimental time and extending the frequency range of the measurements.