Patent Number: 055352537
Section: summary

BACKGROUND OF THE INVENTION The invention relates to the detection and evaluation of the strength of leaks situated on the head of a nuclear reactor vessel filled with pressurised water at high temperature. The leaks are detected and evaluated using a tracer gas: the water vapour itself, which comes from the leak under investigation. One particularly important application of the invention consists in the detection of leaks in penetrations of pressurised water nuclear reactor vessel heads. The penetrations in question are those which allow the control rods of the control rod clusters to pass through. Early detection of such leaks allows the operator to bring the installation to a safe fall-back operating condition. Techniques for detecting leaks in vessel head penetrations are already known, derived from those used for detecting leaks in heat exchangers. Many solutions use monitoring by dilution of a :radioactive tracer. In certain devices, this solution, using monitoring of the particle radioactivity, leads to detection thresholds and integration times which are high because of the extreme dilution of the tracer. Other known solutions do not make it possible to give any direct indication as to the strength of the leak, and have the drawback of being essentially qualitative. SUMMARY OF THE INVENTION The invention aims to provide a method and a device for detecting leaks in penetrations of a water nuclear reactor vessel head which make it possible rapidly to detect the appearance of a leak and to follow its development, while using only simple means and elements which can be adapted to reactors with different powers. The subject of the invention is a device for detecting leaks in penetrations of a nuclear power station vessel head, characterized in that it comprises a circuit for picking off the air flowing in the volume of the chamber where potential leaks emerge, before this air is diluted in the cooling circuit of the control mechanisms of the control rod clusters of the reactor. For this purpose, the invention provides a method according to which the chamber where the potential leaks emerge is swept through by a flow of air, and an increase in the level of tracer gas in this sweeping air, caused by a leak, is detected by picking off. The high sensitivity of the method provided is derived from the fact that the picking off is carried out within the volume of the chamber where the leak emerges. Because of this picking off, the measurement of the level of tracer gas is made before this tracer gas is significantly diluted in the strong flow of air used for cooling the control mechanisms of the control rod clusters. For implementing this method, use is made of a device which is remote-controlled by a computer of the industrial type. The computer manages the various operational sequences of this device, processes the measurements from various sensors and provides diagnostics on the state of operation of the device, on the existence of the leak and its strength. The invention also provides novel techniques which allow, on the one hand, simulation of the leaks in order to estimate the flow rate of sweeping of air passing through the volume of the chamber where the potential leaks emerge, and thus evaluating and following the development of the possible leakage flow, and, on the other hand, checking correct operation of the pick-off and leak detection circuits.