The present invention relates to an apparatus for detecting the sudden evolution of an acoustic signal used for acoustic monitoring and in particular for leak detection. It is more especially used in inspection installations for fast neutron reactors, where it makes it possible to detect leaks in the heat exchange wall of a sodium-heated steam generator. However, it can be used whenever it is desired to detect a leak in a random member.
In a fast neutron nuclear reactor, the steam generator generally comprises steel tubes, within which water and steam circulate under high pressure. Liquid sodium carrying the heat given off by the reactor core circulates outside the tubes. The tubes form the heat exchange walls between the water and the sodium. The water evaporates within the tubes and the thus formed steam is used for driving turbines.
Due to the high chemical affinity between water and sodium, any sealing defect in a tube must be rapidly detected. Thus, any leak of water into the sodium can on the one hand evolve suddenly (leaking at a rate of a few to one hundred grams per second) producing a high pressure field and, on the other hand, can seriously damage the generator through the perforation of adjacent tubes under the action of an erosion--corrosion phenomenon.
At present, two sealing defect detection means are used, constituted by means for detecting hydrogen (in the sodium and in the argon) and safety diaphragms.
A hydrogen detection apparatus continuously measures the hydrogen concentration in the sodium at the outlet from equipment, or in the argon of the covering gases. The sensitivity is excellent, because such an apparatus is able to detect leaks having a flow rate of less than 1 g/sec. However, the response time is long and, in certain cases, may be excessive compared with the breakdown or perforation time of the tubes adjacent to the leak.
Safety diaphragms serve to prevent excessive overpressures within steam generators. They can be fractured by pressure waves produced in the apparatus by the sudden appearance of a leak and by the oscillation of the sodium mass between the argon volumes present in the secondary loop.
More recently a new leak detection procedure has been discovered, which uses acoustic means operating on the following principle. Acoustic sensors are placed on the steam generator to be controlled. Any leak of pressurized steam into the liquid sodium causes the generator walls to vibrate and this is superimposed on the vibration emanating from the different noise sources, i.e. outflow of sodium, outflow of steam, movements of various mechanical parts, equipment vibrations, relaxation of metal stresses, etc. All these vibrations are transmitted through the sodium and the metal parts up to the steam generator walls. The sensors supply a complex electrical signal which reflects all these waves. Its spectral range covers the audible band. The sound wave due to the leak gives rise to a characteristic signal, which is detected by an electronic system connected to the sensors.
Such a procedure is described, for example, in the reports of the ANS Conference, held at Richmond (WA), U.S.A., on Apr. 20-24, 1980, and particularly that of D. A. GREENE, F. F. AHLGREN and D. MENEELY entitled "Acoustic Leak Detection System for Sodium Heated Steam Generators".
Although this procedure has advantages, it is difficult to perform for the following reasons. The background noise detected by the acoustic sensors has an intensity depending on a large number of parameters and particularly the power of the reactor. Thus, if the power passes, e.g., from 20 to 80% of its maximum value, the background noise can be increased by 20 dB, which is considerable and largely masks an increase in the acoustic signal resulting from a leak. Thus, considerable importance is attached to the processing of the acoustic signal, if it is wished to reliably detect a leak and deliberately trigger an alarm.