Patent Number: 040452824
Section: summary

This invention is directed to a method of thermal monitoring of a nuclear reactor core and to a monitoring device for carrying out said method. It is known that, in the case of a nuclear reactor which is in operation, temperature is a physical parameter which has to be checked with care since this latter must not be permitted to depart from a predetermined value at a given monitoring point of the reactor, said parameter being a function of the materials located in the proximity of said monitoring point. Thus the coolant temperatures within the fuel assemblies must always be such that the clad temperature does not exceed a predetermined maximum value under any circumstances. Prior to the present invention, it was the customary practice to carry out thermal monitoring of a reactor core by means of the temperature rise produced within the coolant as this latter passed through the fuel assemblies by detecting successively on the one hand the temperature of the coolant at a point located upstream of the fuel assemblies and on the other hand the temperatures of the coolant at the outlet of each fuel assembly. It is worthy of note that the periodic application of this method did not permit fast detection of a possible sudden temperature rise occurring in a fuel assembly between two successive measurements carried out on said fuel assembly and that the accuracy of a method of this type was also liable to be affected when a substantial temperature variation occurred upstream of the fuel assemblies. The present invention is precisely directed to a method of thermal monitoring of a nuclear reactor core which overcomes the drawbacks mentioned in the foregoing since it has the advantage in particular of permitting continuous monitoring of each reactor fuel assembly. In more exact terms, the method of thermal monitoring of a reactor core in accordance with the invention and comprising "hot", sometimes hereinafter referred to as T.sub.1, and "cold", sometimes hereinafter referred to as T.sub.2, fuel assemblies by utilizing coolant temperatures detected at the outlets of a plurality of fuel assemblies essentially consists: IN ESTABLISHING AT EACH INSTANT ON THE ONE HAND THE MEAN CORE OUTLET TEMPERATURE BY FORMING THE HALF-SUM OF THE MEAN "HOT" (T.sub.1) and "cold" (T.sub.2) core outlet temperatures corresponding respectively to the mean value of the coolant temperatures at the outlet of at least part of the hot fuel assemblies and to the mean value of the coolant temperatures at the outlet of at least part of the cold fuel assemblies and, on the other hand, the difference between said mean "hot" (T.sub.1) and "cold" (T.sub.2) core outlet temperatures, in producing for each fuel assembly aforesaid an analog signal corresponding to the temperature difference between the coolant temperature at the outlet of said assembly and the mean core outlet temperature as initially increased or decreased by a predetermined fraction of the difference between the mean "hot" (T.sub.1) and "cold" (T.sub.2) core outlet temperatures in order to define a state of equilibrium at which said temperature difference is equal to zero, in processing said signal in order to ensure that the appropriate safety actions are initiated by overstepping of threshold values corresponding to temperature variations permitted on each side of the equilibrium zero. The method as defined in the foregoing makes it possible to carry out continuous relative monitoring of the temperatures of a reactor core without entailing the need for continuous comparison with the coolant inlet temperature since it consists in detecting at each instant any potential localized cooling defect in one of the reactor core assemblies by comparison with the cooling at the same instant of the reactor core considered as a whole. Said zero method consists in continuously comparing the coolant outlet temperature of each of the core monitoring assemblies with the mean outlet temperature of the core considered as a whole by initially establishing an analog state of equilibrium such that the difference between these two compared temperatures is reduced to zero by a fraction of the difference between the mean "hot" (T.sub.1) and "cold" (T.sub.2) core outlet temperatures, the fraction aforementioned being intended to constitute a characteristic parameter of each fuel assembly since said fraction is a function of the position of the fuel assembly considered within the reactor core. In more exact terms, the method according to the invention can be presented by way of explanation as follows: if a signal u.sub.k represents the coolant outlet temperature of the k.sup.th fuel assembly which is detected for example by a thermocouple at the outlet of said k.sup.th assembly, if a signal u.sub.c produced by the analog method from a certain number of signals u.sub.k corresponding to "hot" fuel assemblies of the reactor core represents the mean "hot" core outlet temperature, if a signal u.sub.f produced by the analog method from a certain number of signals u.sub.k corresponding to "cold" fuel assemblies of the reactor core represents the mean "cold" core outlet temperature, the method consists in following the progressive variation of the signal: ##EQU1## where u.sub.c + u.sub.f /2 provides a measurement of the mean core outlet temperature U.sub.M and c.sub.k, this characteristic of the k.sup.th fuel assembly being established at the time of an initial adjustment in order to ensure that the signal E.sub.k is of zero value. Any cooling fault which occurs in one of the fuel assemblies and modifies the signal u.sub.k to an appreciable extent results in a state of unbalance which, depending on its magnitude, automatically initiates the necessary safety actions such as a pre-alarm, an emergency shutdown or a control rod drop. It is worthy of note that, if the fuel assembly of the order K which is subjected to abnormal cooling takes part in the generation of the signals U.sub.M and u.sub.c - u.sub.f, these signals are disturbed only to a slight extent. The present invention is also directed to a device for carrying out the method in accordance with the invention which comprises in a general manner: N thermocouples which are equal in number to the number N of reactor core fuel assemblies to be monitored, N measuring channels connected individually to each of the N thermocouples and each comprising in series: at least one amplifier for the signal detected by the thermocouple, PA1 a summing amplifier at the output of the preceding amplifier and also at the output of two associated analog circuits supplied by at least a certain number of the N measuring channels and each comprising means enabling one circuit to deliver a signal corresponding to the opposite value of the mean core outlet temperature and enabling the other circuit to deliver a signal corresponding to the appropriate fraction of the difference between the mean "hot" (T.sub.1) and "cold" (T.sub.2) core outlet temperatures, PA1 at the output of said summing amplifier, devices comprising different thresholds for pre-alarm, emergency shutdown and/or control rod drop, each device aforesaid being intended to deliver command signals for appropriate orders by means of at least one regrouping OR-circuit in the event of overstepping of the corresponding threshold. Further properties and advantages of the invention will become more clearly apparent from the following description of one practical example of the method which is given by way of illustration and not in any limiting sense. In this example of application of the invention, thermal monitoring of the core of a fast reactor cooled by a liquid metal and especially sodium is carried out in a wholly satisfactory manner and offers a considerable advantage since monitoring of the "hot point" of fuel cans is of major importance in this type of reactor. The core of a nuclear reactor of this type is described in particular in the June 1973 issue of "Bulletin d'Informations Scientifiques et Techniques" No 182 published by Commissariat a l'Energie Atomique.