Patent Number: 046541861
Section: description

DETAILED DESCRIPTION The core of the nuclear reactor is crossed in a known manner by a primary fluid which circulates in a primary loop; this fluid absorbs energy as it passes through the core and gives up its energy during its passage through the steam generator to a secondary fluid circulating in a secondary loop. The primary circuit comprises a cold branch and a hot branch, the cold branch being of course placed between the steam generator and the core and the hot branch between the core and the steam generator, in the direction of flow of the fluid. The temperature of the primary fluid is measured at two points in a conventional manner, one of the points being situated on the cold branch and the other point being situated on the hot branch. In FIG. 1, the cold branch temperature signal is designated by 1 and the hot branch temperature signal by 1'. An operator 2 computes, in a conventional manner, the value of the enthalpy at the point of temperature measurement in the cold branch and the point of temperature measurement in the hot branch. The enthalpy can, for example, be determined by a second degree polynomial in T where T, is the measured temperature. To obtain improved precision, the computation can also be carried out using a third degree polynomial. Two signals 3 and 3' are thus obtained for enthalpy in the cold branch and for enthalpy in the hot branch. Two time shift operators 4 and 4' permit the signals 3 and 3' respectively to be delayed. The transfer functions employed are respectively .epsilon..sup.-.tau.op and .epsilon..sup.-.tau.'op where .tau..sub.o and .tau.'.sub.o represent, respectively, the total average time of transit of a molecule of primary fluid between the points of temperature measurement in the cold branch and in the hot branch, and the total average time of transit of a molecule of primary fluid between the points of temperature measurement in the hot branch and in the cold branch (p being the LAPLACE variable). For increased precision, it may be taken into account that the above-mentioned time of transit can be different for two different molecules of water; in particular, the reactor configuration is such that, generally speaking, water molecules have very different speeds at the outlet of the core. It would therefore be possible, using an integrator, to take these different times of transit into account instead of considering only the average time, as shown in FIG. 1. Signals 5 and 5' are obtained at the output of the time shift operators 4 and 4' and are then entered into the registers 6 and 6'. The register 6 produces the difference between the hot branch enthalpy signal 3' and the cold branch enthalpy delayed signal 5. the output signal of the register 6 is designated by 7. The register 6' produces the difference between the hot branch enthalpy delayed signal 5' and the cold branch enthalpy signal 3. The output signal of the register 6' is designated by 7'. Signals 7 and 7' are entered respectively into multipliers 8 and 8' where they are multiplied by the primary flow rate signal 9, the latter being measured in a completely conventional manner. To increase the precision, the primary flow rate signal 9 is filtered at 10 to take account of the variation in the average time of transit of a molecule of fluid through the core. The transfer function of the filter is in this case ##EQU1## where .tau..sub.1 represents the average time of transit of a molecule of primary fluid through the core. At the output of the multiplier 8 a primary thermal power signal 11 is obtained, and at the output of the multiplier 8' a signal 11' is obtained which represents the thermal power absorbed by the steam generator. The dynamics of the primary fluid temperature measurements in the cold branch and in the hot branch are compensated by two identical phase lead correctors 12 and 12' into which the signals 11 and 11' are entered, respectively. The output signals of these correctors are shown as 13 and 13'. The transfer function of these correctors is ##EQU2## where .tau..sub.2 is the time constant of the temperature measurements (measurement corrector) and where .tau..sub.3 is a reduction filter of the transient gain of the measurement corrector. Signals 13 and 13' are then entered into the comparators 14 and 14' where they are compared respectively to a neutron power signal and to a signal representing the thermal power produced by the steam generator. The neutron power signal is obtained in a conventional manner by means of neutron flux measurement chambers which are situated outside the core. The neutron power signal is shown as 15. Signal 15 is corrected as a function of the temperature variations by the use of a correction coefficient K1 between the measurement of neutron flux and the temperature of the annular space in which the neutron power chambers are conventionally situated, it being possible for this temperature to be taken as similar to the cold branch temperature. In FIG. 1, of course, .theta. represents a nominal temperature. In order to respect the signal phase as well as possible in transient operation, the temperature correction is shifted in time by the term .epsilon..sup.-.tau.15p (.tau..sub.15 is the time of transit between the cold branch measurement point and the core entry). To this term may be added a low-pass filter ##EQU3## to allow for the time required by the flux measurement chambers to respond to a variation in the temperature of the cold branch. 16 refers to the neutron power signal which has been corrected for temperature. Signal 16 is made dynamically equivalent to signal 13 by means of a point model 17 of heat transfer between the nuclear flux and the thermal flux of the primary fluid (.tau..sub.4 represents the time constant of heat transfer); the output signal of the model 17 is then entered into a point model of heat transfer of primary fluid in the core corresponding to the time of transit .tau..sub.1 /2 of a molecule of primary fluid from the center of the core to the outlet of the core. The output signal of the model 18 is then delayed by a time shift operator 19 expressing the time of transit .tau..sub.5 of a molecule of primary fluid from the outlet of the core to the point of temperature measurement in the hot branch. The neutron power signal which has been made dynamically equivalent to the signal of primary thermal power is shown as 20. Signal 20 is compared to signal 13 in the comparator 14. The signal 21 which is produced by this comparator is used for correcting the neutron power signal 16 by means of a corrector 22. This corrector 22 comprises an integrator with an integration constant .tau..sub.6 and gain k.sub.2..tau..sub.7 and .tau..sub.8 are respectively, phase lead and phase delay time constants, .tau..sub.8 being smaller than .tau..sub.7. The signal 23 produced by the corrector 22 is added to the signal 16 in the register 24. At the output of the register 24 a normalized signal of neutron power is obtained, and at the output of the model 17 a normalized signal of thermal power is obtained. The device according to the invention thus permits a fast and precise signal of the primary power of the reactor to be obtained by means of only two temperature-measuring sensors and conventional chambers for measuring neutron power. The device according to the invention further permits a fast and precise signal of the secondary power of the reactor to be obtained by means of the same two temperature sensors, as will be described hereafter. To produce a fast and precise signal of the secondary power of the reactor, signal 13' is compared, at 14', to a signal representing the thermal power produced by the steam generator of the cooling loop under consideration; this signal, which is derived from a simplified secondary balance, lacks precision but has the great advantage of being a fast-response signal. This signal 16' is produced in a computer 25 from four signals, namely the temperature and the flow rate of the steam generator feed water, together with the pressure and the flow rate of steam produced by the steam generator. The signal 16' is made dynamically equivalent to the signal 13' by means of a point model 17' of heat transfer between the secondary fluid and the primary fluid (.tau..sub.q is the time constant of heat transfer). The model 17' is followed by a point model 18' of heat transfer of the primary fluid in the steam generator corresponding to the time of transit .tau.Hd 10/2 of a molecule of primary fluid from the center of the steam generator to the outlet of the steam generator. A time shift operator 19' enables the time of transit .tau..sub.11 of a molecule of primary fluid from the outlet of the steam generator to the point of temperature measurement in the cold branch to be taken into account. At the output of the operator 19' a signal 20' is obtained which represents the thermal power produced by the steam generator, this signal being made dynamically equivalent to the signal 13' representing the thermal power absorbed by the steam generator. These two signals 20' and 13' are compared in the comparator 14'. The signal 21' produced by the comparator 14' is entered in a corrector 22'. The signal 23' produced by this corrector is used to correct the signal 16' to which it is added in a register 24'. In this case the corrector 22' is an integrator whose integration constant is .tau..sub.12 and gain k.sub.3. .tau..sub.13 and .tau..sub.14 are time constants of the phase advance and phase lag of the corrector (.tau..sub.14 is smaller than .tau..sub.13). A fast and precise signal of normalized secondary power is obtained at the output of the register 24'. The device according to the invention thus makes it possible to obtain at any time a fast and precise signal of primary power (neutron power and thermal power transmitted at the center of gravity of the core) and of secondary power, using two temperature sensors in the cold branch and the hot branch, neutron power measurement chambers and a simplified secondary balance. This device is of particular advantage during periods of transient operation. It permits instant detection of any variation in the primary or secondary power, permitting possible failures which have caused these variations to be remedied very quickly. The device according to the invention contributes to a proper protection of the core, particularly in the high performance reactors which are constructed at present. The correctors 22 and 22' may be designed differently so as to optimize the response of the correction signal. Furthermore, the registers 24 and 34 could be replaced by multipliers to preserve the measurement zero. The example described relates to a single loop, but the invention can of course apply to a reactor with several loops.