Patent Number: 046997490
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates the nuclear steam supply system of a pressurized water reactor (PWR) nuclear power generating unit which embodies the present invention. The system comprises a nuclear reactor 1 which includes an upright, generally cylindrical reactor core 3 housed in a pressure vessel 5. The core 3 contains fissionable material in which sustained fission reactions occur to generate heat which is absorbed by reactor coolant in the form of light water passed through the core 3. The reactor coolant is circulated in a primary loop which includes a hot leg conduit 7 to convey the heated reactor coolant from the reactor core 3 to the primary side of a steam generator 9 where the heat is transferred to feed water on the secondary side to produce steam. This steam is utilized in a secondary loop (not shown) in a well-known manner to drive a turbine-generator set (also not shown) which produces electric power. The reactor coolant is returned to the reactor core 3 through a cold leg conduit 11 by a reactor coolant pump 13. While one primary loop is shown in FIG. 1 for illustration, in practice a typical PWR has two to four primary loops each serving its own steam generator. Long term adjustment of the reactivity of the reactor core 3 is controlled by disolving a neutron absorbing material such as boron in the reactor coolant which is circulated through the core. The reactor coolant also serves as a moderator to slow the fast neutrons released by the fission reactions down to the energy levels required for sustained fission reactions. A PWR possesses a negative temperature moderator coefficient in that as the water becomes cooler and hence denser, it slows down more fast neutrons to the critical level for fission and thus increases the reactivity of the core. The reactivity of the core 3 is also regulated by control rods 15 made of neutron absorbing material which are inserted into the core 3 vertically from above. The control rods 15 are positioned by a control rod drive system 17 under the direction of a control system 19. Since the control rods move in the axial direction within the core, they have an affect on the axial distribution of core power. Some of the control rods 15 are part length rods which in some installations are used to help control the axial distribution of power within the core. In all installations, the positioning of the control rods is managed by the control system 19 to maintain the axial distribution of power within prescribed limits. It has long been recognized that the power generated by the reactor 1 is proportional to the fast neutron flux escaping from the core 3. Hence, the power is typically measured by elongated neutron detectors 21 (one shown) extending vertically at spaced locations around the pressure vessel 5. These detectors have upper and lower sections 21a and 21b which provide separate indications of the power in the upper and lower portions of the core 3 respectively. The usual practice is to provide four such neutron detectors spaced evenly around the pressure vessel to generate four independent measurements of the neutron flux. The redundancy provided by the mulitple detectors assures the reliability required for protection and control purposes. The separate flux measurement made by the upper and lower halves 21a and 21b of the neutron detectors are transmitted to the control system 19 over lines 23 and 25 respectively. The control system 19 processes the neutron flux signals from the detectors 21a and 21b to calculate the axial offset in accordance with the formula: ##EQU1## This axial offset is a measurement of the skewing of power within the core in the axial direction. A typical scheme for operating a PWR is to maintain the axial offset at a preselected value, which changes through the fuel cycle, during normal operation of the reactor. A typical target value for the axial offset expressed as a percentage is +2 to 3% with an operating band which typically ranges upwards to 7%. If the axial offset drifts outside of this band, the power is reduced to bring it back within limits. As previously mentioned, skewing of the power can also be measured in terms of axial shape index which reverses the terms in the numerator in the above equation thereby referencing the index to the bottom of the core. Under normal operating conditions, radial power distribution within the core is not a concern because movement of the control rods is synchronized to provide symmetry about the longitudinal axis of the core. Provision is made, however, for the possibility that that symmetry could be broken for instance by a dropped rod. Due to the physics of a PWR, the dropped rod will cause an immediate decrease in the power generated in the vicinity of the dropped rod which will initially result in a reduction in the total power generated by the core. The reactor will then attempt to meet the load placed upon it by the demand for steam in the secondary loops by increasing power in the remainder of the core, which as mentioned previously, could lead to local overheating elsewhere in the core. The present invention addresses this problem by detecting the dropped rod condition but shutting the reactor down only when conditions warrant. Our analysis has shown that unprogrammed insertion of certain rods has more affect on other portions of the core than other dropped rods. The effects of the dropping of the various rods into the core were considered for the worst case condition which occurs when there is a large amount of reactivity, available through dilution of the boron in the reactor coolant, through a drop in reactor coolant temperature and through withdrawal of partially inserted control rods. The limiting factor is the departure from nucleate boiling ratio (DNBR) limits. We have determined that as long as the axial offset remains within identifiable limits, applicable to all dropped rod conditions, the DNBR limits for all other localities in the core will not be exceeded and it is not necessary therefore to shutdown the reactor. The situation is aided to some extent by the fact that the dropping of multiple control rods reduces the reactivity to the extent that the power is automatically reduced to a safe level even with the resultant radial peaking of power. In the present invention, a dropped rod condition is detected in a manner similar to that used in U.S. Pat. No. 4,399,095, i.e. by monitoring the negative rate of change of the fast neutron flux. Novel means are then used to determine whether the dropped rod condition warrants shutting down the reactor. FIG. 2 illustrates schematically a portion of the control system 19 of FIG. 1 for implementing the invention. A similar circuit is provided for each of the detector channels 21. The output of each half 21a and 21b of each fast neutron detector is applied to a neutron flux processor 27 which sums the two outputs to generate a power signal on line 29 and generates an axial offset signal which is calculated according to the formula set forth above and applied to line 31. The power signal is applied to a conventional dynamic rate-lag compensation circuit 33 which generates an output representative of the rate of change in the power. If the output of the dynamic rate-lag circuit is negative enough to exceed a preselected setpoint characteristic of a dropped rod, a negative rate bistable 35 will change state. The change of state of the bistable 35 is stored by a memory unit 37 which maintains the stored bistable output until manually reset. Thus, while the flux will subsequently rise as other portions of the core respond to meet the load demand, the indication of the dropped rod condition is preserved. The output of the memory unit 37 is applied to an AND circuit 39. The axial offset signal on lead 31 is compared with two axial offset setpoint signals X.sub.1 and X.sub.2 in bistables 41 and 43 respectively. The output of bistable 43 is applied to an AND gate 45 together with a signal from a part length rod out indicator 47. If the part length rods are out and the axial offset exceeds the set point value X.sub.2, a signal is applied by AND gate 45 to an OR gate 49. On the other hand, if the part length rods are in, and the axial offset exceeds the set point value X.sub.1, bistable 41 will apply a signal to OR gate 49. Either signal is applied by OR gate 49 to AND gate 39 together with the output of the memory circuit 37. If the memory 37 has been set to indicate a dropped rod condition, AND 39 gates either signal indicating that an axial offset limit has been exceeded to an OR gate 51 which, in turn, applies a signal to a two out of four voting logic circuit 53 together with similar signals from the other channels. If a signal is also applied to the two out of four voting logic circuit 53 by any other channel, a reactor trip signal is generated. The reactor trip signal is applied to the rod drive system 17 to insert all of the control rods into the core to shut the reactor down. The output of the rate lag circuit 33 is also compared with a second set point signal in a bistable 55. The value of the set point signal applied to the bistable 55 is selected to be indicative of a rate of decrease in reactor power so severe as to require immediate shutdown of the reactor. Thus, if this set point value is exceeded, the signal generated by the bistable 55 is stored in a memory unit 57 and is applied through the OR gate 51 to the two out of four logic circuit 53 until the memory is manually reset. Again, at least one other channel must also apply a signal to the two out of four voting logic circuit 53 in order to generate a reactor trip. This trip generated when the decrease in flux rate exceeds the set point applied to the bistable 55 is the conventional high flux rate trip signal currently provided in PWRs, and is shown here to illustrate how this trip interfaces with the invention. As mentioned, the set point value for the decrease in flux rate applied to bistable 35 is selected to indicate a dropped rod condition. An exemplary value for this set point corresponds to a value obtained when a 0.05 to 0.1% reactivity worth control rod is dropped into the core. Typical values for the axial offset set point signals applied to the bistables 41 and 43, respectively, which would cause a reactor to trip under a dropped rod condition are approximately +10% with the part length rods out and approximately 0% with the part length rods in. With the present invention, the reactor is only shutdown if axial offset limits indicative of unacceptable local power peaks are exceeded and the reactor can often be run at full power with a dropped control rod. Thus, the invention reduces the likelihood of interruption of reactor operation when a control rod is dropped and allows greater utilization of the reactor under these conditions. While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teaching of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.