Patent Number: 045487847
Section: description

The power control system of the present invention applied to a pressure tube type nuclear reactor is now explained below. FIG. 1 shows a cross-sectional view of the nuclear reactor. A number of pressure tubes 2 are arranged in the nuclear reactor core tank 1. While not shown, all of the pressure tubes 2 have a calandria tube and fuel assemblies. In order to measure a power of the reactor, neutron detectors 3 are mounted between the pressure tubes 2 at several points in a heavy water moderator region 4 in the reactor core tank 1. Power flattening control rods 5 for averaging the power distribution of the reactor and automatic power control rods 6 for automatically controlling the power level of the nuclear reactor based on the measurement by the neutron detectors 3 are disposed in the nuclear reactor. FIG. 2 shows a longitudinal sectional view of the nuclear reactor. In addition to the power flattening control rods 5 and the automatic power control rods 6 described above, a safety rod 7 for emergency shutdown of the nuclear reactor in case of an accident of the nuclear reactor is provided as a control rod. The control rods 5 and 6 and the safety rod 7 are all inserted between the pressure tubes 2 to reduce the power. Signals from the detectors 3 uniformly distributed within the heavy water moderator 4 are summed and averaged in a summing and averaging circuit 8 and calibrated in a calibration circuit 9 with a thermal output of the nuclear reactor determined by a periodic thermal heat balance calculation. In a normal operation, this signal is compared in a sampling adjuster 10 with a preset nuclear reactor power signal A requested by an operator of the nuclear reactor. Assuming that the former signal is positive and the latter signal is negative, when a differential signal amplified by an amplifier 17 is positive, a control rod drive circuit 11 produces a control rod withdrawal signal so that the automatic power control rods 6 are withdrawn from the reactor by a control rod drive mechanism 13 and a control rod drive motor 12 until the signal reaches zero. If the difference between the positive signal and the negative signal is negative, the automatic power control rods 6 are inserted into the reactor so that the power level of the nuclear reactor is automatically controlled to the predetermined level. If the safety rod 7 drops into the nuclear reactor tank 1 by some reason, for example, by an improper operation or a failure in the control rod drive mechanism 13, the signal of the summing and averaging circuit 8 which sums and averages the signals from the nuclear detectors 3 becomes smaller than the signal A (e.g. 100% output) preset by the operator of the nuclear reactor. A difference between the averaged measurement of the nuclear detectors and the preset level A is compared in a comparator 15 with a critical signal level B (e.g. 5% output) for a power reduction which is also preset by the operator of the nuclear reactor. Only when the difference is larger than the preset level B, the control rod withdrawal protection signal circuit 14 produces a control rod withdrawal protection signal to prevent the withdrawal of the automatic power control rods 6. In the prior art control system which has no such control rod withdrawal protection means, the drop of the safety rod is compensated by the withdrawal of the automatic power control rods 6 so that the power level is automatically recovered to the original 100% power level. FIG. 3 shows a change in the nuclear reactor power and a change in position of the automatic power control rods 6. In FIG. 3, it is assumed that an accident of the drop of the control rod such as the safety rod into the nuclear reactor takes place at a time zero (second). In this case, the nuclear reactor power drops to a 75% power level in two seconds from the occurrence of the accident. As a result, the average of the signals from the detectors 3 shown in FIG. 3 becomes smaller than the preset level A (100% output) for the power of the nuclear reactor and the automatic power control rods 6 are withdrawn from the nuclear reactor 0.5 second after the occurrence of the accident due to a time delay in the calibration circuit 9 so that the power recovers to its original level (100% output) in approximately ten seconds. However, the power distribution in the nuclear reactor is not flattened in this case and the maximum linear heat generating rate which is no more than 17.5 kw/ft in order to prevent the fuel in the nuclear reactor from becoming molten and the minimum critical heat flux ratio which is no less than 1.9 in order to prevent the cladding from being burnt out change. Consequently, when the control rod (including the safety rod) other than the optimum designed power flattening control rod is inserted into the nuclear reactor, the power distribution results in a large distortion as shown by a broken line in FIG. 4, in which the area at which the drop accident has taken place shows a low power distribution and the other areas show a high power distribution. As a result, the heat limitations such as the maximum linear heat generating rate and the minimum critical heat flux ratio exceed the design limits at the high power areas and the fuel may become molten and fail. FIG. 5 shows changes in time of the maximum linear heat generating rate and the minimum critical heat flux ratio as the control rod drops. As a result of the reduction of the power by the drop of the control rod such as the safety rod 7, the maximum linear heat generating rate becomes small and the minimum critical heat flux ratio becomes large for about one second after the accident. The power thereafter increases to compensate for the reduction of the power in the low power area as shown in FIG. 3 so that the maximum linear heat generating rate in the high power area changes largely while the minimum critical heat flux ratio changes in a small amount. Those values overshoot and undershoot, respectively, approximately nine seconds after the accident, and when the nuclear reactor output recovers to the 100% power level, the maximum linear heat generating rate assumes a value of 21 kw/ft and the minimum critical heat flux ratio assumes a value of 1.5. The reason why those values are larger and smaller than the pre-accident values 17.5 kw/ft and 1.9, respectively, is because the power distribution is remarkably distorted by the drop of the control rod. On the other hand, when the withdrawal of the control rod is protected in accordance with the embodiment of the present invention, the nuclear reactor power and the position of the automatic power control rods change as shown in FIG. 6 when the control rod drops into the nuclear reactor. In FIG. 6, as the control rod drops, the power is reduced from 100% power to 75% power in a short time (approximately two seconds). However, since the amount of reduction (25%) is larger than the preset amount B (5% power), the automatic power control rods 6 are withdrawn only by the amount corresponding to the time delay in the comparator 15 and the calibrator 9, when the withdrawal of the automatic power control rods 6 is protected by the control rod withdrawal protection signal. As a result, the recovery of the nuclear reactor power level stops at 80% power level. Consequently, even if the power distribution of the nuclear reactor is distorted as shown by the broken line in FIG. 4 by the drop of the control rod, the power level recovers only to as much as 80% and hence the maximum linear heat generating rate for the heat factors rises only to 16 kw/ft as shown in FIG. 7. Similarly, the minimum critical heat flux ratio falls only to 2.1. Since those values satisfy the limitations of no more than 17.5 kw/ft and no less than 1.9, the integrity of the fuel is maintained. The preset level B is determined taking an external disturbance of the nuclear reactor into consideration because it is necessary that the automatic power control fully functions for the external disturbance which may occur during a normal operation of the nuclear reactor and which does not disturb the operation of the nuclear reactor. Thus, an effective value for the preset level B is 5%. With such a preset level, the withdrawal of the control rod is not prevented by the external disturbance, provided that an abnormal condition of the control rod per se such as the drop of the control rod does not take place. If a response value of the dropped control rod is so small that the power changes only as much as five percent when the control rod drops, the power level recovers to the original 100% power level by the withdrawal of the automatic power control rods. In this case, however, since the response value of the dropped control rod is small, the distortion in the power distribution when the control rod drops is small and the integrity of the fuel is maintained. While the safety rod was assumed as the control rod which may drop in the description set forth above, it should be understood that the control rod may be the power flattening control rod or liquid poison. The means for automatically controlling the power level of the nuclear reactor may be concentration control of liquid poison included in the heavy water moderator. In this case, the function of a liquid poison remover may be stopped when the power level is lowered below the preset level B.