Nuclear reactor power control system

In a nuclear reactor power control system, when a control rod or a safety rod drops into a nuclear reactor by a failure of a control rod drive mechanism, a power of a summing and averaging circuit which sums and averages measurements of neutron detectors is smaller than a value A preset by an operator of the nuclear reactor. The averaged value of the neutron detector outputs is compared with the preset value A, and if the difference therebetween exceeds a preset limit signal level B for the output drop, automatic withdrawal of the control rod is stopped, and if the difference is smaller than the limit signal level B for the power drop, the control rod is withdrawn by the amount corresponding to the power drop.

The present invention relates to a nuclear reactor power control system, 
and more particularly to a system for automatically controlling a power 
level of a nuclear reactor. 
In a prior art control system for a power distribution or a power level 
such as that disclosed in Japanese Patent Application Laid-Open No. 
46594/78, signals from power detectors uniformly distributed in a nuclear 
reactor are summed and averaged and the averaged signal is fed back to 
automatic power control rods to control the power. In such a nuclear 
reactor which uses such a power control system, when a safety rod or a 
power control rod is inserted during the operation by reason of drop of a 
control rod or another, the power of the nuclear reactor is abruptly 
reduced and then the power recovers as a result of the withdrawal of the 
automatic power control rod by the action of the power control system. 
When the above power control takes place, the power is raised by the 
automatic power control rods over the entire area of the reactor so that 
the power distribution exhibits a large distortion such that the area at 
which the insertion accident of the safety rod or the control rod has 
taken place shows a low power distribution while the other areas show a 
high power distribution. As a result, thermal limitations such as maximum 
linear heat generating rate and minimum critical heat flux ratio may 
exceed design limitations for the high power area and fuel may become 
molten and fail. 
It is an object of the present invention to provide a power control system 
which can assure safe operation of a nuclear reactor even when the power 
is reduced by an accident or improper operation. 
According to a feature of the present invention, a nuclear reactor power 
control system is provided with a withdrawal protection function to 
prevent the movement of the automatic power control rod by the power 
control system when the power is reduced below a predetermined level by 
some reason. 
The inventor of the present invention considered that the insertion of the 
safety rods or the power control rods into the nuclear reactor when the 
accident or the improper operation takes place results in a local 
reduction of the power level due to one to several safety rods or control 
rods, the increase of the power by the withdrawal of the control rods is 
improper and that it is proper to stop the automatic power control 
movement when the power is reduced with a remarkable unbalance in the 
power distribution. 
According to the present invention, even if the control rods or the safety 
rods drop into the nuclear reactor during the operation of the nuclear 
reactor, the rise of the reactor power level which prevents the integrity 
of fuel is avoided, and a highly safe nuclear reactor is provided.

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.