Anomaly diagnosis system for a nuclear reactor core

An anomaly diagnosis system for a nuclear reactor core comprises an anomaly detecting unit incorporated into a fuel assembly of the nuclear reactor core, and a transmitter-receiver provided outside the reactor vessel, which transmits a signal by wireless to the anomaly detecting unit and receives an echo signal generated by the anomaly detecting unit by wireless. When the anomaly detecting unit detects an anomaly in the nuclear reactor core, such as an anomalous temperature rise in the fuel assembly, the mode of the echo signal deviates from a reference signal mode, and then the transmitter-receiver detects the deviation of the mode of the echo signal from the reference signal mode and gives an anomaly detection signal to a plant protection system, and thereby the nuclear reactor can be shut down without delay.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an anomaly diagnosis system for a nuclear 
reactor core, which gives an anomaly detection signal to a control rod 
driving mechanism or the like upon the detection of an anomaly in the 
nuclear reactor core, such as an excessive temperature rise in the reactor 
core, to shut down the nuclear reactor. 
2. Description of the Prior Art 
When an anomalous condition occurs in the fuel system of a nuclear reactor, 
a control rod is inserted in the core for safety control such as emergency 
shut down. 
Conventionally, the anomalous reaction of the fuel in a nuclear reactor is 
diagnosed by detecting the flow rate and temperature of the coolant by a 
flow meter and a thermometer provided near the outlet of the coolant 
within the reactor vessel and connected through a cable to an external 
diagnosis unit, and the neutron flux density of the fuel assembly by a 
neutron detector inserted in the gap of the fuel assembly and connected 
through a cable to the external diagnosis unit. 
Such a conventional diagnosis system, however, has the following problems. 
First, the detection and warning of an anomalous condition by the detector 
provided at the exit of the reactor vessel or in the gap of the fuel 
assemblies delays the implementation of safety countermeasures. 
Secondly, it is impossible to easily change the detector provided near the 
coolant outlet of the reactor vessel or in the gap of the fuel assemblies. 
Thirdly, it is difficult to extend a cable connecting the detector to the 
outside reactor vessel and the deterioration of the cable reduces the 
reliability of anomaly diagnosis. 
Fourthly, when the detector is inserted in the gap of the fuel assemblies, 
the detected neutron flux density and the detected temperature of the fuel 
assembly are dependent on the position and condition of the fuel assembly 
and are liable to deviate from true values, and thereby the reliability of 
anomaly diagnosis is reduced. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a highly 
reliable anomaly diagnosis system for a nuclear reactor core, eliminating 
the foregoing disadvantages of the conventional anomaly diagnosis system, 
capable of implementing safety countermeasures without an unacceptable 
delay, using detectors capable of being easily changed, eliminating 
difficulties in extending cables for anomaly detection, less liable to 
cause the cables to deteriorate, and capable of obtaining correct detected 
values regardless of the position and condition of the fuel assembly. 
To achieve the object of the invention, the present invention provides an 
anomaly diagnosis system for a nuclear reactor core, comprising: an 
anomaly detecting unit provided in the fuel assembly, the anomaly 
detecting unit being capable of generating an echo signal which is varied 
upon the detection of an anomaly in the nuclear reactor core; and a 
transmitter receiver which gives a signal to the anomaly detecting unit to 
make the anomaly detecting unit provide an echo signal and gives an 
anomaly detection signal to a plant protection system upon the reception 
of an echo signal representing an anomaly in the nuclear reactor core from 
the anomaly detecting unit. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description taken 
in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
First Embodiment (FIGS. 1, 2 and 3) 
Referring to FIGS. 1 through 3, a plurality of fuel assemblies 2 are 
disposed within a reactor vessel 1. As shown in FIG. 3, each fuel assembly 
2 comprises a duct 3 and a plurality of fuel pins 4 contained in the duct 
3. A coolant is introduced into the duct 3 through an inlet 5 formed at 
the bottom of the fuel assembly 2 and exchanges heat with the fuel pins 4 
to cool the fuel pins 4 as the coolant flows toward an outlet 6 formed at 
the upper end of the fuel assembly 2. An anomaly detecting unit comprises 
thermocouples 7a and 7b for detecting the temperature of the coolant, and 
an echo capsule 8 connected to the thermocouples 7a and 7b. The 
thermocouples 7a and 7b are provided respectively near the inlet 5 and 
outlet 6 of the fuel assembly 2. 
As shown in FIG. 1, the echo capsule 8 is associated with an external 
transmitter-receiver 9 for wireless communication therebetween by using 
radio waves. The external transmitter-receiver 9 transmits a radio wave to 
the echo capsule 8 and receives a radio wave echo of a frequency from the 
echo capsule 8. The normal frequency of radio wave echo provided by the 
echo capsule 8 varies when the echo capsule 8 detects ar anomaly in the 
nuclear reactor core. The echo capsule 8 comprises an oscillating circuit 
consisting essentially of an exciting coil 10, a variable-capacity diode 
11, a capacitor 12, and a terminal assembly 13 for connecting the echo 
capsule 8 to the thermocouples 7a and 7b. The variable-capacity diode 11 
has a fixed electrostatic capacity C corresponding to the electromotive 
force V of the thermocouples 7a and 7b, and the echo capsule 8 has a 
resonance frequency Hf corresponding to the electrostatic capacity C of 
the variable-capacity diode 11. 
When the normal frequency of the echo capsule 8 varies, the external 
transmitter-receiver 9 gives an anomaly detection signal through a cable 
14 to a plant protection system 15, for example, for a control rod driving 
mechanism. 
In operation, the external transmitter-receiver 9 transmits a pulse radio 
wave of the resonance frequency Hf to excite the echo capsule 8. When no 
anomaly is found in the nuclear reactor core, the echo capsule 8 transmits 
a radio wave echo representing a normal condition. The external 
transmitter-receiver 9 receives the radio wave echo and counts the 
frequency of the radio wave echo. 
Suppose that an anomaly has occurred in the nuclear reactor core and the 
temperature difference between the coolant at the inlet 5 and the coolant 
at the outlet 6 has been caused to deviate from a reference temperature 
difference, the difference in thermoelectromotive force between the 
thermocouples 7a and 7b varies and the electrostatic capacity of the 
variable-capacity diode 11 varies. Consequently, the resonance frequency 
of the echo capsule 8 varies and hence the frequency of the radio wave 
echo varies. Upon the detection of frequency variation of the radio wave 
echo, the external transmitter-receiver 9 gives the anomaly detection 
signal through the cable 14 to the plant protection system 15 for the 
control rod driving mechanism to shut down the nuclear reactor. 
In case the echo capsule 8 malfunctions, the frequency of the radio wave 
echo is stopped. Therefore, the malfunction of the echo capsule 8 can be 
detected through the detection of the stoppage of the frequency of the 
radio wave echo. 
When the respective fuel assemblies 2 are provided with echo capsules 8 
respectively having different resonance frequencies, an anomalous fuel 
assembly 2 among the plurality of fuel assemblies 2 can be identified. 
Second Embodiment (FIG. 4) 
In the second embodiment, the echo capsule 8 has an oscillating circuit 
incorporating an oscillator having a frequency of normal mode of 
vibration, such as a quartz oscillator 16, the frequency of normal mode of 
vibration varies according to temperature. 
Third Embodiment (FIGS. 5, 6 and 7) 
An anomaly diagnosis system, in a third embodiment, according to the 
present invention, has an external ultrasonic wave emitter-receiver 17 
provided outside a reactor vessel 1. An ultrasonic wave reflecting member 
18 as means for detecting an anomaly is provided on the top of each fuel 
assembly 2 so as to reflect an ultrasonic wave emitted from the ultrasonic 
wave emitter-receiver 17 toward a fixed point on the ultrasonic wave 
emitter-receiver 17. The ultrasonic wave reflecting member 18 is formed of 
a shape memory alloy. When the ultrasonic wave reflecting member 18 
deforms when heated to a predetermined shape memory restoration 
temperature, and thereby the reflecting direction of the ultrasonic wave 
reflecting member 18 is caused to change. 
In a modification, the ultrasonic wave reflecting member 18 may be attached 
to one of the fuel pins 4 as shown in FIG. 7. The ultrasonic wave 
emitter-receiver 17 decides that an anomalous condition occurred in the 
fuel assemblies 2 when the same is unable to receive the echo ultrasonic 
wave. 
In operation, all the ultrasonic wave reflecting members 18 reflect the 
ultrasonic wave emitted from the ultrasonic wave emitter-receiver 7 toward 
the ultrasonic wave emitter-receiver 17 as an echo ultrasonic wave. When 
the temperature of the coolant rises to the predetermined shape memory 
restoration temperature due to an anomaly in the nuclear reactor core, the 
ultrasonic wave reflecting members 18 deform to change the respective 
directions of reflection thereof. Consequently, the ultrasonic wave 
emitter receiver 17 is unable to receive any echo ultrasonic wave. Thus, 
the ultrasonic wave emitter-receiver 17 detects the anomaly and gives an 
anomaly detection signal to a plant protection system 15. 
When the ultrasonic wave reflecting member 18 is formed of a shape memory 
alloy containing a fissile material dispersed or sandwiched therein, the 
temperature of the ultrasonic wave reflecting member 18 is caused to rise 
to the shape memory restoration temperature by the anomalous increase of 
neutron flux density as well as the rise in the temperature of the 
coolant. 
It is also possible to identify an anomalous condition in the fuel assembly 
2 by searching the fuel assembly 2 from which the ultrasonic wave 
emitter-receiver 17 is unable to receive the echo ultrasonic wave. 
As apparent from the foregoing description, the anomaly diagnosis system 
for a nuclear reactor core, according to the present invention has the 
following effects. 
(1) The anomaly detecting unit incorporated into the fuel assembly detects 
an anomaly in the fuel assemblies directly, and a signal indicating an 
anomaly is transmitted by wireless from the echo capsule to the 
transmitter-receiver. Accordingly, the anomaly in the nuclear reactor core 
can immediately be detected and emergency countermeasures for shutting 
down the nuclear reactor can be implemented without delay. 
(2) Incorporated into the fuel assembly, the anomaly detecting unit can be 
taken outside the reactor vessel and can easily be changed. 
(3) Since the anomaly detecting unit communicates with the 
transmitter-receiver by wireless, the anomaly detecting unit and the 
transmitter-receiver need not be interconnected by a cable, and hence the 
anomaly diagnosis system eliminates difficulties in laying a cable and 
problems attributable to the deterioration of the cable and the 
reliability of anomaly diagnosis is improved. 
(4) Since the diagnostic function of the anomaly diagnosis system is not 
affected by the position and condition of the fuel assembly, the anomaly 
diagnosis system has high reliability. 
(5) Even if the anomaly detecting unit should fail to function properly, 
the malfunction of the anomaly detecting unit can be found through the 
detection of disappearance of the echo signal. 
(6) Emergency countermeasures can immediately be implemented without delay 
to shut down the nuclear reactor in case an anomaly occurs in the nuclear 
reactor core, and hence the higher burnup can be obtained without 
entailing any danger to improve the fuel cost economy of the nuclear 
reactor. 
Although the invention has been described in its preferred form with a 
certain degree of particularity, obviously many changes and variations are 
possible therein. It is therefore to be understood that the invention may 
be practiced otherwise than specifically described herein without 
departing from the spirit and scope thereof.