GB2235841A

GB2235841A - Microwave detection apparatus

Info

Publication number: GB2235841A
Application number: GB8918611A
Authority: GB
Inventors: James Fitzpatrick
Original assignee: SOUTH OF SCOTLAND ELECTRICITY
Current assignee: SOUTH OF SCOTLAND ELECTRICITY
Priority date: 1989-08-15
Filing date: 1989-08-15
Publication date: 1991-03-13
Also published as: GB8918611D0

Abstract

Apparatus for detecting the presence or absence of objects at a predetermined location. particularly suitable for use in a hostile environment such as a nuclear fuel dismantling cell, comprises a microwave transmitter/receiver (34), a first waveguide (24) for directing a transmitted microwave signal to the location, and a second waveguide (26) for directing a return signal to a detector (40) forming part of the transmitter/receiver. A comparator (54) compares the returned signal with a threshold, the result of the comparison indicating the presence or absence of an object at the location and generating a control signal. The apparatus preferably used Q-band microwaves. The second waveguide (26) can be omitted and the apparatus adapted to respond to a return signal reflected along the first waveguide (24) from the location being monitored. <IMAGE>

Description

'Aicrowave betection Appraù5n ticrwåve'Deecin Apparaus" This invention concerns an apparatus for detecting objects in a hostile environment, and is particularly, but not exclusively, concerned with the detection of objects such as fuel elements, being handled within a nuclear power installation.

There is a requirement for a reliable means of detecting objects being handled by mechanical apparatus within nuclear power installations. For example, in an irradiated fuel dismantling cell in a nuclear reactor, graphite cylinders containing fuel elements must be transferred from one location to another. The graphite cylinders are arranged in stacks in cylindrical housings and are removed from their housings (eg for transfer to a storage pond) by means of a hydraulic ram located at the bottom of the housing which pushes the whole stack upwards so that the topmost cylinder projects out of the housing and may be engaged by a transport device.

The transport device is a massive steel block which is movable around the top surface of the fuel housings, and has a circular bore formed therethrough which may be positioned above a fuel housing such that a graphite cylinder located therein may be pushed upwards into the bore, where it is gripped by clamping means and can be transported to another location. Since the cylinder to be transported must be pushed fully into the bore of the transport device, and since there is a narrow clearance between the device and the top of the reactor, the top of the next cylinder in the stack from which the topmost cylinder is being removed must project out of its housing and into the bore in order to push the topmost cylinder fully home.Thus, before the transport device can be moved away, the hydraulic ram in the fuel housing must lower the stack again to bring the cylinder which is now the topmost member of the stack back within the housing. There is a possibility that this cylinder will foul the sides of the bore or the housing when the stack is lowered and may become stuck. If this occurs and the transport device is moved, the stuck cylinder would be damaged, causing the dismantling cell to become heavily contaminated.

Accordingly, some means is required for detecting the presence of objects obstructing the gap between the transport device and the fuel housing. Conventional detection devices, such as optical or ultrasonic devices, will not operate reliably in a radioactive environment. It is also necessary for any electronic components to be positioned remote from the irradiated area, and for any access openings in the wall surrounding the irradiated area to be as small as possible.

It is an object of the present invention to provide a detection apparatus suitable for the purpose discussed above, which will operate reliably and, preferably, in a fail-safe manner in a radioactive environment.

A detection apparatus in accordance with the present invention comprises at least a first waveguide means having first and second ends, transmitter means for injecting a microwave signal into said first end of said first waveguide means, receiver means for receiving a return signal from said first waveguide means, and comparator means for comparing the transmitted and received microwave signals, said second end of said first waveguide means being positionable adjacent a location which is to be monitored and said comparator means being adapted to generate a control signal if comparison of said transmitted and received signals indicates the presence of an object at the location being monitored.

Both the transmitter and receiver means may be connected to the first waveguide means such that the apparatus responds to a signal reflected back down the waveguide to the receiver means (via a suitable directional coupling), however it is preferred that the receiver means is connected to a first end of a second waveguide means having its second end facing the second end of the first waveguide means with a gap therebetween such that the device responds to attenuation of the return signal, indicating the presence of an obstruction in said gap.

The frequency of the microwave signal is preferably in the range 33.0 to 50.1 GHz (the Q band), which allows the physical size of the waveguides to be minimised and also exhibits very high noise rejection.

The output from the apparatus may control any type of alarm or control system. In the case of the transport device discussed above, the output signal is preferably used to control the power supply of the device and to disable it if an obstruction is detected.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing, which schematically illustrates an apparatus embodying the invention and its application in detecting objects in an irradiated fuel dismantling cell.

Referring now to the drawing, an irradiated fuel dismantling cell is generally indicated by a wall 10 which encloses the cell, a fuel housing indicated by top plate 12 having a circular aperture 14 formed therein, and a transport device including a steel plate 16, which is movable relative to the plate 12 and which also includes a circular aperture 18 of dimensions similar to aperture 14.

The plates 12 and 16 are substantially parallel and spaced apart by a narrow vertical gap 20.

The fuel housing contains a stack of graphite cylinders, of which only a topmost cylinder 22 is shown, each of which encapsulates a plurality of fuel elements.

The stack is movable vertically by means of a hydraulic ram (not shown) located at the bottom thereof, such that the topmost cylinder 22 may be pushed upwards through the apertures 14 and 18 to be gripped by clamping means (not shown) forming part of the transport device.

In order to avoid contamination of the fuel cell by damage to one of the graphite cylinders, it is essential that the plate-16 is not moved while a cylinder is traversing the gap 20 between the plates. The drawing further shows a detection apparatus, embodying the present invention, adapted to monitor the gap 20 adjacent the aperture 14 and to disable the transport device if an obstruction is detected.

The detection apparatus comprises first and second rectangular waveguides 24 and 26, constituting a transmit line and a receive line respectively, each extending into the cell through an aperture 28 in the wall 10 and into the gap 20. The ends of the waveguides 24 and 26 inside the cell terminate in pyramidal horn antennae 30 and 32 respectively, which are disposed so as to face one another directly across a diameter of the aperture 14 as shown.

The opposite ends of the waveguides 24 and 26 are connected to a microwave transmitter/receiver unit 34. The unit 34 includes microwave transmitter means 36 coupled to the first waveguide 24 by a further short waveguide 38, and receiver means coupled to the second waveguide 26 by another short waveguide 42. The unit 34 may thus be located externally of the cell wall 10 and remote therefrom. The waveguides 38 and 42 of the unit 34 may additionally be fitted with directional couplings 44 for measuring signal power during calibration and/or testing of the apparatus.

The transmitter 36 is preferably a Q band (ie 33.0 to 50.1 GHz) microwave transmitter, suitably a Millitech GDM22-02 Gunn Source giving a signal frequency of 42.1 GHz.

The microwave signal is preferably modulated by modulation means 46. In this example the signal is modulated by a 500 Hz square wave so that the signal injected into the waveguides 38,24 is a series of pulses of one millisecond duration (as shown at A).

The receiver 40 is preferably a "crystal" detector, such as a germanium diode device (suitably a Flann 2315 PJ Detector).

In use then, a signal transmitted along the waveguides 38 and 24 will traverse the aperture 14 and be returned to the receiver 40 via waveguides 26 and 42. If an object such as the cylinder 22 is obstructing the gap 20 between the antennae 30,32, then the return signal will be substantially attenuated or eliminated completely. The variation in the return signal detected by the receiver 40 may then be used to generate a control signal which may be used for any wide variety of purposes.

In the present example, the return signal is used to control a relay 48 which is adapted to be maintained in an energised condition, thus enabling the transport device of the dismantling cell, so long as the microwave signal is unobstructed, and to be de-energised, so as to disable the transport device, if the signal is obstructed or if any part of the apparatus fails. The apparatus thus controls the transport device in a fail-safe manner.

In order to control the relay 48, the 500 Hz carrier of the output signal from the receiver 40 is demodulated and amplified by a modulation amplifier 50 and filtered by filter means 52. The signal is then passed to a threshold comparator 54. If the amplified signal exceeds a preset threshold level (indicating that the transmitted signal was unobstructed), then it is amplified up to a constant limiting level by a limiting amplifier 56, rectified by rectifier means 58 (preferably comprising a diode pump circuit) and finally employed to maintain the relay 48 in an energised state.

As an alternative to the embodiment described above, the apparatus could be adapted to respond to a signal reflected from an object obstructing the gap. In this case only a single waveguide run would be required for the transmitted signal and the reflected, return signal, the return signal being bled off to the receiver means using a directional coupling. In this case the control apparatus would be adapted to maintain the relay 48 energised so long as no return signal was detected. The arrangement illustrated in the drawing is preferred, however, since it is inherently more reliable.

A 0 band microwave signal is particularly preferred, since this allows the cross-sectional dimensions of the waveguides to be minimised. In this example both the waveguides 24 and 26 may pass through a 1/2 inch (1.27cm) diameter aperture in the cell wall, which may be sealed easily and effectively by pressure windows. The 0 band also exhibits high noise rejection characteristics.

The invention as hereinbefore described provides a highly reliable, fail-safe detection system suitable for use in a radioactive environment, the microwave signal and the waveguides within the irradiated area not being adversely affected by their surroundings and the electronics of the device being positionable remotely at a safe distance.

Claims

1. Detection apparatus for detecting the presence or absence of an object at a predetermined location, comprising at least a first waveguide means having first and second ends, transmitter means for injecting a microwave signal into said first end of said first waveguide means, receiver means for receiving a return signal from said first waveguide means, and comparator means for comparing the transmitted and received microwave signals, said second end of said first waveguide means being positionable adjacent a location which is to be monitored and said comparator means being adapted to generate a control signal in response to the result of comparison of said transmitted and received signals.

2. The apparatus of claim 1, wherein both the transmitter and receiver means are connected to the first waveguide means such that the apparatus responds to a signal reflected back down the waveguide to the receiver means via a suitable directional coupling.

3. The apparatus of claim 1, wherein the receiver means is connected to a first end of a second waveguide means having its second end facing the second end of the first waveguide means with a gap therebetween such that the device may detect attenuation of the return signal, indicating the presence of an obstruction in said gap.

4. The apparatus of any preceding claim, wherein the frequency of the microwave signal is in the range 33.0 to 50.1 GHz (the Q band).

5. The apparatus of any preceding claim, wherein said comparator means generates said control signal if the comparison of said transmitted and received signals indicates the presence of an object at the location being monitored.

6. The apparatus of claim 5, wherein said apparatus is adapted for use in monitoring movement of nuclear fuel elements by a transport device in a fuel dismantling cell of a nuclear power installation.

7. The apparatus of claim 6, wherein the control signal is used to control the Bower supply of the device and to disable it if an obstruction is detected at the location being monitored.

8. Detection apparatus for detecting the presence or absence of an object at a predetermined location, substantially as hereinbefore described with reference to accompanying drawing.