GB2114299A

GB2114299A - Ultrasonic temperature- measuring probe

Info

Publication number: GB2114299A
Application number: GB08301959A
Authority: GB
Inventors: Louis Maurice Campana; Ernst Emanuel Siegfri Paetzold; Walter Leonard Arloff
Original assignee: European Atomic Energy Community Euratom
Current assignee: European Atomic Energy Community Euratom
Priority date: 1982-01-28
Filing date: 1983-01-25
Publication date: 1983-08-17
Also published as: FR2520505A1; DE3202744A1; DE3202744C2; GB8301959D0

Abstract

The invention relates to an ultrasonic temperature-measuring probe with a sensor and a sheath surrounding the latter. According to the invention, the sensor (9) is momentarily and regularly displaced longitudinally from its rest position against the force of a compression spring (17), thus preventing or obviating, even at high temperatures, any otherwise unavoidable sticking of the sensor wire to the sheath tube (6). A particular application of the probe is in the measurement of high temperatures in nuclear reactors. <IMAGE>

Description

SPECIFICATION Ultrasonic temperature-measuring probe The invention relates to an ultrasonic temperature-measuring probe having an elongate sensor wire, a casing or sheath surrounding the latter and having a magnetic system by way of which, on the one hand, ultrasonic pulses are emitted to one end of the sensor wire and, on the other hand, ultrasonic echoes are transmitted by this wire to an electronic analysing system, the time lag between the echoes from set reflection points of the sensor being evaluated as a measure for the temperature to be measured.

Ultrasonic thermometers are used for measuring very high temperatures, e.g. above 1 500etc, such as occur, for example, in the fuel columns of nuclear reactors. Thermometers of this type are known, for example, from US PS 4 020 692. The sensor wire is protected by a sheath against harmful environmental influences. At the operating temperatures in question, it is possible for material transformations and diffusion phenomena to occur, which lead to "sticking" or bonding of the sensor wire to its protective sheath and thus impairing of the ultrasonic signal propagation along the sensor wire. The interference echoes produced thereby can heterodyne the measurement results and jeopardize their evaluation. This sticking of the sensor wire to the sheath rapidly led in the past, at high temperatures, to the thermometer becoming unusable and having to be replaced.

In the mentioned US PS it is thus recommended to arrange spacers between the sensor wire and the sheath at those places where in any case a signal reflection is intended to occur or where an interference echo can be tolerated. Accordingly, uncertainty is removed regarding the location at which the sticking can occur. However, such a measure can only be applied to a limited extent, since sufficient places are not always present on the sensor wire at which an echo can be tolerated. Moreover, as a result of the spacers the undesirable sticking of the sensor wire in the sheath is directly promoted.

Another solution to this problem is described in DE OS 27 35 908. Here the thermal expansion of the sensor wire and of the sheath is utilised benefically through the relative movement between them during jumps in temperature. As a result of these relative movements, any sticking is loosened again so that the thermometer can be used again. However, this method is only successful if the temperature in the vicinity of the thermometer can be varied at regular intervals. In the case of a thermometer for a nuclear fuel rod, this means shutting down the nuclear reactor.

The present invention seeks to provide an ultrasonic thermometer of the initially-mentioned type, which prevents or obviates sticking between the sheath and the sensor wire, while dispensing with spacers and jumps in temperature.

In accordance with the present invention there is provided an ultrasonic temperaturemeasuring probe having an elongate sensor wire, a sheath tube surrounding this latter and a magnetic system by way of which, on the one hand, ultrasonic pulses are emitted to one end of the sensor wire and, on the other hand, ultrasonic echoes are transmitted by this wire to an electronic analysing system, the time lag between the echoes from set reflection points of the sensor being evaluated as a measure for the temperature to be measured, wherein the sensor wire is extended rearwards through the magnetic system and is connected to a movable diaphragm of a mechanical impulse transmitter which is adapted periodically to deliver mechanical impulses in such a way that the sensor is momentarily displaced longitudinally from its rest position against the force of a compression spring, as a result of the impulses.

The displacements of the sensor in the longitudinal direction preferably take place outside the measurement cycles and at regular time intervals. In this way, bridges between the sensor wire and the sheath are broken in good time or are prevented from occurring.

Preferably the impulse transmitter is designed as a cylindrical bellows unit which is supplied with air-pressure impulses via a tube.

Preferably the centre axis of the bellows unit is slightly displaced relative to the axis of the sensor wire, and wherein the diaphragm is designed as a centering head lying concentric with the axis of the sensor wire.

The invention will be explained in more detail below on the basis of a preferred example of embodiment, with reference to the single dawing which shows a thermometer according to the invention in section.

The drawing illustrates a fuel column of a nuclear reactor, which consists of a stack of cylindrical fuel pellets 1 of annular crosssection in a fuel tube 2. The tube has a vertical centre axis 3 and is closed at the bottom by an end plug 4. At the top, the fuel tube 2 is welded to a plug 5 in which cylindrically a thermometer sheath tube 6 is securely inserted via a short guide tube 7.

This sheath tube penetrates the entire pellet stack and ends at the bottom just above the closure plug 4.

Between the plug 5 and the uppermost fuel pellet 1 a compression spring 8 is fitted concentrically around the guide tube 7, which compression spring makes possible a variation in length of the pellet stack as a result of temperature changes.

The sheath tube is traversed over its entire length by a sensor wire 9 which has constric tions or waisted sections at those places where an echo is to be produced. In the illustrated example of embodiment, only one constriction 10 is shown, which is situated just above the pellet stack. The mean temperature of the pellet stack can be deduced from the transit time of ultrasonic pulses between this constriction 10 and the lower end of the sensor wire.

Above the plug 5, the sensor wire passes through a magnetic system which comprises a permanent magnet 11 and an electromagnet 12. The coil of the electromagnetic 1 2 concentric to the axis 3 is, on the one hand, acted upon with ultrasonic pulses which are intended to pass through the sensor wire and on the other hand, serves as a measuring coil for the echo signals. The analysing circuit and the pulse transmitter are omitted from the drawing for the sake of simplicity. They are connected to the coil via a connecting cable 1 3. The sensor wire 9 consists of magnetostrictive material in the region of the magnetic system.

The upper end of the sensor wire 9 is provided with a centering head 14 which is mounted axially movably in a thermometer housing tube 15, which extends the fuel tube 2 upwards, and at the same time forms the movable diaphragm of a bellows 16. A spring 1 7 bears against the magnetic system 1 2 and against the centering head 14 and, in the absence of other forces, presses the latter against stops 1 8 which are arranged on the inner wall of the housing tube 1 5.

The bellows 1 6 are essentially cylindrical and terminate at the top in a plug 1 9 which is secured in the housing and which is penetrated by a capillary tube 20. In operation, pressure impulses are directed through this capillary tube into the bellows, so that the centering head is momentarily displaced longitudinally from its rest position as a result of the pressure impulses.

The bellows are not arranged coaxially in the housing tube but slightly eccentrically, so as to create space for the cable 1 3 of the magnetic system. For this purpose, the bellows are secured eccentrically to the central centering head. The housing tube is closed at the top by an end cap 21 through which the cable 1 3 and the capillary tube 20 pass.

In operation, pressure impulses are emitted, as required, in shorter or longer intervals to the capillary tube 20, as a result of which the sensor wire is momentarily displaced in the longitudinal direction, so that possible bridges between the sensor wire 9 and the sheath tube 6 are broken. Of course, temperature measurements are not possible during the duration of the pressure impulses. However, the pressure impulses do not last long so that the thermometer is available practically unlimitedly.

Within the scope of the present invention, the bellows could also be replaced by another mechanical impulse transmitter, for example an electromagnetic system, and the sensor wire could have more than one constriction 10, possibly so as to determine an entire temperature profile over the height of the reactor core. Since there are no supports between the sensor wire and the sheath tube, the thermometer according to the invention makes possible high local resolution of such a profile.

Claims

1. An ultrasonic temperature-measuring probe having an elongate sensor wire, a sheath tube surrounding this latter and a magnetic system by way of which, on the one hand, ultrasonic pulses are emitted to one end of the sensor wire and, on the other hand, ultrasonic echoes are transmitted by this wire to an electronic analysing system, the time lag between the echoes from set reflection points of the sensor being evaluated as a measure for the temperature to be measured, wherein the sensor wire is extended rearwards through the magnetic system and is connected to a movable diaphragm of a mechanical impulse transmitter which is adapted periodically to deliver mechanical impulses in such a way that the sensor is momentarily displaced longitudinally from its rest position against the force of a compression spring, as a result of the impulses.

2. A probe as claimed in claim 1 wherein the impulse transmitter is designed as a cylindrical bellows unit which is supplied with airpressure impulses via a tube.

3. A probe as claimed in claim 2 wherein the centre axis of the bellows unit is slightly displaced relative to the axis of the sensor wire, and wherein the diaphragm is designed as a centering head lying concentric with the axis of the sensor wire.

4. A probe as claimed in claim 1 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.