GB2059058A - A device for measurement of radioactive particles in gases - Google Patents

Abstract

A device for measurement of radioactive particles in gases comprising a turntable (3) driven by a drive device (5) and having at least one opening for filter units (12), a filter supply container (6) arranged on one side of the turntable (3) a gas supply tube (7) on one side of the turntable (3) and a detector (10) with a gas outlet tube (8) on the other side, a filter discharge means (9), a switching unit for stopping the drive device (5) when the turntable (3) reaches a measuring position and a limit value unit for initiating a change of filter (12) when a certain filter contamination level is reached. <IMAGE>

Description

SPECIFICATION A device for measurement of radioactive particles in gases The invention relates to a device for measuring radioactive particles in gases.

When nuclear plants are monitored, in particular nuclear power stations, the degree of contamination of the effluent and also the aerosol content in the air and the inert gas content in the outlet air are measured. In addition the presence of radioactive iodine has to be detected since it presents a particular danger to the environment.

During normal operation, even very small activity levels are to be detected so that highly sensitive detectors are required. On the other hand when there is a fault it should still be possible to carry out realistic measurement so that the range of measurement should extend to several powers of ten. Aerosols (chemical agents that remain in suspension in the air) and iodine are detected in special filters so that the detectors measure the actual increase in activity in the filter media.

In previous cases where there has been a fault, it has been shown that a quite considerable activity builds up very rapidly in the filters and that the measuring device shows the maximum reading. In this case the maintenance personnel would have to change the filters straight away in order to be able to investigate the activity levels which are emitted to the environment realistically. In the case of a fault, the maintenance personnel is under such pressure to carry out other tasks however, that changing the filters is left undone. It must also be expected that the location where the detectors provided with the filters are located is inaccessible or only accessible at great risk because of the high radiation. Therefore there might be an omission in monitoring and this cannot be permitted.

The invention seeks to prevent this deficiency and automate the measurement process to such an extent that all of the activity levels may be detected clearly and without manual intervention.

According to the invention, there is provided a device for intergrated measurement of radioactive particles in gases comprising a filter system and a detector with a limit value unit connected thereafter, wherein the device further comprises a turntable provided with a drive device and having at least one receiving opening for filter units; a filter supply container arranged above the turntable, a gas supply tube on one side of the turn table and a detector having a gas outlet tube on the other side of the turn table, a filter discharge shaft, a switching unit designed to stop the drive device when the measuring position is reached and a limit value unit designed to initiate a change in the filter when a certain filter contamination level is reached.

In a preferred embodiment of the invention, the turntable, filter supply container and the ejection shaft may be encapsulated in a pressure-tight manner so that the measuring device may be housed outside the containment vessel and may be attached to this containment vessel by means of a bypass line in order to monitor the enclosed atmosphere.

The invention will now be described in greater detail, by way of example, with reference to the drawings in which: Figure 1 shows a plan view on to a measuring unit in accordance with the invention having three filter receiving positions in a table; Figure 2 is an enlarged cross-section along the line A-B of Fig. 1; Figure 3 shows a turntable having three receiving positions, and Figure 4 shows a cross-section through this turntable.

In Fig. 1, a square baseplate is designated 1 and a circular cover plate 2 is fixed to the said baseplate. A turntable 3 is mounted between these plates and its outer edge is constructed as a gear rim 3a. A pinion 4 engages this gear rim 3a and is seated on the output shaft of a drive device 5. A geared motor with incorporated end contacts is suitable for the drive device. The turntable 3 is provided with three circular recesses 3b, 3c, 3d which are offset by 120 in each case.

A filter supply container 6 is arranged above the recess 3d and is fixed via a flange 6a to the cover plate 2. A gas supply tube 7 is inserted into the cover plate 2 above the recess 3c. This tube is connected via a flange 7a to the test gas source.

A detector 10 is arranged beneath the recess 3c. An outlet is arranged laterally of the detector and is connected via a tube 8 and a flange 8a to a suction source.

Beneath the recess 3d, a discharge shaft 9 is fixed to the base-plate 1 and extends at an inclined angle downwardly to the right.

In the vertical section in Fig. 2, are shown the baseplate 1, the cover plate 2, the turntable 3 lying therebetween, the gear rim 3a, the recesses 3b, 3c, 3d, the drive device 5, the filter supply container 6, the flanges 6a, the pipe or tube 7 with a flange 7a, the inclined discharge shaft 9 and the detector 10. The detector is surrounded by lead screening 11 in order to suppress the effect of extraneous radiation. Filter units 12 are stacked in the filter supply container 6, the lower one (1 2a) already lying in the recess 3b, of the turntable 3. A further filter unit 1 2b is arranged in the recess 3c in a measuring position above the window 1 0a of the detector 10. A filter unit 1 2c is also shown in non-continuous lines in the recess 3d before it slides down into the discharge shaft.The turnable 3 slides round the bearing 1 3 which is inserted centrally between the base plate 1 and the cover plate 2.

A filter unit 1 2b is shown in section in Fig.

2. It comprises a largely rounded support frame 121, the actual filter element 122 and lattice plates 123, 1 24 for taking up the air drag.

All of the particles of test gas more particularly the particles providing radiation, accumulate during the exposure time of the filter 1 2b, in the filter element 1 22 so that an integral measurement is able to take place in the detector 1 0.

When a filter contamination level, which may be set as a limit value, is reached, a starting command is passed to the drive device and the contaminated filter unit is transported in a clockwise direction together with the turntable until it is located above the discharge shaft 9 after a 120 rotation. The lower highly rounded off portion of the ring 121 causes the ring 121 to slide through the inclined discharge shaft 9 without tilting or turning over. At the same time the next filter unit intended for measurement has been transported in the recess 3b to the measurement position above the detector. The empty recess has also moved beneath the filter supply container 6 during the transportation period, where it picks up a new filter unit.The drive device 5 or the turntable 3 respectively are provided with location contacts such that the motor is stopped after a 120 rotation in each case until the next starting command is received. The exposure time between two starting commands may be weeks or months when there when there is fault-free operation but it there is a severe fault it may be of the order of magnitude of a few minutes.

Fig. 3 shows the turntable 3 with the gear rim 3a and the recesses 3b, 3c, 3d.

Fig. 4 shows a cross-section through the turntable shown in Fig. 3.

If the filter units are numbered and the change of filters or the exposure time respectively are indicated remotely, the maintenance personnel can easily inform themselves of these processes at the test area.

Measurement or test devices of the type described are preferably incorporated, with a gas pump, a throughflow quantity controller and cut-off valve into the bypass of a nuclear power station containment vessel. Since this containment vessel has to be designed according to international rules for its design for five bar excess pressure at 1 20 C, the measuring device must also match these requirements.

In order to provide a seal between the baseplate and the cover plate and the filter supply container, the discharge shaft and the detector, O-rings 14, 15, 16, 17, 18, 19 are provided. A further sealing device (not shown) is provided on the shaft of the pinion 4. In addition the filter supply container and the discharge shaft must have pressure-tight closure elements.

If, when there is a fault new filter units have to be refilled while there is excess pressure in the containment vessel and the irradiated filter units have to be removed, then the cut-off valves in the bypass line should be shut off beforehand. Usually however, it may be assumed that, in the event of a fault, the control room personnel will not notice a change in the filters.

The filter units once removed may be exactly associated with each operation by marking them with numbers. Subsequent selective measurements of the radioactive particles in the laboratory can therefore contribute considerably to finding the cause of a fault.

It seems advisable to indicate the quantity of throughflow gas during the investigation time and in addition to incorporate a constant current controller for the throughflow of gas which compensates for contamination of the filters.

Moving away from the embodiment shown, the turntable may be provided only with one recess. The switching unit is then reprogrammed so that the turntable performs a complete rotation in each case.

In order to be able to monitor the functional reliability of the measuring devices a reference radiation device may additionally be arranged in front of the detector after each individual change or after every third change in the filter so that calibration measurement takes place regularly. The reference radiation device may either be pivoted electromagnetically in front of the detector window or may be closed in an additional recess in the turntable so that the detector emits a control or checking pulse as the reference radiation device passes.

When volume filters are used, e.g. when measuring iodine, the turntable is correspondingly thicker.

A scintillation counter or an ionization chamber adapted to the respective task of measurement may serve as the detector 10.

Claims (11)

1. A device for intergrated measurement of radioactive particles in gases comprising a filter system and a detector with a limit value unit connected thereafter, wherein the device further comprises a turntable provided with a drive device and having at least one receiving opening for filter units; a filter supply container arranged above the turn table a gas supply tube on one side of the turntable and a detector having a gas outlet tube on the other side of the turn table, a filter discharge shaft, a switching unit designed to stop the drive device when the measuring position is reached and a limit value unit designed to initiate a change in the filter when a certain filter contamination level is reached.

2. A device according to claim 1, wherein each filter unit comprises a filter element a support ring and lattice plates reinforcing the filter surfaces.

3. A device according to claim 2 wherein the support ring is rounded at its underside such that the filter- unit slides into the inclined discharge shaft without any tilting motion.

4. A device according to claim 2 or 3, wherein the edge of the turn table is formed as a gearwheel and is connected to a geared motor via a pinion.

5. A device according to claim 4, wherein the device lies in a bypass for the cgntainment vessel of a reactor, which bypass is provided with a pump, and the turntable with the pinion, the filter supply container and the filter ejection shaft are all sealed off so as to be pressure-tight.

6. A device according to any one of claims 1 to 5 wherein the turntable has one receiving opening and the switching unit stops the turntable after one complete rotation in each case.

7. A device according to any one of claims 1 to 5, wherein the turntable has three recesses which are offset by 120 ; a filter supply container, a detector and filter discharge shaft are also offset by 1 20t and the switching unit triggers an 120 rotation in each case.

8. A device according to any one of claims 1 to 7 wherein a remote indication device is provided for indicating the number of contaminated filters per monitoring period.

9. A device according to any one of claims 1 to 8 wherein an indication device is provided for indicating the period of exposure of the filter.

1 0. A device according to claim 9, wherein a measuring device is provided for measuring the throughflow quantity during the period of exposure.

11. A device according to claim 9, wherein a constant flow controller is provided for the gas flow, which controller compensates for contamination of the filter.

1 2. A device according to claim 6 or 7, wherein the turntable has an additional recess with a reference radiation device installed therein; and that the switching unit triggers a calibration reading after each change or after every third change in the filter.

1 3. A device according to any one of claims 1 to 1 2 wherein the turntable is constructed to accommodate substantially thicker volume filters.

1 4. A device according to claim 1 3 wherein the filters are constructed for measuring iodine.

1 5. A device according to any one of claims 1 to 14, wherein a scintillation counter is used as a detector.

1 6. A device according to any one of Claims 1 to 14, wherein an ionization chamber serves as a detector.

1 7. A device for measurement of radioactive particles in gases substantially as ds scribed herein with reference to the drawings.