EP0036163B1 - Tritium detection in gases - Google Patents

Description

The present invention relates to a device for the continuous detection of very small amounts of tritium in a gas mixture, for example in the product gas of a central gas supply, in particular a nuclear-heated coal gasification plant. The device is primarily intended for methane-containing gases, but is also suitable for gases containing CO _z , C _Z H _s or C _s H _s .

In the case of nuclear-heated coal gasification plants, it is to be ensured that radioactive gases, in particular tritium, from the nuclear reactor cannot reach the consumer with the product gas on the way through the gas lines. It is also of interest in the case of non-nuclear-heated coal gasification plants or those gas supply plants which process both natural natural gas and methane produced from coal with nuclear energy and pass it on to a consumer whether the product contains detectable amounts of tritium or other radioactive gases.

Proportional flow meters are known for the detection of the β radiation emitted by tritium, in which a constant methane stream which is completely free of tritium is passed through a closed counting space in which several counting wires are suspended in an electrically insulated manner. Compared to the gas to be tested, these counting rooms have a window that is closed with a very thin metal foil. The ß-radiation entering the counting area through this window ionizes the methane present in the counting area and briefly allows an electrical current to flow between the counting wires and the housing.

Devices are also known in which a mixture of methane and the gas to be tested flows through a counting space at atmospheric pressure and at low speed. For example, DE-A No. 2500510 discloses a device for the continuous detection of very small amounts of tritium, the device consisting of a counting space through which the gas can flow, in which one or more counting wires are spanned and the counting area is surrounded by a heater is. The arrangement described there is a laboratory measuring device which is designed in particular for the selective detection of tritium between other radioactive substances. In principle there is an arbitrarily long measuring time available because the device described is a laboratory measuring device and not an operating measuring device adapted to certain conditions. If you e.g. If the goal is to be able to detect a tritium concentration of 0.37 Bq / g gas significantly, the known measuring devices result in measuring times of around 1 hour to ensure a clear separation from the background radiation that is always present. However, since the passage time of the product gas in a nuclear coal gasification plant from the reactor to the entry into the supply network is only 10 minutes, the measurement time that can be achieved with this would be too long.

The object of the present invention is a device for the continuous detection of very small amounts of tritium in a gas mixture, in particular in the product gas of a central methane supply. The device should have the shortest possible response time, i.e. the measurement time required for the significant detection of a certain low radioactivity of the gas should be as short as possible.

To solve this problem, a device according to the main claim is proposed. The device consists of a counting space through which the gas mixture can flow, in which a plurality of counting wires are spanned and the counting area is surrounded by a heater. The counting space is surrounded by a housing that is designed for high pressures, for example 6 bar. In addition, the counting space is separated from the housing by a ceramic molded body. Many very thin counting wires are arranged in parallel planes in the counting area, so that the counting area is used as cheaply as possible with regard to the distribution of the electric field strengths. In addition, the counting space is constructed in such a way that the gas can flow through it in a straight line, the counting wires everywhere being aligned as precisely as possible parallel to the direction of flow.

The design of the counter tube according to the invention enables very precise measurements despite the increased pressure in the counter tube. Although the mean free path of ions and electrons generated by radioactive radiation is reduced until the next collision due to the pressure increase, this can be compensated for by the embodiment according to the invention. Because the wires are very thin, there is an electrical field strength in their vicinity which is sufficient to form ion cascades and thereby generate counting pulses. The counting space can be better utilized by using many counting wires. Since ion pairs generated by radiation may recombine before generating a cascade, it is necessary to maintain high electric field strengths as evenly as possible in the counting space. The remaining features of the main claim complete the functionality of the counter tube according to the invention. In particular, the very thin counting wires must be protected from tearing by the gas flow, which is done by the fact that the flow is guided as precisely as possible parallel to the counting wires. The possibility of being able to measure at increased pressure in the payment room brings considerable advantages for the sensitivity and response time. The ratio of radioactive decays to be measured increases by a corresponding factor compared to the always present background radiation, and the absolute number of decays in the volume naturally also increases in proportion to the pressure with the radioactive concentration remaining the same. In addition, the counter tubes according to the invention are much easier to shield against background radiation than, for example, counter tubes with a correspondingly increased volume. For The above-mentioned tritium concentration of 0.37 Bq / g of gas now results in a measuring time of about 10 minutes for a gas mixture pressure in the counter tube of, for example, 6 bar, which is sufficient for the intended use. The desired high pressure can be generated with a compressor, which takes a partial flow of the gas mixture from the main line and feeds it back into the main gas line after the measurement. It seems more appropriate not to provide the measuring device for a product gas system immediately after the production system, as usual, but only where the product gas is compressed to the desired network pressure and is accordingly available under a high pressure. The measuring line from the main gas line to the measuring device should be as short as possible so that part of the tritium does not diffuse out through the pipe wall in the measuring line and thus no additional time is lost. The pressure, temperature and speed of the gas in the counting room should be controllable. In order to simplify the evaluation of the measurement, it seems advisable to keep these values constant. All three values are important for the gas flow flowing through the counting area. Maintaining a maximum and minimum gas velocity in the counting room is also important because on the one hand the thin and very sensitive counting wires must not be damaged and on the other hand the gas transport time is important for the measuring time. Furthermore, it appears appropriate to provide the counting room with a shield against the natural radiation from the environment, because the tritium activity to be measured here is already in the range of the natural radiation level. Falsification of the measurements by tritium deposits in the area of the counter tube can be avoided if the counter is freed from radioactive deposits at intervals by heating and similar measures. In addition, by appropriate choice of material for the ceramic molded body, the adsorption of tritium can be kept small, the ceramic molded body also electrically and thermally insulating the counter from the housing.

In a further embodiment of the invention it is proposed in claim 2 that the counting wires of one level are each connected to one another in an electrically conductive manner. This simplifies the connection of the supply lines and makes good use of the counter tube volume with regard to the electrical field strength. Depending on the operating requirements, a different connection of the counting wires is also possible.

In an additional embodiment of the invention it is proposed in claim 3 to arrange a perforated plate in the flow direction in front of the counting space. Since turbulent flows at high pressure could destroy the necessarily very thin counting wires, this perforated plate is a way of reducing the load on the counting wires by making the flow more even.

• Fig. 1 zeigt die konstruktive Ausführung eines Tritiumzählers im Längsschnitt.
• Fig. 2 zeigt einen Querschnitt durch Fig. 1.
• Fig. 3 zeigt in schematischer Darstellung eine Anordnung von drei parallelen Tritiumzählern und eine Schaltung zur Auswertung der Messsignale.

1 to 3 show possible embodiments of the invention.

• Fig. 1 shows the construction of a tritium counter in longitudinal section.
• FIG. 2 shows a cross section through FIG. 1.
• 3 shows a schematic representation of an arrangement of three parallel tritium counters and a circuit for evaluating the measurement signals.

The cylindrical housing 1 is connected with a cover 2 to the inflow 3 and with a cover 4 to the outflow 5 and contains an outer, likewise cylindrical, ceramic molded body 6, which encloses a metallic, rectangular counting space 7. This counting space is open at both ends and contains, in three planes parallel to the direction of flow, a plurality of counting wires 8, 9 and 10 which are fastened in an insulated manner in the counting space 7. The counting wires of one level are each conductively connected to one another and connected to a line which is led to the outside through a high-voltage-resistant insulation 11. Outside the counting space 7, but inside the ceramic molded body 6, an electrical heater 12 is provided, the connections of which are led out of the housing 1 through insulation 13. A circular perforated plate 14 is arranged in the flow direction in front of the molded body 6 and is intended to make the flow more uniform.

In Fig. 3, three complete detection devices are connected in parallel in terms of flow. A compressor 31 is arranged in the main line 30 and compresses the product gas emerging from a gas factory, for example at 1.7 bar, to the pressure of a long-distance gas line of, for example, 70 bar. Each of the three housings 1 is equipped both on its inflow side with a pressure regulating valve 32 and a shut-off valve 33 and on its outflow side with a pressure regulating valve 34 and a shut-off valve 35. In addition, the connecting lines to the main line 30 can each be closed with a shut-off valve 36 and 37. This circuit ensures that the three devices are constantly supplied with the necessary gas pressure as long as the compressor 31 supplies product gas to the gas supply network. This means that these devices are independent of special compressors. The parallel connection of three devices in the example ensures that a single device can be replaced and inspected while the other two devices continue to monitor the tritium content. Not shown in FIG. 3, but for practical operation it is very useful if two devices are connected in series instead of a detection device. With this arrangement, the display of both devices can be compared with one another and conclusions can be drawn about measurement errors if the displays are different. In addition, one of the two devices can be tested with an artificial radiation source, for example.

For the rest, the construction and operation of proportional flow meters in _" Nuclear Physics Measurement Methods" by Hubert Neuert, published in 1966 by G. Braun-Verlag, Karlsruhe, is described, in particular on pages 328,410 ff Description of an electrical and electronic circuit for evaluating the measurement signals can be omitted.

Claims (3)

1. Apparatus for continuously detecting very small amounts of radioactivity comprising tritium in a gas mixture which is suitable as a filling for a tube counter, wherein the device consists of a counter chamber (7) through which the gas mixture can flow and in which one or more counter wires (8, 9, 10) are clamped and wherein the counter chamber is surrounded by a heater, characterised by the following features-

(a) the counter chamber (7) is surrounded by a housing (1, 2, 4) which is designed for high pressures, for example 6 bar;

(b) the counter chamber (7) is separated from the housing (1, 2, 4) by a moulded ceramic body (6);

(c) in the counter chamber (7), a number of very thin counter wires (8, 9, 10) are arranged in parallel planes;

(d) the gas supply and the arrangement of the counter wires (8,9,10) in the counter chamber (7) are such that the counter wires (8, 9, 10) are everywhere aligned to be as parallel as possible to the direction of flow.

2. A device as claimed in Claim 1, characterised by the following feature: the counter wires of one plane are in each case electrically conductively connected to one another.

3. A device as claimed in Claim 1 or 2, characterised by the following feature: an apertured plate (14) is arranged in front of the counter chamber (7) in the direction of flow.

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