Patent Number: 
Section: description

Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a measurement device 2, or measuring cell, arranged around a component, a line segment or medium tube 1. The measurement device forms a part of a nuclear power station. The medium tube 1 forms a part of a coolant loop, for example of a primary circuit system. The measurement device 2 comprises a first concave casing part 4 and a second concave casing part 6. The respective inner wall 8 and the respective outer wall 10 of the first casing part 4 and the second casing part 6 preferably consist of austenitic steel. A cadmium screen 12 in the form of cadmium sheeting is fitted on the respective inner sides of the outer walls 10 in order to stop thermal neutrons. A source chamber 14 is provided in the first casing part 4 to accommodate a neutron source 16 which serves as the emitter. A counter-tube chamber 17 is arranged in the second casing part 6 to accommodate two parallel counter tubes 18 which serve as the receiver. The wall 19 of the counter-tube chamber 17 lying toward the outer wall 10 also has a cadmium screen 12. Embodiments having a plurality of emitters and/or receivers are also possible, and may for example be used to pick up concentration drops or signal differences. In order to protect the neutron source 16 and the counter tubes 18 against high temperatures of the medium tube 1, a cooling duct 20 is arranged between the source chamber 14 and the inner wall 8, or between the counter-tube chamber 17 and the inner wall 8. The length of the cooling duct 20 in the vertical direction corresponds to at least the length of the measurement device 2. Depending on the embodiment of the medium tube 1, the cooling duct 20 may, for example, be annular or elongate in design. Coolant 22 flows through the cooling duct 20 in the direction of the arrows 23. The coolant 22, for example air, is propelled through the cooling duct 20 by means of a fan 24. The fan 24 is, for example, arranged at the lower end of the first casing part 4. A temperature sensor 26 for determining the temperature of the coolant 22 is arranged at the opposite end of the first casing part 4. A signal representing the measured value determined by the temperature sensor 26 is fed to a non-illustrated temperature control system. The temperature control system guarantees that the temperature of the coolant 22 does not exceed an upper threshold or fall below a lower threshold. To this end, the power of the fan 24 and the resulting flow of cooling air are controlled. If appropriate, the first casing part 4 and the second casing part 6 also comprise, between the inner wall 8 and the cooling duct 20, an insulation layer 28 in addition to the cooling duct 20. Air is used as the insulator. Similar to the cooling duct 20, the insulation layer 28 may, for example, be of annular or elongate design. The gap formed between the inner wall 8 and the outer wall 10 of the first casing part 4 is filled with an absorbing moderator 30. Polyethylene is thereby used as the absorbing moderator 30. Similarly, the gap in the second casing part 6 is also filled with absorbing moderator 30. The absorbing moderator 30, the cadmium screen 12 consisting of neutron-absorbing material, and the outer wall 10 consisting of austenitic material form a layered shield 31 against the radiation produced by the neutron source 16. Spacers 32 that are resistant to temperature and that do not expand, are incorporated in the radial direction in the insulation layer 28 and in the cooling duct 20. The spacers 32, for example support devices, serve to prevent a thermally induced change in the measurement geometry, in particular the length of the measurement path. For example, ceramic or mica glass is used as the material resistant to temperature and expansion. FIG. 2 shows the measurement device 2 in cross section. The first casing part 4 and the second casing part 6 are connected to one another by means of a number of externally fitted fastening elements 34. In this regard, the measurement device as a whole encloses or clamps the medium tube 1. The fastening elements 34 are, for example, designed as clips, screws or clamping devices. From this view it can be seen that two counter tubes 18 are again provided; it is also possible for more than two counter tubes to be provided. The neutron source 16 and the two counter tubes 18 are arranged in the source chamber 14 and in the counter-tube chamber 17, respectively. In order to cool the neutron source 16 and the two counter tubes 18, the insulation layer 28 and the cooling duct 20 run concentrically around the medium tube 1. Further, the spacers 32 are fitted at regular intervals in the insulation layer 28 and the cooling duct 20. Similarly to FIG. 1, the outer walls 10 and the wall 19 of the counter-tube chamber 17 respectively have a cadmium screen 12. Using the neutron source 16, a neutron flux 36 is sent through the coolant 38 flowing in the medium tube 1. The neutron flux 36 passes through the boron-enriched coolant 38. The neutron flux 36 is attenuated in dependence on the boron concentration in the coolant 38. The altered neutron flux 36 is determined by means of the neutron detectors, i.e. the counter tubes 18. Signals representing the measured values formed by the counter tubes 18 are transmitted to an evaluation unit 40. From the count rate and the temperature of the coolant 38 (the measuring sensor is not illustrated for reasons of clarity), the evaluation unit 40 determines the concentration of boron or boric acid. Since the neutron source 16 is arranged diagonally opposite the two counter tubes 18, the neutron flux 36 passes through the coolant 38 over the entire width of the diameter d of the medium tube 1. Accordingly, a substantially straight-line measurement path is formed between the neutron source 16 and the counter tubes 18 in the medium tube 1. Because of its highly effective, active thermal insulation using the controllable cooling air flow in the cooling duct 6, the described measurement device 2 exhibits good behavior in terms of thermal influences when determining the boron concentration. The measurement device 2 is therefore suitable, in particular, for direct use on the primary loop of a reactor plant, where temperatures of up to 380xc2x0 C. may occur. The measurement device 2 is mechanically constructed in such a way that even strong temperature fluctuations do not cause any geometrical changes, and therefore do not have any effects on the measuring method. Any possible remaining dependency of the method involving the measurement of neutron absorption on the thermodynamic state of the coolant 38 and the hydraulic system procedure in the primary cooling circuit can be eliminated by computer-assisted evaluation methods in the evaluation unit 40. In order to improve accuracy and obtain a fast display, further process information relevant to the measurement of the boron concentration is thereby also used in addition to the measurement signal from the measurement device 2. This information is processed in the evaluation unit 40 by using model-based plausibility and balancing algorithms. In addition, the radiation-screening construction of the measurement device 2 precludes significant exposure of the operating personnel to radiation. It will be understood that in the context of large-surface components 2 it is possible to use a similar measurement device 2 but with the neutron source 16 and the counter tubes 18 arranged in a one-piece casing. In the case of such a device, it is favorable to use a reflection measurement signal. In this case, the signal put out by the neutron source 16 is reflected inside the component 1 and then picked up by the counter tubes 18.