Sensors and Achutors B, 13-14 (1993) 693694 693

Chemical sensors in hydrogen diagnostic systems in nuclear power

Sergey S. Yakimov Department of Mohdar Physics, Russian Scientific Center, ‘Kiuchatov Institute’, Kwchatov Square 123182 Moscow (Russian Fedemtion)

1. Intruductlun

The diagnostics of hydrogen in a reactor installation is one of the central problems in the complex approach to increasing their safety. The use of semiconductor sensors as primary transducers (having some advantages over traditional methods of hydrogen detection) is very promising for the diagnostic systems. First of all, they can be manufactured on the basis of batch microelec- tronic technology, ensuring the formation of gas-sen- sitive layers with reproducible properties and thickness. The transducers also have minimal weight and dimen- sions as well as relatively high thermal and chemical stability, which allows them to be used directly in places where there is a danger of hydrogen leakage.

2. Gas sensors based on the MOS structure

MOS structures with a metallic Pd layer have a high sensitivity to hydrogen contained in an ambient gaseous medium, which makes it possible to consider these structures as a basis for the development of a semi- conductor sensor [l]. The result of measuring by a silicon MOS sensor the concentration of hydrogen in a gas mixture from the derivative 8(AV)/& in the initial region of the transient process is presented in Fig. 1. The accuracy reached in these measurements is better than 10% and several times better than in the case when the concentrationP, is measured from the steady state of the shift AK The above results were obtained when detecting the hydrogen content of inert gas. Analogous dependences were achieved when measuring the hydrogen content of air, but in this case the sensitivity is an order of magnitude lower.

3. Principal schemes of diagnostic systems

3.1. Diagnostic of hydrogen in a nuclear reactor installation

Hydrogen separated from the process medium by a membrane is transported into the measurement chamber

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initial part of the response curve

i , 1 ‘, 50 loo 150 200 t, s

Fig 1. Initial part of the sensor response curves concentrations of Hz in He.

to

containing the sensitive element and placed beyond the process medium [2]. The radiation stability of the membranes is sufficiently high; the permeability of the membranes to hydrogen remains practically unchanged during their long-term operation. Figure 2 gives the block diagram of the diagnostic system. The concen- tration of hydrogen in water at a temperature of about 180 “C was reliably detected at a level of 1 cm3/l. The sensor method was also applied to detect a change of the order of lo-’ ppm in the concentration of H2 in the testing facility, imitating the operation conditions in the secondary liquid-sodium circuit of an W-type reactor within the range 5 X 10-*-3X 10-l ppm.

3.2. Diagnostic of hydrogen in nuclear fuel The presence of hydrogen in nuclear-reactor fuel

elements causes the embrittlement of the fuel cladding when H, is released from the UOz pellets. It has been established that for normal operation of the reactor the hydrogen content of UO,-based nuclear fuel must be less than 2 ppm. This means that in analyzing a UOz pellet several grams in weight, the control method must guarantee an Hz sensitivity of about lo+’ g. The usefulness of the method was demonstrated experi-

8 1993 -Else&r Sequoia. AU rights reserved

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_ gas mixture from 3 membrane

- He + 10% air

3 -

tionally-used coulometry. The sensor measurement method is sufficiently simple in operation and the data obtained are computer-processable. The time of analysis can be reduced to several seconds when using laser techniques or puked high-temperature extraction of Hz gas.

_ calibration gas mixture

Fig. 2. Block diagram of Hz diagnostics in technological media of nuclear reactor units: 1, sensor chamber; 2, valve; 3, flow stabilizer.

mentally by measuring the Hz content of the U02 pellets used for nuclear power plant (NPP) fuel elements using a single fuel specimen (about 5 g in weight) and high- temperature extraction of H, [3]. The H, content of the UO, pellet was measured at various temperatures of H, extraction from the specimen in the temperature range 400-1600 “C. The minimum concentration of Hz experimentally detectable (*lo-” g) seemed to be three orders of magnitude lower than that for tradi-

4. Conclusions

The parameters of the developed experimental in- stallations are superior in some cases to the analogous parameters of installations operating on the basis of traditional techniques developed earlier. This indicates the prospects for using sensor diagnostic systems in solving the problem of ensuring the safety of NPP operation.

Acknowldgements

The experimental investigations were carried out with the following co-workers of the Russian Scientific Center ‘Kurchatov Institute’: D. Arnold, A. Babulevich, A. Babichev, V. Chaplanov, V. Goncharov, V. Grabchak, V. Filippov, S. Lazarev, V. Malyshev, A. Rzhanov and A. Terentjev.

References

1 I. Lundstrom and C. Svensson, in 1. Janata and R. J. Huber, (eds.), Solid Stafe Chemical Sensors, Academic Press, New York, 1985, Cb. 1, pp. 246.

2 V. Chaplanov, V. Filippov, A. Rzhanov, A. Terentjev and S. Yakimov, System to detect hydrogen in water, Sensors and Achiarom B, 5 (1991) 185-186.

3 A. N. Babichev, N. E. Babulevich, S. D. Lasarev and S. S. Yakimov, A new method of diagnostic of hydrogen content in the samples of NPP fuel, At. Energ, 68 (1990) 186-388.