Development of In-Situ Gas Analyzer for Hydrogen Isotopes in Fusion Fuel Gas Processing

Transcription

1 Journal of NUCLEAR SCIENCE and TECHNOLOGY, 28[6], pp. 509~516 (June 1991). 509 Development of In-Situ Gas Analyzer for Hydrogen Isotopes in Fusion Fuel Gas Processing Kenji OKUNO, Tatsuhiko UDAt, Shigeru O'HIRA and Yuji NARUSE Japan Atomic Energy Research Institute* Received December 27, 1990 To develop a real time and in-situ process gas analyzer for fusion fuel gas processing systems, application study of laser Raman spectroscopy was performed by measurement of hydrogen isotopes. Using an Ar ion type laser of which wavelength 488 nm, power 0.7 W, and single pass irradiation method, Raman spectra of hydrogen isotopes were measured and intensities of the Stokes rotational lines and Q-branch were quantitatively analyzed. The Stokes rotational lines at 587, 443 and 415 cm-1 were selected as suitable ones for quantitative analysis of H2, HD and D2. Normalizing the Raman intensity of partial pressure H2 as 100, relative Raman intensity ratio of H2: HD: D2 was obtained as 100: 58: 47. The detection limit for hydrogen was estimated as 0.05 kpa in partial pressure and 500 ppm in concentration. But multiple pass method further improved the detection limit to 100 ppm. KEYWORDS: tritium, hydrogen isotopes, fusion reactors, fuel gas, tritium gas processing, laser Raman spectroscopy, in-situ gas analysis, performance, sensitivity I. INTRODUCTION To establish fusion fuel gas processing system for an experimental reactor, tritium processing gas measurement system must be developed. Much research and development have been done regarding tritium processing and related technology, at the Tritium Process Laboratory (TPL) of the Japan Atomic Energy Research Institute (JAERI)(1). To develop a real time, in-situ measurement systems is significant for implementation of the fusion fuel gas process operation and control, and for the tritium fuel gas management and safety. For example, in operation control of the hydrogen isotope separation system (ISS), it is necessary to analyze the hydrogen isotopes composition in the process gas(2). The detection limit, desired for operation control of the ISS, should be less than 100 ppm or 0.1 kpa at partial pressure. For the purpose of adopting in-situ analysis, representative measurement methods were compared. Then feasibility study has been performed by laser Raman spectroscopy, considered to be suitable choice. II. COMPARISON OF ISOTOPIC HYDROGEN METHODS M EASUREMENT which Currently the quantitative analysis of hydrogen isotopes is based on two distinct techniques of gas chromatography and mass spectrometry. The other methods considered are optical ones, namely infrared and Raman scattering spectroscopy. If relative tritium ratio in hydrogen isotopes is previously known, ionization chamber must be also applicable. In Table 1 various measurements methods are compared. Ionization chamber"" is suitable for an in-situ monitor but it is difficult to * Tokai -mura, Ibaraki-ken Permanent address : Energy t Research Laboratory Hitachi, Ltd., Moriyama-cho, Hitachi-shi

2 510 J. Nucl. Sci. Technol., measure inradioactive gases. While the instrumentation for gas chromatography(5)(6) is relatively simple and cheap, much time is needed (typically over 30 min) and much carrier gas is consumed, introducing radioactive waste gas in the process. Mass spectrometry(6)(7) also requires much time for measurements and calibrations, and analyzed gas is effluent as radioactive waste gas. Further, separation of the individual peaks of, H+2 and D+, or He+ and T+ which have almost the equal 3 mass numbers, is difficult. Finally, both methods have the disadvantage of being a batch process and not a real time, in-situ analysis. Table 1 Comparison of tritium process gas measurement methods A : Applicable or detectable, N. A Not applicable or difficult Optical methods such as infrared and Raman scattering spectroscopy, may be suitable choice for in-situ measurement. Although infrared spectra of liquid and solid H2, D2 and T2 mixture had been measured by Souers et al.(8)(9) molecular gases of hydrogen isotopes are not active in the infrared. But Raman scattering can be observed for the homonuclear molecular gases. Isotopic hydrogen gas analysis using laser Raman spectroscopy has been studied by several authors(10)~(12). Accurate Raman line positions for molecular hydrogen have been obtained by theoretical calculations and experiments by Veirs et al.(13) However, to implement the in-situ analytical method for the nuclear fusion reactor fuel gas system, further detailed analytical study must be made about the characteristics of laser Raman spectroscopy, including detection limit, spectral resolution and peak identification. Such a study requires determination of the gas cell structure and the optical arrangement. In the nuclear fusion plasma exhaust gas, in addition to isotopic hydrogen species, some other gaseous impurities may be included. Major impurities are considered to be methane, other carbon compounds(14), and nitrogen compounds(15). These gases are removed by the fuel cleanup system before the isotope separation system. To allow an experimental evaluation of laser Raman spectroscopy for the fusion fue gas Iprocessing systems isotopic hydrogens and compounds must be detected separately in spectrum. To apply the laser Raman spectroscopy to the process isotopic hydrogen gas measurement, H2 and D2 mixed gas was preliminarily analyzed. III. EXPERIMENTAL METHODS Hydrogen (purity >99.9%), and deuterium (purity 99%) were prepared for the Raman spectroscopic analysis experiments. A pure sample of each gas, of fixed partial pressure, was injected into the gaseous loop shown in 1. To get an isotopic equilibrium mixture gas of H2-HD-D2, H2 and D2 passed Fig. through the equilibrator, in which platinum black coated aluminum catalyst was packed. The sample gas was introduced to a gas cell (15 mm I.D.; 140 mm length) made of Pyrex glass. As a Raman excitation source, a single wavelength (488 nm) argon ion laser was irradiated into the gas cell. Usually the laser was irradiated by a single pass method. To improve the laser irradiation 24

3 Vol. 28, No. 6 (June 1991) 511 Fig. 1 Isotopically equilibrated H2 gas preparation loop and laser Raman spectroscopic analyzer efficiency and detection limit, a multiple pass method was also tried. The laser was an argon ion (Ar+) type (GLS-3200, Nihon Denki Co. Ltd.), with a normal maximum power 2W. The spectrometer used was the Czerny-Turner type double monochromator (NR-1100, Japan Spectroscopic Co. Ltd.). The monochromatic light was detected with a photomultiplier (R-464, Hamamatsu Photonics Co. Ltd.). During the experiments, the wavelength of the exciting laser and its power were fixed at 488 nm and 700 mw, respectively. IV. RESULTS 1. Raman Spectrum of Hydrogen Isotopes The Raman spectrum of isotopically equilibrated H2-HD-D2 gas is shown in Fig.2. Hydrogen has two groups of Raman lines. The first group are Stokes pure rotational, S0-branch lines, observed below 1,000 cm-1, the second group are the vibrational Q-branch lines between 2,900 and 4,200 cm-1. The observed Raman intensity of the Stokes pure rotational line is larger than that of the Q-branch. Pressure : 80 kpa, Spectral resolution : 4.6~7.5 cm-1 2 Raman spectrum of isotopically equilibrated H2 and D2 mixture Fig. gas Figure 3 shows the detailed spectrum of the Stokes rotational lines. Many Stokes rotational lines relate to the initial rotational energy levels of the molecular hydrogen and the transitions allowed by change of the rotational quantum number J->J+2. Each peak of the hydrogen isotopes were observed individually. Then these rotational lines considered to be suitable for accurate quantitative analysis of hydrogen isotopes. 25

4 512 J. Nucl. Sci. Technol., These bands are allowed by rotational quantum number change J->J+2 (J=0 or an integral number). Fig. 3 Raman spectrum of Stokes rotational bands of H2 and D2 mixture gas effect of spectral resolution on peaks separation and Raman intensity. While improving the spectrum resolution made some lines being detected separately, their intensities decreased. The Q-branch lines relate to the initial rotational energy level of the molecular hydrogens and transition allowed by change of the vibrational quantum number v->v+1. When analyzing the processing gas, a complicated Raman spectrum of rotational lines might be observed, because many kinds of molecules will be contained in the gas. For quantitative analysis, it is necessary to select the optimum lines by considering the chemical form of molecular gases contained. 2. Spectral Resolution Effect To select the optimum spectral resolution for quantitative analysis, the effect of spectral resolution on the observed Raman intensity was evaluated as shown in Fig.5. Although the Q-branch lines are observed as small peaks, each peak of the isotopic hydrogens is well separated. Figure 4 shows the detailed Raman spectrum of the Q-branch of deuterium. This figure also represents the Sample gas : H2, Partial pressure : 34 kpa, Raman band, Stokes rotational : 587 cm-1, Laser wavelength : 488 nm, Power : 700 mw 5 Effect of spectral resolution Fig. on Raman intensity measurement Fig. 4 Effect of resolution on Raman spectrum peaks separation for Q-branch of D2 gas Generally, the observed Raman intensity is determined by spectral resolution, because the spectral resolution is proportional to the optical slit width, which is proportional to the mechanical slit width of the spectrometer. When the optical slit width is less than the actual optical width of the Raman lines, the measured Ramanintensityisexpressedas =I0(W/W0)2,(1) Im 26

5 Vol. 28, No. 6 (June 1991) 513 where /0: Raman intensity in slit width Wo (arbitrary units) W: Mechanical slit width (mm). The calculated and observed Raman intensities are shown in Fig.5. The observed Raman intensity is smaller than the calculated one above 4 cm-1, and it tends to saturate above 10 cm-1. This is because the actual Raman lines have a limited optical width, namely broadness. If the objective is to identify the fine chemical structures, or to find separately various molecules and isotopes in the mixture gas, a high resolution spectrometer might be desired. For the purpose of quantitative analysis, a sufficiently large Raman intensity is required rather than high resolution spectroscopy. Then in the following experiment, a resolution of 5 cm-1 was selected as the optimum measurement condition. Even though complex molecular gas mixtures may be present, their spectral lines are expected to be separated in some degree on this resolution. 3. Calibration for 112 and D2 Gases Figure 6 relates the partial pressure of H2 or D2 gas and Raman intensity of their Stokes rotational lines. The linearity is confirmed between the Raman intensity and the partial pressure from 0.05 to 100 kpa. Assuming that detection limits are twice as much as the signal noise, they are estimated to be about 0.05 kpa for H2 and 0.1 kpa for D2. The partial pressure of 0.05 kpa at normal atmospheric Laser wavelength : 488 nm, Resolution : 5 cm-1 6 Relationship between partial pressure Fig. of H2 or D2 and observed Raman intensity pressure gas is equivalent to 500 ppm in concentration. 4. Effect of Multiple Pass Method To detect lower concentrations of hydrogen isotope gases, a multiple pass method(16) was tried. In the multiple pass technique the irradiation laser beam is returned to the gas cell many times. The optical equipment is shown in Fig.7. Ideally the gain is the same as the reflection times from the mirrors, but in practice it is less due to additional reflection and transmission losses per pass, especially at the Brewster faces of the gas cell. Fig. 7 Schematic of Raman multiple-pass system The maximum gain has been reported elsewhere as about 20 times(17)'. Then it was expected that the detection limit would be improved at least 1 decade. The Raman spectra of H2 detected with multiple pass and single pass methods are compared in Fig.8. By removing the gas cell, the multiple pass gave about a 20 times larger intensity than the single pass, but when inserting the gas cell, it was only six times larger. The estimated lower detection limits for H2 and D2 are 10 and 20 Pa, respectively. Although the multiple pass is one of the more effective methods for increasing the sensitivity, it was not easy to set an optical alignment with accurate reproducibility. So that common experimental measurement was performed by single pass. The other methods to improve the detection limit would be to get higher signal to noise ratio by increasing the accumulation period or by decreasing the dark current of a photodetector. 27

6 514 J. Nucl. Sci. Technol., analysis was made experimentally using the pure standard gas. But it is difficult to obtain the standard gases of isotopically exchanged hydrogens or their compounds. It is presumed that the composition of isotopically exchanged hydrogen gases might vary according to temperature changes, catalytic effect by the structural materials of the process gas line or by radiation. Using isotopically equilibrated H2-HD-D2 gas, we tried to measure the partial pressure of HD gas by substituting H2 and D2 partial pressures from the total isotopic hydrogen pressure. The partial pressures of H2 and D2 gases were quantitatively analyzed by calibration with standard gases. The total isotopic hydrogen pressure was given measuring the pressure of the initially filled H2 or D2 gas. Then to confirm the equilibrium composition made in H2 and D2 complex gas, the equilibrium constant K was calculated by Sample gas : H2, Partial pressure : 34 kpa, Raman band, Stokes rotational : 587 cm-1, Resolution : 5 cm-1 8 Fig. Effect of multiple-pass on increasing sensitivity for Raman intensity measurement V. DISCUSSION 1. HD Gas Analysis Calibration for H2 or D2 gas quantitative K=P2HD/PH2PD2, (2) where Px: Partial pressure of molecule x. Measured H,, HD and D2 partial pressures and calculated equilibrium constant K are shown in Table 2. The equilibrium constants obtained are almost the same value as reported elsewhere(18). Table 2 Measurement of H2, HD and D2 partial pressures by laser Raman spectroscopy and estimated equilibrium constants The equilibrium constants increased slightly on heating the gas from room temperature to 520 K. They were measured at several temperatures and were agreed well with reported values(19). This then verifies the feasibility of a monitor for precise quantitative analysis of isotopic hydrogens by laser Raman spectrometry. 2. Estimation of Relative Absolute Raman Intensity Major problems for quantitative analysis by Raman spectrometry might be that the 28

7 Vol. 28, No. 6 (June 1991) 515 laser output power tends to change, though slowly, as time proceeds, and optical alignment of the spectrometer tends to shift with temperature changes or mechanical stress to the spectrometer. Fluctuation of Raman intensity caused by laser power changes can be corrected for by monitoring the real irradiation power. But fluctuation caused by shifts of the optical alignment should be corrected by some other method. If the relative Raman intensity ratios for all kinds of isotopic molecular gases are previously known, although alignment shift of the spectrometer occurs, the sample gases could be evaluated easily by quantitatively measuring one representative gas as a reference and relatively comparing with. The observed relative Raman intensity ratios for H2 HD and D2 are shown in Table 3. Normalizing the Raman intensity of 587 cm -1 Stokes rotational line of H2 as 100, the relative intensities for HD and D2 were obtained as 59 and 48, respectively. Then the Raman intensity Ix of each isotopic hydrogen gas x2 is expressed by Ir=kC,(3) where k: Relative intensity coefficient (100, 59 and 48 for H2, D2 and HD respectively) C: Concentration of x2. If sensitivity is varied due to, for example a minor change of the gas cell setting place, or optical alignment shift of the spectrometer, it can be corrected by calibrating with the standard H2 gas. Table 3 Relative Raman intensities observed from gaseous H2, HD and D2 (Intensities are normalized to H2 Stokes rotational band as 100) t Evaluated absolute intensity from photomultiplier sensitivity depend on the Raman scattering wavelength. The relative Raman intensity ratio measured at all wavelengths should not agree with the absolute intensity ratio, because the sensitivity of the photodetector used in this experiment depends on the wavelength of Raman scattering, namely the wave number of the Raman shift, as shown in Fig.9. In case of measuring the Stokes rotational S0-branch lines of wave numbers 450~600 cm-1, the photomultiplier sensitivity variance is less than 2% and has only a little effect. But it has a significant effect on measurement of Q-branch lines, (wave numbers 2,900~4,200cm-1), and the sensitivity decreases 60~90% compared to sensitivity for measurement of Stokes lines. The corrected Raman intensity ratio of hydrogen isotopes, which might be equiva- Laser wavelength : 488 nm (wave number, 20,491.8 cm-1) ; Photomultiplier type : R Photomultiplier sensitivity Fig. dependence on wave number 29

8 516 J. Nucl. Sci. Technol., lent to the absolute Raman intensity ratio, is 1-12 : HD: D2=100: 58: 47 as shown for the corrected values in Table 3. It seems better to select a photodetector which is more sensitive and has less sensitivity dependence on wavelength. However such a photodetector is not now available. Although reported values of the absolute Stokes rotational intensities could not be found, Q-branch intensities of H2 and D2 have been reported elsewhere(12). Our observed relative Q-branch intensities are IH, : ID2=51 : 40 and the ID2/IH2 ratio is The reported Q- branch intensity ratio ID2/IH2 was 0.8, which agree well with the present value. From this result, the observed HD Raman intensity ratio is considered to be accurate. Thus this feasibility study has shown that Raman spectroscopy seems suitable for an in-situ monitor of gases in the fusion reactor fuel gas process. In the next phase, the characteristics on tritium T2 gas, and impurity ex- gases like methane and isotopic hydrogen changed compounds should be studied. VI. CONCLUSION To develop an in-situ and real time analytical monitoring system for a nuclear fusion fuel gas process, isotopic hydrogen gases H2, HD and D2 were measured by laser Raman spectroscopy. According to the following summarized results, Raman spectroscopy was expected to be applicable for measurement of the processing gases : (1) As the suitable Raman lines for quantitative analysis, Stokes rotational lines at 587, 443 and 415 cm-1 were selected for H2, HD and D2. (2) For exciting laser wavelength of 488 nm, power of 700 mw, and using a single pass cell, H2 or D2 Raman intensity was in proportional to its partial pressure under 100 kpa. Relative Raman intensity ratio for II, : HD: D2 was obtained as 100 : 58 : 47. (3) The lower detection limits for H2 and D2 were 0.05 kpa and 0.1 kpa. These values correspond to 0.05 and 0.1% as concentration at normal atmospheric pressure gas. Using the multiple pass cell, the detection limit was six times better than in the single pass cell. REFERENCES (1) NARUSE, Y., MATSUDA, Y., TANAKA, K.: Fusion Energ. Des., 12, 293 (1990). (2) KINOSHITA, M., BARTLIT, J.R., SHERMAN, R.H. : Nucl. Technol./Fusion, 5, 30 (1984). (3) COLMENARECE, C. A.: Nucl. Instrum. Methods, 114, 269 (1974). (4) JALBERT, R. A.: A new tritium monitor for the tokamak fusion test reactor, LA-UR , (1985). (5) GENTY, C., SCHOTT, R., LEFEVRE, H., FROMENT, G., SANSON, C.: Determination of tritium in an analytical chemistry laboratory, Proc. of Tritium Symp. Las Vegas, 102 (1971), Messenger Graphics. (6) GENTY, C., SCHOTT, R.: Anal. Chem., 42, 7 (1970). (7) ELLEFSON, R. E., CAIN, D., LINDSAY, C. N.: J. Vac. Sci. Technol., A5, 134 (1987). (8) SOUERS, P. C., FEARON, D., GARZA, R., KELLY, E. M., ROBERTS, P. E., SANBORN, R. H., Tsu- GAWA, R. T., HUNT, J. L., POLL, J. P.: J. Chem. Phys., 70, 1581 (1979). (9) SOUERS, P. C., FUENTES, J., FEARON, E. M., ROBERTS, P. E., TSUGAWA, R. T., HUNT, J. L., POLL, J. P. : ibid., 72, 1679 (1980). ETCHELL, R. E., OTTESEN, D. (10) K.: S The potential use of Raman spectroscopy in the quantitative analysis of hydrogen isotopes, SAND , (1974), (11) CARAVAN, S. M., GILL, J. T.: Proposed implementation of laser Raman scattering spectroscopy for analysis of hydrogen isotopes, MLM- 2583, (1979). (12) HARNEY, R.C., BLOOM, S.D., MILANOVICH, F.P.: Stable isotope ratio measurement in hydrogen, nitrogen and oxygen using Raman scattering, UCRL-76743, (1979). (13) VEINS, D. K., ROSENBLATT, G. M.: J. Mol. Spectrosc., 121, 401 (1987). (14) YAMADA, R. : J. Nucl. Mater., , 359 (1987). 5) GLUGLA, (1 M., PENZHORN, R. D., ANDERSON, J. L., BARTLIT, J. R.: Fusion Technol., 14, 683 (1988). (16) KIEFER, W., BERNSTEIN, H. J., WIESER, H., DANYLUK, M.: J. Mel. Spectrosc., 43, 393 (1972). (17) HILL, R.A., MULAC, A.J., HACKETT, G.E. : App. Opt., 16, 2004 (1977). (18) PYPER, J. W., SOUERS, P. C.: The chemical equilibria relating the isotopic hydrogens at low temperatures, UCRL-52104, (1976). (19) MURPHY, W. F., HOLZER, W., BERNSTEIN, H. J.: Appt. Spectrosc., 23, 211 (1969). 30