Enrichment of tritium by a combined gaschromatography—isotopic decomposition process

OOD708X 83440687-05SO3.00~0 Copyrighr 0 1983 Pergamon Press Ltd

Inc. J. .4ppl. Rodiur. Isot. Vol. 34. No. 1. pp. 687-691. 1983 Printed in Great Britain. All rights rcssrvcd

Enrichment of Tritium bY a Combined Gaschromatography-Isotopic Decomposition Process MASAKAZU TANASE and MINE0 KATO Japan Atomic Energy Research lnstitute.‘Tokai Research Establishment, Tokai-Mura. Naka-Gun. Ibaraki-Ken. 319-I I, Japan (Receired 9 Alrgrrst1982; in recisedfonn 7 Seprerlrher 1982) A novel method for tritium enrichment, based on a combined process of gaschromatographic separation of Hz, HT and Tz with MnC12-coated alumina and decomposition of HT to H2 and T, on U turnings. has been studied. A mixture of Hz, HT and T1 was separated with an alumina column dipped in liquid N2. after which the HT fraction was passed over U turnings heated at 1073 K to be decomposed according to the equilibrium reaction of ZHT = H2 + T2. The decomposed gas was again subjected to gaschromatography. This cyclic process was repeated several times. After some preliminary experiments with H-D mixed gas, tritium was successfully recovered as T, with a yield higher than 80”” from H-T mixed gas including only 5.0 x lob2 atom’, of tritium.

1. Introduction WITH THE aim of furnishing

fuel tritium to controlled thermonuclear fusion experiments, technology for tritium production by irradiating ‘LiAI alloy as target material in a nuclear reactor has been developed in the Japan Atomic Energy Research Institute!” Since gaseous tritium (TL) extracted from the neutron-irradiated ‘LiAI alloy by heating in a vacuum furnace is usually accompanied by isotopic impurities of Hz and HT which are probably released from the target or the furnace material, it must be purified prior to use in fusion experiments. Several methods have been proposed or adopted’*-s) for enriching tritium in the crude gas. Gaschromatography is one of the most, promising techniques because of its high separation factor and a method using paladinized alumina as column packing has been developed in France.“j’ However, further enrichment of HT species can not be achieved by a gaschromatographic separation only. In order to improve the enrichment technique, a novel method of combining a decomposition process of ZHT--H, + T2 and gaschromatographic separation in series has been devised. As a material for promoting the decomposition of HT. U turnings were utilized instead of Pt, I” Pd”’ or nicrome wire”’ because of the easy decomposition of HT on such turnings. As column packing, MnC12-coated alumina have been applied, as was used for the analysis of deuterium by Kwan et CI~.“~.’” After an experimental apparatus was set up, the optimal flow rate of He carrier gas was determined 687

for the separation of Hz, HD and D1. Then the heating temperature of a U turnings column suitable for the decomposition of HD was determined by passing either HD or an Hz/D, mixture (1: I in molar ratio) through the column. Under optimal conditions, enrichment of deuterium was tried with H-D mixed gas. After this, a sample gas of Hz, HT and T, was separated and tritium was also enriched from the gas.

2. Experimental

and Results

2.1. Materials (1) Hz: A product of Nihon Sanso Co. with a purity of over 99.9999996. (2) HD: A product of Merk Sharp and Dohme Co. with a purity of over 98 atom% of deuterium. (3) D, : A product of Showa Denko Co. with a purity of over 99.5’4. The above gases were used without any further purification. (4) He: A product of Nihon Sanso Co. with a purity of over 99.999%. The gas was used as a carrier after purification by passing through a MolecularSieve (described below) column cooled with liquid N2. (5) T, was obtained from a commercial target (IAEA) by heating it under vacuum. (6) Tritium standard gas: A product of Johnson Laboratories Inc. with a concentration of 139 kBq.L-’ (298 K, 1 atm, CH,T in N2) packed in a gas cylinder.

Masakaru Tanase and Mineo Kato

688

I

7.

He cylinder

2. Cold 3-5.

8. Electric

Trp

6-way

6. Sampling

U turnings

cock port

column furnace

I I Thermal conductlvlty detector I2 lonlzarlon

ChamDer

9. Thermocouple

13. HTWDI

trap

IO. MnCI, - coated alumina column

14 T, CO2 1 trap

FIG. I. Apparatus for enrichment of deuterium or tritium. (7) Molecular Sieve jA: A product of Gasukuro Kogyo Co. of 60/80 mesh. (8) MnCl,-coated alumina: Activated alumina (6OiSO mesh). a product of Wako Chemical Ind., coated with 10 wto/oMnCI,. (9) U turnings: Supplied by Mitsubishi Atomic Power Industrie Co. They were used after washing with a HNOs solution, water and acetone successively.

-7.2 Experimetltal

set-up

*The experimental apparatus (Fig. 1) consisted mainly of a U turnings column (7). a separation

column filled with MnCl+oated. alumina (10). a TDC (11). a small ionization chamber (12). and two traps (13 and 14). The U turnings column (quartz, 0.4cm i.d., 5 cm long) was filled with about 0.8 g of U turnings and placed in an electric furnace. The separation column (304-SS, 0.3 cm i.d. 3 m long) containing about 15 g of MnCl,-coated alumina was held in liquid Nf. The TCD incorporated into a gaschromatograph GC-6AM (Shimadzu Corp.) was used for detection and determination of Hz, HD and DZ. For the detection of tritium, an ionization chamber with a coaxial cylindrical cathode was manufactured Brass

Tritium

“0”

“0” ring

aas

ring

Brass

Hi

Cathode

I

FIG. 2. Design drawing of ionization

chamber.

/

guard

Tritium enrichment b.v gaschromatographr

by Applied Engineering Inc. (Fig. 2). The cathode and the guard were made of brass plated with chromium and they were insulated with polychlorotrifluoroethylene. The chamber has an effective volume of about 2Ocm’ and was operated at 500 V. Performance of . the chamber connected to a vibrating reed electrometer (Takeda Riken Industry Co.) was tested with tritium standard gas. The traps (304~SS of 0.2 cm i.d. 30cm long) were filled with about 1 g of Molecular Sieve 5A and kept in liquid N2 to collect HT (or HD) and T2 (or D,). The components were connected with 3046s tubes of 0.1 cm i.d. Six-way cocks were furnished for changing the flow paths of gases.

~

689

R.T.

1E\

2.3. Enrichment of deuterium in H-D mixed gas 2.3.1. Separation of HZ, HD and Dz. The optimal flow rate of carrier He to separate the isotopes of interest was determined. First the alumina was activated by keeping it at 373 K for 15 h under He flow. About 0.3 cm3 (STP) of mixed gas of Hz, HD and D, (about 1: 1:l in molar ratio) was injected to the sampling port and the retention time of each species was determined in relation to the flow rate ranging from lOcm’.min-’ to 200cm3.min- ‘. Temperature and current of the TCD were kept at 373 K and 84.7 mA, respectively. A separation factor to be compared was calculated frbm retention time ratio of HD and D, against that of Hz. As no clear influence of the flow rate on the separation factor was observed in the range lO-200cm3min- *, the flow rate of 70cm3.min-’ was chosen in further studies. Figure 3 shows a typical chromatogram of the separation of Hz, HD and Dt with a flow rate of 70 cm. min- I. All compounds were satisfactorily separated and the whole separation took about 12 min. 23.2. Decomposition of HD by U turnings. In order to determine the optimal heating temperature of the turnings to decompose HD in Hz + Dt effectively, about 0.3 cm3 of either HD or a mixture of H2 and D, (1.0: 1.0) were passed through U turnings heated at predetermined temperatures lower than 873 K, and molar ratios of HI, HD and D, were compared. The results of experiments with HD are shown in Fig. 4. The peak area of Hz and D, increased with the temperature of the U turnings. At 873 K, the molar

IO 5 0 Time ofter sample introduction, FIG. 4. Decomposition of

min

HD on U turnings.

ratio of H2:HD:D2 was estimated to be 4.9:4.3:1.0 from the calibration curves for H2, HD and D2. Similarly, the results (Fig. 5) with a mixture of H2 and D, indicate that the product ratio became a constant 4.9:4.1: 1.0 at temperatures higher than 773 K. The

R .T.

I 373

K

I

02

H,HD 0

I ‘ 6 7 9 9 10 II Time after sample introduction,

FIG. 3. Typical gaschromatogram

I2 min

I3

of Hz, HD and D>

I

I 0

5 Time

after

02 I IO

sample introduction,

I I5 min

FIG. 5. Reaction of Hz and D2 on U turnings.

Masakax

690 TABLE

1.

Tannaseand Mineo Karo

Enrichment of deuterium in H-D mixed gas Final gas

Original gas Number of

Atom

enrichment

D1

on of D

cycles

0.20 1.0s 0.91 2.M 2.23

6.2 6.7 6.7 6.7 6.7

-I 7 7 7 8

Run NO.

(j%)

H2

1 Z 3 4 5

43.8 62.4 82.5 169.9 185.5

5.60 6.74 10.8 21.7 23.9

Recovery of D, (/lmol)

2.86 3.96 MO 11.8 13.3

Atom

(“2

“a of D

92.3 89.0 91.6 91.5 93.8

>98.0* >98.5* 98.1 > 99.5. 98.4

* These values were obtained by an additional purification of D2 fraction. consistency of both experiments shows that the U turnings column heated at 873 K attains decomposition equilibrium for HD under the He flow rate of 70 cm’min- ‘. Therefore, the column was kept at this temperature in deuterium enrichment. 2.3.3. Enrichment of deuterium in H-D mixed gas. Sample gas mixtures of HZ, HD and D, were newly prepared before every run by passing a mixture of Hz and D, (about 50: 3) on the U turnings column heated at about 873 K. The molar ratio of HZ: HD:D, in the resultant gas was roughly 100: 10: 1. A sample gas of 1.1 cm3 to 4.7 cm3 was added first into the U column with the He flow rate of 70cm3.min-I, while the U turnings column was kept at 873 K. The cyclic process was repeated until HD could no longer be detected by the TCD. The results are summarized in Table 1 together with the experimental conditions. Deuterium of the initial concentration of about 7% was enriched successfully to an isotopic purity higher than 98% with a yield of about 92%. The ratio of the numbers of D against those of H in the final gas, was over 650 times the ratio in the initial gas. Typical chromatograms of original and final gases are shown in Fig. 6. The whole procedure (8 repetitions) took about 2.5 h. 2.4 Enrichment of tritium

in

H-T mixed .gas

2.4.1. Preparation of H-T mixed gas. Sample gas was prepared by mixing tritium released from both the Ti target and Hz. The chemical composition and

molar ratio of the gas were determined by radiogaschromatography. From the radiogaschromatogram (Fig. 7) of the initial sample gas, the constituents were identified to be Hz, HT and T2, and their molar ratios was determined to be 1: 10m3:3 x lo-‘. The atomic percent of tritium in the initial gas was also estimated to be 5.0 x lO-2>;. 2.4.2. Enrichment of tritium in H-T mixed gas. Mixed gas of Hz, HT and T, was injected through sampling part 6 in Fig. 1 with a “Pressure-Lok” gas syringe (Precision Sampling Corp.), and carried by He flow onto the alumina column where each of the constituents was separated mutually. After Hz was eluted, HT and T2 were collected in each of the cold traps. The HT was recovered from the trap by replacing the liquid Nz bath with an oven kept at 573 K, and introduced into the heated U turnings column. The resulting mixed gas of HZ, HT and T, was separated again through the alumina column. By repeating this cycle, TZ was accumulated in the T, trap. The enrichment was carried out with the use of the sample gases ranging from 0.3 1 to 4.4 cm3. The rate of He was kept at 70cm3.min-’ as in the case of the deuterium experiment, but the U turnings column was heated at 1073 K, and a preliminary process of HZ removal by gaschromatography was added before the tritium enrichment cycle. The heating of the tumings column at 873 K and the presence of too much Hz in the initial gas reduced the formation of the fraction of T2 in the decomposition process because a

HT \r Tz

I 6 Time

I 7 after

I 8 sample

I 9

I IO

I II

introduction,

I I2

I I3

min

FIG. 6. Gaschromatograms of original and final gases in deuterium enrichment.

IS.’

IO Time

after

sample

FIG. 7. Radiogaschromatogram

26

20 intruducth,

min

of initial H-T mixed gas.

Tririum

enrichment

691

by gaschromarograph!

TABLE 2. Enrichment of tritium in H-T mixed gas

Run

Volume of

Number of Hz-removal

Number of enrichment

no.

HL(T). cm’

cycles

cycles

1 2 3 1

3.1 x lo-’ 6.1 x 10-l 1.5 4.4

2 I

7

Recovery of T. “0 29 40 61 81

6 9 8

4 4

be shortened by omission of the Hz removal process in case the initial gas contains more tritium (e.g. 90% or so) as is expected in the actual tritium production run.

3. Conclusion

Time after

sample introduc:ion,

min

FIG. 8. Radiogaschromatogram of final gas in tritium enrichment.

of Hz was left on the turnings after the run. The cycle of enrichment process was repeated until the recovery percentage of T, per one cyclic run became below 3% of intial tritium. The results of tritium enrichment are shown in Table 2, together with the experimental conditions. The increase of sample gas volume enhances the recovery yield of tritium which reached 81% in the experiment with 4.4cm3 (including 200 MBq of T) of sample gas. Almost all of the unrecovered tritium were left as HT in the trap. The final gas obtained by an additional purification of T2 fraction with h&Cl,-coated alumina was radiogaschromatographed (Fig. 8). The chromatogram did not show any trace of HT. Because the product did not form HT when passed through the U turnings column heated at 1073-K no Hz was present. The T/H ratio in the finalgas increased over lo3 times. Although the whole procedures at run No. 4 took 6 h, the time can

Deuterium and tritium in H-D and H-T mixed gases were successfully enriched with yields higher than 8Oy; as D, and as TZ by a combined method of gaschromatographic separation and isotopic decomposition. As this method was thought to be applicable to the enrichment of tritium on a practical scale without any essential difficulties, further engineering studies, aimed at handling about 1 g of tritium, are in progress.

References

small amount previous

I. ABET.,

YAMAGL’CHI K., Kuw H., TANASEM.. SHIKATA E.. UVEI H.. TACHIKAWAK. and TANAKA K. ANS. CONF-800427. 367 (1980). 2. LW K. H. ORiVL-TM-3976 (1972). 3. BARTLITJ. R., SHERMANR. h.. S&Z R. A. and DENTON 4.

5.

6. 7. 8.

9.

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Cryogenics

19. 275 (1979).

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(1961). 10. FUJITA K. and KWAN T. Bunseki Kagaku 12. 15 (1963). 11. YOSHIKAWAY., SHIZIOZAKX A. and ARITA K. ibid. 24.45 (1975).