富山大学水素同位体科学研究センター研究報告25 : 63‑70, 2005.

In‑situ measurements of tritium desorption from a vanadium alloy by

β‑ray‑induced X‑ray spectrometry (BIXS)

Ⅹiang Liul, H･ Ho‑a2, y Hatano2 and M･ Matsuyama2

I southwestem Institute of Physics, P･0･ Box 432, Chengdu 610041, Sichuan, China
2Hydrogen Isotope Research Center, University ofToyama, Gofuku 3190,Toyama

93018555, Japan(Received January 25, 2006; Accepted March 30, 2006)
Abstract

Thermal release of tritiumfrom a vanadiumalloy was investigated by means of
β‑ray‑induced X‑ray spectrometry (BIXS). The sample tested was a Vl4Cr‑4Ti alloy

recrystallized by amealing at 1000 oC f♭r 2 h. A tritium gas diluted with hy血ogen wascharacterized with a quadmpole mass spectrometer and a small ionization chamber, andthe concentration of tritium was evaluated to be O･14%･ The sample was exposed to血etritium gas at 400 oC. The total concentration of hydrogen isotopes absorbed in thesample was O･17 at%,and the corresponding tritiumconcentration was 2.4 appm,
respectively･ Tritiumstarted to desorb at 350 oC. The duration of time required f♭rdesorption was much longerthanthat expected from the diffusion coefrlCient of tritiumin the alloy, indicating that the rate of tritium release was controlled by a surface

recombination process.

I. Introduction

Vanadiumalloys, in particular V‑4Cr‑4Ti alloy, are considered as promlSlng
candidates of structural materials for fusion reactors l1]. Trappingandthermal
desorption behaviors of hydrogen isotopes will be crucial issues fわr their applications infusion energy systems because the behaviors of hydrogen isotopes will be closely

related to the D‑Tfuel controland the mechanical properties of the alloys. BIXS(P‑ray‑induced X‑ray spectrometry) [2,3] is a newly developed teclmique to detect the63Ⅹiang Liu ･ H. Homma ･ Y. Hatano ･ M. Matsuyama

tritium distribution in the su血ce layers (here the su血ce layers denote the region of theescaped depth of P particles in materials)and in the bulk of materials (the distancedepends on the attenuation ofX‑rays in the materials andusually exceeds 1 OOトLm)･

Up to now, many kinds of materials containing tritium have been examined byBIXS to characterize the X‑ray spectraand the depth profiles of tritium, e･ g･ carbon
based materials l4], stainless steel l5]and tungsten l6]･ Vanadium alloys, however, havenot been investigated so払r･ In the present work, thermal desorption of tritium from aV4Cr‑4Ti alloy was examined by uslng tritium gas diluted with hydrogen･ Firstly, thetritium gasused was characterized with a quadrupole mass spectrometer (QMS)and asmall ionization chamber. The recrystallized Vl4Cr14Ti alloy sample was exposed to thetritium gas at 400 oC,and the depth proflle of tritium concentration was measured

in‑situ by a teclmique of BIXS･ Finally, the release behavior of tritiumin the alloy wasinvestigated by raising the sample temperature step by step fiom room temperature (RT)to450oC.

2. Experimental

2.I Characterization of tritium gas

A quadrupole mass spectrometer (QMS) was used to characterize the tritiumgas
released from a getter material(Zr‑Ni alloy) at 250 oC･ Fig･ 1 shows the mass
spectrum of the tritium gas･ This figure shows a small peak at mass number 3and large
one at mass number 2,0 7よV543'l100000000indicatlng tritium was
7!tJn'qJt!Ji)!su3tul

diluted wi血hydrogen.

From the intensity ratio
between the peaks atmass numbers 2 and 3,
the concentration of51015

was roughly estimated to64

2530

354045Fig･l Mass spectmm oftritium gas diluted withhydrogen.

be 0.1%.

20

Mass number

tritium in the released gas

In･situ measurements of tritium desorptionfrom a vanadium alloy by β‑ray･induced 冗‑ray spectrometry (BIXS)

In order to estimate the tritium concentration more accurately, another technique
was employed. In this case a small ionization chamber was used as the tritium monitor.

The detailed experimental arrangementand processing method canbe found in Ref. [7].

Table 1 shows the experimental conditions and results. TritiumConcentration wasevaluated with an emplrlCal fbmula obtained by standard samples. Based on thecharacterization tests, tritiumContent in the hydrogen isotope gas released kom the各etter material at 250 0C was detemined to be about 0.14 %.

Table 1 Tritium concentration in the gas released fTrom getter material (Zr‑Ni alloy).
Getter&W77W&R

Current 稗ﾖ6Temperature (oC) 着250

鉄3#270

都cs290310

59.50

3#F柳bA) 窒R

30.00

#c"

V蹠&

100.003CbC

157.00S

2.2 Hydrogen isotope absorption and thermal release experiments

A sheet of V14Cr‑4Ti alloy (NIFS‑HEAT‑2) (99.5 % cold workedand 0.5 mm inthickness) was supplied by the National Institute fわr Fusion Science, NIFS. The samples

with size of 15 x 10 x 0.5 mm3 were cut from this sheet･ After mechanical polishing by
A1203 powder with size'of O･06トImand cleamng with acetone inanultrasonic bath, thesamples were amealed at1000 oC f♭r 2 h in vacuum f♭rrecrystallization. Finally, one､,こ､≡or the samples was placed ona thin molybdenum heatermounted on a transfTer rod,andthen thi s device was■ヽ■ヽ人

comected to the main vacuum
Fig. 2system for tritiumdesorption

experimentswith in‑situ BIXS

A sketch drawlng Of apparatus for tritium

desorption experimentsand in‑situ BIXS
measurements.

65

Ⅹiang Liu ･ H. Homma ･ Y. Hatano ･ M. Matsuyama

analysis as shownin Fig. 2. In order to detect low energy P‑particles, a thin berylliumwindow (150 pm in thickness) was set up in the fTront ofa Si detector･ A very thin Au

film was coated on the berylliumwindow to prevent the tritiumcontamination of Be.
ARer heating the getter material to 250 oC, the valves 1 and 2 Were opened, whilethe valves 3and 4 Were closed to make a glVen VOlume oftritiumgas. Then the valve 3
was opened to introduce tritium gas into the sample chamber for absorption･ The sample
temperature was kept at 400 oC. An equilibriumpressure was obtained aRer 2habsorption･ From the extent of pressure change, the total concentration of hydrogenisotopes in the sample and the co汀eSpOnding tritium concentration were evaluated to be

0.17 at%and 2.4 appm, respectively. After cooling downthe sample to room
temperature (RT), the residual hydrogen isotope gas was recovered by the getter
material. BIXS measurements were conducted to measure depth pro丘Ie of tritium in thesample.

In order to investigate the themal desorption of tritium, the sample was heated in
vacuumstep by step舟om 250 to 450 0C with a step of50 oC･ In every cycle the sample

was kept at the glVen temPerature for lh,and then cooled downto RT･ The depth

profiles of tritiumremalnlng in the sample were obtained byanalyzing the observed
X‑ray spectra.

3. Results and discussion
3.I β‑ray‑induced X‑ray spectra of V‑4Cr‑4Ti alloy

Figure 3 shows a typical example of BIXS spectra obtained at RT a鮎r tritium

absorption. The obseⅣed spectnm indicated by dot marks consists of characteristic

X‑ray peaks of V, Tiand Cr (sharp peaks)and broad bremsstrahlung X‑rays･ The
intensities of Cr(Kα)and Ti(Kα) peaks were evaluated to be 4 % of intensity of V(Kα)

peak. These intensity ratios correspond to the composition of the alloy･ Since Cr(Kα)

peak was overlapped with V(Kβ) peak, the intensity of the fわrmer was evaluated bysubtracting the contribution of the latter by assumlng that the intensity ratio ofvanadiumpeaks, I(Kβ)/I(Kα) is 2/15. Solid line in this figure indicates a spectrum

obtained by simulation assumlng unifbm tritium distribution in the specimen. Details

66In'situ measurements of tritium desorptionfrom a vanadium alloy by β･ray･induced 冗‑ray spectrometry (BIXS)T･d)T･SlunO3JN'37tfJh!lunOU

0 0 0 0 .0‑｡淵 淵.0 1 5.01 ｡此 州t･専)I.S 7unO3[NgtuBu!)unou'Observedspectrum0000●■ ● 一一‑∫.‑̲̲■L一■‑̲

1 l 1 0 4 つム一 O QO●‑Simulation/hV 400 0 0 0●12345678●

Energy ofX‑rays, E/ keV

Cr(Ka) + V(Kβ)./024681012141618

Energy of X‑rays, E / keV
Fig･ 3

Typical example ofBIXS spectra ofV‑4Cr14Ti alloy･ Channel width, AE,ofSi detector was 34.18 eVof the simulation method are described elsewhere [8日n this simulation, absorption

coefficients of photons in V‑4Crl4Ti alloy are important parameter to calculate theattenuation of X‑rays, which affect the shapeand intenslty Of X‑ray spectra･ According
to Ref･ [9], the absorption edge of K‑shell in vanadium is 5.4639 keV. The energy

dependence of absorption coefficient in vanadiumgiven in Ref. [10] could be expressed
as 8363･9E ‑2･77503 below the absorption edge and as 78493E ‑2180386 in higher energy
reg10n, Where E is the photon energy･ The spectrum thus obtained by the simulationagreed fairly well with the observation as shownin Fig･ 3･ This agreement indicates that

tritiumdistribution in the sample wasuniformat least up to the depthof about 100 pm(the maximum detection depth or BIXS).

3･2 Thermal desorption of tritium from V‑4Cr‑4Ti alloy

As mentioned in Section 2･2, the sample was heated at temperatures fTrom 250 to450 oC (With step of 50 oC)for 1 h,and the tritiumdesorption was examined by means67ⅩiangLiu ･ H. Homma ･ Y. Hatano ･ M. MatSuyamaof BIXS aRer every^3qSd3C.OtJqCad(｡X)Ah.倉S.J37tZT'一ヽム一ーl505050.一＼t ＼ ＼ ●heating step. The change in■■V(Kα) peak intensity isplotted agalnSt amealing
temperature in Fig. 4 as ameasure of tritium content

inthe sample. No significant

100200

300

400

500

Annealing temperature / Oc
tritium desorption wasFig. 4 Change in V(Kα) peak intensity withobseⅣed below

300oC,

annealing temperature.

while the tritium content
clearly decreased with incTeaSlng temperature in higher temperature reglOn･ A鮎rarnealing at 450 oC, most of tritiumwas released･ This result indicates that a baking
temperature of 400 0C or higher is strongly required for tritiumdecontaminationfrom
this type of alloy. No systematic change was observed in the intensityratios between
Cr(Kα)and V(Kα) peaks,and Ti (Kα)and V(Kα) peaks during desorption･The duration of time necessary for desorption was much longer thanthe criticalFig･ 5

Normalized BIXS spectra ofV‑4Cr‑4Ti alloy afterthermal

desorptlOn process.

68In･situ measurements of tritium desorptiohfrom a vanadium alloy by β･ray･induced X‑ray spectrometry (BIXS)

time of tritiumdiffusion in the sample, which canbe roughly estimated by theexpression of L2/6DT, Where L is the thickness of samples (0.5 mm), and DT is thedi瓜ISion coefficient of tritium. According to Hashizume et al. [11], the dimlSion

coefficient oftritium in the present alloy lS expressed as
DT ‑ (7.5土0.2) × 10L4 exp (‑ 0.13 (eV) /kT),

where k is the Boltsmarm constant. The critical time was evaluated to be about 10

seconds even at 250 0C and much shorter than 1 h. Hence, it was concluded that

desorption rate was controlled not by dimlSion process but by surface recombination
process･ Namely, the trapplng effect in the bulk was weaker thanthe effect of surfaceba汀ier agalnSt reCOmbination under the present conditions. As shown in Fig. 5, theshape of bremsstrahlung X‑ray spectrum after desorpt10n did not show a slgniflCant
change in comparison with the initial one, although the reduction in tritium content due

to desorption resulted in poorer slgnal‑to‑noise ratio. Theuniformdepth proflle oftritium should be maintained during desorption process, if desorption rate isfully
controlled by the surface process. Hence, this observation is consistent with theabove‑mentioned conclusion.

4. Summary

In‑situ measurements of tritiumdesorption fTrom a V‑4Cr‑4Ti alloy were camied out
by using P‑ray‑induced X‑ray spectrometry (BIXS). Tritium started to desorb at 350 oC.

The period of time required for tritium desorpt10n Was much longerthanthat expectedfrom di瓜lSion coefficient. It was therefore concluded that the tritiumrelease rate wascontrolled not by di瓜lSion process but by surface recombination processunder thepresent experimental conditions.

Acknowledgements
This study was perf♭med under the Japan‑China Core University Program. Theauthors express their thanks to ProfTessor T. Murogaand Dr. T. Nagasaka for the supply
of a sheet of NIFSIHEAT‑2 alloy.

69

ⅩiangLiu ･ H. Homma ･ Y. Hatano ･ M. Matsuyama

References[1]

R･ J･ Kurtz, K･ Abe, V. M. Chemov, D. T. Hoelzer, H. Matsui, T. Murogaand G. R.

Odette: J. Nucl. Mater., 329‑333 (2004) 47.
[2]M. Matsuyama, K. Watanabe, K. Hasegawa, Fusion Eng. Des., 39‑40 (1998) 929.

[3]

M. Matsuyama, T. Murai, K Watanabe, Fus. Sci. Techno1., 41 (2002) 505.

[4]

M. Matsuyama, T. Tanabe, N. Noda, V. Philipps, K.H. Finken, K. Watanabe, J. Nucl.

Mater., 290‑293 (2001) 437.
[5]

Y. Torikai, A.N. Perevezentsev, M. Matsuyama, K. Watanabe, Pus. Sci. Teclm01., 41(2002)736.
[6]

M. Matsuyama, T. Murai, K. Yoshida, K. Watanabe, H. Iwakiri, N. Yoshida, J. Nucl.

Mater., 307‑311 (2002) 729.
[7]

M. Matsuyama, K. Watanabe, Fusion Eng. Des., 18 (1991) 91.

[8]

M･ Matsuyama, K. Watanabeand K. Hasegawa, Fusion Eng. Des., 39140 (1998)

929‑936.

[9]

D･L. Lide (editor in chief), CRC Handbook of Chemistry and Physics, 76th Edition,

CRC Press, Boca Raton (1995).
[1 0]

Kagaku‑binran, Kiso‑hen, 2nd Ed., Maruzen, Tokyo ( 1 975). (in Japanese)ll l]

K･ Hashizume, J. Masuda, T. Otsuka, T. Tanabe, private communication.

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