富山大学水素同位体科学研究センター研究報告 21: 61-72, 2001.

論文
SS-316 ステンレス鋼からのトリチウムの除染(I)A. Perevezentsev1)，渡辺国昭2)，松山政夫2)，鳥養祐二2)
1) UKAEA, Culham Science Centre, Abingdon, OX14 3DB, UK
2) 富山大学 水素同位体科学研究センター〒930-8555 富山市五福 3090

Removal of tritium from stainless steel type 316(I)
A. Perevezentsev1), K. Watanabe2), M. Matsuyama2), Y. Torikai2)1)UKAEA, Culham Science Centre, Abingdon, OX14 3DB, UK
2) Hydrogen Isotope Research Center, Toyama University, Gofuku, Toyama 930-8555, Japan(Received November 5, 2002; Accepted December 20, 2002)

Abstract
Tritium distribution in stainless steel type 316 exposed to tritium-containing hydrogenat various temperatures was studied. Sub-surface layer of approx. 20µm thickness enrichedwith tritium was observed for all samples. This layer contributes 20% to 40% to overall
tritium inventory. Thermal desorption study reveals that majority of tritium released fromthe contaminated steel is in a form of water.
Several method of decontamination, such as purge with various gases at elevatedtemperatures and heating with methane–air flame, were tested. Heating with flame allows
removal of largest fraction of tritium inventory and in shortest period of time among thetested decontamination methods.

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A. Perevezentsev・渡辺国昭・松山政夫・鳥養祐二

1. Introduction
Maintenance and decommissioning of deuterium-tritium burning fusion reactors would
require detritiation of stainless steel, which is a most common material of the tritium handlingfacilities [1]. The JET machine presents an example of such need [2]. The feature of stainlesssteel contamination by tritium is a large mobility of the contaminant. For example, tritium wasfound in the bulk of stainless steel exposed to gaseous tritium-containing hydrogen at room
temperature [3]. Tritium demonstrated [4,5] so-called “re-growth” effect of re-appearing at thedecontaminated surface. This indicates that being a mobile contaminant tritium can migrate instainless steel quite easily even at moderated temperatures.
Another feature of tritium contamination of stainless steel is a form in which it is released.
Thermal desorption studies of stainless steel exposed to gaseous tritium-containing hydrogenshow that majority of tritium is released in a form of water vapor [1, 6-10].
Knowledge of tritium behavior in stainless steel is important for safety of operation anddecommissioning of tritium handling facilities and future fusion reactors. Availability of aneffective detritiation technique is a vital for reduction of the decommissioning cost caused byneed for the waste disposal. The most commonly considered methods for detritiation are washing,thermal desorption, isotopic exchange, chemical and electrochemical etching, plasma discharge
and destructive techniques, such as melting [1]. If decontamination of intermediate level waste(ILW) could reduce their activity below low level waste (LLW) threshold, this could lead to a
substantial reduction in cost of the waste disposal. However, where detritiation of ILW using asimple and cheap technique of tritium recovery leads to generation of a sophisticated “mixed”
chemical and radiological “secondary” waste, that needs to be processed as well, the overall costof decontamination process could make it not cost-effective.
This paper reports first results of study of stainless steel type 316 detritiation by nondestructive methods.

2. Experimental
2.1 Samples preparation
Planchetes of SS316 of 15x15mm square shape and 0.6mm thickness (supplied by
Metalfast, UK) were used as delivered. The only treatment was cleaning by ultrasonic bathing in
acetone. The samples were placed in a quartz reactor attached to a high vacuum system. The
reactor was pumped for several days at room temperature and residual gas pressure of ≈10-6Pa.
The samples were heated then at 670K for 3 hours under vacuum of ≈10-5Pa and pumped again
for several days at room temperature. Residual gas analysis shows that main impurity was water,which content was considerably reduced after the reactor heating. Samples were loaded withtritium by exposure to 1kPa of hydrogen containing 32% tritium. Prior to withdraw of thesamples to ambient atmosphere the reactor was pumped for several days at room temperature.
List of samples is given in Table 1.

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SS-316 ステンレス鋼のトリチウムの除染 (Ⅰ)

Table 1 List of samples loaded with tritium.
Sampleseries

I
II
III
IV
Va)

Temperatureand durationofexposure
to tritium≈295K,
20 hours
470K, 3 hours
590K, 3 hours
590K, 3 hours
590K, 3 hours

Heat treatment
after exposureb)

None
None
None
720K, 5 hours
770K, 10 hoursa) Samples were of 10x10mm size with 4.8mm thickness.
b) Samples loaded with tritium were baked-out in evacuated and valved off reactor.
2.2

Analysis of tritium distribution and release

Tritium distribution in the samples was evaluated by their acid etching followed by liquid
scintillation counting of tritium in the acid cocktail. Etching cocktail, which is effective forwhole surface corrosion and oxidation of desorbed hydrogen [8,11], contained 6.6w% HNO3 and10.9w% HCl in water. It was assumed that all tritium, independently of form in which it ispresented in the sample, was oxidised during the etching and converted to water.
Thermal desorption study was used for assessment of chemical forms in which tritium
can be released from the samples. Schematic of the test facility is given in Fig.1. Sample ofstainless steel was loaded into quartz reactor, which was purged with argon of high purity grade,
containing <0.1ppm of oxygen and <0.2ppm of methane. Reactor was heated at rate of 2K/min.
Gas flow rate was 0.1NL/min, which is equivalent to gas velocity of ≈0.01m/s. Gas passes thereactor, a water bubbler, a molecular sieve trap, a reactor with copper oxide maintained attemperature of 970K for conversion of molecular hydrogen and hydrocarbons to water, and asecond water bubbler. Efficiency of HTO trapping in a water bubbler was measured 99.5%. The
facility shown in Fig.1 was also used for measurement of rate of tritium out-gassing from thesamples under air at ambient temperature. Relative humidity of the air was usually of ≈25% at atemperature of 295K. Separate reactors and bubblers were used for samples of large and small
rate of tritium out-gassing.
2.3

Decontamination tests
The following methods of detritiation were tested:a) Purging with ambient air at an elevated temperature.
b) Purging at an elevated temperature with pure argon or argon containing either 10.0% of NH3
or 3.1% of H2.
c) Heating by methane – air flame with simultaneous purging with ambient air.
The facility shown in Fig. 1 was used for decontamination tests with the gas-purge. No copperoxide reactor was employed in the tests with argon containing NH3 or H2. To reduce influence ofa reactor “memory” on results of tritium release measurement, the test reactor was cleaned prior
to each decontamination test by purging with the relevant tritium-free purge-gas at temperature63

A. Perevezentsev・渡辺国昭・松山政夫・鳥養祐二

200K higher than that of the test. Amount of tritium released from the reactor during cleaning
runs never exceeded 1% of amount of tritium released from the sample in the following
decontamination test. For decontamination by flame, the sample was placed in an open-end
quartz reactor purged by pumping via three consecutive bubblers connected to another end of thereactor. Air flow rate through the reactor was ≈4NL/min. The reactor was installed inside
secondary containment. No tritium was observed outside of the secondary containment.

Sample

Cu O

Gas

Bubbler Molecularsieve

Bubbler

JG02.44-1c

T = 970 K

Fig. 1 Schematic of experimental facility.
3. Results and discussion
3.1 Tritium distribution in original samples
Table 2 presents the tritium inventory of the samples, tritium concentration in the surface layer
and in the bulk. The overall tritium inventory was evaluated as a sum of tritium recovered fromthe samples during their decontamination and tritium left in the decontaminated samples andthen measured by their chemical etching. The tritium depth profiles are presented in Fig. 2.
Table 2 Distribution of tritium inventory in samples.
Tritium
concentration,
kBq/cm3
bulk
Surface*)I
0.21≥4.4E4
2E2
II
1.9
7E5
5E3
III
10.1
4.9E6
2E4
IV
3.2
≥4.0E5
6E2
V
1.8
6.5E4
1.8E3
* The concentration was measured for a layer of 0.5µm to 5µm thickness. For a layer ofthickness larger than 5µm, an uncertainty in the tritium concentration is indicated by a mark ≥.
SampleseriesTritium
Inventory MBq

For all the samples loaded with tritium at various temperatures the concentration in the surface
layer was of approximately two order of magnitude larger than that in the bulk. The tritium depth
profiles given in Fig.2 show tested samples having an enriched with tritium sub-surface layer ofapprox. 15÷20µm thickness. Amount of tritium counted in acid etching’s cocktails resulted fromremoval of those layers presents 20% to 40% of the overall tritium inventory in the samples.

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SS-316 ステンレス鋼のトリチウムの除染 (Ⅰ)

The observed tritium distribution is quite different from what could be expected inaccordance with a classical mechanism of hydrogen diffusion in metals. For example, tritiumprofile calculated using the classical equation [3]1 – C/CS = erf (X/2[D⋅t]0.5)(1)(where CS and C are tritium concentrations on the surface and at distance X from the surface, D
is diffusion coefficient [12], t is time) for sample of series III shows that ratio C/CS for a point ofhalf sample’s thickness should be ≈0.9.
Tritium profiles presented in Fig. 7 to Fig. 11 for decontaminated samples also
demonstrate presence of tritium-enriched sub-surface layer.
To explain the observed phenomena, it can be suggested that an unexpectedly large tritium
concentration in a sub-surface layer might be caused by a non-classical mechanism of tritium
penetration into the stainless steel. This penetration occurs even at room temperature, whichmeans that tritium involved in such a process is more mobile than atomic hydrogen diffusing
into a crystalline lattice of the stainless steel. A possible mechanism of the tritium penetration
into a sub-surface layer could be an intergranular migration and trapping in defects of crystalline
lattice, which concentrate at the surface of stainless steel. If this assumption is correct, an
influence of micro-structural properties of stainless steel in sub-surface layer on tritium
distribution might be expected.

JG01.467-9c

Tritium concentration (kBq/cm3)

107

I:
II:
III:
IV:
V:

106

105
III

104
II

103

V
IV

102
0

I
40
20
60
Distance from surface (µm)

80Fig. 2 Tritium depth profile for samples of different series.
The thermal desorption spectrums show that a majority of tritium is released in a form ofwater vapor. This is in agreement with other thermal desorption studies [1,6,7,9,10]. As an
example, the spectrum of the sample of series II is presented in Fig. 3. There were only one peakof HTO and two peaks of HT. The ratio of tritium released as HTO to that of HT was found 40,
150, 260 for samples of series I, II and III, which were loaded with tritium at temperature of≈295K, 470K and 590K respectively. An increase of the ratio HTO/HT with rise of temperatureof tritium loading could be attributed to increasing rate of reaction of oxygen in the “oxide” layerof the steel with hydrogen penetrated into the metal. The HTO/HT ratio for sample of series V,
which was loaded with tritium under the same conditions as sample of the series III but was thenbaked-out at temperature of 770K for 10 hours, was measured 15. A decrease of the HTO/HT
65

A. Perevezentsev・渡辺国昭・松山政夫・鳥養祐二

ratio for the baked-out sample could be caused by a chemical reaction between metal andtritiated water leading to metal oxidation and conversion of water to hydrogen. Another reason
could be thermal desorption of tritium from the sample predominantly in a water form during
the sample’s baking-out followed by absorption of released tritium in reactor’s wall [6,9]. The
decreased tritium inventory for sample of the series IV, V (see Table 2) indicates that tritium
desorption from the sample occurs during its baking-out. The temperature of baking-out was
sufficient [1] for considerable tritium thermal desorption from the samples.
CG01.02/2c

120
100
80

600

65
400

40
200
20
0

400

600
800
Temperature (K)

1000

0

Rate of HT released (Bq/cm2•h)Rate of HTO release (kBq/cm2•h)140

Fig. 3 Thermal desorption spectrum of tritium released from sample of series II.
3.2 Tritium removal from stainless steel
Some results of study of tritium removal from stainless steel by gas-purge have been
already presented elsewhere [13]. The kinetic curves of tritium recovery by the gas-purge attemperature of 570K presented in Fig. 4 show that rate of tritium removal increases in the row of
the purge-gases: Ar < Ar/H2 < Air < Ar/NH3. The results of other decontamination tests are
given in Table 3. It is well known [1] that water vapor is responsible for tritium removal fromsteel by purging with air. This means that oxidation of metal in reaction with oxygen does notcontribute much to the detritiation process. Therefore tritium removal from the stainless steel canbe attributed to two basic mechanisms, i.e. thermal desorption and isotopic exchange of tritiumon the metal surface with hydrogen-containing gaseous species, such as hydrogen, water vapor,ammoniac used in this work. Separate study [14], in which the same rate of tritium removal fromthe stainless steel was observed for purging with argon containing water vapor at different partialpressures of 0.073Pa and 2.6kPa, showed that pressure of water vapor in purge-gas is not thefactor predominantly determining rate of tritium removal. The increase of tritium removal rate
for purge at temperature of 570K in row hydrogen, water vapor and ammoniac can be attributed
to increase of activity of those reagents in isotopic exchange with tritium on the metal surface.

66

SS-316 ステンレス鋼のトリチウムの除染 (Ⅰ)

Table 3 Results* of the decontamination tests.
Sample and decontamination procedureARF
I-a: Ar/NH3-purge at 670K for 5.5 hours
150
0.75
I-b: Air-purge at 670K for 5.5 hours
170
0.85
I-c: Heating by flame for 10 seconds
200≈1II-a: Air-purge at 570K for 5.5 hours
1.7E3
0.89
II-b: Ar/NH3-purge at 570K for 5.5 hours
1.8E3
0.95
II-c: Ar/H2-purge at 570K for 5.5 hours
1.5E3
0.79
II-d: Ar-purge at 570K for 5.5 hours
1.3E3
0.68
II-e: Heating by flame for 10 seconds
1.9E3≈1III-a: Air-purge at 770K for 5.5 hours
6.4E3
0.63
III-b: Air-purge at 670K for 5.5 hours
8.6E3
0.85
III-c: Air-purge at 570K for 5.5 hours
9.1E3
0.9
III-e: Ar/H2-purge at 670K for 5.5 hours
6.4E3
0.64
III-f: Heating by flame for 10 seconds
9.3E3
0.92
IV-a: Heating by flame for 10 seconds
3.2E3≈1V-a: Air-purge at 570K for 5.5 hours
1.2E3
0.67
V-b: Heating by flame for 10 seconds
1.4E3
0.78
* AR is activity (kBq) removed from the sample, F is a fraction of tritium inventory removed
from the sample.
Taking into account small difference between the rates of tritium removal by purging
with Ar/NH3 and air, simplicity of operation and processing of secondary waste, purging with an
air containing a small amount of water vapor was considered as more attractive than purging
with argon containing ammoniac. Tritium on the surface of stainless steel heated under air
atmosphere participates in various reactions of isotopic exchange and recombination leading to
removal of the tritium from the surface. Tritium of the sub-surface layer, which is enriched bytritium, migrates then to the decontaminated surface. This tritium can also migrate from the subsurface layer deeper to the bulk. This means that an optimum temperature should exist for tritium
removal due to competition between mentioned processes. Fig. 5 shows that this temperature isaround 570K.
JG02.44-2c

Ar
Ar – H2
Ar – NH3
Air

Fraction of inventory remained in sample

1.0

0.8

0.6

0.4

0.2

0
0

1

2

3
Time (hours)

4

5

Fig. 4 Kinetic curves of tritium recovery from samples of series II by purge with various gases at
a temperature of 570K.
67

A. Perevezentsev・渡辺国昭・松山政夫・鳥養祐二

JG01.467-12c

0.8

1.0
0.8

0.4

0.6
0.2

0.4
0.2

0
400

500

Fraction of inventory removed

Rate constant, h–1

0.6

0
800

600
700
Temperature (K)Fig. 5 Rate constant of tritium removal and fraction of tritium removed by air-purge.
Tritium removal exhibits different behavior for samples of different thickness andconditions of tritium loading. Fig. 6 shows the kinetic curves of tritium recovery from thesamples of series III and V. Fast detritiation of the sample of series III at the beginning of the
air-purge can be attributed to tritium removal from the sub-surface layer enriched by a mobile
tritium. Tritium removal from the sample of series V, in which larger fraction of hydrogen wastransferred from water to hydrogen and driven more far from the surface, occurs slower than forsample of the series III and is most likely controlled by diffusion of atomic hydrogen from thebulk to the surface.
Review of Table 3 clearly shows that heating by flame allows removal of the largest
fraction of tritium inventory and in a very short period of time. Increase of stainless steel
thickness and baking-out of stainless steel pre-loaded with tritium make tritium removal moredifficult. Acid etching of decontaminated samples revealed trace of internal oxidation forsamples exposed to air at elevated temperatures.

0.8

0.6
2
0.4

0.2
1
0
0

1

2

3
Time (hours)

45

JG01.467-10c

Fraction of inventory remained in sample

1.0

6

Fig. 6 Kinetic curves of tritium removal from samples of series III (1) and V (2) by air-purge at atemperature of 570K.
Tritium depth profiles in the original and decontaminated samples are shown in Fig. 7-11.
The profiles of decontaminated samples demonstrate presence of tritium-enriched sub-surface
layer as it was observed for the non-decontaminated samples. For samples of series I and II,68

SS-316 ステンレス鋼のトリチウムの除染 (Ⅰ)

where tritium in sub-surface layer contributes largely to the overall tritium inventory, tritium
depth profiles are quite similar for samples decontaminated by different methods. For samples ofseries III, IV and V, where tritium is driven deeper to the bulk by loading at elevated temperatureor baking-out of loaded samples, tritium depth profiles are different for samples decontaminatedby different methods.

Original
Thermal desorption
Ar/NH3, 670K
AiR, 670K
Flame

104

103

102

101

10
0

JG02.44-3c

Tritium concentration (kBq/cm3)

105

10

20
30
40
Distance from surface (µm)

5060Fig. 7 Tritium depth profile of the original and decontaminated samples of series I.
106

Tritium concentration (kBq/cm3)

10

Original
Ar, 570K
Ar/H2, 570K
Ar/NH3, 570K
Air, 570K
Flame

5

104

103

102

10
0

JG02.44-4c

101

10

20
30
40
Distance from surface (µm)

5060Fig. 8 Tritium depth profile of the original and decontaminated samples of series II.

69

A. Perevezentsev・渡辺国昭・松山政夫・鳥養祐二

107

Tritium concentration (kBq/cm3)

10

Original
Thermal desorption
Air, 570K, 16h
Air, 670K, 5.5hAir, 770K, 16h
Flame

6

105
104
103
102

JG02.44-5c

101
10
0

10

20
30
40
Distance from surface (µm)

5060Fig. 9 Tritium depth profile of the original and decontaminated samples of series III.
106
Original
Flame

Tritium concentration (kBq/cm3)

10

5

104

103

102

10
0

JG02.44-6c

101

5

10
15
20
25
30
Distance from surface (µm)

3540Fig. 10 Tritium depth profile of the original and decontaminated samples of series IV.
105

104

103

102
0

JG02.44-7c

Tritium concentration (kBq/cm3)

Original
Air, 570K, 12.5h
Flame

10

20
30
40
Distance from surface (µm)

50Fig. 11 Tritium depth profile of the original and decontaminated samples of series V.
70

SS-316 ステンレス鋼のトリチウムの除染 (Ⅰ)

4. Conclusions
1. Tritium distribution between surface and the bulk was studied for samples of stainless steel
type 316 loaded by exposure to gaseous tritium-containing hydrogen at room and elevatedtemperatures. Tritium concentration of approximately two orders of magnitude exceeding theconcentration in the bulk was found in the sub-surface layer of 15 ÷ 20µm thickness. The tritiumof the sub-surface layer contributes 20 to 40% to the overall tritium inventory. Easy tritium
penetration into the sub-surface layer might be attributed to hydrogen intergranular migrationand trapping in defects of crystalline lattice.
2. Detritiation by various methods, e.g. purging with air, argon, argon containing hydrogen orammoniac, heating by methane-air flame, was tested. Heating by flame allows much quicker
detritiation than gas-purging and larger fraction of tritium inventory removed. Two main
mechanisms, i.e. thermal desorption and isotopic exchange with gaseous hydrogen-containing
species, were considered determing the rate of tritium removal from the stainless steel by gaspurge. Activity increases in the row of hydrogen-containing agents, ie hydrogen, water vapor,ammoniac, being responsible for isotopic exchange with tritium on the metal surface. Taking
into account additional efforts needed for processing of “secondary” waste, air was considered asa preferable agent for detritiation by gas-purge. There is an optimum temperature of ≈570K, fortritium recovery from stainless steel by air-purge.

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