J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Evaluation of tritium trap effect produced by high energy proton irradiationin SS316
Hirofumi NAKAMURA1, Kazuhiro KOBAYASHI1, Sumi YOKOYAMA2,
Shigeru SAITOH1, Toshihiko YAMANISHI 1and Kenji KIKUCHI3
1Japan Atomic Energy Agency, Tokai, Ibaraki, 319-1195, Japan
2Left for Fujita Health University,Kutsukake-cho,Toyoake,Aichi.470-1192.Japan
3Left for Ibaraki University, Tokai-mura, Ibaraki-pref., 319-1106, Japan(Received: 29 October 2009 / Accepted: 5 April 2010)

Based on results of tritium measurement in the SS316 specimens irradiated up to 5.9 dpa in the SINQ target(580MeV proton) using a thermal desorption (TDS) method, trap site density and trap energy in the materials
induced by the high-energy proton irradiation have been evaluated by means of the numerical tritium transport
analysis. The results indicate that almost residual tritium in the SS316 specimen exists in the trap site, whose trap
density is maximum 238appm (5.9dpa) and trap energy is >1.4 eV, and that tritium release by the TDS is mainly
attributed to the disappearance of the trap sites by the specimen heating, whose activation energy is about 0.7 eV.
The trap site density seems to be almost proportional to the irradiation dose (dpa). Additionally, irradiation
conditions such as the dose or irradiation temperature do not affect on the trapping mechanism.
Keywords: Tritium, Trap site, Irradiation, SS316, spallation, diffusion

1. Introduction

2. Experimental results

Tritium transport in irradiated materials is one ofinterests for the accurate tritium inventory or tritium
permeation evaluation in a nuclear fusion reactor or an
accelerator-driven nuclear transmutation system. As tothe post irradiation experiment of high-energy irradiatedmaterials, there are some extensive studies for the gas
analysis such as hydrogen isotopes and helium in thematerials irradiated with 580 MeV protons at SINQ [1, 2]
target and 800 MeV proton at LANCE target [3, 4]. The
authors also have measured tritium in the stainless steel
316F (SS316) irradiated in the SINQ target using a thermal
desorption spectroscopy (TDS) method with a constantramp rate [5]. The results indicated that residual tritium
amount in the irradiated SS316 was less than 20% of
estimated tritium generation amount by the spallation
reaction in SS316. While this result agreed with theprevious studies of the results of hydrogen [2, 3], thedetails of mechanism of residual tritium in SS316 were notclear. In order to understand such tritium retention in theirradiated SS316, the numerical tritium transport analysis
based on the one-dimensional diffusion and trapping model
has been performed not only for the TDS experiment but
also for the high-energy proton irradiation and storage
period. This paper presents the results of a numericalanalysis on the tritium retention and release behavior
in/from the irradiated SS316 obtained by the TDS
experiments, and discusses on the tritium trapping andrelease mechanism and its dose effect.

Specification of the specimens, experimental
procedure of tritium measurement in SS316 has been
described in a previous report [5]. We recall some key
data, which are closely related to the tritium transport
analysis. Fig. 1 shows the schematic drawing of thetritium measurement apparatus. The tritium is extracted
from specimens by the TDS method with the constantramp rate of 10 K/min. under the gas flow
(N2+H2(<2vol.%), 250 cm3/min.). Tritium released from
specimen is measured by an ion chamber (1.5L volume)
continuously during the TDS experiment for the real time
tritium measurement. Water bubblers collect all tritium
ThermoController

Tritium
Extraction

Data logger

HeaterIon
Chamber

Mass flow
controller

I.C.

Stack
Tritium
measurement

SampleThermoController

1.5 L

HeaterMass flow
meter

ThermoOxidation Controller
catalyst
N2+H2(<2%)

Bubbler1

Bubbler2

Dryer(Silica gel)

Fig.1.  Schematic drawing of tritium measurement
apparatus.

author’s e-mail: kobayashi.kazuhiro@jaea.go.jp326©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research

H. Nakamura et al., Evaluation of Tritium Trap Effect Produced by High Energy Proton Irradiation in SS316

H, appm

2532

2145

2145

on the measured residual tritium amount (R) and thecalculated tritium generation amount (G), the R/G ratio areevaluated, and those are found to be 15~20% as shown in
Table 1.
Fig. 2 shows a tritium release behavior from theSS316 specimens (#1 ~ #3) obtained by the TDS. The
left and right vertical axis means the tritium concentrationin the carrier gas measured by an ion chamber, andspecimen temperature, respectively. Tritium release fromthe SS316 specimens starts above 523 K, and achieves apeak at about 673K, and additional tritium release is notobserved above 673 K for all specimens. A tritium
release at different irradiation conditions such as #1(5.9dpa) and #2, 3 (5 dpa) shows almost similar behavior
except tritium concentration in the carrier gas. Therefore,
it can be concluded that the starting and peak temperatureof tritium release do not depend on the irradiation
temperature. Additionally, little difference was observed
between #2 and #3. So, it is concluded that the surface
conditions such as oxide layer does not affect the TDS
behavior, and that tritium is not maldistributed in the oxide
layer of the SS316 specimen.

D, appm

150

127

127

3. Tritium transport analysis

extracted from the specimen for quantitative measurements
remained in the specimen.
Table 1 summarizes the specification of the specimens
and results of the tritium measurement. The specimens
used for tritium measurement were three discs of SS316(3mm in diameter, 0.2mm in thickness and 13mg of aweight) irradiated 5 dpa at 363-383 K and 5.9 dpa at
383-433 K, respectively. Before the TDS, only #3 disc
was polished by an emery paper in order to remove theeffect of surface oxide on TDS behavior of tritium. Based
Table 1 Summary of specification of the specimens and theresults of tritium measurement [5]Specimen number

#1

#2

#3
(Polished)

Dose (dpa)(a)
Irradiation temp, K(b)T, appm

5.9

5.0

5.0

433/463363/383

363/383

39

33

33

Generated T (G) (8.6 MBq) (7.3 MBq) (7.3 MBq)
Evaluation

4

He, appm

Residual T(c)Experiment

255

216

216

1.37 MBq 0.91 MBq 0.87 MBq

Residual T(R) (d) 1.75 MBq 1.16 MBq 1.11 MBq

R/G (%) of T

20

16

15

(a) and (b) were evaluated at PSI. (c) is measured values at a timeof 1280th cooling day. (d) is estimated values at a time after
irradiation.

In order to understand observed tritium retention
and tritium release behavior in/from the SS316 specimens,a numerical tritium transport analysis has been carried out.
In this transport, one-dimensional diffusion equation
analysis with trap effect of tritium and other hydrogenisotopes (protium and deuterium, protium represented
deuterium in this analysis) in SS316 was consideredusing TMAP code, which was developed by Idaho
National Engineering Laboratory in USA [6]. In thetransport analysis, hydrogen isotopes transport can be
expressed in the eqs. (1) and (2),nwCTi (x, t) Di
E
§
·
(1)
C (x, t)¨ C  C (x, t) ¸ Q C (x, t) exp( T )3#1(5.9dpa)#2(5dpa)#3(5dpa)(polished))

500

1000

400

900
800

300

700

200

600

100
0
1000

500
2000

3000

O2

i

T0

¦

Ti

0

i
©
¹
wCi (x, t)
w 2Ci (x, t) wCTi (x, t)Di Si (x, t)
wt
wt
wx 2

Ti

kT

(2)

where, C: mobile concentration, C T : trapped
concentration, C T0 : trap site density, S: source, D:
diffusivity, O: lattice constant, Q0: lattice vibration
frequency, k: Boltzmann constant, T: temperature and ET:trap energy for hydrogen isotopes i (T and H),
respectively. Here, ET consists of activation energy ofdiffusion and binding energy of hydrogen isotopes andtrap site [5]. Since it is found that surface oxide does
not act as the tritium release barrier in the results of #2
and #3 specimens, hydrogen isotope transport in theoxide layer is not considered in this analysis.
Boundary condition at the surface is expressed in the Eq.
(3) assuming the Sievert’s law,(3)

Temperature (K)Tritium concentration (Bq/cm )

wt

400
4000

Time (s)Fig.2. Tritium release behavior from the proton
irradiated SS316 specimens (#1 ~ #3) (Ramp rate: 10
K/min., carrier gas: N2-2vol.%H2, flow rate: 250
cm3/min.)C (0, t )327

C (d , t )

Ks i

Pi

H. Nakamura et al., Evaluation of Tritium Trap Effect Produced by High Energy Proton Irradiation in SS316

here, Ks and P means Sievert’s constant, and partialpressure of hydrogen isotopes i, while d means the
thickness of the specimen.
The transport analysis has been performed for thehigh-energy proton irradiation at SINQ and storage(irradiation phase), and the TDS after irradiation andstorage of the specimens (TDS phase).
In the irradiation phase analysis, the transport analysis ofhydrogen isotopes in SS316 was carried out under thetritium and hydrogen generation in SS316 by thespallation reaction, which is given by the SINQ target
irradiation conditions [2]. Table 2 summarizes key
parameters for this analysis such as transport properties
or generation rate of hydrogen isotopes and so on [7, 8].
Fig. 3 also shows time evolution of the specimen
temperature and generation rate of tritium and protium in

SS316 normalized by that at normal beam irradiation
shown in Table 2, respectively for #1 specimen. Here,proton irradiation was carried out for about 1.5 years withan interval of about three month, and the specimens werestored for about four years after the proton irradiation.
While actual specimen temperature and generation rate
were fluttered due to the frequent beam interruption,
averaged constant values were assumed in this analysis.
Trapping parameters such as trap site density and trapenergy were evaluated by fitting between the analyticalresults and the experimental result of residual tritium
amount (R/G) in the specimens.
On the other hand, the TDS analysis was carried out
under the initial condition of hydrogen isotopes
distribution in the specimens derived by the irradiation
phase analysis, and basic diffusion equation andboundary conditions were same with the irradiation phaseanalysis (eqs. (1) and (3)) except trapping effect in thespecimens.
The trapping parameters were also
evaluated by fitting the analytical results with tritium
release behavior from the specimens.
We attempted
two TDS mechanisms to explain observed tritium release
behavior; one is tritium release by a de-trapping from thetrap site shown in the eq. (2) (de-trapping model), andanother is tritium release by the disappearance of the trapsite by annealing of the specimen as shown in the eq. (4)(trap site anneal model).
(4)
wC T 0 ( x, t)
EF

Table 2 Transport properties in SS316 andanalytical conditions for the transport analysis
Transport properties

Value

2

D0=1.6u10-6

Diffusivity (DH) (m /s) [7]

ED=52.9 kJ/mol
2

D0=1.6u10-6/3

Diffusivity (DT) (m /s)

ED=52.9 kJ/mol
KS0=2.57u1023

Sievert’s constant [8]3(atoms/m -Pa)

ES=0.14 eV

v0

1.0u1013㻌 /s䃚

2.8u0-10m

S (atoms/s/m3) (irradiation phase)(Normal beam/Focused beam)*wt

H

7.3u1018/8.9u1018

T

1.6u1017/1.9u1017

S (atoms/s/m3) (TDS phase)

0 (H,T)

Irradiation temperature (K)383/433 (#1)

(Normal beam/ Focused beam)*

4. Analytical results and Discussion
4.1 The de-trapping model analysis

Interval

450

Storage

1.5

Temperature
Production rate

1.0

400
350

0.5

300
250

0

1

2

3

4 5 6 18
7
time (10 s)

19

0.0
20

Production rate (a.u.)

Temperature (K)Irradiation)

Fig.4 shows the analytical results evaluated by thede-trapping model for the TDS behavior compared withthe experimental results. Here, (a) and (b) shows theresults for the 5 dpa irradiated specimens (#2 and #3) and
5.9 dpa irradiated specimen (#1), respectively. In Fig.
4(a), the result of TDS analysis with the trap energy of
1.1 eV and the trap site density of 245 appm shows
almost same tritium release peak temperature around 670
K, and the residual tritium amount in the specimen in thisanalysis is almost same with the experimental results(~0.9 MBq). Oliver et al. reported the trap energy of 1.1
eV in the proton irradiated 316SS up to ~20 dpa [9].
Although derived trap energy is similar to theirvalue, this analytical result indicates that tritium releasestarts at lower temperature (~400K) than the experimental
result (~500K) and the TDS peaks are broader than theexperimental result. The difference could be attributed
to the assumption of the hydrogen isotopes transport in
SS316 during the proton irradiation phase. As to theresults for the #1 specimen, analytical result with the trapenergy of 1.1 eV at the trap site density of 350 appm

363/383 (#2, #3)

Focused beam operation: 484-547th day

Irradiation

kT

here, D and EF means pre-exponential factor andactivation energy of trap site annealing, respectively.

* Normal beam operation: 0-176th and 267- 483rd day,500

 C T 0 ( x , t ) D exp( Fig. 3. Analytical conditions of the irradiation phaseanalysis. (H,T production rate and irradiation
temperature during proton irradiation)

328

3

T concentration (Bq/cm )3

T concentration (Bq/cm )H. Nakamura et al., Evaluation of Tritium Trap Effect Produced by High Energy Proton Irradiation in SS316

400(a) #2,3Trap density
300 245appm

difficult, since we could not obtain consistent trap energy,
trap site density and TDS behavior using the de-trapping
model.

#2#3
ET=1.10eV
ET=1.15eV

4.2 The trap site anneal model analysis

ET=1.20eV

Prior to the trap site anneal model analysis, we
obtained the R/G ratio and the mobile/trapped ratio oftritium in the irradiated SS316 specimens after theirradiation phase with the de-trapping model. Fig. 5
shows the R/G ratio in specimens for different trap site
densities induced by the irradiation for the trap energy of
1.4eV derived from the transport analysis under theconditions shown in Fig. 3 for each specimen. Where,
1.4 eV of the trap energy is determined by the tritium
release starting temperature of the TDS behavior on thebasis of the results of the de-trapping model analysis asshown in Fig. 4(a). Those results indicate that almost
tritium remained in irradiated SS316 is trapped in the trapsites, since the derived mobile/trapped ratio of tritium inthe specimen is negligibly small. It is why mobile
tritium in the specimen had diffused to the surface

ET=1.25eV

200

ET=1.30eV
ET=1.35eV

100
0
400

600
800
Temperature (K)500 (b) #1
Trap density
400 350appm
300

1000

#1
ET=1.10eV
ET=1.15eV
ET=1.20eV
ET=1.25eV
ET=1.30eV
ET=1.33eV

200
100
0
400

30
600
800
Temperature (K)1000#1(5.9dpa, 433/463K)#2(5dpa, 363/383K)

25
R/G (%)

Fig. 4. Fitting results of the TDS analysis for theSS316 specimens using the de-trapping model.
Where, (a) and (b) show the fitting results for #2, 3
specimen and #1 specimens, respectively.

ET=1.4eV

Observed(#1)
Observed
15(#2, #3)
10
20

5

shows that the TDS peak temperature is also almost samewith the experimental result as well as the #2 and #3
specimen as shown in Fig. 4 (b). Here, the trap site
density of 350 appm is just a trial value under theassumption of proportionality between the dose and thetrap site density. But, this result can not satisfy theresidual tritium amount in the specimen, since the derived
residual tritium amount is ~0.7MBq, which is smaller
than the observed value (~1.4MBq). Derived analyticalresults for the #1 specimen are attributed to the higher
irradiation temperature than that of the #2 and #3
specimens. Since a part of tritium trapped in the trapsites should be de-trapped and released from thespecimen during the proton irradiation due to the higher
irradiation temperature, the residual tritium amount in the#1 specimen could be smaller than that in the #2 and #3
specimens irradiated lower temperature under the same
trap site condition. When we assume over thousands
appm of the trap site density in the #1 specimen, residual
tritium amount will be satisfied by the de-trapping modeland the experimental results. But such assumption is
not reasonable, and inconsistency of the TDS peak shapestill remains. Therefore, application of the de-trapping
model to observed TDS behaviors is concluded to be

0
0

50

100

150

200

250

300

Trap site density (appm)

Trap site density (appm)

Fig. 5. R/G ratio in specimens for different trap site
densities induced by the proton irradiation.
300
Present work
200

100

0
0

1

2

3
dpa

4

5

6

Fig. 6. Relationship between derived trap site density
and irradiation dose by the irradiation phase analysis.

329

300 D=1

EF=0.39eV3#1
EF=0.35eV

T concentration (Bq/cm )500 (a) #1
Trap density
400 238 appm

EF=0.37eV
EF=0.38eV
EF=0.40eV

200
100
0
400

600
800
Temperature (K)3400(b) #2,3Trap density
300 168 appm

#2
#3
EF=0.35eV
EF=0.36eV
EF=0.37eV

200 D=1

EF=0.38eV
EF=0.39eV
EF=0.40eV

100
0
400

1000

T concentration (Bq/cm )3

T concentration 㻔Bq/cm 㻕

3

T concentration 㻔Bq/cm 㻕

H. Nakamura et al., Evaluation of Tritium Trap Effect Produced by High Energy Proton Irradiation in SS316

600
800
Temperature (K)

1000

600

(a) #1500 Trap density
238 appm
400
D=500
300

Observed(#1)
EF=0.65 eV
EF=0.69 eV
EF=0.75 eV

200
100
0
400

500

600 700 800 900 1000
Temperature (K)(b) #2, #3
400 Trap density
300
200

168 appm
D=500

EF=0.69 eV
EF=0.75 eV
D=500

100
0
400

Observed(#2)
Observed(#3)
EF=0.65 eV

500

600 700 800 900 1000
Temperature (K)Fig. 8. Fitting results of the TDS analysis for theSS316 specimens using the trap site anneal model for
D=500. Where, (a) and (b) show the fitting resultsfor #1 specimen and #2,3 specimens, respectively.

Fig. 7. Fitting results of the TDS analysis for theSS316 specimens using the trap site anneal model for
D=1. Where, (a) and (b) show the fitting results for#1 specimen and #2,3 specimens, respectively.

associated with D=500 gives good agreement with theexperimental results for all specimens. Additionally,
released tritium amount from each specimen well agrees
with the analytical result. This result indicates that 500
hydrogen isotopes atoms are occupied in one trap site.
When we assume that hydrogen isotopes is trapped in the
bubbles or voids in SS316, the mean void size
corresponds to about 10 nm. According to the report byGarner et al. [10], hydrogen exists as voids, whose mean
size are 37-170 nm, in the stainless steels irradiated in afast neutron reactor up to 17.5 dpa. Therefore, there is apossibility that voids or bubbles in irradiated SS316 of
the hydrogen isotopes acts as trap sites which is annealed
at high temperatures, although the observed and analysis
could be applied to limited irradiation conditions.
On the basis of the results of the tritium transport
analysis in the SINQ irradiated SS316 specimens, tritium
behavior such as retention or TDS in/from irradiatedSS316 can be considered as follows; (1) almost residual
tritium in the SS316 specimens after irradiation exists inthe trap site, whose trap energy is >1.4 eV, (2) tritium
release from the specimens by the TDS is mainly
attributed to the disappearance of the trap sites byannealing, and its activation energy is about 0.7 eV.
Once trap site disappeared, 500 of hydrogen isotopes

enhanced by heating of the specimen during proton
irradiation, and had been released from the specimen.
Derived trapping parameters for the #1 specimen (5.9dpa)
and #2, 3 specimens (5dpa) are CT0 of 238appm and
168appm with ET >1.4 eV, respectively. Fig. 6 shows
the relationship between the irradiation dose of thespecimens and the CT0.
Almost proportional
relationship between the dose and CT0 can be seen.
In the TDS phase analysis, the trap site anneal model
shown in the eq. (4) has been attempted to the
explanation of the TDS behavior shown in Fig. 2. Here,initial distributions of tritium and hydrogen in thespecimens are based on the results of the irradiation
analysis, those are CT0=238appm (#1) and 168appm (#2and #3). Fig. 7 and Fig. 8 shows the fitting results of the
TDS analysis using the trap site anneal model assuming
D=1 and D=500, respectively. Here, (a) and (b) shows
the results for the 5.9 dpa irradiated specimen (#1) and 5
dpa irradiated specimens (#2 and #3), respectively. The
results in Fig. 7 indicated that assumption of D=1 is notsuitable for the TDS mechanism. On the other hand,assumption of D=500 gives good agreement between
analytical results and experimental results as shown in
Fig. 8.
Derived TDS behavior of EF =0.69 eV330

H. Nakamura et al., Evaluation of Tritium Trap Effect Produced by High Energy Proton Irradiation in SS316

atoms will be de-trapped. Therefore, we can estimate
the feature of the trap site produced by the high-energy
proton irradiation in the SS316 specimens as follows,
although observed results can be applied to limited
irradiation conditions:(1) Single kind of trap site exists in the SS316 specimen,
even though the irradiation conditions are different,
since the trap energy is independent of the dose andthe irradiation temperature.
(2) The trap site density increases almost linearly with theirradiation dose.
(3) Although the trap energy of the trap site induced by
the irradiation in the SS316 specimens is relatively
high (~1.4 eV), the trap sites will disappear in lower
temperature by annealing.
(4) A possibility of bubbles as the trap site could be
estimated, because the 500 of hydrogen isotopes
atoms were de-trapped once a trap site disappeared.
There is still an issue on description of theirradiation defects as trap site. Further study on therelationship between production and disappearance of theirradiation defects and release behavior of tritium or othergases from the irradiated materials is desired.

[6] G. R. Longhurst, D. F. Holland, J. L. Jones and B. L. Merrill,
EGG-FSP-10315 (1998).
[7] H. Nakamura, “Investigation on tritium permeation throughplasma facing components of the fusion reactor”, Kyushu
University doctoral dissertation (2002) (in Japanese)[8] B. L. Doyle, and B. K. Brice, Radiation Effects, 89 21(1985).
[9] B. M. Oliver, Y. Dai, R. A. Causey, J. Nucl. Mater., 356 148(2006).
[10] F. A. Garner, E. P. Simonen, B. M. Oliver, et. al., J. Nucl.
Mater., 356 122 (2006).

5. Summary

The trap site density and trap energy in SS316
irradiated in the SINQ target (580MeV proton) up to ~6
dpa have been evaluated by the numerical analysis for the
TDS behavior of tritium from the irradiated SS316. The
results indicated that all of residual tritium in the SS316
specimens is trapped in the trap site, whose trap energy is>1.4 eV, and tritium release by the TDS is mainly
attributed to the disappearance of the trap sites by the
specimen annealing at the TDS, and its activation energy
is about 0.7 eV. Additionally, it is estimated that single
kind of trap site exists in the SS316 specimen eventhough the irradiation conditions is different, since thetrap energy is independent of the dose and irradiation
temperature, and the trap site density increases linearlywith the irradiation dose (dpa) (238appm at 5.9 dpa).

Acknowledgement

The help provided by the Paul Scherrer Institute on
irradiation in the SINQ target is highly acknowledged.

References[1] Y.Dai, G.S.Bauer, J. Nucl. Mater. 296 43 (2001).
[2] Y.Dai, Y.Foucher, M.R.James,B.M.Oliver, J. Nucl. Mater.
318 167 (2003).
[3] F. A. Garnar, B. M. Oliver, L. R. Greenwood, M. R. James,
et al., J. Nucl. Mater. 296 66 (2001).
[4] B. M. Oliver, M. R. James, F. A. Garnar, S. A. Maloy, J.
Nucl. Mater. 307-311 1471 (2002).
[5] K. Kikuchi, H. Nakamura, K. Tsujimoto, K. Kobayashi, etal., J. Nucl. Mater. 356 1571 (2006).

331