J. Plasma Fusion Res. SERIES, Vol. 11 (2015)

Correlation between annihilation of irradiation defects and tritium release
behavior for advanced tritium breeding material Li2+xTiO3
Misaki SATO, Kenta YUYAMA, Yasuhisa OYA and Kenji OKUNO
Graduate School of Science, Shizuoka University(Received: 26 September 2014 / Accepted: 19 January 2015)

The effect of lithium concentration on tritium release behavior for Li2+xTiO3 was investigated and thecorrelation between annihilation of irradiation defects and tritium release was studied. To evaluate the kinetics ofirradiation defect annihilation, Electron Spin Resonance (ESR) measurement was applied for neutron-irradiatedLi2.0TiO3 and Li2+xTiO3 such as Li2.4TiO3. It was found that two irradiation defects, E’-centers and O-centers, wereformed for those samples. The annihilation of E’-centers for Li2.0TiO3 proceeded at around 600 K, although that forLi2.4TiO3 were observed in the temperature range between 400 K and 700 K. In addition, tritium release, namely
Peak 1 and Peak 2, were observed around 470 K and 600 K for Li 2+xTiO3. A crystal structure of Li2+xTiO3 wasfound to be a mixture of Li2TiO3 and Li4TiO4 crystals. Therefore, Peak 1 and Peak 2 were attributed to release oftritium retained in Li4TiO4 and Li2.0TiO3 structures, respectively. These facts indicated that irradiation defects
formed in Li4TiO4 structure was annihilated easily compared to Li2.0TiO3, leading to tritium release at lower
temperature of 400 K as Peak 1 in Li4TiO4.
Keywords: Solid breeding material, blanket, lithium titanate, irradiation defect, ESR, TDS

stability of trapping site and so on.
Especially for understanding tritium releasemechanisms, tritium release behaviors from Li2.0TiO3 andlithium enriched sample (Li2+xTiO3) were investigated bymeans of Thermal Desorption Spectroscopy (TDS) [2, 3]
in our previous study. Tritium TDS spectrum of Li2.0TiO3
consisted of single release stage at around 600 K under theconstant heating rate of 0.5 K/min. The rate-determining
step of tritium release was evaluated as the tritium
diffusion process in Li2.0TiO3 from the results of kinetic
analyses. Tritium TDS spectra for Li2+xTiO3 were consistedof two release stages at around 470 K and 600 K. Thetritium release stage around 600 K was derived from thetritium diffusion in Li2.0TiO3. It was found that the amountof tritium released at the additional release peak (450 K)
for Li2+xTiO3 was increased with the increase in lithium
concentration. It was expected that tritium release forLi2+xTiO3 would be controlled not only by Li2.0TiO3structure but also by Li4TiO4 structure, which was formedby the excess lithium concentration [4]. Recent tritium
TDS measurements for Li4TiO4 showed tritium releasestages were located at 450 K and 600 K, suggesting thattritium release stage at lower temperature for Li2+xTiO3
would be derived from the tritium release from Li4TiO4.
These facts indicated that the tritium release mechanisms
in Li2+xTiO3 would be controlled by the both structures ofLi2TiO3 and Li4TiO4.
For the estimation of tritium inventory in Li2+xTiO3,
the elucidation of tritium trapping and de-trapping

1. Introduction

For the development of D-T fusion reactors, it isimportant to establish an effective fuel recycling system. In
the test blanket module for ITER, lithium titanate (Li2TiO3),
one of ternary lithium oxides, is selected as tritium
breeding materials due to good tritium release, highchemical stability, and so on [1]. Because tritium as fuelmust be produced in D-T fusion reactors, one of the key
roles of Li2TiO3 is the tritium generation. For the steadystate reactor operation, sufficient amount of tritium must begenerated in blanket system to keep fuel cycle of thereactors. However, the concentration of lithium in Li2TiO3will be decreased by the consumption by the reaction of6Li(n, D)T and 7Li(n, nD)T during D-T fusion reactor
operation. The vaporization of lithium will also reduce thelithium concentration in Li2TiO3, resulting in the reductionof tritium generation rate with reactor operation. Therefore,
the use of Li2.0TiO3 with lithium enrichment, such as
Li2.2TiO3, Li2.4TiO3 (Li2+xTiO3) is considered to increase
the tritium generation rate in Li2.0TiO3.
Tritium generated in Li2TiO3 migrates in the grain via
various processes such as trapping and de-trapping attrapping sites, diffusion in the interstitial of lattice andrecombination on the surface. Understanding of tritium
migration mechanisms is quite important for stable
extraction of tritium form the blankets. The interaction
between generated tritium and Li2.0TiO3 will be changed
with an increase in lithium concentration, because of theamount of generated tritium, the change of crystal structure,author’s e-mail: r0332013@ipcipc.shizuoka.ac.jp

53©2015 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchM. Sato et al., Correlation between Annihilation of Irradiation Defects and Tritium Release Behavior for Advanced Tritium Breeding Material Li2+xTiO3

1.5

O-centerE'-center㻌

Li2.4TiO3

1.00.5

㻌

18-1

Amount of Defecrs /10 g [Li2TiO3]

processes in Li2+xTiO3 is also important. The irradiation
defects are considered to act as trapping sites for tritium insolid breeding materials, indicating that tritium inventory
in Li2+xTiO3 would be controlled by the density of theirradiation defects. The irradiation defects would beannihilated with tritium de-trapping, then trapped tritium
would be released. Hence, the influence for the increase oflithium concentration in Li2TiO3 on defect behavior is a
critically important subject. The evaluation for theannihilation kinetics of the defects can contribute tocontrolling the tritium inventory in blanket systems.
Therefore, in this study, ESR experiments for thermal
neutron irradiated Li2.0TiO3 and Li2.4TiO3 were performed
to evaluate the behavior of irradiation defects in thesematerials. Both of isochronal annealing and isothermal
heating were applied to elucidate the annihilation kinetics
for irradiation defects.

0.0-0.5
2.102.05

2.00

1.95

g-value / Fig.1 ESR spectrum for neutron irradiated Li2.4TiO3
consisted of two kinds of irradiation defects, E’-centers andO-centers located at g=2.01 and g=2.05 [8-10], respectively.
E’-centers are the oxygen vacancies trapping one electronand O-centers are interstitial oxygen atoms in interstitial oflattice, indicating that these irradiation defects are pairdefects so called Frenkel pair. It has been considered thatE’-center would act as tritium trapping sites [11]. Fig. 2
shows the annihilation behavior of E’-centers byisochronal heating for Li2.0TiO3 and Li2.4TiO3, respectively.
It was found that O-centers has annihilated in the same
temperature region, indicating that the annihilation
processes of these defects proceeded by recombination ofE’-centers and O-centers. Fig. 2 clearly showed that theannihilation temperatures for E’-centers in Li2.0TiO3 andLi2.4TiO3 were different and annihilation of E’-centers for
Li2.0TiO3 initiated above 600 K, although that for Li2.4TiO3
was shifted toward lower temperature region of 400 - 700
K. Hara et al., reported that Li4TiO4 structure existed inlithium enriched samples [4]. These facts indicated thatirradiation defects formed in Li4TiO4 structure wasannihilated easily compared to Li2.0TiO3 resulting in theshift of annihilation temperature for Li2.4TiO3, suggestingthat interstitial oxygen atoms can easily diffuse in Li4TiO4compared to Li2.0TiO3. Therefore, tritium trapped byirradiation defects in Li4TiO4 can be de-trapped at lower

2. Experimental

Powders of Li2.0TiO3 and Li2.4TiO3 purchased from Kaken
Co. were used as samples. These samples were introduced
into quartz tubes and preheated at 1173 K under He gas
with the pressure of less than a few pascal for 3 hours.
After heating, thermal neutron irradiation was performed at
Research Reactor Institute, Kyoto University (KURRI).
The thermal neutron flux was 5.5 × 1012 n cm-2 s-1. The
thermal neutron irradiation was carried out for 10 min,corresponding to the thermal neutron fluence of 3.3 × 1015
n cm-2. In order to clarify the annihilation behaviors andthe densities of irradiation defects for Li2.0TiO3 andLi2.4TiO3, the isochronal annealing experiments from R.T.
to 773 K were performed with the heating step of 25 K,
with the holding time for 5 minutes. The isothermal
heating experiments at the temperature range of 573 - 673
K for up to 8 hours were also done to evaluate the kinetics
of defect annihilation. After each isochronal and isothermal
heating step, the samples were immediately cooled toliquid nitrogen temperature. Thereafter, the ESR (BrukerBiospin EMX 8/27 EPR Spectrometer) measurements
were carried out at liquid nitrogen temperature toinvestigate the irradiation defects in samples. The amountof defects were estimated using CuSO4࣭5H2O which has aunpaired electron in each molecule [5,6].

3. Result and discussion

Fig. 1 shows the ESR spectrum for neutron irradiatedLi2.4TiO3. It was found that the asymmetric and rather
complex ESR spectrum was observed for theneutron-irradiated Li2.4TiO3. It should be noted that major
defects would be induced by collision with recoiled tritium
and D-particle generated by the nuclear reaction of6Li(n,D)T, as was the case in the previous studies [7]. The
signal of ESR spectrum was enhanced, indicating that theirradiation defects were formed in the sample. It was foundthat the ESR spectrum for neutron irradiated Li2.4TiO3

2.5
2.0Li2.0TiO31.5

㻌The amount of defects / 1018

㻌E'-canter
Li2.4TiO3

1.0E'-centerO -center

300

400

500

600

Temperature / K

700

800

Fig.2 Annihilation behavior of E’-centers in isochronalheating for Li2.0TiO3 and Li2.4TiO354

Tritium release rate / Bq s

Li2.0TiO3Li2.4TiO3

4.0

1.00.80.6

2.00.40.0300

400500 600 700Temperature / K

8000.2900.

Fig.3 Comparison of TDS spectra and annihilation
behaviors of E'-center

Fig.4 Typical result of isothermal experiment forLi2.4TiO3temperature than that in Li2.0TiO3.
Fig. 3 shows the comparison of TDS spectra under theheating rate of 0.5 K/min and annihilation behavior ofE'-center for Li2.0TiO3 and Li2.4TiO3. The TDS spectrumfor Li2.4TiO3 consisted of two tritium release stages. In ourprevious study [2], Peak 1 located at 450 K was attributed
to tritium release from Li4TiO4 structure in Li2.4TiO3 andPeak 2 at around 600 K was attributed to tritium releasefrom Li2.0TiO3 structure. The tritium release temperature
region for Peak 1 and the annihilation temperature regionof irradiation defects formed in Li2.4TiO3 were almost thesame. On the other hand, the annihilation temperature
region of irradiation defects formed in Li2.0TiO3 was higher
than 600 K, indicating that tritium in E-centers of Li2.0TiO3structure were still trapped below 600 K, indicating thattritium release as Peak 1 would be triggered by theannihilation of irradiation defects in Li4TiO4 structure.
The typical result of the isothermal annealing
experiments for Li2.4TiO3 is shown in Fig. 4. The
annihilation rate of E’-centers should be controlled by thefirst order reaction as follows,

N (t ) = N

0elementary process in the annihilation of E’-center. The
annihilation of E’-centers takes place by the oxygen
diffusion in these structures. The oxygen diffuses in thecrystal structure with interacting to the constituent atoms.
The Li4TiO4 and the Li2.0TiO3 structures have the same
constituent atoms although the chemical bonds among
these atoms are different. Therefore oxygen diffusivity may
not be different substantially. The second reason is the ratioof each structure. It can be considered simply that Li2.4TiO3
consists of Li2.0TiO3 and Li4TiO4 with the fraction of 80%
and 20%, respectively. It can be said that the Li2.0TiO3 isthe predominant structure and the contribution of Li4TiO4structure to the apparent activation energy for theannihilation of E’-centers will be small. Consequently, theactivation energy for the annihilation of E’-centers inLi2.4TiO3 was similar to that in Li2.0TiO3.
The annihilation behavior of irradiation defects inLi2.4TiO3 was evaluated. It was found that the irradiation
defects were annihilated at lower temperature region forLi2.4TiO3 compared to those in Li2.0TiO3, although theactivation energy for defect annihilation was almost thesame. The defect annihilation at lower temperature
indicates that tritium recovery can be achieved effectively
at lower temperature. Actually, the tritium release peak wasobserved at lower temperature region for Li2.4TiO3. Inexp( kt)(1), where N(t) is the amount of E’-center in time, N0 is theinitial amount of E’-center and k is the rate constant ofreaction. Kinetics of de-trapping process were defined asfollows,A exp(Ea)
RTRateconstant, ln k / -k  (2), where A is pre-exponential factor. The activation energy
was evaluated for Li2.4TiO3 by the Arrhenius type plots asshown in Fig. 5. The activation energy of annihilation for
E’-canter in Li2TiO3 was evaluated about 0.5 eV [7].
Therefore the activation energies for the annihilation ofE’-centers were about 0.5 eV and were almost the same forthe two samples, although the annihilation temperature ofE’-centers in Li2.4TiO3 was lower than that in Li2.0TiO3.
Two possibilities which can explain the same activationenergy for the annihilation of E’-centers in Li2.0TiO3 andLi2.4TiO3, are considered. The first is due to the same-5.0

E'-centerExp. results
Fitting.

-6.0

㻌

6.0

Normalized ESR peak erea / --1

M. Sato et al., Correlation between Annihilation of Irradiation Defects and Tritium Release Behavior for Advanced Tritium Breeding Material Li2+xTiO3-7.0-8.0

1.5

1.6

1.7

1000/T / K-1

1.8Fig.5 Analytical result for E’-canter in Li2.4TiO355

M. Sato et al., Correlation between Annihilation of Irradiation Defects and Tritium Release Behavior for Advanced Tritium Breeding Material Li2+xTiO3addition, the same activation energy of defect annihilation
for Li2.4TiO3 and Li2.0TiO3 indicates that the increased
lithium concentration would not impact the density and thestability of defects. These facts indicate that largeadvantage of using Li2+xTiO3 in blanket system of D-T
fusion reactor is expected due to easy tritium recovery andtritium inventory reduction in Li2+xTiO3 compared to thosein Li2.0TiO3.

J. Osuo et al., Fus. Eng. Des. 86, 2362 (2011).
K. Moritani et al., J. Nucl. Mater. 325, 169 (2004)
K. Moritani et al., Nucl. Inst. Meth Phys. Res. B 232, 317(2004)[10] Y. Bensimon et al., J. Phys. Chem. 61, 1623 (2000).
[11] M. Oyaidzu et al., J. Nucl. Mater. 329, 1313-1317 (2004).
[7][8][9]4. Conclusion

The effect of lithium concentration on tritium release
behavior was investigated with comparison of irradiation
defects and tritium release behavior. The effect ofirradiation defects on tritium release behavior for Li2.0TiO3and Li2.4TiO3 were studied by means of ESR. Fromisochronal heating experiments, the amount of E’-centers
in Li2.0TiO3 changed slightly in the temperature region
below 600 K and its annihilation started above 600 K.
However, E’-center formed in Li2.4TiO3 were annihilatedaround 400 K to 700 K. Because Li4TiO4 structure existedin Li2+xTiO3, the E’-centers annihilated at lower
temperature in Li2.4TiO3 were formed in other structure,such as Li4TiO4. In addition, the tritium releasetemperature region for Peak 1 and the annihilation
temperature region of irradiation defects formed inLi2.4TiO3 were almost the same. These facts indicated that
tritium release as Peak 1 would be triggered by theannihilation of irradiation defects in Li4TiO4 structure. The
elucidation of tritium release kinetics was carried out bymeans of isothermal annihilation experiments for Li2.0TiO3and Li2.4TiO4 to reveal the dynamics of defect annihilation
for controlling tritium inventory in blanket systems. Fromthe values of the rate constants in various heatingtemperatures, the activation energies were evaluated forLi2.0TiO3 and Li2.4TiO3 to be commonly 0.5 eV. It wasexplained that Li2.4TiO3 consisted of large amount of
Li2.0TiO3 structure, and annihilation process of irradiation
defects in Li2.4TiO3 was controlled by annihilation rate ofirradiation defects formed in Li2.0TiO3. Although theamount of defects was not increased, that of tritium
released at lower temperature was increased with theincrease in lithium concentration. Therefore, it wasindicated that tritium inventory in Li2+xTiO3 will bedecreased compared to Li2.0TiO3 in fusion reactors.

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