Correlation between Tritium Release and Thermal Annihilation ofIrradiation Damages in Neutron-irradiated Ceramics Breeders
OKUNO, KenjiRadiochemistry Research Laboratory, Shizuoka University
836 Oya, Suruga-ku, Shizuoka 422-8529
E-mail: srkokun@ipc.shizuoka.ac.jp

From the beginning, a hot atom means an atom

1. Introduction

formed with energy well in excess of ambient
thermal energy or highly charged via a nuclear

The high energy chemistry [1-3] is definedas the interaction of high energy particles withevent.

Recently, it has been available to studymatter, the nature and reactivity of short-lived

on the hot atom reaction that the hot atom can be

species induced by the action of particle andformed by the non-nuclear reaction as well as byelectromagnetic radiation or hot atoms on

the nuclear event.

substances in their gaseous and condensed states,

have become to include not only the chemistry

and chemical processes initiated in organic andinduced via the nuclear event but also via

inorganic systems by high energy radiation.

non-nuclear reactions using beam method and/or

The hot atom chemistrybeganphotolysis, with the energy scale in the range

from ”Hot atom chemistry” which have beenfrom several tens to hundreds eV as shown inused as informal synonym for “recoil chemistry”

Fig. 1, thus it is called as the “High energy

or “chemical effects of nuclear transformations”.

chemistry”.

Thehighenergy

chemistry

Energy10-2

10-1

Chemical

100

Thermalequilibrium

101

102

103

Hot atom

104105

Physical

Non-thermal
equilibrium

collision → damages

Fusion reactors
Tritium produced
Tritium
producedbby
y nuclear
nuclearprocesses
reaction
Implanted tritium
6Li(n, α)Tin plasma facing components
PlasmaET=2.73 MeV

Our interesting energy region D-D reaction

Fig. 1 Energy of tritium in fusion reactor conditions.

1106 (eV)The high energy chemistry deals withkinetics of thermal species will govern thechemical events caused by the deposition in

behavior of the tritium, although the chemical

single atoms of energy, which is much greater

effects of nuclear transformation obviously enter

than chemical bond and ionization energies.

the scene in the very first stage of exposure toTherefore the high energy chemical reaction

neutron. Energetic tritium can be produced withprogresses without any other energies such as

the neutron capture reactions of lithium asactivation energy in thermal chemical reactions.

follows.

The type of energy conveyed to the atom, andthence to the surrounding atoms and moleculesn + 6Li → T + 4He + 4.78 MeV(1)are: (1) the kinetic energy of recoil from gamman + Li → T + He + n’ – 2.82 MeV

(2)

7

4

ray or energetic particle emissions (for example,
typically of the order of 100 eV from the (n, γ)

It is important to investigate that what kind ofprocess and 100,000 eV from the (n, p) process),

tritium chemical form is induced by high energy(2) the energy of Coulomb repulsion generated

chemical reaction during neutron irradiation

when the high positive charge acquired by an

because these information should be considered

atom as a result of internal conversion of γ rays,in the tritium recovery concepts from theexcitation and/or emission of inner shell electron

blanket.

by X-ray absorption, followed by Auger electron

In the present study, high energy chemistryemission, spreads over the atom in a moleculesof tritium produced in tritium solid-breederand (3) the energy of ionization and molecularmaterials will be present mainly focused on ourexcitation conveyed to molecule of the medium

experimental results for Li2O.

by γ rays, X rays and high energy electrons
emitted.

The recoil energy and massive

2. High energy chemical issues in solid

Coulomb repulsion are different initiators ofbreeding materials

reactions than those found in radiation chemistry,
but the reactions of electrons, atoms, radicals,

2.1 Experimental methodand1) Neutron irradiation

ions

followingtheprimaryenergy

deposition are similar.

The degassed materials（10-20 mg）weresealed in quartz ampoules under vacuum andirradiated in the T- pipe of the JRR-4 reactor of

1.1 Application of high energy chemistry tofusion reactor research

the Japan Atomic Energy Research Institute.

In fusion reactors, there is possibility that

The thermal and fast neutron fluxes at thehigh energy chemistry of tritium would play

irradiation position, monitored with Co and Ni

important roles in breeder materials and plasma

foils, were 4 × 1013 and 3 × 1012 cm-2s-1,

facing components, as shown in Fig. 1.

respectively. The temperature at the target

In the blanket, the recoil tritium is produced

surface was about 320 K ． The irradiatedby the Li (n, α) T and Li (n, n’α) T reactions.

materials were stored in a container cooled withIts final states would be independent of primaryliquid nitrogen.

67species, and classical thermodynamics and2

procedures were described in the previous paper2) Analytical procedures
The tritium species (T+,T- and T0) existing[5 ].

in Li2O, LiOH and LiH crystals were analyzed
with a radiometric method based on the2.2 Tritium existing states [5-9]following reactions [4]. When the irradiated

In neutron-irradiated Li2O crystals, tritiummaterial is dissolved in heavy water, the labile

was found to exist in the T+, T- and T0 states.

tritium (T+) in the solid is converted to tritiated

The abundance of these species is listed in Table

water

1, together with other data to be compared. The

throughtheimmediatedexchangevalue is an "initial" abundance obtained for thereaction,
T+ + D2O -> D+ + DTO

material which was not subjected to any heat(3)

treatment after the neutron irradiation.

orOT- + D2O -> OD- + DTO.

Although the most abundant species was T+(4)

in the Li2O crystals irradiated with neutrons, it isTritide anion (T-), which would be bonded to Li+,to be noticed that the initial abundance of T+ wasgives rise to gaseous DT on hydrolysis,

obviously low compared with the value obtained
for unirradiated Li2O crystals in which tritium+ -+-Li T + D2O -> Li (OD) + DT.

was thermally doped at 470 K under equilibrium(5)with gaseous HT.
0

In the latter case more thanThe exchange between tritium (T ), which

99.99% of tritium was found in the T+ state.

would be in the form of molecular hydrogen (HT,Also in neutron-irradiated LiOH crystals, nearlyT2) as well as hydrocarbons, and deuterium of

100% of tritium was in the T+ state as listed inheavy water is very slow in absence of catalysts,

Table 1. On the contrary, almost all of the tritium

0so that the T species is evolved in tile form ofwas found in the T- state in LiH crystals

HT, T2 and hydrocarbon-T.

Consequently theirradiated with neutrons. The T+ species in LiOH

tritiumaqueousphasecrystals is considered to be in the form of OT,corresponds to the T+ species. The T- species iswhile the T- state of tritium in LiH crystals

observed as gaseous DT and can be separated

would be bound to Li+ ions. The presence of T-

from the T0 species which is evolved as HT, T2in neutron-irradiated Li2O crystals suggests thatand hydrocarbon-T.

there should be positive sites which have strong

remainingintheaffinity for T- ions in the crystal.

Details of the apparatus and the analytical

Table 1 Tritium species existing in lithium oxide crystals irradiated with neutrons.

3

To understand the nature of T+ and Tspecies in Li2O crystals irradiated with neutrons,
thermal behavior of the tritium species wasinvestigated. Fig. 2 illustrates the relative
abundance of T+, T- and T0 species in Li2O
crystals as a function of the heating temperature.
Thesample

washeatedatprescribedtemperatures for 10 min, without breaking thequartz ampoule used for the neutron irradiation.
A district temperature dependence was observedin the abundance of all but T0 species. The T+
fraction in the crystals irradiated to 2.5x1015 and2.5x1016 cm-2 increased to 93% or more when
the crystal was heated at 570 K. Quite a reversefeature was true for the T- species and its
abundance became only 5% at 570 K. The same
trend was observed for the crystals irradiated to17

8.3x10-2cmFig. 2 Abundance of T+、T- and T0 species inLi2O as a function of the heating temperature.
The samples were heated for 10 min at eachtemperature after neutron irradiation to 2.5x1015(○,●), 2.5x1016 (□,■) and 8.3x1017 cm-2(△, ▲).

although the curves shifted tosomewhat higher temperature. In the present
experiment, the upper temperature of isochronalheating was limited to 570 K because theconsiderable amount of tritium was released
from the crystal at higher temperatures. Anyway
it was the experimental evidence that T+ was themost stable state in Li2O crystals. Alternatively,
radiation damages induced by the neutron
irradiation would possibly contribute to theexistence of the T- state of tritium.

It isinteresting to note that the difference between
the initial and maximum abundance of T-, ∆A=Amax-Ainit,

becamelarger

withincreasing

neutron fluence, although no distinct neutron
fluence dependency wasabundance of T-.

observed in the initialHigh energy tritons (2.7MeV) and helium ions (2.1 MeV) can contributeto the production of radiation damage in Li2O

Fig. 3 Isothermal change of T- abundancein Li2O irradiated with neutrons.

crystals. The typical radiation damage inducedby the 6Li(n, α')T reaction in Li2O has beenreported to be F+ centers (oxygen-ion vacancy4occupied by one electron) [10]. The F+ centerof the recovery of F+ centers through thermal

can act as a positive site where tritium is trappedannealing. The presence of T+ species wouldin the T- state.

becomeFig. 3 illustrates the T- abundance in thelessprobablewithdecreasing

concentration of F+ centers. In fact, Noda et al.

Li2O crystal irradiated to 2.5x1015 cm-2 as a[10]function of the isothermal heating time. In Li2O

measurements that the F+ center produced incrystals heated below 500 K, the T- abundance

Li2O is annihilated by heating above 400 K.

have

reportedfromtheirESRInT-Fig. 4, their experimental result is superimposed

above

on the change in the abundance of T+ and T-

approximately 500 K showed a maximum.

species obtained for the Li2O crystals irradiatedAppearance of the maximum in these curves

to approximately the same neutron fluencesuggests the presence of two competitive(3.8x1016 cm-2 for the former and 2.5x1016 cm-2

reactions; the T+-to-T- conversion and its inverse.

for the latter).

Although reaction rates of both the conversions

500 K seems to be closely correlated to thebecame higher with increasing temperature, theannihilation of F+ centers in Li2O crystals. The

rate of the T--to-T+ conversion was enhanced

increase of the T- fraction in the temperature

above 500 K and the maximum appeared therange between 400 and 500 K of the isochronalearlier, the higher the temperature. The T+-to-T-

heating for 10 min (Figs. 2 and 4) reflected theconversion, which took place in preference topreferential T+-to-T- conversion as observed atthe T--to-T+ conversion, might be attributed toearly time of the isothermal heating (Fig. 3).

increased

monotonica11y,abundance

inthewhilecrystal

theheated+The T--to-T+ conversion above

In conclusion, the presence of the T- species

the interaction of tritium with F centers.
On the other hand, the T--to-T+ conversion

in Li2O crystals irradiated with neutrons isobserved above 500 K can be explained in terms

ascribed to the interaction of tritium with F+centers. The T- species has a nature to beconverted to the stable T+ species through heat
treatments above 500 K. In relation to thethermal

releaseoftritiumfromneutron-irradiated Li2O above 570 K, it ispossible to conclude that the tritium diffuses inthe T+ form, which would be bound to O2- ionsin the crystal.

Fig. 4 also suggests that tritium

behavior in Li2O crystal could be correlated withannihilation of the irradiation damages.

In ourrecent studies, the similar correlation has
observed for other lithium oxides [11-14].
Fig. 4 Cmparison of the change in T+ andT- abundances in Li2O with the feature ofF+ center annihilation.
5

3. Energetic Tritium release mechanism

temperatures below 570 K, however, the rate of[15-20]reaction (6) to the right is still lower than that of
the T+ diffusion, and the overall tritium release
rate is controlled by the thermal decomposition

The fundamental tritium release behavior ofneutron-irradiated

Li2O

crystals

canof LiOT at the surface.

beinterpreted in the following mechanism (Fig. 5),

At higher temperatures (>570 K), the+which consists of two major processes: the T

thermal decomposition of LiOT proceeds much

diffusion in the crystal and the evolution of T2O

faster than the bulk diffusion of T+, so that themolecules as a result of the reaction

HTO molecules are evolved as soon as T+
reaches the surface. Consequently, the bulk

2LiOT(s) -> Li2O(s) +T2O(g).

diffusion of T+ becomes the rate-determining(6)

step of the T2O(g) release process above 570 K.
Tritium produced by the Li(n, α)T reaction

At this time, however, it is to be noted that thehas 2.73 MeV of initial kinetic energy associatedreversible reaction of eq. (6) is controlled by thewith the nuclear recoil event. This high-energy

partial pressure of T2O(g) or H2O(g) over thetriton loses its kinetic energy penetrating throughsolid.

Li2O crystals as far as 30.8±2.5 µm. The tritium

molecules would occur. under the conditionis thermalized in the crystal sharing threewhere the partial pressure of the water species is6+-0The

immediate

evolutionofT2O

maintained below the equilibrium pressure.

different valences (T , T and T ), although
distribution of the valence states depends uponIn addition to the above mentioned major

the irradiation conditions. The most abundant

processes through which most of the tritium

species in Li2O crystal is T+, and its fraction

produced in Li2O crystals is released in theincreases with increasing neutron fluence.

chemical form of T2O, there are minor processes

Internal heat and radiation damage induced byleading to the T2 evolution. A fraction of the T-

the neutron irradiation would play a role inspecies which might be bonded to Li+ is released

determining the abundance of T+ species.

via the reaction analogous to LiH(s) -> Li(s, l) +If the neutron-irradiated Li2O crystals are

1/2H2(g). The T0 species which might be present

heated under vacuum, the T2O(g) releaseas molecular tritium in solid is also a source of

becomes apparent around 500 K. The origin ofT2.

the T2O molecule is the T+ species which might
be in the form of OT- or LiOT in the vicinity ofthe solid surface. The T2O molecule is evolved
via reaction (6).
At temperatures above 500 K, the tritium
species other than T+ existing ill the crystal areconverted into the T+ species and begin tomigrate outward, interacting with O2- the matrix.
The diffused T+ species forms LiOT at theFig. 5 Tritium release mechanism from Li2O.

surface and is also released via reaction (6). At6be[11] S. Akahori, E. Tega, Y. Morimoto,applicable to explain the fundamental behavior

K. Okuno, M. Nishikawa, K. Munakata,of tritium release not only from Li2O crystals butH.

also from other lithium oxides.

M. Okada, J. Radioanal. Nucl. Chem., 255

The mechanism presented here could

Moriyama,K.

Kawamoto,and(2), 257 (2003).
[12] M. Oyaidzu, Y. Morimoto, H. Kodama,

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