富山大学研究推進機構水素同位体科学研究センター研究報告 39：25－35，2019.

技術報告
水素雰囲気中性子照射用キャプセルのための材料選択
波多野雄治 1，外山健2

1

富山大学研究推進機構水素同位体科学研究センター〒930-8555 富山県富山市五福 3190
2
東北大学金属材料研究所附属量子エネルギー材料科学国際研究センター〒311-1313 茨城県東茨城郡大洗町成田町 2145-2

Selection of Capsule and Hydrogen Source Materials forNeutron Irradiation under Hydrogen Gas Atmosphere
Hatano Yuji1, Toyama Takeshi2
1

Hydrogen Isotope Research Center, Organization for Promotion of Research,
University of Toyama,Toyama 930-8555, Japan2
International Research Center for Nuclear Materials Science,
Institute for Material Research, Tohoku UniversityIbaraki 311-1313, Japan(Received May 19, 2020 ; accepted August 7, 2020)
Abstract
Tungsten is a primary candidate of plasma-facing material (PFM) of a future

fusion reactor.As PFM, tungsten will be exposed to deuterium-tritium plasma while
irradiated with high energy neutrons. Trapping of tritium at neutron-induced defects
needs to be understood for accurate evaluation of tritium inventory in a vacuum vesselof a reactor. From these viewpoints, the conceptual design of the capsule to realize
neutron irradiation of tungsten in an existing fission reactor in a hydrogen gas
atmosphere was proposed in this report. The irradiation conditions were supposed to be25

波多野 雄治・外山 健

400 ℃ and 0.1 MPa. Vanadium hydride was selected as a hydrogen source from theviewpoint of equilibrium hydrogen pressure, reaction rate and thermal stability.
Molybdenum was chosen as the capsule material due to its low hydrogen permeability.

1. Introduction
Tungsten is a primary candidate of plasma-facing material (PFM) of a future
fusion reactor because of favorable physical properties such as high melting point andhigh thermal conductivity [1]. PFM will be exposed to plasma of deuterium (D), tritium(T) and helium under irradiation of high energy (14 MeV) neutrons. Here, D and T are
fuel, and helium and neutrons are products of fusion reactions. T is a radioisotope ofhydrogen, and therefore the inventory of T in the vacuum vessel of a fusion reactor
should be limited to a certain level. Neutron irradiation results in formation of latticedefects via displacement damages. It is known that radiation-induced defects in tungsten
act as strong traps against hydrogen isotopes [1-5]. Hence, the inventory of T intungsten could significantly increase after neutron irradiation.
The effects of neutron irradiation on hydrogen isotope retention in tungsten
have been examined by using research fission reactors [2-5]. A significant increase in
the hydrogen isotope retention was indeed observed after neutron irradiation. Vacancies
and vacancy clusters are considered the main trap sites [5]. In these studies, tungsten
specimens were first irradiated with neutrons in a helium gas atmosphere withouthydrogen isotopes, and then exposed to D plasma or D2 gas. Thereafter, deuterium
retention
analysis.

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was determined by thermal desorption spectrometry or nuclear reaction

水素雰囲気中性子照射用キャプセルのための材料選択

Hautojärvi et al. have examined the growth of vacancy clusters both inhydrogen-free and in hydrogen-doped niobium and tantalum samples damaged withhigh energy electrons [6]. They reported that the hydrogen can be bound to vacancies
and retard vacancy migration. Similar retardation of vacancy migration has been
observed also for Fe [7]. Recently, Markelj et al. have performed simultaneous
irradiation of tungsten with Wions and D ions and compared the results with thoseobtained by sequential irradiation with Wions and D ions [8]. They reported that D
retention after simultaneous irradiation was larger than that after sequential irradiation
with W ions and D ions. These observations indicate that the presence of hydrogenisotopes can affect the development of microstructures upon irradiation. However,neutron irradiation of tungsten in the presence of hydrogen was not carried out, sincefrom a safety point of view it is difficult to introduce a large amount of hydrogen into anuclear reactor.
It is obvious that the plasma-facing material of a fusion reactor will beirradiated with neutrons under exposure to D-T plasma, i.e. in the presence of hydrogenisotopes. Therefore, it is necessary to develop a technique for irradiating specimens withneutrons in a hydrogen atmosphere.
In this report, a basic concept of an irradiation capsule is proposed to realize
safe neutron irradiation in a hydrogen gas atmosphere at a reasonable cost. The
proposed capsule contains a metal hydride as a source of gaseous hydrogen. The
candidate capsule material is also discussed in terms of hydrogen permeation.

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波多野 雄治・外山 健

2. Conceptual design of irradiation capsuleSpecimens for neutron irradiation should be as small as possible in order tominimize the amount of radioisotopes generated by neutron-induced transmutation. The
sizes of specimens used in the previous study [2-5] were 6 mm in diameter and 0.2 or
0.5 mm thick. These specimens were enclosed in a metal capsule in an atmosphere ofgaseous helium to prevent oxidation during neutron irradiation and then installed in a
research reactor.
During reactor operation, the specimens are heated by absorbing radiation. The
specimen temperature is determined by the balance between heating by radiation andheat loss controlled by the transfer of heat from the specimens to the capsule wall andfrom the capsule wall to coolant water.
The conceptual design of the capsule for neutron irradiation in an atmosphere of
Metal hydride
1 atm He gas

W(a) Before irradiation~25 oCH2 gas H2 gas

H2 gas

H2 gas H2 gas

H2 gas(b) During irradiation
at elevatedtemperature(c) After irradiation~25 oC

1 atm He gas+ 1 atm H2 gas

1 atm He gas+ residual H2 gas

Fig. 1. Conceptual design of irradiation capsule to realize neutron irradiation oftungsten specimens in a hydrogen gas atmosphere. Hydrogen gas is reversibly
supplied and absorbed by a metal hydride.

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水素雰囲気中性子照射用キャプセルのための材料選択

gaseous hydrogen is shown in Fig. 1. Initially, hydrogen is stored in metal hydride diskswith a size similar to the size of the specimens, as shown in Fig. 1(a). The operation of
the reactor leads to an increase in the temperature of both specimens and metal hydride
disks. Hydrogen gas is released with increasing temperature of metal hydride, and thepartial pressure of hydrogen gas in the capsule becomes constant if the temperature of
the hydride reaches a stable value and equilibrium is attained between the hydrogen gas
and the metal hydride (Fig. 1 (b)). When cooling after the completion of neutronirradiation, hydrogen gas is absorbed by the metal hydride and therefore the partialpressure of hydrogen gas in the capsule decreases, as shown in Fig. 1 (c). Therefore, thecapsule can be opened safely.
Typically, metal hydride in powder form is used to supply hydrogen to increase
the reaction rate by increasing the surface area and reducing the diffusion distance.
However, metal hydride becomes radioactive after neutron irradiation via transmutation.
Powdered radioactive material has a higher risk of contamination. Therefore, it is better
to use bulk and reliable metal hydride in the form of, for example, thin disks or wires.

3. Metal hydride
The requirements for metal hydride are listed below.
(1) High thermal stability; the melting point of the metal hydride and the metal itself
should be sufficiently higher than the irradiation temperature. The vapor pressure ofmetal should be low enough at the irradiation temperature to prevent reaction withtungsten specimens.
(2) High reaction rate with hydrogen; as discussed in the previous section, it is better touse bulk material rather than powder. In order to realize sufficiently high rates of29

波多野 雄治・外山 健hydrogen desorption and absorption, large hydrogen diffusivity is required for themetal hydride. As reported in [9], metals with a body-centred cubic (bcc) structure
has a greater diffusion ability of hydrogen than metals with face-centred cubic (fcc)and hexagonal close-packed (hcp) structures. In addition, the reaction rate on thesurface should be sufficiently high.
(3) The appropriate value of the heat of hydride formation or hydrogen solution; theequilibrium pressure of hydrogen gas is determined by the heat of hydride formation
if the hydride retains its structure at the irradiation temperature. If the hydride phasedecomposes into the solid solution phase at the irradiation temperature, theequilibrium pressure is controlled by the heat of the hydrogen solution and thefraction of hydrogen in the metal. It is expected that tungsten will be used in therange between the ductile-brittle transition temperature (DBTT, ~300 ℃) and therecrystallization temperature (~1300 ℃). The trapping of a hydrogen isotope by aradiation-induced defect is an exothermic reaction and, therefore, more significant at
lower temperatures. Therefore, irradiation tests at 300–400 ℃ are a priority. The
metal hydride should have an equilibrium pressure of about 0.1 MPa (1 atm) in thistemperature range with a moderate hydrogen content (for example, [H]/[M] ~ 0.1).
The hydrogen content should not be too high because metal hydride can be brittle
with high hydrogen content.
The metals of group 5 (vanadium, niobium and tantalum) and group 6(chromium, molybdenum and tungsten) have a bcc structure and preferred thermalproperties. However, group 6 metals do not form a hydride, and the hydrogen solubility
is very low [10]. Thus, group 5 metals appear to be the best candidates. In a previous
study [11], we examined the surface reaction rates of hydrogen gas for vanadium,30

水素雰囲気中性子照射用キャプセルのための材料選択

niobium and tantalum and found that vanadium has the highest desorption andabsorption reaction rates among these metals. In addition, it is known that the highest
surface reaction rate can be obtained by heat treatment at 400–700℃ by dissolving theoxide layers in the bulk without surface segregation of other elements such as sulfur[12].
According to the pressure-composition isotherms of the V-H system [13], thehydride phase decomposes into the solid solution phase at ~200 ℃. An equilibriumpressure of about 0.1 MPa at 400℃ is achieved at the fraction of hydrogen atoms tovanadium atoms [H]/[V] ~ 0.3. Therefore, vanadium hydride with a hydrogen content of[H]/[V] ~ 0.3 seems to be a good source of hydrogen.
The number of vanadium hydride disks required to supply hydrogen gas wascalculated assuming an irradiation capsule with inner volume of 2 cm3 and containing
20 tungsten specimen disks. Exposure to D plasma after neutron irradiation (sequentialexperiment) showed that the fraction of D in tungsten can reach 1 at.% due to trappingat radiation-induced defects [2-4]. If tungsten specimen disks absorb hydrogen to acomparable fraction during neutron irradiation in a hydrogen gas atmosphere, therequired amount of hydrogen for filling the capsule with 0.1 MPa H2 gas at 400 ℃ is
3.6×10-4 mol H (3.0×10-4 mol H in tungsten and 6×10-5 mol H in a gas phase as H2molecules). The vanadium hydride disk of the same size as the tungsten disk, and with afraction of hydrogen [H]/[V] = 0.3, contains 5.0×10-4 mol H. The change in thehydrogen fraction in the vanadium hydride disks should be reduced to minimum tomaintain a constant partial pressure of hydrogen gas. A set of 10 disks of vanadium
hydride serves this purpose if the loss of hydrogen due to permeation through thecapsule wall is negligible (see the next section).

31波多野 雄治・外山 健

4. Capsule material
The damage rate by neutron irradiation is relatively low. Consequently, it takesa relatively long time to accumulate damage to a level relevant to fusion conditions. For
example, it usually takes about 1 month to reach a damage level called 1 displacement
per atom (dpa) corresponding to the level at which each tungsten atom in the bcc lattice
is displaced from its original position 1 time as a result of collision with neutron.
Therefore, if the capsule material has high permeability at the irradiation temperature, asignificant amount of hydrogen may be lost from the capsule.
The material of the capsule should have a sufficiently high mechanical strengthand corrosion resistance against cooling water at the expected irradiation temperature.
Consequently, austenitic stainless steels, nickel-based alloys and molybdenum-based
alloys may be candidates.
If the diffusion process of hydrogen in a metal is the rate-limiting step, thepermeation rate J can be expressed as follows:
𝐽 = −𝐷= −𝐷𝑃 . ,where D is the diffusion coefficient, Ci is the hydrogen concentration in the subsurface
region of inner wall of the capsule, d is the thickness of the wall, KS is the solubility ofhydrogen, and P is the partial pressure of hydrogen in the capsule. From the diffusivity
and solubility data reported in [14], the permeation rate in type 316L stainless steel at
400 ℃ is evaluated to be 4.3×10-12 mol m-1 Pa-0.5 s-1. If the diameter and length of thecapsule are 8 mm and 50 mm (surface area is 1356 mm2 ), the wall thickness is 1 mm,
the partial pressure of hydrogen is 0.1 MPa and the irradiation time is 30 days (2.6 Ms),
then 5×10-3 mol H will be lost during irradiation by permeation. The amount ofhydrogen lost is significant compared to the value mentioned in the previous section.

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水素雰囲気中性子照射用キャプセルのための材料選択

Data for a nickel-based alloy (Inconel 625) yielded a comparable value of the hydrogenpermeation. On the other hand, the permeability in molybdenum was evaluated to be
2.2×10-13 mol m-2 Pa-0.5 s-1; this value was 20 times lower than that for type 316L
stainless steel due to lower hydrogen solubility. Hence, the amount of hydrogen that canbe lost by permeation is also less by an order of magnitude. It was therefore concluded
that molybdenum and molybdenum-based alloys are the best candidate of capsulematerial.

5. Summary
A conceptual design of a capsule for neutron irradiation of tungsten inhydrogen gas atmosphere is proposed. The irradiation conditions are supposed to be400 ℃ and 0.1 MPa. Vanadium hydride is selected as the hydrogen source from theviewpoint of the equilibrium hydrogen pressure, reaction rate and thermal stability.
Molybdenum is selected as the capsule material due to its low hydrogen permeability.

Acknowledgements
This study has been supported in part by JSPS KAKENHI Grant Number
18H03688 and JP20H02661.

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