Fusion Engineering and Design 197 (2023) 114054

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Fusion Engineering and Design
journal homepage: www.elsevier.com/locate/fusengdes

T production using a high-temperature gas-cooled reactor for the DEMO
fusion reactor: Li rod structure for the initial irradiation test
Hideaki Matsuura a, *, Taisei Abe a, Kanta Kitagawa a, Motomasa Naoi a, Hiromi Kawai a,
Kazunari Katayama b, Teppei Otsuka c, Minoru Goto d, Shigeaki Nakagawa d, Etsuo Ishitsuka d,
Shimpei Hamamoto e, Kenji Tobita f, Satoshi Konishi g, Yuki Koga h, Ryoji Hiwatari h,Youji Someya h, Yoshiteru Sakamoto haDepartment Applied Quantum Phys. Nucl. Eng., Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan
Department Advanced Energy Eng. Sci., Kyushu University, 6-1 Kasugakouen, Kasuga city, Fukuoka 816-0811, Japanc
Department Electrical and Electronic Eng., Kindai University, 3-4-1 Kowakae, Higashi-Osaka city, Osaka 577-0818, Japand
Japan Atomic Energy Agency, 4002 Narita-cho, Higashi-Ibaraki-gun, Oarai-machi, Ibaraki 311-1393, Japane
Blossom Energy Inc., 5-25-18 Hongo, Bunkyo-ku, Tokyo 113-0033, Japanf
Department Quantum Sci. Energy Eng., Tohoku University, 6-6-01-2 Aoba, Sendai City, Miyagi 980-8579, Japang
Kyoto Fusioneering Ltd., 1-6-1 Otemachi, Chiyoda-ku, Tokyo 100-0004, JapanhNational Institutes for Quantum and Radiological Science and Technology, Rokkasho, Aomori 039-3212, Japanb

A R T I C L E I N F O

A B S T R A C T

Keywords:
Tritium
Zirconium
Ni coating
Irradiation test module
GTHTR300
HTTR
DEMO fusion reactor

This work proposes tritium (T) production using a high-temperature gas-cooled reactor (HTGR) and studies it for
the initial T inventory in a demonstration fusion reactor and a prior engineering test with T handling. At thisstage, the aim is to investigate the compatibility between electricity and T production, which makes the stableconfinement of T in the Li-loading rods during the HTGR operation period a crucial issue. The total T outflow into
the helium gas during the reactor operation period was attempted to be reduced using Zr spheres with Ni coating
as a T absorption material to avoid an increase in the inner T pressure. The basic hydrogen absorption propertiesof the Zr spheres with Ni coating, which coexist in an environment with Al2O3 and/or LiAlO2 oxides, were
measured. The structures of the Li-loading rod for a typical commercial HTGR and irradiation test were studied
using the obtained data. In addition, an outline of the irradiation test is reported.

1. Introduction
The tritium (T) supply method must be prepared for the initial T
inventory in a demonstration (DEMO) fusion reactor [1–5]. Tritium is
produced in Canadian Deuterium Uranium (CANDU) reactors using a D(n,γ)T reaction [6]. However, uncertainties in the T-supply scenario for
future fusion DEMO reactors appear to increase continuously. The
cross-section of the 6Li(n,α)T reaction is almost six orders of magnitudelarger in the thermal neutron energy range than that of the D(n,γ)T re­action in CANDU reactors. In this study, we propose the use ofhigh-temperature gas-cooled reactor (HTGR) [7,8] for T production. In
standard HTGR designs, a large amount of neutron absorption material(e.g., 6B) is loaded into the core as a burnable poison (BP) to control
neutron criticality. In our proposed method, a Li compound, instead of aboron compound, is loaded into the reactor core as a BP.

High-temperature gas-cooled reactors (HTGRs) offer several advan­
tages. The main core structure comprising an HTGR is graphite, which is
chemically stable and does not react with the Li compound. Typically,
the BP in an HTGR is inserted separately from the fuel component in thesolid state (i.e., as B4C). Therefore, B4C can be easily replaced with the Licompound, and both electricity and T can be efficiently and simulta­
neously produced without significantly changing the original structuraldesign of the HTGR. Although the large core size of an HTGR may not be
attractive from an economic viewpoint, it provides sufficient space forloading the Li compound without 6Li enrichment along with materials toprevent T leakage.
At this stage, we aim to produce T to be compatible with electricity
production and heat sources for hydrogen production using an HTGR.
Hence, the stable confinement of T in a Li-loading rod during the HTGR
operation period is a crucial issue. Herein, we investigated a Li-loading* Corresponding author.
E-mail address: matsuura@kyudai.jp (H. Matsuura).
https://doi.org/10.1016/j.fusengdes.2023.114054
Received 7 October 2022; Received in revised form 7 October 2023; Accepted 28 October 2023
0920-3796/© 2023 Elsevier B.V. All rights reserved.

H. Matsuura et al.

Fusion Engineering and Design 197 (2023) 114054

rod structure for containing T using dense-quality Al2O3. If the HTGR is
to be operated in a higher temperature range (i.e., rod temperaturereaching ⁓900 ◦ C) to improve the electricity generation efficiency andto obtain heat source for hydrogen production, the T permeability of the
Al2O3 layer in the Li rod would increase [9,10] along with the T leakage.
We reduced the T outflow during this operation by attempting to adopt
Zr in the rod as a T absorption material to avoid an increase in the inner
T partial pressure [9–14]. Coating with an antioxidant material isnecessary to prevent the tritium absorption capability from decreasing
owing to oxidization via interactions with coexisting oxides (i.e., LiAlO2
and Al2O3). Fig. 1 illustrates an example of a Li-loading rod, including Zr
spheres with a Ni coating, for a 300-MWe gas-turbine high-temperature
reactor (GTHTR300) [15]. We are trying to contain the tritium produced
inside the Li-loading rod as much as possible; however, the outflow
tritium from the Li-loading rod can be recovered from He gas using atechnique similar to that of the fusion blanket system.
We experimentally measured the basic hydrogen absorption prop­
erties of a Zr sphere with a Ni coating under conditions similar to thoseof HTGRs. The T outflow from the Li-loading rod to the helium coolant
gas was estimated by assuming that GTHTR300 operates at a nominal
capacity. A prototype test module was used for the initial irradiation testof the Li-loading rod. The current typical specifications of the test
module and irradiation test outline are reported.

Fig. 2. Photos of (a) the Zr spheres with Ni coating in the quartz tube, (b) SEM
image of the Zr sphere with Ni coating, and (c) the cross-section.

(b) quartz containers and (c) the image photograph of the Al2O3
container, including the Ni-coated Zr spheres and the LiAlO2 powder.
Fig. 4 shows a schematic diagram of the experimental apparatus. An
Al2O3 container was introduced in the center region of a quartz tube
heated to the preset temperature of 600–900 ◦ C. These temperatures
were kept constant throughout the measurements. Hydrogen gas flowed
into the quartz tube after the evacuation of the interior. Pressure dec­
rements caused by absorption by the Zr spheres with or without the Ni
coating were continuously observed.
2.2. Structure of the irradiation test moduleThis section introduces an example of the test module currently
considered for use in the irradiation test to examine the T production
and containment properties. Fig. 5 depicts the Al2O3 container con­taining LiAlO2 powder and Zr spheres with Ni coating. In the irradiation
test, a Ti-0.05w%Co wire are included in the test module to specify theneutron flux. The Al2O3 container is further contained in a quartz
container (Fig. 3(b)) along with the Zr spheres with the Ni coating. Zr
spheres outside the Al2O3 container are used to measure the amount of T
exiting the Al2O3 container. It is assumed that the amount of T producedin the irradiation test was contained in a quartz container.

2. Experimental apparatus and analysis model
2.1. Hydrogen absorption experiment
Experiments were performed to evaluate the hydrogen absorption
properties of Zr spheres coated with Ni. Fig. 2 presents photographs of(a) the whole appearance of tens of Ni-coated Zr spheres in a quartz tube,(b) a scanning electron microscopy (SEM) image of a Ni-coated Zr
sphere, and (c) an SEM image of the cross-section. The Ni-coated Zr
sphere had approximately 600-μm diameter and was covered withapproximately 5 μm of Ni layer. In the experiments, Zr spheres with orwithout a Ni coating were placed in Al2O3 or quartz containers alongwith LiAlO2 powder. Fig. 3 displays the photographs of the (a) Al2O3 and2.3. Analysis modelTwo types of analysis models are presented in this section: (A) the
first model is used for the experiment to obtain the hydrogen absorption
properties of the Zr spheres with Ni coating (Fig. 2), and (B) the second
one is for the irradiation test using the test module explained in Section
2.2. In experiment (A), the pressure in the quartz tube (Fig. 4) was
expressed as follows:
dp(t)
dnZr
H (t) RT=−,
dt
dt 2Vc(1)where p(t) and nZr
H (t) represent the hydrogen pressure and the molnumber, respectively, of the hydrogen atoms absorbed into the Zr
sphere; dnZr
H (t)/dt is the hydrogen absorption rate into the Zr sphere withor without the Ni coating; and R, T, and Vc are the gas constant, thetemperature averaged in the hydrogen existing space, and the total
volume of the quartz tube, gas tank, and flexible tubes, respectively. We
obtained the hydrogen absorption rate by simultaneously solving thefollowing diffusion equation for the Zr sphere with a Ni coating:
(
)
∂cZr
1 ∂
∂cZr (t, r)
H (t, r)= DZr 2
r2 H
(2)
r ∂r
∂t
∂r
with the initial and boundary (based on Sieverts’ law) conditions of

1/2
Zr. Here, cZr
cZr
H (0, r) = 0 land cH (t, r = routside ) = SZr p(t)H is the H con­centration, SZr and DZr represent the H soluvility and diffusivity in the Zr
sphere. The mol number of the hydrogen atoms existing in the Zr sphereat t (i.e., nZr
H (t)) was estimated as
∫
3
nZr
cZr
(3)
(t)
=
N
Zr
H
H (t, r)d r,
VZr

where VZr and NZr denote the volume and number of Zr spheres in theFig. 1. Schematic view of the Li-loading rod example.
2

H. Matsuura et al.

Fusion Engineering and Design 197 (2023) 114054

Fig. 3. Photos of the (a) Al2O3 and (b) quartz containers and (c) view from the top, including Zr spheres with Ni coating and LiAlO2 powder.

Fig. 4. Illustration of the experimental device.
Fig. 6. Conceptual diagram of the T diffusion model in an irradiation test
module. The orange curve corresponds to the T concentration.

dpin (t)=dt(dpout (t)=dtZr(in)

ST −

dnT

dt

(t)

−

⃒ )
2 O3
dnAl
(t)
dcAl2 O3 (t, r)⃒⃒RT
T,
+DAl2 O3 A T
⃒
dt
dr
2Vin
BD

⃒
(
)
Zr(out)dcAl2 O3 (t, r)⃒⃒
dnT
(t)
RT
− DAl2 O3 A T
−
,
⃒
dr
dt
2V
out
BD

(4)
(5)

where pin(t) and pout(t) represent the pressures inside and outside theZr(in)

Zr(out)

2 O3
2 O3
Al2O3 container (Fig. 5), respectively, and nT , nT
, nAl
, cAl
,A
T
T
denote the mol numbers of T inside/outside the Zr sphere and the Al2O3
container and the T concentration in the Al2O3 layer and surface area of
the Al2O3 container, respectively. The mol numbers were estimated in amanner similar to that shown in Eq. (3). DAl2 O3 represents the diffusivityof T in Al2O3, while Vin and Vout represent the volumes inside andoutside the Al2O3 container, respectively. The T inlet speed (ST) corre­
sponds to the T production rate in the test module and is evaluated asfollows:∫ ∫1
ST (t) =Σn− Li (t, →r , E)d3 rdE,
r , E)ϕ(t, →(6)NAFig. 5. Schematic view of the Al2O3 container containing the Zr spheres with Ni
coating and LiAlO2 powder with the Ti-0.05w%Co wire as a neutron
flux monitor.

Li rodwhere Σn− Li represents the 6Li(n, α)T macroscopic cross-section, and ϕ isa neutron flux in the Li-loading region. In the Al2O3 container, thediffusion equation for the Al2O3 cylinder should be solved simulta­
neously to estimate the amount of T permeance from inside to outside,( Al2 O3
)
2 O3
∂cAl
1 ∂
∂c
(t, r)(t, r)T= DAl2 O3
r T
(7)
r ∂r
∂t
∂r

quartz tube, respectively.
In Experiment (B), we considered the pressure balances in two re­gions (i.e., inside and outside the Al2O3 container). Fig. 6 shows aconceptual diagram of the T-diffusion model in the irradiation module.
The orange curve corresponds to the T concentration. T produced in
LiAlO2 quickly diffused in the hollow portion. We assumed that the Tpressure in the voids (15 % of porosity) of the LiAlO2 and the hollowportion layers reached the same value. Some parts of the T penetrated
the Al2O3 container and flowed out toward the outside region of the
Al2O3 container. The T that flowed into the outside region was absorbedby the Zr spheres placed outside the Al2O3 container.
The pressure balances inside and outside of the Al2O3 container are
respectively written as

2 O3
2 O3
with the initial and boundary conditions of cAl(0, r) = 0, cAl(t, r =
TT

2 O3(t, r = routside ) = SAl2 O3 pout(t)1/2.
rinside ) = SAl2 O3 pin(t)1/2, and cAlT
Hydrogen permeated through the Al2O3 tube in atomic form. The
hydrogen concentration was proportional to the square root of the input
hydrogen pressure [11]. Hence, we determined the boundary conditionof Eq. (7) at the inner and outer surfaces of the Al2O3 container based on

3

H. Matsuura et al.

Fusion Engineering and Design 197 (2023) 114054

Sieverts’ law. The diffusivity, DAl2 O3 , and solubility, SAl2 O3 , coefficients of
Al2O3 for the hydrogen isotope were obtained from the work of
Katayama [11].
3. Results and discussion
3.1. Measurement of the basic hydrogen absorption properties of Zr
spheres with Ni coating
Fig. 7 illustrates the time variation of the pressure in the quartz tube
(Fig. 4) caused by hydrogen absorption by the Zr spheres with/without
the Ni coating at several temperatures. In this experiment, Zr sphereswith or without Ni coating were preset in the quartz tube. The dottedlines represent the experiment without the Ni coating, whereas the solidlines denote the experiment with the Ni coating. Error bars represent thestandard deviation. At 900 ◦ C, the hydrogen absorption property did not
significantly depend on whether there was Ni coating. However, if the Ni
coating is not adopted, the hydrogen absorption capability of the Zr
sphere without Ni coating will rapidly deteriorate as the temperature
decreases. At 600 ◦ C, no hydrogen absorption was observed for theuncoated Zr sphere. The result could be understood that the role of Ni as
an antioxidant material would be reduced as hydrogen’s diffusion co­efficient increases in the high-temperature range. In addition, the dif­ference between with and without Ni coating becomes small because thecapacity of absorption is also reduced due to decrement in the solubility.
Let us now look at the hydrogen absorption properties when the Zr
spheres coexist with oxides (i.e., LiAlO2 and Al2O3). Fig. 8 represents thetime variation of the pressure in the quartz tube after hydrogen wasinserted into the quartz tube at 900 ◦ C. In these experiments, Zr sphereswith or without a Ni coating were included in the quartz tube along with
LiAlO2 and Al2O3. Zr spheres were first added to an Al2O3 container
containing LiAlO2 powder. The Al2O3 container was placed in a quartz
tube. The mass ratio of LiAlO2 powder to Zr spheres in the current
experiment when 50 mg LiAlO2 powder was assumed to be approxi­
mately 20 times greater than that assumed in the GTHTR300 operation.
When the Zr sphere was coated with a Ni layer, high hydrogen ab­sorption ability was still observed, even if the Zr spheres with the Ni
coating were contained in the Al2O3 container along with 5–50 mg of
LiAlO2 powder. In contrast, the uncoated Zr spheres exhibited a large
variation in their hydrogen absorption capabilities. A Ni coating is stillrequired when Zr coexists with oxides, even though the diffusion
property of hydrogen increases, and the solubility decreases in the hightemperature range. The Zr spheres could absorb hydrogen somewhat
more stably because of the role of the antioxidant material compared tothe case without the Ni coating.
We compared the experimental data with the simulation data Eqs.

Fig. 8. Temporal behavior of the H2 pressure in the cylindrical quartz tube
(Fig. 4) at 900 ◦ C. An Al2O3 container, in which 30 Zr spheres with (solid line)/
without (dotted line) Ni coating and 5, 10, and 50 mg LiAlO2 powder are
included, is set in the quartz tube.

(1)–((3)) to determine apparent diffusivity and solubility, respectively.
Fig. 9 shows the results obtained at several temperatures. The solid lines
denote the simulation results, and the dotted line depicts the fitting of
the experimental data. The blank squares represent the experimental
data. In this experiment, 30 Zr spheres were placed in an Al2O3 container
along with 50 mg LiAlO2 powder. The absorption capacity of hydrogen
was well maintained at a temperature higher than 800 ◦ C, regardless ofthe temperature. At 600 ◦ C, the hydrogen absorption performanceclearly changed compared with that in the experiment involving atemperature higher than 800 ◦ C. However, no significant deterioration
in solubility was observed. At 600 ◦ C, the pressure reached an equilib­
rium state of approximately 240 Pa. This occurred approximately 25 hafter hydrogen insertion. At 800 and 900 ◦ C, the numerical simulations
provided good explanations for the pressure variation. In the simulation,
we chose a diffusivity of D = 2.4 (0.54) × 10− 11 m2/s and a solubility ofS = 244 (556) mol/m3/Pa1/2 to reproduce the measured data for 900(800) ◦ C temperature. In a cooperative project, decrease in diffusivity
has been experimentally examined using a Ni-coated Ti sphere [16].
This was then measured such that the diffusivity was determined fromthe Ni property and reduced by ⁓1/2000 [16] compared with previ­
ously reported values [17–20]. The diffusivity and solubility values
obtained here seem to be within a consistent range compared to previ­
ously reported data [13,14,17–20].

Fig. 9. Comparison of the experiment and the simulation for the temporal
behavior of the H2 pressure in the cylindrical quartz tube (Fig. 4) for several
temperatures. An Al2O3 container, in which 30 Zr spheres with Ni coating and
50 mg LiAlO2 powder are included, is set in the quartz tube. The dotted line
represents the fitting curve of the experimental data.

Fig. 7. Temporal behavior of the H2 pressure in the cylindrical quartz tube
(Fig. 4) for several temperatures. Only 30 Zr spheres with (solid line)/without(dotted line) Ni coating are set in the quartz tube.
4H. Matsuura et al.

Fusion Engineering and Design 197 (2023) 114054

The hydrogen absorption capability of Ni-coated Zr may deteriorate
when it coexists with Al2O3 and LiAlO2 oxides for a longer period. We
next performed a similar measurement as Fig. 9, after leaving the Nicoated Zr in an Al2O3 container for 3 days with 50 mg LiAlO2 powder
keeping 900, 800, or 600 ◦ C temperatures. The results are shown in
Fig. 10 along with the simulations. The solid lines denote the simulation
results, whereas the dotted line depicts a fitting of the experimental
data. The blank squares represent the experimental data. In the simu­
lation, we chose a diffusivity of D = 9.2 (0.5) × 10− 13 m2/s and a sol­ubility of S = 114 (588) mol/m3/Pa1/2 to reproduce the measured datafor 900 (800) ◦ C temperature. For both 900 and 800 ◦ C temperatures,
because of changes in the surface condition due to oxidization, etc., thediffusivity was decreased by approximately 2 orders. However, thisdecrease in solubility was not significant. At 600 ◦ C, the pressure
reached an equilibrium state at approximately 300 Pa. This occurred
approximately 30 h after hydrogen insertion.

LiAlO2 was adjusted such that the produced T becomes approximately
600 g after a 360-day operation. The adjustment was performed bychanging the filling fraction of the LiAlO2 pebbles (with 15 % porosity)in the rod to 17.4 %. The amount of T produced along with the neutron
multiplication factor during the 360-day GTHTR300 operation was
estimated using the continuous-energy Monte Carlo transport code,
MVP-BURN [23], which assumed the entire core model. Nuclear data
were taken from JENDL-4.0 [24]. The fuel region temperature was1100 ◦ C, and the Li rod temperature was the same as that of themoderator (900 ◦ C). All the control rods were assumed to be pulled out.
The neutron multiplication factor was 1.06 after the 360-day operation.
The statistical errors of the effective multiplication factor and reaction
rate were less than 0.1 %, which is sufficiently accurate for our
discussion.
The loaded LiAlO2 in the initial irradiation test was determined in thesame manner by considering the regulation values in a hot laboratory toanalyze the test module after irradiation. The amount of T produced was
suppressed to a level lower than that produced in standard HTTR
operation [25]. The mass of T absorbed by the Zr spheres in a test
module after irradiation was estimated by simultaneously solving Eqs.
(2)–(7), using the diffusivity and solubility shown in Fig. 10. For thesimulation of the irradiation test, the quartz container had a height anddiameter of 30 and 14 mm, respectively, with a thickness of 2 mm; thus,
Vout was assumed to be 1.3 × 10− 6 m3. The 40 (2) Ni-coated Zr sphereswith 600 μm diameter were included outside (inside) of the Al2O3
container. In the simulation temperature of the irradiation test module
was assumed as 800 ◦ C, and the irradiation time was 90 days. Table 2
summarizes the results along with the loaded masses of the LiAlO2 andZr spheres.

3.2. Estimation of the typical T outflow to the He coolant in GTHTR300
and HTTR
The amount of T outflow from the Li rod to the He coolant during a
360-day GTHTR300 operation was estimated based on the obtainedhydrogen absorption data of the Zr sphere with Ni coating. The typicalstructure of the Li-loading test module is also presented.
The GTHTR300 core comprises 90 fuel columns arranged in anannular ring. Each fuel column comprises eight vertically piled hexag­
onal graphite blocks. The hexagonal fuel block was 407 mm wide acrossthe flats, including 1-mm gaps on both sides and 1000 mm high, witharrays of 57 fuel channels and three BP insertion holes. A Li rod wasinserted into the BP insertion holes, creating 2160 Li rods loaded into thewhole core. The optimization of the efficient production of T is currently
being studied.
To evaluate the T production and containment properties, we plan to
conduct an irradiation test using an irradiation fission reactor (e.g.,
HTTR [21] and/or Japan Research Reactor-3 (JRR-3) [22]). In thisstudy, we introduce an example of a test module structure assuming the
HTTR (Fig. 11) and/or JRR-3. An irradiation column was prepared, andLi compound loading was permitted in the HTTR. At this point, weconsidered almost the same test module structures for the HTTR and
JRR-3 as an initial experiment. Table 1 presents the basic structure of thetest module and irradiation conditions.
To estimate the T produced in the GTHTR300, the loaded amount of

4. Concluding remarks
In this study, we measured the hydrogen absorption properties of Nicoated Zr spheres. The ability of the hydrogen absorption of the Zr
sphere w/o Ni coating significantly deteriorated at a temperature rangeof less than ~600 ◦ C. At ~900 ◦ C temperature range, by adopting the Ni
coating as an antioxidant material, the hydrogen absorption ability is
continuously maintained in the available range, even if Zr coexists withoxides (i.e., LiAlO2 and Al2O3). In a typical GTHTR300 operation shownin Tables 1 and 2, we attempt to absorb 100~150 μg T per 1 g of Zr
spheres; the data shown in Fig. 10 at 900 ◦ C shows several times greater
T absorption ability per unit Zr mass.
The T containment properties of the typical Li-loading module were
evaluated based on the apparent diffusivity and solubility obtained byassuming (A) T production in GTHTR300 and (B) the initial irradiation
test using the HTTR. In GTHTR300, the T outflow from the Li-loadingrod could be suppressed by less than 1 % of the produced T. A proto­
type test module was produced and reported for the irradiation test.
The hydrogen absorption properties were measured within a few
days at the experimental scale. In long-term operation, the dissolution of
the coated Ni into Zr may be problematic, depending on the Zr sphereradius and Ni layer thickness [26]. Subsequent long-term hydrogen
absorption experiments are currently underway. The effect of alloying
on the hydrogen absorption capability and its characteristics should beclarified. Further improvements may be necessary considering theirradiation test results. We investigated the T containment in a Li rod. As
an alternative, a continuous T recovery system containing He gas might
be considered in future research [27]. The performance of the HTGR for
T production, for example, the criticality performance and/or T pro­duction rate, is affected by how to load and design the Li-loading rod, as
well as the choice of the Li compound [7,10,27]; thus, a continuous
discussion of this point is necessary.
In our experiments, hydrogen was used instead of tritium. Since
diffusion properties of hydrogen in Zr and Zr alloys are strongly influ­
enced by crystallographic microstructures and additive elements [28],well-known isotopic effects could be neglected within the experimental

Fig. 10. Temporal behavior of the H2 pressure in the cylindrical quartz tube
(Fig. 4) after leaving the Ni coated Zr in an Al2O3 container for 3 days with 50
mg LiAlO2 powder at 900 or 800 ◦ C. Comparison of the experiment with thesimulation was made. An Al2O3 container, in which 30 Zr spheres with Ni
coating and 50 mg LiAlO2 powder are included, is set in the quartz tube. The
dotted line represents the fitting curve of the experimental data.
5H. Matsuura et al.

Fusion Engineering and Design 197 (2023) 114054

Fig. 11. (a) Horizontal cross-section of the HTTR core and the irradiation columns (green three hexagons). (b) Fuel block with three BP (Li rod insertion) holes.

the work reported in this paper.

Table 1
Design parameters of the Li-loading rods for GTHTR300 and HTTR.

Electric output [MW]Thermal output [MW]Temperature [ ◦ C]
Neutron flux [cm− 2s− 1]
Operation (irradiation) time[days]Height of the Li rod (Al2O3
container) [mm]Outer radius of the Li rod (Al2O3
container) [mm]Inner radius of the Li rod (Al2O3
container) [mm]T production(GTHTR300)

Initial irradiation test
(HTTR)

300
600
900
~1014
360

–
30
600–900~6.0 × 1013
30–90

950

14

22

4

15.6

2.5

Data availability
Data will be made available on request.
Acknowledgments
This work was supported by JSPS KAKENHI Grant-in-Aid for Scien­tific Research (B) JP18H01200, JP21H01065 and cooperative programwith BA (The Joint Implementation of the Broader Approach Activities
in the Field of Fusion Energy Research) reactor design team in Japan.
The authors thank the staffs of the ultramicroscopy research center at
Kyushu University for helpful assists in the observation and preparationof samples.
References

Table 2Masses of the initial loading of the 6Li and Zr spheres and T produced and flowed
out in the initial irradiation test and the T production in GTHTR300.
T production(GTHTR300)

Initial irradiation test
(HTTR)

Mass of the loaded 6Li
Mass of the loaded Zr spheres
Mass of the T production
Mass of T absorbed by the Zr
spheres

4.2 kg4.7 t
0.60 kg
0.60 kg

21 μg
30 mg
1.1 μg0.93 μg (inside)

Mass of the T outflow into the
He gas

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errors in practical cases. We have assumed T is produced by 6Li(n,α)T
reaction in the chemical form of T2, not HTO. Leakage through the Al2O3
layer estimated may be overestimated to some extent. When tritium is
produced in HTGRs, all materials are exposed to intensive radiationfrom gamma rays and neutrons. In such cases, tritium diffusion may be
regarded owing to the tritium trapping effect at the radiation defects[29]. On the other hand, enhancement of hydrogen diffusion by radia­
tion for oxide. including Al2O3 has been reported [30]. Oxides are po­tential candidates for permeation barriers, and radiation-induced
diffusion may deteriorate the barrier effect [31]. At this point, themechanism of deterioration has not been fully understood, and an
irradiation test for the Li rod is necessary.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence6H. Matsuura et al.

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