Fusion Engineering and Design 169 (2021) 112441

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

The T-containment properties of a Zr-containing Li rod in ahigh-temperature gas-cooled reactor as a T production device forfusion reactors
Hideaki Matsuura a, *, Takuro Suganuma a, Yuki Koga a, Motomasa Naoi a, Kazunari Katayama b,
Teppei Otsuka c, Minoru Goto d, Shigeaki Nakagawa d, Shinpei Hamamoto d, Etsuo Ishitsuka d,Kenji Tobita e, Satoshi Konishi f, Ryoji Hiwatari g, Youji Someya g, Yoshiteru Sakamoto gaDept. Applied Quantum Phys. Nucl. Eng., Kyushu Univ., 744 Motooka, Fukuoka 819-0395, Japan
Dept. Advanced Energy Eng. Sci., Kyushu Univ., 6-1 Kasugakouen, Kasuga City, Fukuoka 816-0811, Japanc
Dept. Electrical and Electronic Eng., Kindai Univ., 3-4-1 Kowakae, Higashi-Osaka City, Osaka 577-0818, Japand
Japan Atomic Energy Agency, 4002 Narita-cho, Oarai-machi, Higashi-Ibaraki-gun, Ibaraki 311-1393, Japane
Dept. Quantum Sci. Energy Eng., Tohoku University, 6-6-01-2 Aoba, Sendai City, Miyagi 980-8579, Japanf
Institute for Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japang
National Institutes for Quantum and Radiological Sci. Tech., 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
Nickel coating
Lithium-Loading rod
HTTR
GTHTR300
DEMO fusion reactor

The production of tritium (T) using high-temperature gas-cooled reactors (HTGRs) has been studied for a priorengineering research with T handling and initial T possession in demonstration fusion reactors. Stablecontainment of T in Li-loading rods during HTGR operation is a critical issue. This study investigates this for anirradiation test to examine T-containment performance in Li-loading rods and develops an analytical model of
evaluating the amount of T outflow to a He coolant. The hydrogen absorption characteristics, including thedeterioration of the hydrogen absorption speed after Zr has sufficiently absorbed the hydrogen, is experimentallymeasured assuming an HTGR setting. We present an analytical model of evaluating the T outflow from a Li rod
and, on the basis of this model, estimate the total T outflow, assuming the presence of a gas-turbine high-tem­
perature reactor of 300 MWe with a nominal capacity and a high-temperature engineering test reactor. It isdemonstrated that, by loading a sufficient amount of Zr into the Li rod, the T outflow can be suppressed to lessthan a small percent of the total T produced during 360 days of reactor operation.

1. Introduction

compound as a burnable poison (BP) instead of a boron compound. The
main core structure in an HTGR is graphite, which is chemically stableand does not react with the Li compound. Although the large core size ofan HTGR is not attractive from an economic viewpoint, it provides
enough space for loading the Li compound without 6Li enrichment,along with structural materials to prevent leakage of T. In an HTGR, theBP is typically inserted in a solid state (i.e., as B4C), separately from thefuel component; therefore, the Li compound can be loaded into the re­actor’s core without significantly changing the original structuraldesign. This treatment is efficient in terms of both (1) the increment ofavailable BP for T production and (2) T recovery. The nuclear charac­
teristics and fuel temperature condition were analyzed to confirm thenuclear and thermal feasibility of a lithium (Li)-loaded HTGR [4], and it

In technical tests of tritium (T) circulation and initial T possession ina demonstration (DEMO) fusion reactor, it is important to prepare the Tsupply method using an outside source [1]. The T has often been pro­
duced by means of a D(n,γ)T reaction in Canadian Deuterium Uranium(CANDU) reactors [2]; however, uncertainties in the T-supply scenario
in the case of DEMO reactors seem to be continuing to increase. Because
the cross-section of the 6Li(n,α)T reaction is almost six orders ofmagnitude larger across the thermal-neutron energy range comparedwith the D(n,γ)T reaction in CANDU reactors, and high-temperature
gas-cooled reactors (HTGRs) have several advantages (outlined
below), we use an HTGR [3] to analyze T production by inserting a Li

* Corresponding author.
E-mail address: matsuura@kyudai.jp (H. Matsuura).
https://doi.org/10.1016/j.fusengdes.2021.112441
Received 1 November 2020; Received in revised form 14 February 2021; Accepted 1 March 2021
0920-3796/© 2021 Elsevier B.V. All rights reserved.

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Fusion Engineering and Design 169 (2021) 112441

is demonstrated that the analysis results satisfy the design requirements.
Therefore, the nuclear and thermal feasibility of a Li-loaded HTGR that
produces thermal energy and T was confirmed [4].
At this stage, we are aiming for compatibility between electricity and
T production in an HTGR. We currently consider that the amount of T
flowing out from Li rods to form helium gas should be suppressed to alower level during operating periods. We investigate the Li-loading rod
structure to contain T using dense quality Al2O3. Numerical simulations
have predicted that if we can operate the HTGR in a low-temperature
range, keeping the rod temperature below 500 ◦ C, the leakage of T
from the Li rod with the Al2O3 layer can be suppressed to less than 1% of
the total amount produced [5]. However, if we intended to operate the
HTGR in a much higher temperature range (i.e., the rod temperaturereaching ~900 ◦ C) to improve the electricity generation efficiency, the T
permeability of the Al2O3 layer in the Li rod would increase [6], and theleakage of the T would rapidly increase. To reduce the outflow of the T
during such an operation, we attempt to adopt Zr in the rod as a T ab­sorption material to avoid increasing the inner T partial pressure [7–11].
The T pressure in the Li rod during HTGR operation could be determinedby the pressure balance between the production rate, absorption by Zr,
and outflow rates from the rod. The T production rate can be estimatedusing the neutron flux in the reactor core, which is evaluated by theneutron transport and nuclear burning analyses; the T absorption isgoverned by the properties of the Zr. The T outflow rate can be estimatedfrom diffusion theory with respect to the Al2O3 layer, which depends on
the T pressure itself. If we could keep the T pressure at a lower level
during the reactor operation period, the T outflow would also be sup­pressed to a lower level. It is well known that the T absorption propertyof Zr depends on the preabsorbed (contained) T state in the Zr. This
property is also affected by the oxidation of the surface area. It is
important to specify the deterioration of the hydrogen absorption rate in
Zr under the HTGR conditions during operation. For development of an
irradiation test in the HTGR, a tentative analysis model is required.
In this study, we experimentally measure the basic hydrogen ab­sorption properties of Zr in an HTGR setting and estimate the T outflow
ratio from the Li-loading rod to the He coolant, assuming the 300 MWe
gas-turbine high-temperature reactor (GTHTR300) and hightemperature engineering test reactor (HTTR) to be operating at nomi­
nal capacity [12,13].

Fig. 1. Illustration of the experimental apparatus.

termination with hydrogen absorption by Zr were continuously
observed.
2.2. Neutron transport and nuclear burning simulation for GTHTR300
and HTTR cores
The estimations of the amount of produced T (production rate) andthe effective multiplication factor during the GTHTR300 and HTTR
operations were made using the continuous-energy Monte Carlo trans­
port code, MVP-BURN [14], which used the entire core model.
Throughout the calculations, nuclear data were taken from JENDL-4.0[15]. The fuel region temperature was 1,100 ◦ C, and the Li rod tem­
perature was the same as that of the moderator (900 ◦ C).
Fig. 2(a) presents a horizontal cross-section of a gas-turbine hightemperature reactor with a 300 MWe nominal capacity (GTHTR300)
core [12]. The core consists of 90 fuel columns arranged in an annularring; 55 inner and 36 outer removable reflector columns; 18 inner and
12 outer control rod guiding columns; and 8 outer fixed reflector sectors.
Each fuel column was composed of eight hexagonal graphite blocks,
which were piled up vertically. The hexagonal fuel block was 407 mm
wide across the flats (including 1 mm gaps on both sides) and 1,000 mm
high, with arrays of 57 fuel channels and 3 BP insertion holes (see Fig. 2(b)). The fuel rod placed in the fuel channel was enclosed in a graphite
block and comprised fuel compacts and a graphite sleeve of a 26 mm
diameter. The diameter of each fuel channel in the fuel block was 39mm, and helium gas flowed through the space between the walls of thefuel channel and a fuel rod as a coolant.
The horizontal cross-section of the HTTR core is also illustrated inFig. 3(a) [13]. The core consists of two regions, namely the actual coreand reflector regions. The actual core consisted of 30 fuel and 7 control
rod guide columns, each of which was composed of a stack of 5 fuel and4 reflector blocks. The actual core was surrounded by the reflector re­gion, i.e., replaceable and permanent reflector blocks and 9 control rod
guide blocks. The core was 2.9 m in height and 2.3 m in diameter. Eachhexagonal block was 360 mm wide across the flats and 580 mm high[see Fig. 3(b)]. The fuel block contained 31 or 33 fuel channels and 3 BP
insertion holes. Li rods that were 480 mm high with 15 mm Al2O3 caps
on both the upper and lower sides were loaded into the BP insertion
holes.
In this study, we assumed a 360-day reactor operation period. All
control rods were assumed to be pulled out. The time steps in the burnup simulations were taken as 0, 1, 5, 30, 60, 120, 180, and 360 days. For
each of the time steps, 6 million neutrons were generated. The statisticalerrors of the effective multiplication factor and reaction rate were lessthan 0.1 % in all calculations, which is sufficiently accurate for our
discussion.

2. Experimental apparatus and analysis model
2.1. Hydrogen absorption experimentHydrogen absorption experiments were carried out using a Zr cyl­
inder (see Table 1) with a diameter of 15.8 mm and a height of 15 mm (1mm thickness). A schematic illustration of the experimental apparatus ispresented in Fig. 1. A Zr cylinder was introduced in the center region of aquartz tube, which was heated to the preset temperature of 900 ◦ C; thistemperature was kept constant during the measurement procedures. To
control the hydrogen inlet speed, a mass-flow controller was introduced.
After the inside of the quartz tube had been evacuated, hydrogen gas
flowed into the tube controlling the inlet speed. After the inner hydrogenpressure reached the target values, the hydrogen inflow was halted. The
pressure increments during hydrogen inflow and reduction after

Table 1
Properties of Zr.

2.3. Analysis model of T outflow to the He coolant

Impurity
C (wt%)
FeCr (wt%)
H (wt%)N (wt%)
O (wt%)
Zr+Hf (wt%)

0.015
0.083
<0.0003
0.006
0.143
99.5

In the experiment, the pressure of hydrogen in the quartz tube (seeFig. 1) was determined using the following pressure balance equation:
(
)
dp(t)
dnZrC (t) RT= S0 −
(1)
dt
dt
2Vc
2

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Fusion Engineering and Design 169 (2021) 112441

Fig. 2. (a) Horizontal cross-section of the GTHTR300 core; (b) fuel block with BP (Li rod insertion) holes.

Fig. 3. (a) Horizontal cross-section of the HTTR core; (b) fuel block with BP (Li rod insertion) holes.

where p(t) and nZrC(t) represent the hydrogen pressure and the numberof hydrogen atoms absorbed into the Zr cylinder, respectively, dnZrC/dt
is the hydrogen absorption rate into the Zr cylinder, and S0 the hydrogen
inlet rate, which is controlled by the mass-flow controller (see Fig. 1). R,
T, and Vc are the gas constant, temperature, and total volume of thequartz tube, gas tank, and flexible tubes, respectively. Here, to obtain
the hydrogen absorption rate, we simultaneously solve the following
diffusion equation for the Zr cylinder:()∂cZrC (t, r)
1 ∂∂cZrC (t, r)
= DZr
r
(2)
r ∂r
∂t
∂r

(

)

∂cAl (t, r)
1 ∂∂cAl (t, r)
= DAl
r
r ∂r
∂t
∂r

(5)

with the initial and boundary conditions, cAl(0,r) = 0, cAl(t,r = rinside) =SAlp(t)1/2, and cAl(t,r = routside) = 0. It has already been confirmed thathydrogen permeates through the Al2O3 tube in an atomic form (i.e., thehydrogen concentration is proportional to the square root of an input
hydrogen pressure [6]), so we determined the boundary condition of Eq.
(5) at the inner surface of the Al2O3 tube on the basis of Sieverts’ law.
The number of T atoms absorbed in the Al2O3 at time t, i.e., nAl(t), can be
estimated as∫(6)nAl (t) = cAl (t, r)d3 r

with the initial and boundary (on the basis of Sieverts’ law) conditions,cZrC(0,x) = 0 and cZrC(t,x = xboudary) = SZrp(t)1/2. The number ofhydrogen atoms at t absorbed into Zr, i.e., nZrc(t), can be estimated by∫
nZrC (t) =cZrC (t, r)d3 r
(3)

VAl

where VAl is the volume of the cylindrical Al2O3 layer. The outflow of T
from the outer surface of the Al2O3 cylinder into the He coolant wasestimated from a nonequilibrium solution of the above T diffusion
equation for the Al2O3 cylinder as follows:⃒∂cAl ⃒⃒
J = − AAl DAl(7)∂r ⃒boundary

VZrC

where VZrC is the volume of the cylindrical Zr absorber.
It should be noted that Eq. (1) can be extended to express the pres­sure balance in the Li-loading rod in the HTGR core. In such a case, the T
inlet speed (S0) corresponds to the T production rate in a Li-loading rod,which can be evaluated as∫ ∫Σ n− Li (→r , E)ϕ(t, →r , E)d3 rdE(4)
S0 (t) =where AAl is the outer surface area of the Al2O3 cylinder. The diffusivity
DAl and solubility SAl coefficients of Al2O3 for a hydrogen isotope weredrawn from the work of Katayama [6].
To estimate the T absorption rate, Eq. (2) is also solved at the sametime. When we consider the T absorption by the Zr pebbles included in
the center portion of the Li-loading rod, we can simultaneously solve thediffusion equation for the Zr pebbles in the spherical coordinate:()∂cZrS (t, r)
1 ∂∂cZrS (t, r)= DZr 2
r2
(8)
r ∂r
∂t
∂r

Lirod

where Σ n− Li represents the 6Li(n,α)T macroscopic cross-section, and ϕ isa neutron flux in the Li-loading region, which is evaluated by the Monte
Carlo simulation using the MVP, as described in the previous section, so
as to satisfy the critical condition of the reactor. In the reactor, the quartz
tube should also be replaced by the dense quality Al2O3 cylinder, and, toestimate the amount of T outflow to the He coolant, the diffusion
equation for the Al2O3 cylinder should be simultaneously solved:

with the initial and boundary conditions, cZrS(0,r) = 0, cZrS(t,r = routerside)= SZrp(t)1/2. In the same manner as Eq. (1), the number of T atoms in a Zr
3

H. Matsuura et al.

pebble is defined as nZrS(t):∫nZrS (t) =cZrS (t, r)d3 r

Fusion Engineering and Design 169 (2021) 112441

(9)

VZrS

loading rod. The yellow curve corresponds to the T concentration.
In this study, we ignored the function of Zr as an antipermeationmaterial (Zr only exists to absorb the T and keep the pressure at a lower
level). This treatment can estimate the T leakage on the safe (larger)
side. T produced in the LiAlO2 layer diffuses quickly in the hollow
portion, and we assume that the pressure of T in the LiAlO2 and thehollow portion layers reaches the same value (see Fig. 4). Therefore, thepressure balance in the reactor can be written as(dp(t)
dnZrC (t)dnZrS (t) dnAl (t)= S0 −− NZrS
−
dt
dt
dt
dt
+ DAl AAl)⃒
dcAl (t, r)⃒⃒RTdr ⃒boundary 2Vp

Fig. 5. Comparison between experiment and simulation. Temporal behavior of
the pressure in the cylindrical quartz tube for several hydrogen inlet speeds.

(10)

depends on the preabsorbed (contained) hydrogen status in the Zr cyl­
inder. We terminated the hydrogen inlet after the pressure had increased
to 1,600 Pa. The decrease in pressure after the termination is illustrated
in Fig. 6 for several inlet speeds. Until the pressure reached 1,600 Pa, thetime duration was almost inversely proportional to the inlet speed,which implies that almost the same number of hydrogen atoms is
involved in the Zr cylinder when the inlet is terminated, not dependingon the inlet speed. Fig. 6 illustrates the difference in hydrogen absorp­
tion speed, in spite of the number of hydrogen atoms involved in the Zr
cylinder being almost the same. This is because the gradient of thehydrogen density in the Zr cylinder differs depending on the inlet (ab­sorption) speed. The simulations explain the experiment.
The hydrogen absorption property of Zr also depends on the quantityof preabsorbed T. The temporal pressure decrease after termination of
the inlet is plotted in Fig. 7 for several different initial pressures. The
dotted lines represent the least-square fitting for an exponential func­
tion. In the experiment, the quantity of preabsorbed hydrogen grewlarge as the initial pressure increased. It was found that as the pre­absorbed hydrogen increased, the absorption speed decreased. From thefitting lines, we could estimate based on the absorption times that thepressure decreased to 1/e from the initial values, i.e., 6.75, 9.93, and12.4 s for 400, 800, and 1,600 Pa initial pressures.

where Vp is the sum of the volumes of the hollow portion (without Zr
pebbles) and porosity in the LiAlO2 layer (see Fig. 4). NZrS represents thenumber of Zr pebbles, which is determined from the volumes of both thecentral hollow portion and the Zr pebbles and their filling ratio. The
above time-dependent, non-linear differential equations, i.e., Eqs. (2)–(10), are solved simultaneously, satisfying the T mass balance in the
GTHTR300 and HTTR cores, along with the neutron transport and nu­
clear burning equations to satisfy the reactor criticality condition. The
porosities of Al2O3 and the LiAlO2 layers are assumed to be 0 and 37 %,respectively, and the filling ratio of the Zr pebbles is 54 %.
3. Results and discussion
3.1. Measurement of basic hydrogen absorption properties of Zr
Fig. 5 illustrates the time variation of the pressure in the quartz tube
for several inlet speeds (see Fig. 1). The red circles, green squares, blue
triangles, and black diamonds represent the measured data for the inlet
speed of 20, 10, 5, and 2.5 sccm, respectively. The red diamonds are themeasured data when the Zr cylinder does not exist in the quartz tube for2.5 sccm inlet speed. The error bars represent the standard deviation. In
the viewgraph, it can be seen that the fraction of hydrogen absorption toinlet one increases as the inlet speed decreases. The solid lines denote
the result of the simulations. In the simulation, we choose the diffusivity
D = 2.6 × 10− 9 m2/s and solubility S =108 mol/m3/Pa1/2 of the Zr toreproduce the measured data. The values of diffusivity and solubility are
somewhat smaller but seem to be within the consistent range comparedwith the previously reported data [16–19].
Next, we look at how the hydrogen absorption property of Zr

Fig. 6. Comparison between experiment and simulation. Temporal behavior of
the pressure in the cylindrical quartz tube after termination of the hydrogen
inlet at 1,600 Pa for several inlet speeds.

Fig. 4. Conceptual diagram of the T diffusion model in a Li-loading rod. The
yellow curve corresponds to the T concentration.
4

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Fusion Engineering and Design 169 (2021) 112441

Fig. 7. Temporal behavior of the decreased hydrogen pressure in the cylin­
drical quartz tube due to absorption by Zr after reaching 400, 800, and 1,600 Pahydrogen pressure.

3.2. Estimation of the T outflow to the He coolant in the GTHTR300 and
HTTR
In this section, we estimate the T outflow from the Li rod to the He
coolant during the 360-day operations of the GTHTR300 and HTTR usedin this study. Throughout the simulations, we assumed the Li-loading
rod structures as indicated in Table 2. When we used the porosities of
Al2O3, the LiAlO2 layers, and filling ratio of the Zr pebbles presented
above, the cumulative weight of the amount of produced T was esti­
mated to be ~610 (~30) g for GTHTR300 (HTTR). The properties of the
Li-loading rods in the GTHTR300 and HTTR are listed in Table 2.
The amount of T outflow from the Li-loading rods for GTHTR300 and
HTTR was evaluated by simultaneously solving Eqs. (2)–(10). With Eq.
(7), the weight of T flowing out from all rods was estimated. From eqs.
(3), (6), and (9), the amount of T existing in the Zr and Al2O3 layer canalso be evaluated. From the pressure p(t), the amount of T2 gas existingin a hollow portion and property of the LiAlO2 layer is obtained, in
accordance with the ideal gas equation. The weights of T existing in Zr,
Al2O3, the hollow portion, the LiAlO2 layers, and the outflow into the He
coolant are presented in Fig. 8 as functions of the reactor operation timefor (a) GHTTR300 and (b) HTTR. The temperature was assumed to be
900 ◦ C. In the simulations, the diffusivity D = 2.6 × 10− 9 m2/s andsolubility S =108 mol/m3/Pa1/2 of the Zr are assumed. The bold blacklines show the cumulative weight of T produced in the reactor M(t),which is calculated as

Fig. 8. Temporal behavior of the cumulative weights of T produced, absorbed
in the Zr and Al2O3 layers, remains as T2 gas and is flown out of the Li-loadingrods in the (a) GTHTR300 and (b) HTTR cores. The diffusivity (D = 2.6 × 10− 9m2/s) and solubility (S =108 mol/m3/Pa1/2) for Zr were adopted for thesimulations.

∫t

GTHTR300

HTTR

300
600
950
44

30
450
14

15.6
14.6
12.2

5.2
5.0
2.3

0.5

0.5

S0 (t )dt

′

(11)

o

We confirmed that the sum of weights, i.e., T in Zr, the Al2O3 layers,
the hollow portion, and outflow, agreed with M(t). If the Zr properties
measured in our experiment were retained during reactor operation, theoutflow T can be estimated at less than 0.1 % (0.5 %) for GTHTR300(HTTR) after 360 days of operation. The total production of T was 610(30) g for GTHTR300 (HTTR), so the weight of outflow T was almost 0.5(0.15) g. In standard HTGRs, T is always produced by the ternary fission
reaction of fuel particles, with reactions between neutrons and impu­
rities contained in the core graphite, e.g., 6Li, in the helium coolant; 3He,in the control rod; and 10B, even if we do not artificially load the 6Li into
the core [20]. The natural weight of T generation in GTHTR300 (HTTR)
was estimated at ~20 (0.24) g per year [21]. The evaluated T outflow
was at a lower level compared with that in the natural generation.
In the HTGR, a large core size made it possible to load a significant
amount of Zr into the Li rod. The atomic ratio T/Zr could be estimated tobe less than 0.1 at most, even after the 360-day operation. In such a case,as was shown by Zuzek [22], the capability of T absorption in Zr would
be sufficiently high to reduce the partial T pressure, as long as the ideal
condition was satisfied during operation. In the HTGR, the T absorption

Table 2
Design parameters of the Li-loading rods for the GTHTR300 and HTTR.
Electric output [MW]Thermal output [MW]Height [mm]Li rod diameter [mm]Inner radius [mm]Al2O3 layer
Zr layer (/w Ni)
LiAlO2 layer
Zr pebbles with Ni coating [mm]radius′M(t) =–

5

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Fusion Engineering and Design 169 (2021) 112441

capability would be reduced by oxidization through interaction withcoexisting oxides (i.e., LiAlO2 and Al2O3). To prevent such a deteriora­tion, coating with an antioxidant material is necessary [7]. Accordingly,
we coated the Zr with Ni. In this case, the T absorption property was
reduced. In our study of a cooperative project, the decrement was
experimentally ascertained [23], and it was predicted that the diffu­
sivity was determined from the Ni property, so the diffusivity was
reduced by ~1/2,000 from that of the previously reported values.
Yamanaka [24] reported that the relative reduction in solubility due tooxidization was restricted to 1/3–1/2 in the HTGR condition when the
O/Zr atomic ratio was within 0.25–0.398. In our experiments, the sig­
nificant deterioration in the solubility was also not observed.
To examine the degree of deterioration at which the diffusivity andsolubility induces an increment of the outflow ratio larger than 1% of thetotal T production, simulations were made that assumed a solubility of
1/100 (1/10), in addition to the deterioration of diffusivity of 1/2,000
for GTHTR300 (HTTR). In the current Li-rod design (see Table 2) the Zr/
T atomic ratio is much larger in the GTHTR300 than in the HTTR. This is
the reason why the reduction in the solubility (to reach the outflow ratio
above 1%) is different in the two devices in Fig. 9. The results are dis­played in Fig. 9 for (a) GTHTR300 and (b) HTTR. For GTHTR300, when
the diffusivity D = 1.3 × 10− 12 m2/s and solubility S =1.08 mol/m3/Pa1/
2
are assumed, the T outflow ratio reaches 1.6 % (~10 g), which is still
smaller in comparison to natural production.
4. Concluding remarks
In this paper, we described an analytical model for evaluating T
leakage from an Li rod and for developing a suitable rod structure. On
the basis of this model, T outflow from the rods was estimated using themeasured diffusivity and solubility of Zr, assuming GHTR300 and HTTR.
By utilizing the Ni-coated Zr in the Al2O3 cylinder, the T outflow could
be suppressed to less than 1% of the T produced in both reactors. If thesolubility deteriorated by 1/100 (1/10) from the measured value, in
addition to the deterioration in the diffusivity (1/2,000), the outflow
ratio would reach 1.6 (4.0)% to the weight produced ~610 (~30) g
during 360 days of operation in GTHTR300 (HTTR).
We are currently planning an irradiation test of the Li-loading rod(demonstration of T production), using HTTR, in the near future. In thefirst stage of the irradiation test we will use a smaller-size test module tosuppress the large amount of T production. The present model would bea tentative proposal for discussion. By comparing the predicted values
with the measured data, the presented model could be improved. If the T
outflow ratio increases, modification of the Li-loading rod structure
would be necessary, i.e., an increment in the quantity of Zr and/or Al2O3
in a Li rod would be entailed. In this paper, we have considered
containment of T in the Li rod. As another option, a continuous T re­covery system contained in the He gas could be considered in future
research [25]. Ongoing discussion is important.
Throughout the discussion, we have ignored the antipermeation
property of the Zr. In our study of a cooperative project, anotherexperiment using the Zr as an antipermaeation material has also been
progressed; and good performance was reported [9,17,26]. This may
further improve the T containment performance in HTGRs, and thereflection of the results to the design of the Li-loading rod structure is
expected.
To recover the T from the Li rod after the HTGR operation, we must
increase the temperature of the Li rods in a considerably high vacuum
state [27]. To recover the remaining small amount of T, the Li rods might
be dissolved in concentrated hydrochloric acid [28–30].

Fig. 9. Temporal behavior of the cumulative weights of T produced, absorbed
in the Zr and Al2O3 layers, remains as T2 gas and flows out of the Li-loading rods
in GTHTR300 and (b) HTTR cores. The diffusivity (D = 1.3 × 10− 12 m2/s) andsolubility (S = 1.08 (10.8) mol/m3/Pa1/2 for Zr in the GTHTR300 (HTTR) wereadopted for the simulations.

considering the tritium supply using the high-temperature gas-cooled
reactor (HTGR). This is a first attempt in the world. The HTGR has aunique reactor core structure, and which has attractive and profitablefeatures as a tritium production device. We are currently planning anirradiation test of the tritium production using HTGR in 2022 at theearliest. In this paper, we show a tentative analysis model to analyze thetritium production rate and outflow rate from the Li-loading rods to the
He coolant, considering the criticality in the fission reactor nuclear
burning. We believe that the development of the system would provide
many useful knowledges to the fusion community.
Acknowledgments
This work was supported by JSPS (Japan Society for the Promotionof Science) KAKENHI Grant-in-Aid for Scientific Research (B)JP18H1200 and cooperative program with BA (The Joint Implementa­tion of the Broader Approach Activities in the Field of Fusion Energy
Research) reactor design team in Japan.

Declaration of Competing Interest

References

In the fusion reactor development, establishment of the tritium
supply scenario is required for initial startup of DEMO reactor and priorengineering test of the reactor system with tritium circulation. We are[1] M. Nishikawa, H. Yamasaki, H. Kashimura, S. Matsuda, Effect of outside tritium
source on tritium balance of a D-T fusion reactor, Fusion Eng. Des. 87 (2012)
466–470.

6

H. Matsuura et al.

Fusion Engineering and Design 169 (2021) 112441[16] M. Nagasaka, T. Yamashina, Solubility of hydrogen and deuterium in titanium andzirconium under very low pressure, J. Less-Common Metals 45 (1976) 53–62.
[17] J. Izumino, K. Katayama, H. Matsuura, et al., Study on hydrogen absorption in Zr
power used for tritium confinement in a production system of tritium for fusion
reactors with a high-temperature gas-cooled reactor, Nucl. Mater. Energy 17(2018) 289–294.
[18] W.M. Albrecht, W.D. Goode, The Diffusion of Hydrogen in Beta Zirconium, BMI1373, 1959, https://doi.org/10.2172/4247005.
[19] M. Someno, Solubility and diffusion of hydrogen in zirconium, J. Jpn. Inst. Met. 24(1960) 249–253 (in Japanese).
[20] B.W. Gainey, A review of tritium behavior in HTGR system, GA-A13461, General
Atomic (1976), https://doi.org/10.2172/7359491.
[21] H. Ohashi, N. Sakaba, T. Nishihara, et al., Analysis of tritium behavior in very hightemperature gas-cooled reactor coupled with thermochemical iodine-sulfur process
for hydrogen production, J. Nucl. Sci. Tech. 45 (2008) 1215–1227.
[22] E. Zuzek, J.P. Abrlata, A. San-Martin, F.D. Manchester, The H-Zr (HydrogenZirconium) system, Bull. Alloy Phase Diag. 11 (1990) 385–395.
[23] T. Otsuka, S. Yokoyama, Y. Akahoshi, et al., T-containment performance of Li rod
in High-temperature gas-cooled reactor for T production: (1) hydrogen absorption
properties of Ni coated zircaliy-4 Co-existing with Li oxide, AESJ Annual Meeting(2020) (in Japanese).
[24] S. Yamanaka, T. Tanaka, M. Miyake, Effect of oxygen on hydrogen solubility inzirconium, J. Nucl. Mater. 167 (1989) 231–237.
[25] H. Matsuura, H. Nakaya, Y. Nakao, et al., Evaluation of tritium production rate in a
gas-cooled reactor with continuous recovery system for fusion reactor, Fusion Eng.
Des. 88 (2013) 2219–2222.
[26] D. Henzen, K. Katayama, H. Matsuura, Evaluation of tritium confinement
performance the aassembly composed of zirconium and alumina simulating
lithium rod, Fusion Eng. Des. 168 (2021), 112372.
[27] K. Ashida, Y. Hatano, W. Nishida, et al., Recovery of hydrogen isotopes by Pdcoated ZrNi from inert gas atmosphere containing impurities, J. Nucl. Sci. Tech. 38(2001) 952–958.
[28] S. Sato, Y. Verzilov, K. Ochiai, et al., Neutronics experimental study on tritium
production in solid breeder blanket mockup with neutron reflector, J. Nucl. Sci.
Tech. 44 (2007) 657–663.
[29] A. Klix, Y. Verzilov, K. Ochiak, et al., Recent tritium breeding experiments withlithium titanate mock-ups irradiated with DT neutrons, JAERI-Conf 2004-012(2004) 209–214.
[30] R. Dierckx, Direct tritium production measurement in irradiated Lithium, Nucl.
Inst. Methods 107 (1973) 397–398.

[2] P. Gierszewski, Tritium supply for near-term fusion devices, Fusion Eng. Des. 10(1989) 399–403.
[3] H. Matsuura, S. Kouchi, H. Nakaya, et al., Performance of high-temperature gascooled reactor as a tritium production device for fusion reactors, Nucl. Eng. Des.
243 (2012) 95–101.
[4] M. Goto, K. Okumura, S. Nakagawa, et al., Nuclear and thermal feasibility oflithium-loaded high temperature gas-cooled reactor for tritium production forfusion reactors, Fusion Eng. Des. 136 (2018) 357–361.
[5] H. Nakaya, H. Matsuura, K. Katayama, et al., Study on a method for loading a Li
compound to produce tritium using high-temperature gas-cooled reactor, Nucl.
Eng. Des. 292 (2015) 277–282.
[6] K. Katayama, H. Ushida, H. Matsuura, et al., Evaluation of tritium confinement
performance of alumina and zirconium for tritium production in a hightemperature gas-cooled reactor for fusion reactors, Fusion Sci. Tech. 68 (2015)
662–668.
[7] K.A. Burn, E.F. Love, C.K. Thornhill, Description of the Tritium-Producing Burnable
Absorber Rod for the Commercial Light Water Reactor, TTQP-1-015, 19, PNNL,
2012, p. 22086.
[8] S. Nagasumi, H. Matsuura, K. Katayama, et al., Study on tritium production using ahigh-temperature gas-cooled reactor for fusion reactors: evaluation of tritium
outflow by non-equilibrium diffusion simulations, Fusion Sci. Tech. 72 (2017)753–759.
[9] K. Katayama, J. Izumino, H. Matsuura, S. Fukada, Evaluation of hydrogen
permeation rate through zirconium pipe, Nucl. Mater. Energy 16 (2018) 12–18.
[10] Y. Koga, H. Matsuura, Y. Ida, et al., Study on Lithium rod test module andirradiation method for tritium production using high temperature gas-cooled
reactor, Fusion Eng. Des. 136 (2018) 587–591.
[11] H. Matsuura, R. Okamoto, Y. Koga, et al., Li-rod structure in high-temperature gascooled reactor as a tritium production device for fusion reactors, Fusion Eng. Des.
146 (2019) 1077–1081.
[12] T. Nakata, S. Katanishi, S. Takada, et al., Nuclear Design of the Gas Turbine High
Temperature Reactor (GTHTR300), JAERI-Tech, 2002, 2002-066.
[13] S. Saito, T. Tanaka, Y. Sudo, et al., Design of High Temperature Engineering Test
Reactor (HTTR), JAERI, 1994, p. 1332.
[14] Y. Nagaya, K. Okumura, T. Mori, M. Nakagawa, MVP/GMVP II: General Purpose
Monte Carlo Codes for Nuetron and Photon Transport Calculations Based on
Continuous Energy and Multigroup Methods, JAERI, 2005, 1348.
[15] K. Shibata, O. Iwamoto, T. Nakagawa, et al., JENDL-4.0: a new library for nuclear
science and engineering, J. Nucl. Sci. Tech. 48 (2011) 1–30.

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