Nuclear Engineering and Design 415 (2023) 112665

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Nuclear Engineering and Design
journal homepage: www.elsevier.com/locate/nucengdes

Loading method of Li rods for tritium production using High-Temperature
Gas-Cooled reactor for fusion reactors
Yuki Koga a, *, Hideaki Matsuura a, Kazunari Katayama b, Teppei Otsuka c, Minoru Goto d,
Shimpei Hamamoto d, Etsuo Ishitsuka d, Shigeaki Nakagawa d, Kenji Tobita e, Youji Someya f,
Yoshiteru Sakamoto faDepartment of Applied Quantum Physics and Nuclear Engineering, Kyushu University, 744 Motooka Nishi-ku Fukuoka-shi Fukuoka, 319-0395, Japan
Department of Advanced Energy Engineering Science, Kyushu University, 6-1 Kasugakouen Kasuga-shi Fukuoka, 816-0811, Japan
Department of Electrical and Electrtonic Engineering, Kindai University, 3-4-1 Kowakae, HigashiOsaka city Osaka, 577-0818, Japand
Japan Atomic Energy Agency, 4002 Narita-cho Oarai-machi Higashi-Ibaraki-gun Ibaraki, 311-1393, Japane
Department of Quantum Sci. Energy Eng., Tohoku University, 6-6-01-2 Aoba, Sendai City, Miyagi 980-8579, JapanfNational Institutes for Quantum Science and Technology, Rokkasho, Aomori 039-3212, Japanbc

A R T I C L E I N F O

A B S T R A C T

Keywords:
Fusion reactor
Tritium production
HTGR
Tritium production module
Optimization Prepared for submission to
Nuclear Engineering and Design

An initial tritium inventory is required to start fusion DEMOnstration Power Station (DEMO) reactors. However,a method to supply adequate tritium has not been determined yet. Tritium production via 6Li(n,α)T reaction byloading Li rods into the burnable poison (BP) holes of a high-temperature gas-cooled reactor (HTGR) has been
proposed to address this problem (Matsuura et al., Nucl. Eng. Des. 243 (2012) 95 − 101). In previous preliminary
studies, Li rods loaded in all BP holes were assumed to have the same design. This study evaluated whether the
performance of Li rods can be improved for future optimization by adjusting the Li rod arrangement and theamount of Li compounds in them. The amount of tritium produced for gas turbine high-temperature reactor 300(GTHTR300) was evaluated, while the total amount of Li compounds was maintained and the amount of loaded
Li compounds changed depending on the layers and fuel regions. The maximum amount of tritium produced did
not increase during the evaluations when reactor feasibility was satisfied. This implies that it is possible to reduce
the number of Li rods while maintaining the amount of tritium produced for optimization, thereby reducing thecosts of manufacturing Li rods and tritium recovery. GTHTR300 can produce 800 g of tritium in 360 days ofoperation using 2160Li rods. The results showed that the same amount of tritium could be produced by loading
720Li rods with the same number of fuel blocks. In addition, the effective multiplication factor, burn up, andpower density of GTHTR300 were not significantly influenced during the operation.

1. Introduction
A method for supplying tritium using an external source is required
to precisely plan an engineering test of tritium circulation and to prepare
initial tritium in the DEMOnstration Power Station (DEMO) (Botter
et al., 1986). Tritium for the International Thermonuclear Experimental
Reactor was produced in the Canadian Deuterium Uranium (CANDU)
reactor via the D(n,γ)T reaction (Gierszewski, 1989). However, uncer­
tainty regarding the tritium supply for DEMO reactors is increasing.
Tritium production using high-temperature gas-cooled reactors(HTGRs) has been proposed as a method for supplying tritium (Gotoet al., 2018). Tritium production analyses were conducted assuming that

Li compounds were loaded in the HTGR as burnable poisons (BPs)instead of boron compounds (B4C). HTGRs have several advantages fortritium production. First, the cross-section of 6Li(n,α)T used for the
HTGRs is almost six orders of magnitude larger across the thermal
neutron energy range compared to the D(n,γ)T reaction used for the
CANDU reactors. Second, the HTGR structure consists of graphite, which
is chemically stable and does not react with Li compounds. Third,
although the large reactor size of the HTGR is not attractive from theperspective of economy, there is sufficient space to place sufficient Licompounds and tritium barrier without 6Li enrichment. B4C (genuineBP) in the standard design is loaded in the solid state separately from thefuel components. Therefore, Li compounds can be loaded into the* Corresponding author.
E-mail address: koga.yuki@qst.go.jp (Y. Koga).
https://doi.org/10.1016/j.nucengdes.2023.112665
Received 22 March 2022; Received in revised form 9 December 2022; Accepted 5 October 2023
0029-5493/© 2023 Elsevier B.V. All rights reserved.

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Nuclear Engineering and Design 415 (2023) 112665

reactor core without significantly changing the original structuraldesign. The nuclear characteristics and fuel temperature conditions
were analyzed to confirm the thermal and nuclear stability of Li-loaded
HTGRs (Hollenberg, 1986); which confirmed that the safety re­quirements of the design were satisfied. Currently, an HTGR is being
researched that produces tritium with a power generation performance
similar to that of the standard specification (when B4C was loaded asBP). A schematic of the Li rod structure of the HTGRs is shown in Fig. 1.
The Li rods have Ni-coated Zr and LiAlO2 in sealed cylinder-shaped highdensity Al2O3 to decrease the leakage of tritium during hightemperature operation (approximately 900 ℃).
The amount of tritium produced from 2160Li rods loaded in gas
turbine high-temperature reactor 300 (GTHTR300) (Katayama et al.,2015) was simulated to be 500–800 g over 360 days of operation(Kawamura et al., 1992). In previous studies, it has been assumed that Li
rods loaded in all BP holes have the same design for preliminary eval­uations. Additionally, the characteristics of the effective multiplication
factor (keff) were different from those of the standard specifications(Katayama et al., 2015). It has been considered that the amount oftritium produced in the GTHTR300 is 500–600 g/year by loading same
design Li rods with natural abundance 6Li into whole of the reactor on
the real operation, and keff was analyzed to be about 1.05 after 360 daysof operation. We have been focused on 800 g/year tritium production on
our studies to challenge furthermore performance. The purpose of thisstudy is to evaluate whether the performance of Li rods and the reactor
characteristics of GTHTR300 can be improved for future optimization by
the Li rod arrangement and the amount of Li compounds in them.

horizontally, as shown in Fig. 2. Each fuel region was given an identi­
fication number: 1–1 to 8–3. 1–2 implies the fuel regions 2 in layer 1 forexample.
The design of proposed Li rod for GTHTR300 is shown in Fig. 3(a)(Koga, 2022). These Li rods were 1000 mm in height and 44 mm indiameter. They consisted of a hollow portion, Li compound (LiAlO2)
with 85 % theoretical density (Kunitomi et al., 2004), Zr layer/particles,
and an Al2O3 layer. The concentration of 6Li in LiAlO2 is naturally
abundant. The inserted Zr particles occupied 60 % of the hollow portion
excluding Zr layer and LiAlO2 volume. This Li rod design is referred to asa layer type Li rod to distinguish from following one. There is a limit to
the amount of 6Li that can be loaded into the Li rod if the intent is toincrease the amount of LiAlO2. This was due to the natural abundance of6
Li in the Li rods. Therefore, a Li rod model with high enrichment 6Li was
utilized. The Li rod model for the GTHTR300 used in this study is shownin Fig. 3(b). The enrichment of 6Li was 90 at%, and LiAlO2 was particle
shaped. It was assumed that Li with an enrichment of 6Li lower than 90
at% would not be manufactured for fusion reactors. The amount ofloaded Zr was increased by decreasing the LiAlO2 volume withincreasing 6Li concentration. This Li rod model is referred to as a particle
type Li rod.
2.2. Nuclear calculation
Nuclear calculations from the continuous-energy Monte Carlotransport code MVP-BURN (Matsuura, 2017; Matsuura, 2021) using theJapanese Evaluated Nuclear Data Library-4.0 (JENDL-4.0) (Matsuura
et al., 2012) were utilized to evaluate the amount of tritium produced in
the Li rods and the effective multiplication factor keff. It was assumed
that the GTHTR300 core system had Li rods loaded into all BP holes, andthe operation period was 360 days. In addition, the amount of loaded
LiAlO2 can be changed in each fuel region. The average moderator
temperature of the Li rods for GTHTR300 was set to 1170 K. It was
assumed that all control rods were pulled up during the simulations. The
keff on day 360 must be greater than 1.02, because this is the minimum
amount of keff required for the actual reactor operation. The time steps
for the burn-up simulation were set to 0, 1, 5, 30, 60, 120, 180, and 360
days. The thermal output of the GTHTR300 throughout the operation
was set to 600 MW. Additionally, 6 000 000 neutron particles weregenerated at each time step such that the statistical error was guaranteed
to be less than 0.1 % for the keff and reaction rates of nuclides to beburned or transmuted.

2. Calculation model
2.1. HTGR design and Li rod structures
A schematic of the GTHTR300 core is shown in Fig. 2. The
GTHTR300 is an HTGR design with a thermal output of 600 MWt (300MWe), created by the Japan Atomic Energy Agency. The GTHTR300
core had a ring-shaped structure consisting of 90 fuel columns, 30
control rod guide columns, and 46 replaceable reflector columns. The
core was surrounded by permanent reflector columns both inside andoutside. There were eight core layers between the two reflector layers
vertically. The core has a height of 8 m and diameter of approximately
5.6 m. The height and width across the flats of the hexagonal block were1,000 and 405 mm, respectively. The fuel blocks contained 57 fuel rods
and three BP holes. The enrichment of 235U was 14 wt% in all the fuelrods. The fuel columns in the core are divided into three fuel regions

2.3. Tritium absorption model for Zr
Diffusion calculations using the diffusion equation and Sieverts’ law
were conducted to evaluate the tritium absorption rate of Zr. The Zr
particles and Zr layer were converted into a Zr cylinder with the same
surface area for simpler evaluation. The diffusion equation used to
calculate the tritium concentration distribution in Zr for a cylindricalcoordinate system is expressed as follows:()∂CZr (rZt , t) DZr ∂∂CZr (rZt , t)
rZr
(1)
=
∂t
rZr ∂rZt∂rZr
where CZr , t, DZr , and rZt are the tritium concentration in Zr (mol/m3),
elapsed time (s), diffusion coefficient of Zr (m2/s), and distance in theradial direction (m), respectively. Sieverts’ law was used as theboundary condition for the Zr cylinder surface and is expressed as√̅̅̅
CZr0 = SZr P(2)
where CZr0 , SZr , and P are the tritium concentration in the Zr cylindersurface (mol/m3), the solubility coefficient of Zr (mol/m3/Pa1/2), andthe tritium partial pressure (Pa), respectively. Subsequently, CZr was
integrated into the Zr volume and differentiated in time to evaluate theamount of tritium absorbed in Zr using the following equation:

Fig. 1. Schematic of Li rod structure.
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Nuclear Engineering and Design 415 (2023) 112665

Fig. 2. Schematic of the GTHTR300 core. The GTHTR300 consists of eight core layers. The fuel columns in the core were divided into three fuel regions in this study.

Fig. 3. Schematics of proposed Li rod designs for the GTHTR300. Layer type Li rod (b) Particle type Li rod.

∫

JZr = −∂C (t, r)d3 r∂t Vzr Zr(mol/m3), elapsed time (s), diffusion coefficient of the Al2O3 layer (m2/s), and distance in the radial direction (m), respectively. Sieverts’ law
was used as the boundary condition, similar to the Zr tritium absorption
model, and is expressed as√̅̅̅
CAl0 = SAl P
(5)

(3)

where JZr and VZr are the tritium flux from the Zr surface (mol/s) and thevolume of the assumed Zr cylinder (m3), respectively. Matsuura et al.
obtained the DZr and SZr values at 1170 K (Matsuura et al., 2019).
Furthermore, DZr was reduced by 1/2000 to consider the reported in­fluence of Ni coating and LiAlO2 (Nagaya, 2017). The temperature effectof this reduction was small on the four temperature conditions in theexperiment, and it was neglected for this study.

where CAl0 , SAl , and P are the tritium concentrations in the inner surfaceof the Al2O3 layer (mol/m3), solubility coefficient of the Al2O3 layer(mol/m3/Pa1/2), and tritium partial pressure in the hollow portion (Pa),respectively. The DAl and SAl values were obtained by Katayama et al.
(Nakata, 2002). Fick’s law was used to calculate the amount of perme­
ated tritium in the inner and outer surfaces of the Al2O3 layer, which isexpressed as⃒∂CAl ⃒⃒
Jin,out = − Ain,out DAl(6)∂rAl ⃒in,out

2.4. Tritium leakage model for Al2O3 layer
To evaluate the tritium leak rate, a tritium diffusion equation incylindrical coordinates was used, which is expressed as()∂CAl (rAl , t) DAl ∂∂CAl (rAl , t)
rAl
(4)
=
∂t
rAl ∂rAl∂rAl

where Jin andJout are the tritium fluxes from the inner surface of the Al2O3
layer and inside the Al2O3 layer to inside the Al2O3 layer and outerwhere CAl , t, DAl , and rAl are the tritium concentration in the Al2O3 layer
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surfaces of the Al2O3 layer (mol/s), respectively. Ain and Aout are the innerand outer surface areas of the Al2O3 layer (m2), respectively. Jin repre­
sents the tritium leakage rate. The tritium in the outer surface of the
Al2O3 layer was assumed to immediately leak into the He coolant;
therefore, the tritium concentration in the outer surface was set at 0.
Finally, Eqs. (3)–(6) were combined with the tritium balance equation
to evaluate tritium absorption and leakage:dP
GRT
JZr RT
Jin RT=++
dt Vhp+Li(15) Vhp+Li(15) Vhp+Li(15)loaded LiAlO2 was reduced in the particle type Li rod. The keff wasalmost same in each step for the two cases. The cumulative amounts ofleaked tritium throughout the 360 days of operation for the two cases
are shown in Fig. 5. The amount of leaked tritium decreased by 27 %because more particle Zr was loaded into the particle type Li rods. Next,
the fuel burnup and power density were compared to those when B4C
BPs were loaded. It was important to confirm the reactor feasibility for
operation, as in the research conducted by Goto et al. (Hollenberg,1986), because the loading method of the Li rods was changed. The fuelburnups in each fuel region when the B4C BPs or Li rods are loaded areshown in Fig. 6. The numbers on the horizonal axis denote layer and fuelregion, respectively. For example, 1–2 implies the fuel regions 2 in layer
1. There was no significant difference in fuel burnup with changes in BP.
The burnup for the particle type Li rods was almost the same as theresults for the layer type; therefore, they were omitted. The power
densities in each fuel region when the B4C BPs are loaded are shown in
Fig. 7. The numbers on the horizonal axis denote the same as those in
Fig. 6. The maximum power density was approximately 9 W/cm3, which
did not exceed the design data of GTHTR300 (Nishikawa et al., 2012).
The power density distributions were different because the control rods
were inserted into the reactor in the calculations of the design data.
Therefore, the distributions of the fuel burnup and power density when
B4C BPs are used should be similar to those when Li rods are used. To
guarantee reactor feasibility, the fuel burnup and power density in eachfuel region must not exceed 42 GWd/t and 9 W/cm3, respectively. The
power densities in each fuel region when the layer type Li rods were
loaded are shown in Fig. 8. The power densities for the particle type Li
rods were similar to the results for burnup, and hence they were omitted.
The power density values and distributions were similar to those of the
B4C BPs. The feasibility of the reactor was confirmed when the particle
type Li rods were uniformly loaded. The particle type Li rod was used for
subsequent evaluations.

(7)

where G, R, and Vhp+Li(15) are the tritium molecule generation speed at
the tritium production rate (mol/s), gas constant (J/K mol), and volumeof the hollow portion and void of the LiAlO2 layer in the Li rod (m3),respectively. The anti-permeation property of Zr, which prevents tritiumfrom permeating into the Al2O3 layer, was ignored for conservativeevaluation. This tritium leakage calculation is sufficiently accuratecompared to a highly accurate evaluation (Matsuura et al., 2019).
3. Results and discussion
3.1. Performance evaluation of particle type Li rod
In this study, the amount of tritium produced, and reactor charac­
teristics were evaluated by changing the amount of LiAlO2 in the particle
type Li rods depending on the layers and fuel regions. There is a possi­
bility that the amount of tritium produced and reactor characteristics for
the GTHTR300 is affected by replacing the layer type Li rods (Fig. 3(a))
to the particle type Li rod (Fig. 3(b)) because the self-shielding effect isweakened by uniformly loading particle LiAlO2 into Li rods. First of all,the influences on produced/leaked tritium, keff, fuel burnup, and power
densities for the GTHTR300 while using particle type Li rod were
evaluated in this section. The cumulative amounts of tritium produced
and keff throughout 360 days of operation for the GTHTR300 when thelayer type or particle type Li rods were loaded are shown in Fig. 4. The
total amount of loaded 6Li for the layer type and particle type Li rods inthe entire reactor were 6554 and 6249 g, respectively, which produced
approximately 800 g of tritium after 360 days of operation. keffdecreased sharply at the beginning of the operation by generation of Xe.
The negative reactivity for Li rods decreases with the amount of 6Li bytritium production. However, the reactivity less decreased than that for10
B contained in B4C BPs with operation time and keff decreased
straightly. The transmutation performance of the particle type Li rod
was better than that of the layer type because particle LiAlO2 was loaded
more uniformly with Zr particles: effective 6Li density for neutrons
reduced and self-shielding effect was weaken. Therefore, the amount of

3.2. Evaluations of the effect on tritium production performance bychanging LiAlO2 distribution
The amount of tritium produced and reactor characteristics can be
influenced by changing LiAlO2 distribution on in the vertical (layers)
and radial direction (three fuel regions). These influences were evalu­
ated when the loaded LiAlO2 distribution was changed on fuel regions of
the GTHTR300 while maintaining the total amount of LiAlO2 loaded
into the entire reactor constant in this section. At first, the influenceswere evaluated when the loaded LiAlO2 distribution was changed invertical direction. The loading ratios of LiAlO2 (%) on the layers of the
GTHTR300 for each calculation case are listed in Table 1. The

Fig. 4. The cumulative amount of produced tritium and the keff value
throughout the 360 days of operation for the GTHTR300 with Li rods shownin Fig. 3.

Fig. 5. The cumulative amounts of leaked tritium throughout the 360 days ofoperation for the Li rods shown in Fig. 3 in the operation of Fig. 4.
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Nuclear Engineering and Design 415 (2023) 112665

Fig. 6. The fuel burnups in each fuel region for the GTHTR300 when the B4C BPs or layer type Li rods were loaded. The numbers on the horizonal axis denote layer
and fuel region, respectively. 1–2 implies the fuel regions 2 in layer 1, and so on. The results for the particle type Li rods were almost identical to those shown ingreen. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. The power densities in each fuel region for the GTHTR300 when the B4C BPs were loaded. The numbers on the horizonal axis denote the same as thosein Fig. 6.

because the increase in transmutation rate of 6Li in the center layers dueto the addition of LiAlO2 was less than the decrease in transmutation
rate in the outer layers due to the reduction of LiAlO2. Therefore, keffstarted increasing from Case 9 owing to the decrease in the amount oftritium produced.
Next, the influence of the loaded LiAlO2 distribution in the radial
direction was evaluated. The loading ratios of LiAlO2 (%) in the radial
direction of the GTHTR300 for each calculation case are listed inTable 2. The loaded LiAlO2 was biased towards the center (1) or outside(2, 3) of the fuel regions. Cases 17 was the same conditions to Case 6.

percentage of loading ratios was set to be 100 % when the amount of
LiAlO2 was the same as that the LiAlO2 distribution was uniformly.
Therefore, Case 6 is the same condition as that of section 3.1. The loaded
LiAlO2 was biased towards the center or outside of the layers, because
the neutron flux was higher in the center layers. The amount of tritium
produced and keff after 360 days of operation in Cases 1–11 are shown in
Fig. 9. With the increase in LiAlO2 loading in the center layers, theamount of tritium increases and keff decreases. However, it can beobserved that the amount of tritium that was produced decreasedslightly from Case 10, and keff was the minimum in Case 9. This is5

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Fig. 8. The power densities in each fuel region for the GTHTR300 when layer type Li rods. The horizonal axis denotes layer and fuel region, respectively. The results
for the particle type Li rods were almost identical to these.
Table 1The loading ratios of LiAlO2 (%) on the layers of GTHTR300 for each calculation case.
Calculation case
Layer

1, 8
2, 7
3, 6
4, 5

1

2

3

4

5

6

7

8

9

10

11

115
105
95
85

112
104
96
88

109
103
97
91

106
102
98
94

103
101
99
97

100
100
100
100

97
99
101
103

94
98
102
106

91
97
103
109

88
96
104
112

85
95
105
115

increase in transmutation rate of 6Li in the fuel regions due to theaddition of LiAlO2 was less than the decrease in transmutation rate in theother regions due to the reduction of LiAlO2. Therefore, keff started
increasing from Case 16.
Overall, the maximum amount of tritium produced was approxi­
mately 800 g when the amount of LiAlO2 changed, depending on thelayers and fuel regions, while maintaining the total amount of Li. In
addition, the amount of tritium produced reached approximately 800 g
in all cases if the operation time was adjusted until keff decreased to 1.02.
This demonstrates that effective optimization can occur while main­taining the amount of tritium produced.
3.3. Evaluations of the effect on tritium production performance byreducing the number of loaded Li rods
3.3.1. Reducing the number of Li rods in all fuel regions uniformly
Reducing the number of Li rods while maintaining the amount oftritium produced was considered for one of the optimizations. The
amount of loaded 6Li to produce 800 g/y tritium by a smaller number of
Li rods and reactor characteristic were evaluated. It is clear that the
distributions of burnup and power density were extremely distorted, andreactor feasibility was not assured for operation if there were fuel re­gions without Li rods. Therefore, it is noted that the amounts of loaded
LiAlO2 were uniform in all Li rods and at least one Li rod was placed in
the fuel block during the subsequent examination. The amount oftritium produced and keff throughout the 360 days of operation based on

Fig. 9. The amount of produced tritum and keff values after 360 days ofoperation when the LiAlO2 distribution was changed in layers of
the GTHTR300.

The amount of tritium produced and keff after 360 days of operation inCases 12 − 22 are shown in Fig. 10. The amount of tritium produced
increased with increasing loading of LiAlO2 outside and decreased after
the peak in Case 16. However, keff was at a minimum in Case 16, which
was inversely proportional to the produced tritium. This is because the6

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Table 2The loading ratios of LiAlO2 (%) in the radial direction of the GTHTR300 for each calculation case.
Calculation case
Fuel region

1
2
3

12

13

14

15

16

17

18

19

20

21

22

179.2
87.5
75

133.3
90
80

147.5
92.5
85

137.7
95
90

115.8
97.5
95

100
100
100

84.2
102.5
105

68.3
105
110

52.5
107.5
115

36.7
110
120

20.8
112.5
125

Fig. 10. The amount of tritium produced and keff values after 360 days of operation when the LiAlO2 distribution was changed in the radial direction of
the GTHTR300.

the amount of loaded 6Li when two Li rods were loaded in each fuelblock (1440 loaded Li rods) were evaluated as shown in Fig. 11. The
amount of LiAlO2 loaded in one of 1440Li rods is as 1.5 times as that of
2160Li rods when the total amount of LiAlO2 loaded is same. In addi­tion, the amount of particle Zr loaded decreased by the adjustment. The
amount tritium produced tritium and that of 6Li loaded were propor­
tional but keff and 6Li were inverse proportional as past research showed.
The maximum amount of tritium produced was 800 g when 8187 g of 6Li
was loaded into the reactor. The amount of loaded 6Li to produce 800 gof tritium increased when two Li rods were loaded in each fuel block.
This is because the total transmutation performance of the Li rods
decreased by reducing the number of Li rods, and more 6Li was required
to produce 800 g of tritium during 360 days of operation. The amount oftritium produced and keff throughout the 360 days of operation ac­
cording to the amount of loaded 6Li when one Li rod was loaded in eachfuel block (720 loaded Li rods) were shown in Fig. 12. The amount of
LiAlO2 loaded in one of 720Li rods is as 3 times as that of 2160Li rods
when the total amount of LiAlO2 loaded is same. The transition of the
produced tritium and keff was more unclear than Fig. 11 despite enoughstatistic errors. Because the increase of 6Li loaded was much smaller forthe total amount of it for the evaluation condition. The maximum
amount of tritium produced was 801 g when 39 059 g of 6Li was loaded
into the entire reactor. The total transmutation performance of the Li
rods was significantly decreased by reducing two Li rods in each fuelblock, which required a much larger amount of 6Li to produce 800 g oftritium.
The cumulative amount of tritium produced and keff throughout the360 days of operation, when two Li rods (one Li rod) were loaded andthe total amount of loaded 6Li was 8187 g (39 059 g), are shown in
Fig. 13 of red lines (blue lines). The keff for one Li rod exceeded that for
two Li rods because the transmutation performance was inferior in earlytime of operation by self-shielding effect: Significant operation of con­trol rods is required to suppress reactivity when the number of Li rods is
reduced. The fuel burnups and power densities for both cases weresimilar to those when three Li rods were loaded (Figs. 6 and 8), which
satisfied reactor feasibility. Their transitions were similar to thoseobserved when the three Li rods were loaded. The reactor feasibility was
also satisfied when one Li rod was loaded into each fuel block. The
amounts of tritium leaked when one, two, and three Li rods were loadedin each fuel block are shown in Fig. 14. The amount of Zr decreased withan increase in the amount of LiAlO2 (6Li) to maintain the amount oftritium produced to be 800 g when the number of Li rods was reduced.
Compared with the result when three Li rods were loaded, the amount of
leaked tritium increased by 441 % and 58 % when one and two Li rods
were loaded in each fuel block, respectively. This is because reducing theamount of Zr caused tritium to leak more from Al2O3 layer by tritium
leakage-absorption balance.
3.3.2. Reducing the number of Li rods in specific fuel regions
There is a possibility that tritium production and containment per
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Fig. 11. The amount of tritium produced and keff values after 360 days of operation based on the amount of loaded 6Li when two Li rods were loaded in each fuelblock of the GTHTR300.

Fig. 12. The amount of tritium produced and keff values after 360 days of operation based on the amount of loaded 6Li when one Li rod was loaded in each fuel blockof the GTHTR300.

formance are improved by reducing the number of Li rods in specific fuelregions. The amount of tritium produced and reactor characteristics
were evaluated by reducing the number of Li rods in specific fuel re­gions. GTHTR300 has a vertical temperature distribution direction, andthe lower layers have a higher temperature (Katayama et al., 2015). The
influence of reducing Li rods in the upper layers was evaluated based on
our research, which suggested that the tritium absorption performanceof Zr was better at higher temperatures (Okumura et al., 2000), and thetritium containment performance of Li rods was superior at higher
temperatures. The number of Li rods in each layer when the Li rods werereduced in the upper (lower temperature) layers is shown in Table 3. At
least one Li rod was loaded into each fuel block in layers 1 − 4 to ensurethat the fuel burnup and power density were not distorted. The amountof tritium produced and keff throughout the 360 days of operation, basedon the amount of loaded 6Li under the conditions listed in Table 3, were
evaluated using the method on section 3.3.1. The amounts of loaded
LiAlO2 were uniform in all Li rods for each evaluation. The maximum
amount of tritium produced was 570 g when the amount of loaded 6Li
was 10 624 g and keff was over 1.02. Under these conditions, 800 g oftritium could not be maintained. The cumulative amount of tritium
produced and keff throughout 360 days of operation for 10 624 g ofloaded 6Li are shown in Fig. 15 (red lines).Their tendencies with respect
to the operation time were similar to those when the Li rods were uni­
formly loaded (Fig. 4, for example), except the keff significantly
decreased throughout the time because the transmutation performanceof the Li rods worsened. However, the fuel burnups and power densities
8Y. Koga et al.

Nuclear Engineering and Design 415 (2023) 112665

Fig. 13. The cumulative amount of tritium produced and the keff value throughout the 360 days of operation for the GTHTR300 when two (one) Li rods were loadedin each fuel block and the total amount of loaded 6Li was 8187 g (39 059 g).

expected limit in the upper layers, and these distributions were distorted
in the upper layers. Therefore, this loading method does not achieveadequate feasibility and tritium production.
Considering that the neutron flux is generally higher in the center
layers and more tritium is produced there, the influence of reducing the
Li rods in the outer layers was also evaluated. The number of Li rods ineach layer when the Li rods were reduced in the outer (lower neutronflux) layers is listed in Table 4. The amount of tritium produced and keffthroughout the 360 days of operation for loaded 6Li under the conditions
listed in Table 4 were also evaluated using the method described above.
The amounts of loaded LiAlO2 were also uniform in all Li rods. The
maximum produced tritium was 902 g when loaded 6Li was 29 372 g andkeff was over 1.02. This loading method increased the amount of tritium
produced at 100 g. The cumulative amount of tritium produced and kefffor 29 372 g of loaded 6Li throughout the 360 days of operation are
shown in Fig. 15 (blue lines). The tendencies of these transitions weresimilar to those of uniformly loaded Li rods, except that keff decreased
more after 120 days than before 120 days. The fuel burnups and power
densities exceeded the expected limit in the upper and lower layers, andthese distributions were distorted in the upper and lower layers.
Therefore, this loading method increased the amount of tritium pro­
duced from 800 g/year; however, the reactor feasibility could not besatisfied.

Fig. 14. The cumulative amounts of leaked tritium throughout the 360 days ofoperation when one, two, and three Li rods were loaded in each fuel block andapproximately 800 g of tritium was produced.
Table 3The number of loaded Li rods in each layer when the number of Li rods werereduced in the lower temperature layers.
Layer

The number of Li rods per fuel block

The number of Li rods in layer

1
2
3
4
5
6
7
8

1
1
1
1
2
2
3
3

90
90
90
90
180
180
270
270

4. Concluding remarks
This study evaluated whether the performance of Li rods and thereactor characteristics of the GTHTR300 can be improved for future
optimization by adjusting the Li rod arrangement and the amount of Li
compounds in them. The maximum amount of tritium produced did notincrease from 800 g/year in the evaluations when the amount of loaded
Li compounds changed, depending on the layers and fuel regions, while
maintaining the total amount of Li compounds. Next, the effect of
reducing the number of loaded Li rods was evaluated as an example of
optimization. By uniformly reducing the number of Li rods, themaximum amount of produced tritium was maintained at 800 g. How­ever, the amount of loaded Zr decreased and the amount of leaked
tritium increased about 5 times and 1.6 times when one and two Li rods
were loaded in each fuel block, respectively, compared to three loaded Li

are completely different from those when Li rods were uniformly
reduced. Even when one Li rod was loaded in each fuel block of theupper layers, both the results significantly increased and exceeded the9

Y. Koga et al.

Nuclear Engineering and Design 415 (2023) 112665

Fig. 15. The cumulative amount of tritium produced and the keff value throughout the 360 days of operation for the GTHTR300 when the number of Li rods was
reduced in the lower temperature layers (the outer layers) and the total amount of loaded 6Li was 10 624 g (29 372 g).

the Li rods are removed, should be evaluated.
Measures are required to reduce the number of Li rods while pre­venting tritium leakage, which increases the amount of loaded Zr.
Increasing the radius of Li rods and making the Al2O3 layer thinner while
maintaining the Al2O3 layer tritium containment performance andphysical strength can be one of the measures. This can also increase thetransmutation performance by weakening the self-shielding effect anddecreasing the required amount of 6Li. The amount of tritium produced
could not be significantly increased while satisfying the reactor feasi­
bility in this study. However, research on optimization should continue
to develop a method to increase the amount of tritium produced byburning the fuel uniformly.

Table 4
The number of loaded Li rods in each layer when Li rods were reduced in outer(lower neutron flux) layers.
Layer

The number of Li rods per fuel block

The number of Li rods in layer

1
2
3
4
5
6
7
8

1
1
2
3
3
2
1
1

90
90
180
270
270
180
90
90

CRediT authorship contribution statementrods, because more 6Li was required on reducing the Li rods. However,
the ratio of T/Zr increased to 1.99 at% from 0.54 at% (layer type Li rod)at maximum and tritium partial pressure was evaluated to be a few
pascals. Therefore, The Zr + T phase was in β regime in the any evalu­
ations. He partial pressure caused by 6Li(n,α)T reaction was evaluatedMPa but its magnitude balanced with He coolant pressure for the HTGR.
The GTHTR300 reactor characteristics were not significantly influenced
during operation; therefore, the feasibility of the reactor was satisfac­
tory. Additionally, investigations were conducted to ascertain whether
reducing the number of Li rods at specific loading positions can improve
the tritium production and containment performance. The amount oftritium produced was increased using this approach, but the reactor
feasibility could not be satisfied. Therefore, uniformly reducing thenumber of Li rods from 1440 to 720 is an effective optimization method
for tritium production by GTHTR300.
It is concerned whether Li rods can keep feasibility with effected byphenomena such as swelling in thermonuclear environments. We
assumed the amount of Zr loaded conservatively for that. The swellingfor LiAlO2 was reported to be very small (Otsuka, 2020; Shibata, 2011).
The Li rod feasibility will be confirmed by an irradiation examination by
an HTGR.
The keff characteristic of HTGRs with Li rods is expected to be close to
the standard specifications to avoid changing the operation method
when Li rods are loaded. The simultaneous use of B4C BPs and Li rods is
considered to make the keff characteristic closer to the standard speci­
fications for further optimization. The characteristic and amount oftritium produced, when B4C BPs are loaded into the BP holes from which

Yuki Koga: Conceptualization, Methodology, Software, Writing –original draft, Writing – review & editing. Hideaki Matsuura:
Conceptualization, Supervision, Project administration, Funding acqui­sition, Validation, Writing – review & editing. Kazunari Katayama:
Validation, Writing – review & editing. Teppei Otsuka: Validation,
Writing – review & editing. Minoru Goto: Validation, Writing – review& editing. Shimpei Hamamoto: Validation, Writing – review & editing.
Etsuo Ishitsuka: Validation, Writing – review & editing. Shigeaki
Nakagawa: Validation, Writing – review & editing. Kenji Tobita:
Validation, Writing – review & editing. Youji Someya: . Yoshiteru
Sakamoto: .
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data availability
The authors do not have permission to share data.
Acknowledgments
This work was supported by JSPS (Japan Society for the Promotion
10

Y. Koga et al.

Nuclear Engineering and Design 415 (2023) 112665

of Science) KAKENHI Grant-in-Aid for Scientific Research (B)JP21H01065, cooperative program with BA (The Joint Implementationof the Broader Approach Activities in the Field of Fusion Energy
Research) reactor design team in Japan, and JST SPRING Grant Number
JPMJSP2136.

Kunitomi, K., Katanishi, S., Takada, S., Yan, X., Tsuji, N., 2004. Reactor core design of gas
turbine high temperature reactor 300. Nucl. Eng. Des. 230 (1-3), 349–366.
Matsuura, H., 2017. Tritium production for a fusion reactor by a high-temperature gascooled reactor. J. Plasma Fusion Res. 93 (10), 457–464.
Matsuura, H., et al., 2021. The T-containment properties of a Zr-containing Li rod in ahigh-temperature gas-cooled reactor as a T production device for fusion reactors.
Fusion Eng. Des. 169, 112441.
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Nakagawa, S., Nishikawa, M., 2012. Performance of high-temperature gas-cooled
reactor as a tritium production device for fusion reactors. Nucl. Eng. Des. 243,
95–101.
Matsuura, H., Okamoto, R., Koga, Y., Suganuma, T., Katayama, K., Otsuka, T., Goto, M.,
Nakagawa, S., Ishitsuka, E., Tobita, K., 2019. Li-rod structure in high-temperature
gas-cooled reactor as a tritium production device for fusion reactors. Fusion Eng.
Des. 146, 1077–1081.
Y. Nagaya et al. MVP/GMVP Version 3: General Purpose Monte Carlo Codes for Neutronand Photon Transport Calculations Based on Continuous Energy and Multigroup
Methods JAEA-Data/Code 2017 2016 28.
T. Nakata et al. Nuclear Design of the Gas Turbine High Temperature Reactor(GTHTR300) JAERI-Tech 2002 2002 266.
Nishikawa, M., Yamasaki, H., Kashimura, H., Matsuda, S., 2012. Effect of outside tritium
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