Annals of Nuclear Energy 166 (2022) 108830

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Annals of Nuclear Energy
journal homepage: www.elsevier.com/locate/anucene

Design of Cesium target assembly with YH2 moderator and Gd thermal
neutron shielding to produce 135Cs for LLFP transmutation study using
the experimental fast reactor Joyo
Yoshihisa Tahara a,1, Peng Hong Liem a,b,⇑, Naoyuki Takaki a, Satoshi Chiba cabcTokyo City University, 1-28-1, Tamazutsumi, Setagaya, Tokyo 158-8557, Japan
Nippon Advanced Information Service (NAIS Co., Inc.), 416 Muramatsu, Tokaimura, Ibaraki 319-1112, JapanTokyo Institute of Technology, 2-12-1 N1-12, Ookayama, Meguro, Tokyo 152-850, Japana r t i c l e

i n f oArticle history:
Received 2 November 2020
Received in revised form 6 November 2021
Accepted 9 November 2021

Keywords:
LLFP
135
Cs
Fast reactor
Joyo
HydrideGadolinium
Irradiationa b s t r a c t
In this design study, we investigated the feasibility of producing a sufficient amount of 135Cs long-lived
fission product (LLFP) in the Joyo fast reactor by irradiation of natural cesium (133Cs). The irradiation is
expected to contribute to providing 135Cs sample for transmutation cross-section measurements forenhancing the accuracy of the LLFP nuclear data. Two irradiation fields, one located in the outer reflector
and another one in the inner reflector which was adjacent to the core were investigated, and appropriate
target subassemblies were designed by using the MCNP and FISPACT-II codes. In the inner reflector case,
the YH2 moderator material was adopted for the neutron spectrum adjustment within the target subassembly. The gadolinia thermal neutron shielding is proposed to suppress the power peak of the adjacent fuel assembly. The result confirmed that a sufficient amount of 135Cs can be produced at 1200
Effective Full Power Day (EFPD) in Joyo reactor (Mark IV core, 100 MWth).
Ó 2021 Elsevier Ltd. All rights reserved.

1. Introduction
Radioactive nuclides such as minor actinides (MAs) and fission
products (FPs) are produced in nuclear reactors. MAs are mainly
produced by the radioactive capture reaction. FPs are producedby fission, and nuclides such as 79Se, 93Zr, 99Tc, 107Pd, 129I, 135Cs
have long half-lives (more than 200,000 years) and are called
long-lived fission products (LLFPs). FPs and MAs are categorized
into high-level radioactive waste (HLW) and are subject to geological disposal in Japan, therefore, it would be beneficial to transmute
LLFPs into short-lived radionuclides or stable elements to reduce
the risk of radiation exposure (Partitioning and transmutation: P& T) (IAEA, 1982). The latest safety case report of Japan for the deep
geological repository has shown that 129I and 79Se are the primarycauses of potential radiation exposure in the far future, dependingon the assumed scenario (NUMO, 2018a; NUMO, 2018b).
In the past, many simulation studies on the transmutation of
LLFPs using nuclear reactors have been conducted (Arie et al.,⇑ Corresponding author.
E-mail address: liemph@nais.ne.jp (P.H. Liem).
Present address: Tokyo Institute of Technology, 2-12-1 N1-12 Ookayama,
Meguro-ku, Tokyo 152-850, Japan1

https://doi.org/10.1016/j.anucene.2021.108830
0306-4549/Ó 2021 Elsevier Ltd. All rights reserved.

2009; Ikegami et al., 2001; Naganuma et al., 2006; Sekimoto
et al., 1998; Takaki et al., 2002, 2003, 2004; Yang et al., 2004).
Recently, it has been shown that the transmutation of LLFPs is possible without isotope separation using a Monju (714MWt) classfast reactor (Chiba et al., 2017; Wakabayashi et al., 2019a;
Wakabayashi et al., 2019b; Wakabayashi et al., 2020). A sodiumcooled fast reactor was selected in the studies because of its highexcess neutrons and high neutron flux available for the transmutation. In this series of studies, the support ratio (SR: Destruction/
Production) is used as a performance index to represent the transmutation capability of the nuclear reactor. A nuclide with an SR
value of more than 1.0 can be reduced in the system, and on thecontrary, if it is less than 1.0 it cannot be reduced by transmutation. The studies showed that the SR values for all LLFP nuclides
are greater than 1.0, however, if the nuclear data uncertainties of
1r are taken into account, there are possibilities that SR values
for 79Se, 93Zr, and 135Cs may be lower than 1.0 (Yamano et al.,2021). In other words, if the uncertainty of the SR (which originated from the nuclear data uncertainty of the capture crosssection) is too large then even when the best estimate of SR is justabove 1.0, there is a possibility that the actual SR is less than 1.0
and the reactor system does not reduce the LLFP (but increase
the LLFP). Therefore, the accuracy of the nuclear data of these LLFPY. Tahara, Peng Hong Liem, N. Takaki et al.

Annals of Nuclear Energy 166 (2022) 108830

flux environment, we considered another irradiation field adjacentto the core, i.e. loading the test assembly in the inner reflector
region in contact with the core most outer fuel subassembly. As
will be shown later, we also incorporate moderator materials suchas YH2 and YD2 in the target subassembly for spectrum adjustment. The use of YH2 that has a strong moderating capability will
induce higher power peaks in the adjacent fuel assembly. To
resolve the problem, we propose the use of gadolinia thermal neutron shielding in the target subassembly design. The main goal or
purpose of our design study is to obtain an irradiation test assembly (hereafter, test-assembly) loaded into an appropriate irradiation field that can provide a sufficient amount of 135Cs. We
expect this will contribute to the cross-section measurements for
improving the nuclear data accuracy.
The structure of the paper is as follows. In Chapter 2 the Joyoreactor is briefly described. Chapter 3 describes the irradiation target, test assemblies, and loading positions proposed in the present
design. In Chapter 4, the design and analysis method used are first
explained, and then the analysis results are discussed. The conclusions of the present work are given in Chapter 5.

nuclides should be enhanced by measurements. To provide an adequate amount of samples for the measurement, one needs to investigate the LLFP target, irradiation assembly as well as irradiation
fields in the useable fast reactor (in this case Joyo reactor).
To obtain reliable nuclear data for LLFP transmutation, one has
to measure the neutron capture cross-section using the time-offlight method, etc., and for the measurements, several hundred
mg to 1 g of LLFP target material is required (Shcherbakov et al.,2005). In the case of LLFPs, however, it is difficult to procure such
an amount of LLFPs. To cope with the problem, an irradiation testof natural cesium containing LLFP (135Cs) in the burnup chain
was devised. This study was carried out by simulation analysis
assuming irradiation in the Japanese sodium-cooled fast experimental reactor ‘‘Joyo” (Maeda et al., 2005). In Joyo, an irradiation
subassembly was loaded in the outer reflector region of the twolayer reflector to form an appropriate irradiation field (Aoyama
et al., 2005).
In this paper, we report the results of the design study on thecesium target subassembly for two irradiation fields in the Joyoreactor. The first irradiation field is in the outer reflector region
same as in the previous study (Aoyama et al., 2005). Loading thetarget subassembly into this location is recommended by the Joyoreactor group since it would not affect the reactor characteristics(as will be shown later), however, the neutron flux level is not highenough, especially for our cesium case. To provide a high neutron

2. Experimental fast reactor Joyo
Joyo is the first FBR in Japan. It was constructed as an experimental reactor and achieved the initial criticality in 1977. The main

Fig. 1. Core configuration of Mark-IV and the loading positions for the test assemblies. The symbols of 5D1 etc. represent locations in the reactor core of Joyo.
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Table 1Main parameters of the Joyo Mark-IV core.
Parameters (Unit)

ValuesReactor thermal power (MWt)
Maximum number of fuel subassemblies*
Days per operation cycle (day)Operation cycles per year
Active core height (cm)
Initial fuel density (%TD)
Maximum Pu Content: Pu/(Pu+241Am + U) (wt%)
Fissile Pu content: (239Pu+241Pu)/(Pu+241Am + U) (Inner/Outer) (wt%)
O/M ratio
Primary coolant temperatures (Inlet/Outlet)(oC)
Reflector /Shielding
Control rod requirements (%Dk/k)

100
79
60
5
50
94
32
16/211.97
350/456
SUS/B4C
Maximum excess reactivity 2.0(burnup1.4 + operating margin 0.6)
Temperature and powercompensation 1.49 (100 degrees C  rated power)
1200
Cs2CO3 (4.24 g/cm3)
YH2, YD2
95
8.5/7.5
0.5/ SUS

Irradiation period (EFPD)
The chemical form of the target
Moderators
Moderator volume ratio in the target pellet (％)
Diameter of the target rod (Outer/Inner) (mm)Cladding thickness (mm) and material of the target rod
Length of target pellets and number of target rods in the irradiation test-assemblies
Test-assembly A
Test-assembly B*

2.0 cm/ 1 rod
5.0 cm/ 37 rods

Including the number of irradiation test fuel subassemblies

ing the fuels consists of three layers of stainless-steel
subassemblies (one inner reflector and two outer reflectors).
Shielding subassemblies with B4C are loaded in the two outermost
layers. The Mark-IV core configuration is shown in Fig. 1 and themain parameters of the core specifications are listed in Table 1.
The irradiation fields at the positions where the irradiation test
subassembly (hereafter, test-assembly) is loaded, i.e. Position-1
and Position-2 shown in Fig. 1 were examined.

role of Joyo is the improvement of technology for a fast reactor,
irradiation of fuel and material, and demonstration of innovativetechnology for putting a fast reactor into practical use (Maeda
et al., 2014). In the future, reducing the volume and toxicity ofradioactive waste (reducing environmental burden), studies on
the safety of fast reactors, and basic technology as a neutron irradiation reactor, etc. are planned as operational targets of Joyo
(Aoyama et al., 2005; Itagaki et al., 2017). Hence, the present study
on irradiation experiments related to LLFPs is in line with the operational targets of Joyo. The transmutation experiments of MA/LLFP
were also studied assuming irradiation in the prototype fast reactor Monju (Kitano et al., 2003), however, it was decided that Monju
would be decommissioned (Nagaoki, 2020).
Joyo is planned to restart with enhanced safety levels reflecting
the lessons learned from the Fukushima Daiichi Nuclear Power
Plant accident. Its core is redesigned and called Mark-IV (100MWt) (Itagaki et al., 2017). The core consists of two fuel regions(inner and outer regions), the reflector region and the shielding
region. The fuel region has an equivalent diameter of approximately 78 cm and a height of 50 cm. The reflector region surround-

3. Irradiation target, test assemblies, and loading positions
3.1. Irradiation target
Since LLFPs are produced from fissions in a nuclear reactor andextracted through chemical separation and refinement, it is difficult
to get a sufficient amount of LLFP needed for an irradiation experiment such as a nuclear cross-section measurement. Accordingly,
we decided to irradiate a natural element that produces an LLFP
nuclide as its isotope. Stable nuclides included in such a natural ele-

Fig. 2. Neutron capture and decay chains of Cs and Ba isotopes.

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Annals of Nuclear Energy 166 (2022) 108830

Fig. 3. Irradiation test assemblies.

capture cross-section and resonance integral of the LLFP 135Cs is8.302 (b) and 53.52 (b), respectively.
Cesium carbonate (Cs2CO3) was selected as the chemical formof cesium because of its high dissociation temperature. Yttrium
hydride (YH2) was used as a moderator to soften the neutron spectrum and increase the transmutation rate, and the irradiation target was assumed to be a homogeneous mixture of Cs2CO3 and YH2.
The volume ratios of Cs2CO3 and YH2 were determined to be 5%
and 95%, respectively, by a pin cell calculation with a buffer region
simulating Joyo’s in-core neutron spectrum to maximize theamount of 135Cs produced.

ment need to have large capture cross-sections to produce a sufficient amount of LLFP. Also, to produce the LLFP nuclide efficiently,
the maximum number of times of consecutive neutron capture reactions to reach the LFPP nuclide should be limited to two.
As the element and the LLFP nuclide which can meet suchrequirements, we selected natural cesium (which consists of only
133Cs) and 135Cs (See Fig. 2). Based on the JENDL-4.0 nuclear data
(Shibata et al., 2011), the thermal capture cross-section of 133Cs
is 28.90 (b) and that of 134Cs is 140.6 (b), while the resonance integral of 133Cs is 446.2 (b) and that of 134Cs is 72.50 (b). The thermal

Fig. 4. Core configuration around Position-1.
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three surfaces face the reflector subassemblies made of stainlesssteel. Yttrium hydride YH2 is employed as a moderator to achievea high transmutation rate even in a narrow region like the test
assembly with a width of about 8 cm. However, some of the moderated neutrons leak out of the test assembly and significantly
increase the power of the surrounding fuel rods. To avoid such
power peaks, gadolinia rods for shielding thermal neutrons areplaced in the outermost layer of the test-assembly A. Fig. 4 shows
the detailed core configuration around Position-1.
As can be seen from Fig. 5, the neutron capture cross-sections of155
Gd and 157 Gd are very large in the thermal energy region, but
small in the fast energy region. This means that fast neutrons fromthe fuel region enter the inside of the test assembly without being
absorbed by the gadolinia (Gd2O3) rods (hereafter, Gd rods) and areused for transmutation after being thermalized but thermal neutrons leaking from the test assembly without contributing to thetransmutation are absorbed by them. As a result, the power peak
due to the thermal neutrons can be suppressed.
As shown in Fig. 1, Position-2 is relatively far from the fuel subassembly compared to Position-1, so the moderator has a small
effect on fuel pin power, and various types of moderators can beused (Aoyama et al., 2005). Fig. 6 shows the core configuration
around the test-assembly B loaded at Position-2. The total neutron
flux at this position is 1/3 of that at Position-1. Therefore, atPosition-2, the utilization of intermediate energy neutrons issuitable for transmuting without decreasing neutron flux. For thisreason, aluminum is selected as the moderator. Fig. 7 shows the
cross-sections of 27Al and 133Cs. From this figure, it can be seen thataluminum has a small scattering cross-section that is almost constant but has a relatively large resonance scattering cross-section

3.2. Irradiation test assemblies and loading positions
The two test assemblies designed for neutron irradiation are
shown in Fig. 3. The test-assembly A is loaded in Position-1 (5D1in Fig. 1) and the test-assembly B is loaded in Position-2 (7D1 in
Fig. 1). Since Position-1 is next to the fuel subassembly the highneutron flux can be utilized for transmutation. The three surfacesof the test assembly face the fuel subassemblies and the otherFig. 5. Neutron capture cross-sections for 155Gd, 156Gd, and 157Gd (JAEA, 2021).

Fig. 6. Core configuration around Position-2.

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Annals of Nuclear Energy 166 (2022) 108830

in the energy range exceeding 104 eV. From this, it is expected thataluminum slows fast neutrons down into the resonance energy
region and forms an intermediate energy spectrum.
Six aluminum blocks (Al) are placed around the test-assembly B
to form an intermediate energy neutron spectrum. Furthermore,
yttrium deuteride (YD2), which has a smaller neutron capture
cross-section than YH2, is used as the moderator for testassembly B so that the neutron flux would not decrease.

4. Design and analysis results
4.1. Design and analysis method
The neutronics calculations were performed in a threedimensional, fully heterogeneous core model using the Monte
Carlo code MCNP6.2 (LANL, 2017) and the nuclear data library
JENDL-4.0. The number of effective neutron histories is 50 million.
Under this calculation condition, the worst uncertainty of the total
flux tally at the Cs targets is less than 0.83 % and 0.35 % for the irradiation Position-1 and 2, respectively, while the uncertainty of theFig. 7. Neutron capture and scattering cross-sections for 27Al and 133Cs (JAEA,
2021).

Fig. 8. Schematic flow diagram of burnup calculation.
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Fig. 9. Fine spatial divisions and groupings of Gd rods for tallying in the test-assembly A.

Fig. 10. Change in Cs isotopes with the irradiation time at Position-1.

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Annals of Nuclear Energy 166 (2022) 108830

Fig. 11. Change in Ba isotopes with the irradiation time at Position-1.

keff ¼ 0:99847  0:00008 for test-assembly A in Position-1,

effective multiplication factor (keff) is about 8 pcm. The fuel compositions are those at the beginning of the equilibrium cycle of
the Joyo Mark-IV core. It should be noted that for a typical fast
reactor like Joyo, the core compositions at the beginning and theend of the equilibrium cycle will not differ significantly, therefore,
the core composition at beginning of the equilibrium cycle is taken
as the representative composition for the present study.
The compositions of Cs, Ba, and Gd were calculated withFISPACT-II 3.20 (Fleming et al., 2018) using the VITAMIN-J 175energy-group spectra (Sublet et al., 2016) and the flux levels, whichwere obtained from the MCNP core calculations. As a self-shielding
method, the universal curve model in FISPACT-II was used in thecalculation. Since the isotopic compositions change with burnup,
the fuel rod power peaks of the subassemblies adjacent to testassembly A were calculated by MCNP using the isotopic compositions obtained after each burnup calculation. Thus, the MCNP corecalculation and the subsequent FISPACT-II burnup calculation are
repeated to proceed with burnup. Fig. 8 shows a schematic flow
diagram of the burnup calculation using MCNP and FISPACT-II.
The irradiation period is up to 1200 effective full power days
(EFPDs). This irradiation time corresponds to 4 years under theconditions of 60 days per cycle at rated power and 5 cycles per
year. The burnup step is determined by first observing the time
behaviors of the LLFP and gadolinium isotopes under a constantneutron flux spectrum and level. If the nuclides change rapidly
during the time interval initially set, it is changed to the shorterone. In real calculations, these variable time steps are adopted to
produce accurate burnup calculation results.

keff ¼ 1:00017  0:00008 for test-assembly B in Position-2.
The reactivity of the core in which the test-assembly A is loaded islow by 0.25 % against the reference core. This reactivity loss is within
the reactivity margin of 0.6% set for control rod maneuver as shownin Table 1 and thus can be compensated by the control rods.
4.3. Burnup analysis results
4.3.1. Results at Position-1
The material configuration in the test-assembly A loaded at
Position-1 and fine spatial divisions and groupings of gadolinia
rods are shown in Fig. 9. This figure reflects the MCNP calculation
model. The center pin contains the homogeneous mixture of
Cs2CO3 and the moderating material YH2 and is surrounded byeighteen moderator pins, which have only YH2. In the outermost
layer, eighteen gadolinia rods are loaded.
Special attention was paid to the gadolinium burnup calculation
because the gadolinium isotopic compositions vary very rapidly
with burnup and their distributions in the pellet strongly dependon the spatial self-shielding of neutron absorption. Accordingly,
each gadolinia pin was divided into five equal-volume areas astally regions in a burnup calculation to obtain a reliable result. Finetime steps were employed for the neutron spectrum and flux
reevaluation in each region in each pin at the early stage of theburnup calculation. Furthermore, since the burnup of gadolinium
isotopes strongly depends on the thermal flux, the Gd rods weregrouped into three as shown in Fig. 9. The neutron population
was tallied in each group in burnup calculations.
The change in Cs isotopes with the irradiation time is shown in
Fig. 10. This figure shows that 135Cs nuclide is produced to theamount of about 10% of the initial 133Cs (natural cesium) at 600
EFPDs, and at 1200 EFPDs it becomes comparable to the amountof the residual 133Cs.
The change in Ba isotopes with the irradiation time is shown in
Fig. 11. The production of 134Ba at 1200 EFPDs is almost equal to
the residual amount of 133Cs. The production of 135Ba and 136Ba
is lower than that of 134Ba approximately by a factor of 10. The
measured amounts of 134Ba､135Ba､ and 136Ba can be used as sup-4.2. Criticality analysis results
The test assembly is loaded into the position of either Position-1
or Position-2. The core without any test assemblies is the reference
core which consists of 75 fuel subassemblies. The control rods areat the position of 505 mm from the bottom of the core; the control
rod position fully withdrawn is 650 mm. The control rods were
fixed to this position in all the core calculations. The effective
multiplication factors (keff) are shown below,
keff ¼ 1:00100  0:00008 for no test-assembly (Reference),8

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Fig. 12. Gadolinium isotope concentrations in the test-assembly A as a function of the irradiation time. Group 1, 2, and 3 indicate the grouping of Gd rods shown in Fig. 9.

plementary data in the evaluation of the (n, c) reaction rates of133
Cs and 134Cs.
Fig. 12 shows the change in 155Gd and 157Gd isotopes as a function of the irradiation time. The 155Gd concentration decreases toabout half of its initial value at the burnup of 40 EFPDs andbecomes less than 1/10 at 1200 EFPDs. On the other hand, the157
Gd concentration becomes half after the burnup of about 14
EFPDs, which is earlier than 155Gd, but its residual amount at
1200 EFPDs is larger than that of 155Gd. Thus, it is seen that thechange of 157Gd with burnup is not large than that of 155Gd. Thisis because the depletion of 157Gd is compensated by the production
from 156Gd (n, c) reactions in the burnup chain of Gd isotopes asshown below.

64152Gd ! 64153Gd ! 64154Gd ! 64155Gd ! 64156Gd
ðn;cÞ

ðn;cÞ

ðn;cÞ

ðn;cÞ

 ! 64157Gd ! 64158Gd !
ðn;cÞ

ðn;cÞ

ðn;cÞ

From these, it is found that the role of Gd to suppress the power
peaking can be maintained for a considerably long period.

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Fig. 13. The axial power distribution of the hot assembly. The mean power of this assembly is normalized to 1.0.

Table 2
Evaluation of peaking factors and maximum linear power densities.
Burnup(EFPD)

0
0
540
1200
*
**
***

Peaking factor*

Linear powerDensity

FA

FRod

FR

Fz

FQ

q’max(W/cm)

1.006
0.890
0.918
0.924

1.784**
1.086***
1.126**
1.149**

1.795
0.967
1.034
1.062

1.146
1.146
1.146
1.146

2.057
1.108
1.185
1.217

432
233
249
256

Remarks

Without Gd rods
With Gd rods

The definitions of the peaking factors are shown in Sec. 4.3.
The peak power occurs in fuel neighboring to the test-assembly A (Fig. 14).
The power peak appears on the opposite side of the fuel subassembly surface on which the test-assembly A is neighboring (Fig. 15).

Since the Mark-IV core accommodates 75 fuel subassemblies
and each fuel subassembly has 127 fuel rods, the mean linear
power density q’ is, assuming the total thermal energy is generated
in the inner and outer fuel subassemblies for conservativeevaluation,

The power peak occurs in the fuel subassembly (4D1 in Fig. 1)
next to the test-assembly A and its value will increase with burnup
because of 155Gd and 157Gd depletion. The linear power density,
however, must not exceed its design limit of 330 W/cm in Joyo
to avoid fuel melting. Therefore, the linear power densities were
evaluated using the power peaking factors obtained by the simple
synthesis method.

hq0 i ¼

F A ¼ Assembly relativ e power in the core ðhot subassemblyÞ

100ðMW Þ  106
127ðrods=subassemblyÞ  75ðsubassemblyÞ  50ðcmÞ¼ 210 W=cm

F Rod ¼ Maximum rod power normalized in the subassembly

Using the power peaking factor and the mean linear power density, the maximum linear power density is evaluated as

F R ¼ Radial peaking factor ¼ F A  F Rod
F Z ¼ Axial peaking factor
Maximum linear power density of hot subassembly¼
Mean linear power density of hot subassembly
F Q ¼ Power peaking factor ¼ F R  F Z ¼ ðF A  F Rod Þ  F Z

q0max ¼ hq0 i  F Q
The power peaking factors and the maximum linear power densities are summarized in Table 2. In the case without Gd, all the Gd
rods were replaced with YH2 moderator rods. The normalized pin
power (distribution) is shown in Fig. 14 and the maximum one(FROD) is as high as 1.784. This large FROD causes a large linear
power density of 432 W/cm, which exceeds the design limit of
330 W/cm. On the other hand, when Gd rods are used as shieldingof thermal neutrons, the power of the same fuel rod reduces to

The power peaking factors FA, FRod, and FZ are obtained from the
MCNP calculation results. The axial power peaking factor FZ isdefined as the ratio of peak to average assembly power density
for the hot subassembly having the power peak, and its value is
1.146 from the axial power distribution shown in Fig. 13. Thisvalue of Fz was used for all the power peaking factor calculations.

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Fig. 14. Pin power distribution in the hot assembly when the test-assembly A has no Gd rods. (Gadolinium rods are replaced with YH2 moderator rods.)

4.3.2. Results at Position-2
The placement of the test-assembly B and Al blocks at Position2 is shown in Fig. 18. The test-assembly B has 37 target pins that
contain the mixture of Cs2CO3 and YD2, and it is surrounded by
six aluminum blocks. The core power peak is not an issue because
this position is far from fuel subassemblies and the yttrium deuteride (YD2) having lower slowing down power than YH2 is used
as a moderating material. As expected, the power peaking factor
was as low as 1.19.
The change in Cs isotopes with the irradiation time is shown in
Fig. 19. The production of LLFP 135Cs nuclide is about 1/1000 times
the initial amount of 133Cs (natural cesium) at 600 EFPDs and at
1200 EFPDs it reaches 1/100. The change in Ba isotopes with theirradiation time is shown in Fig. 20. Compared to Fig. 11 at
Position-1 the production of the Ba isotopes is 1/10 to 1/100. The
reason why the production is so small is due to the low neutron
flux. That is, the total flux at Position-2 is 6.0  1014 cm-2s1 andabout 3 times lower compared to that of 1.8  1015 cm-2s1 at
Position-1. The neutron spectra are shown in Fig. 21.
The production of 135Cs (initial loading and production) is summarized in (Table 3). The production rate of 135Cs is 7.17 %/EFPY at

1.026 as shown in Fig. 15. In this case, the peak power of this fuelassembly appears on the opposite side and FROD is 1.086 from
Fig. 15. From the burnup calculation results in Table 2, it is seenthat the linear power density is 256 W/cm even at 1200 EFPDs(although the gadolinium rods have already been irradiated considerably) and meets the design requirement. This demonstrates
that the utilization of Gd rods is effective in suppressing powerpeaks while achieving a high transmutation rate.
Neutron spectra of the target pellet (YH2 + Cs2CO3) in energy
and lethargy representations are shown in Fig. 16. The neutron
spectrum in energy representation increases monotonically in logarithmic scale in the energy range from 1 MeV to about 0.6 eV. Further, since a small thermal peak appears in the energy range
between 0.01 eV and 0.1 eV neutrons are found to be thermalized.
On the other hand, the neutron spectrum in the lethargy representation simply decreases with decreasing neutron energy in thesame energy region. Observing neutron spectra within the Gadolinia pellets shown in Fig. 17, the flux level of the neutron spectra ismuch lower than that shown in Fig. 16 for neutron energies below
1 eV and have no thermal peak. From this fact, one can conclude
that gadolinia effectively works as thermal neutron shielding.

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Fig. 15. Pin power distribution in the hot assembly when the test-assembly A has Gd rods.

Fig. 16. Neutron spectra of the target pellet (Cs2CO3 + YH2) of the test-assembly A.

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Fig. 17. Neutron spectra within the Gd pellets belonging to Group-1. Neutron spectra are denoted by u1 to u5 from the inside of the pellets toward the outside.

Fig. 18. Test-assembly B and the Al blocks at Position-2. The symbols of 6D1 etc. represent locations in the reactor core of Joyo.

increased by (1) using all 19 rods of YH2 + Cs2CO3, (2) increasing
the target height from 2.0 cm to the needed length, to provide
an adequate amount for cross-section measurements. Extending
the irradiation time more than 1200 days may not increase

Position-1 and 0.22 %/EFPY at Position-2. As mentioned before, theproduction rate at Position-2 is low because of the low flux leveland may not be considered further in the future. On the other hand,the 135Cs production capability at Position-1 can be potentially13

Y. Tahara, Peng Hong Liem, N. Takaki et al.

Annals of Nuclear Energy 166 (2022) 108830

Fig. 19. Change in Cs isotopes with the irradiation time at Position-2.

Fig. 20. Change in Ba isotopes with the irradiation time at Position-2.

can be produced in the Position-1 irradiation field adjacent to thecore with a specially designed Test-assembly A for 1200 EFPD irradiation time. Consistent with the previous study, the neutron flux
level in the outer reflector (Position-2) is not adequate consideringa realistic irradiation time. The use of YH2 moderator material
inside the test-assembly A is found effective for the neutron spectrum adjustment. More importantly, we proved that the newly
proposed gadolinia thermal neutron shielding adopted in testassembly A can suppress the power peaks of the adjacent fuelassembly while maintaining a high transmutation rate of natural
cesium into 135Cs. Our study also shows that the reactivity of theinsertion of the test-assembly A to the irradiation field adjacentsignificantly the production capability (cf. Fig. 10). Measurement ofisotopes of Cs and Ba in PIE will allow validating the cross-sectionsof the 133Cs(n,c)134Cs and 134Cs(n,c)135Cs reactions through theburnup analysis.
5. Conclusions
Two irradiation fields in the inner (Position-1) and outer reflector (Position-2), and their corresponding target subassembly (Testassembly A and B) designs to produce a sufficient amount of 135Cs
in the Joyo reactor (Mark-IV core, 100 MWth) were investigated.
The investigation results show that a sufficient amount of 135Cs
14

Annals of Nuclear Energy 166 (2022) 108830

Y. Tahara, Peng Hong Liem, N. Takaki et al.

Fig. 21. Neutron spectra in the target pellets (Cs2CO3 + YD2) of the test-assembly B and the surrounding moderators (Al).

Table 3
Summary of the production of 135Cs.

***

Test-assembly(Loading position)

Moderator material

Target height

Number of targets*

Initial loading (133Cs)

Production(135Cs)
Production rate**
360 EFPDs

1200 EFPDs

Assembly-A(Position-1)
Assembly-B(Position-2)

YH2
YD2

2 cm
5 cm

1
37

152.8 mg14.14 g

7.7 mg5.11 %/EFPY
12.6 mg0.09 %/EFPY

36.0 mg7.17 %/EFPY
102.2 mg0.22 %/EFPY

The target is the mixtures of Cs2CO3 and moderator, whose volume ratios are 5 % and 95 %, respectively.
1 EFPY = 365.25 EFPDs.

to the core can be compensated by the Joyo control rods. The
results of our investigation and target subassembly design are
expected to contribute to resolving the problem of providing a sufficient amount of 135Cs for cross-section measurements to enhance
the LLFP cross-section accuracy in the future.

by fast reactors”, entrusted to the Tokyo Institute of Technology
by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).

CRediT authorship contribution statement

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Yoshihisa Tahara: Investigation, Writing – original draft. Peng
Hong Liem: Methodology, Software, Writing – review & editing.
Naoyuki Takaki: Writing – review & editing. Satoshi Chiba:
Supervision.
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.
Acknowledgments
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