Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 3, pp. 279-282, 2005

Transmutation of Technetium in the Experimental Fast Reactor “JOYO”
Takafumi Aoyama,* Shigetaka Maeda, Yukimoto Maeda, and Soju Suzuki
Japan Nuclear Cycle Development Institute 4002, Narita, O-arai, Ibaraki 311-1393, JAPANReceived: July 31, 2005; In Final Form: September 30, 2005
The present study examines the potential for the demonstration of fission product transmutation in the experimental fast reactor JOYO at JNC’s Oarai Engineering Center. The possibility of creating a highly-efficient transmutation irradiation field of JOYO by loading neutron moderating subassemblies in the reflector region was investigatedin a series of scoping calculations. A cluster of reflector subassemblies was replaced with beryllium or zirconium
hydride (ZrH1.65) moderated subassemblies. These moderated subassemblies surrounded one central test subassembly that would contain 99Tc or 129I target material. With this modification, a high fast neutron flux can be tailored to epithermal or thermal energies. A comparison of reaction rates among the spectra is made illustrating theintegral effects on selected target reactions. The 99Tc transmutation rate achieved was 21.0% using ZrH1.65 and27.8% using beryllium as moderator. As a result of this study, basic characteristics of LLFP transmutation inJOYO using relevant moderator materials were clarified and the future feasibility was shown.

TABLE 1: Main Core Parameters of JOYO MK-III

1. Introduction
One of the most important roles of Partitioning and Transmutation (P&T) is to reduce the practical risk ascribed to thedisposed nuclear waste. Based on the current safety evaluation
for a deep geologic repository, the transmutation of Long-Lived
Fission Product (LLFP) could be more effective at reducing risk
than minor actinide transmutation due to the higher mobility ofLLFP in groundwater. In the feasibility study on commercialized fast reactor cycle systems, transmutation technology ofLLFP using fast reactors has been studied.1
The present study examines the potential to demonstrate thetransmutation of fission products such as technetium in theexperimental fast reactor JOYO at JNC’s Oarai Engineering
Center. 99Tc and 129I were considered as the target LLFP to betransmuted in this study because these nuclides have high fission yield, high dose risk, and a fast reactor is suitable for theirtransmutation.
2. Description of JOYOThe experimental fast reactor JOYO was constructed as thefirst step in sodium cooled fast reactor development in Japan.
Its current missions include improvements in fast reactor safety
and operation, and irradiation testing of advanced fuels andmaterials. The JOYO reactor attained initial criticality in 1977
with the MK-I breeder core. From 1983 to 2000, JOYO wasoperated at 100 MWt for thirty five operational cycles with theMK-II irradiation test core.
In 2003 the JOYO reactor upgrade to the 140 MWt MK-IIIcore was completed to increase the irradiation testing capabilityof JOYO. The MK-III core2, 3 incorporates significant changes
to the core size and arrangement, fuel enrichment, and reactor
power level compared to the MK-II core. This upgraded JOYOMK-III has been operated since 2003 with the primary missionof developing fuels and materials considered in the feasibility
study on commercialized fast reactor cycle systems.
The main parameters of the MK-III core are tabulated inTable 1. The MK-III core specifications were determined suchthat the fast neutron flux would be increased and a large number*Corresponding author. E-mail: aoyama.takafumi@jaea.go.jp.
FAX: +81-29-267-7481.

Specification
Reactor Thermal Power(MWt)
Maximum Number of Driver Fuel Subassemblies*
Equivalent Core Diameter(cm)
Core Height(cm)
235
U Enrichment(wt%)
Pu Content: Pu/(Pu+U)(wt%)
Fissile Pu Content: (239Pu+241Pu)/(Pu+U)(wt%)
Maximum Linear Heat Rate of Fuel Pin(W/cm)
Maximum Burn-up of Fuel(Pin Average)(GWd/t)Total Neutron Flux(n/cm2·s)
Fast Neutron Flux(n/cm2·s)
Number of Control Rod
In 3rd Row
In 5th Row
Reﬂector/Shielding
Primary Coolant Temperature (Inlet/Outlet) (˚C)
Operation Period per Cycle(day)
Operation Cycle per Year(cycle)

Data
140
85
80
50
18
23/30**
16/21**
42090
5.7 × 1015
4.0 × 1015
42
SUS/B4C
350/50060
5*Including Number of Irradiation Test Fuel Subassemblies**Inner Core /Outer Coreof irradiation rigs could be irradiated simultaneously. The fuelregion in the MK-III core is divided into two radial enrichment
zones to flatten the neutron flux distribution. The active core iscylindrical and about 80 cm in equivalent diameter and 50 cm inheight. There is a radial reflector region of stainless steel surrounding the core that is 25 to 30 cm thick. Shielding subassemblies with B4C are loaded in the outer two rows of the reactor
grid, replacing radial stainless steel reflector subassemblies.
As a result, the fast neutron flux is up to 1.3 times higher thanthat of the MK-II core, with 140 MWt reactor power. Also, theirradiation test field space with a high neutron flux is approximately double that of the MK-II core.
Four neutron environments are available for experiments inthe upgraded JOYO MK-III, ranging from the reactor core to
the ex-vessel irradiation hole as follows.
(1) Fuel Region: High temperature and high neutron flux(2) Reflector Region: Softer neutron spectrum and two orders
lower neutron intensity than fuel region© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2005

280 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005

Aoyama35

Core CenterBeTran smu tatio n rate of Tc-99(%/yr)

30

AZrH1.65

2520
151052
Irradiation Test RigReflector

Inner Driver Fuel

B 4 C Shielding

Outer Driver FuelModerator

Control Rod

Target Subassembly00

1.0E+15

Total neutron flux (n/cm2·s)(3) Upper Core Region: Higher temperature and softer neutron
spectrum than reflector region(4) Ex-vessel Region: Lower energy neutron dominated

3.1. Neutron Moderator and Target Subassembly. The
possibility of creating a highly-efficient transmutation irradiation field by loading neutron moderating subassemblies in thereflector region was investigated in a series of scoping calculations.
Figure 1 shows a layout of the JOYO core for this study,
where a cluster of reflector subassemblies was replaced withnew moderator and target subassemblies. Many materials wereconsidered for use as a fast reactor neutron moderator, including hydride materials, B4C enriched in 11B, graphite, and beryllium (Be).
Among hydride materials, the investigation indicated that
ZrH1.65 is the most promising candidate from the view point of
temperature resistance and small neutron capture cross section.1 However, hydride materials have the problem of dissociation of hydrogen, so further investigation is required.
On the other hand, beryllium has better transmutation performance than 11B4C and we have experience using berylliumin JOYO as a neutron source material by means of the Be (γ, n)reaction. Beryllium metal or zirconium hydride (ZrH1.65) werepreliminary selected for the moderator.
Six of the row 7, 8 and 9 reflector subassemblies were replacedwith beryllium or ZrH1.65 moderated subassemblies. The structure of a moderator subassembly is similar to the other corecomponents. The wrapper tube is made of stainless steel (SUS).
The moderator fraction ratio (moderator/(moderator + sodium +SUS)) was varied in the range from 0% to 100%. The moderator subassemblies surrounded one test subassembly that would
contain 99Tc or 129I target material. As the purpose of this study
was to investigate neutronic feasibility of the LLFP transmutation, the form of the target material was not considered.
This core configuration was based on the fact that the volume of a single subassembly is not large enough to contain sufficient moderator material. Therefore, the concept of separating
moderator and target subassembly has a clear advantage for theirradiation tests. The radial reflector between the driver fueland the moderator subassemblies works as a thermal neutron
buffer that splits the thermal neutron region of the moderator

6080100

Figure 2. 99Tc transmutation rate as a function of moderator fraction.
1.2E+15

3. Neutron Flux Tailoring

40

Moderator fraction (%)

Figure 1. Target locations in JOYO core (1/3rd core model).

In the fuel region, the total flux is 3 to 5 × 1015 n/cm2·s and thespectrum index (φ E>0.1MeV /φ total) is approximately 0.6 to 0.7. In
the ex-vessel irradiation hole, the total neutron flux magnitude
is about 1012 and the spectrum index is on the order of 0.01.

20

8.0E+146.0E+144.0E+14

Be
2.0E+140.0E+00

ZrH1.65

020

40

60

80

100

Moderator fraction (%)

Figure 3. Neutron flux change as a function of moderator fraction.

subassemblies from the fast neutron region of the nearby driver
fuel subassemblies. This avoids the thermal spike that may
occur near the boundary between extremely different neutron
spectrum regions.
3.2 Analysis Method. The neutron flux distribution in thecore was calculated in 70 energy groups using the three-dimensional diffusion code CITATION4 in this study. The geometricmodels used homogeneous representations for all the core component subassemblies. The JFS-3-J3.2R cross section set basedon the JENDL-3.2 library5 was used for the neutron spectrum
and transmutation calculation. The absolute value of the neutron flux was determined from the heat balance at the 140 MWt
full power condition.
The ORIGEN2 code6 was used to provide transmutation rates
and one group cross sections were collapsed using the calculated 70 group neutron spectrum and the JFS-3-J3.2R cross sections without considering neutron self shielding effects. Five
operational cycles per year were assumed in the ORIGEN2 calculation, where each cycle runs 60 effective full power days.
The change of neutron flux distribution associated with fuelburn-up and core arrangement change due to refueling is notconsidered here, however it does not affect the major transmutation characteristics.
4. Results
4.1. Moderator Composition. Figure 2 shows the calculated
results for the 99Tc transmutation rate. This could be increased
by increasing the moderator fraction of beryllium due to the large
resonance in the 99 Tc (n, γ) cross section around 6 eV. When
ZrH1.65 is used for the moderator, however, the transmutation rate
has a peak at about 30% fraction caused by a cancellation between
the capture reaction increase due to neutron moderation and theneutron shielding effect, as shown in Figure 3.

J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005 281

Transmutation of Technetium in the Experimental Fast Reactor “JOYO”
1.0E+16

Neutron Spectrum (n/cm2 s/Lethargy)

Transmutation rate of I-129(% /yr)10864Be

20ZrH1.65020406080

100

1.0E+15
1.0E+141.0E+13
1.0E+12

1.0E+101.0E+09
1.0E+08
1.0E-09

Moderator fraction (%)

Fraction Ratio: ~60%

1.0E-07

1.0E-03

1.0E-05

1.0E-01

1.0E+01

Neutron Energy (MeV)

Figure 4. 129I transmutation rate as a function of moderator fraction.

Moderator

8.15 cm

SUS(Base)Be(90%)
ZrH1.65(60%)
ZrH1.65(30%)
Core Region(1st Row)

1.0E+11

Figure 6. Comparison of neutron spectrum in various locations ofJOYO.

TABLE 3: Energy Dependent Neutron Flux

Fraction Ratio: 90%

Rad. reﬂector

Neutron Flux (n/cm 2 ·s)
Ratio (%)
Total
> 0.1 MeV < 1 keV < 0.312 eV
8.71 × 1014
49.7
9.9
0.0

Be (90%)9.95 × 1014

16.8

48.6

6.4

ZrH1.65 (30%)

7.69 × 1014

24.5

51.0

18.2

ZrH1.65 (60%)

5.68 × 1014

17.8

65.7

36.8

Core region

5.53 × 1015

70.6

0.4

0.0

Figure 5. Cross section view of moderator subassemblies.

TABLE 4: Transmutation Rate at the Core Mid Plane
For irradiating 129I targets, the transmutation rate showed thesame tendency as 99Tc. However, using ZrH1.65 moderator indicated a peak at around 60% fraction as shown in Figure 4. Thisis due to the different neutron energy sensitivity to the capture
cross section between 99Tc and 129I. Higher 129I transmutation
could be achieved with a softer neutron spectrum than 99Tc.
The cross section views of the subassemblies with the optimized moderator fraction are illustrated in Figure 5. Table 2
shows the main design parameters of the moderator subassemblies. The one structure with a fraction under 60% is similar
to the B4C shielding subassembly and the other fraction of 90%
is similar to the outer radial stainless steel reflector. Both B4C
shielding subassembly and outer radial reflector are routinely
fabricated for use in JOYO.
TABLE 2: Moderator Subassembly Parameters
Subassembly length2970 mmModerator stack length

650 mmModerator

BeZrH1.65

Number of element
Fraction ratio of the moderator material

190%

730, 60%

4.2. Neutron Spectrum and LLFP Transmutation rate.
Figure 6 shows the 70 group neutron spectrum and flux values
in the target subassembly. The spectra averaged over the 5 cm
axial height at core mid-plane were shown and compared withthose in the driver fuel and radial reflector regions. In thesecases, the moderator fractions in the moderator subassemblies
were the optimized results above. The neutron flux in four different energy ranges for the range of cases examined is summarized in Table 3. These data show that the installation of themoderator and target subassemblies could tailor the spectrum
to epithermal or thermal energies.
Table 4 compares the corresponding transmutation rates atthe core mid-plane. The 99 Tc transmutation rate was 21.0%

Fractional transmutation rate (%/yr)
99129
Tc
I
Rad. reﬂector

3.0

1.7

Be (90%)

27.8

6.6

ZrH1.65 (30%)

21.0

6.8

ZrH1.65 (60%)
Core region

17.1
5.4

7.7
3.4

using ZrH1.65 and 27.8% using beryllium as moderator.
4.3. Moderator Effect on Core Components. The effect ofadding moderation extends to the neighboring core componentsof driver fuel and B4C shielding subassemblies. Moderated neutrons leaking from the moderator subassembly can reach andbe absorbed in the adjacent fuel pins. This causes fuel pin
power to increase since the 239Pu fission cross section is higher
at thermal energies than at fast energies.
Figure 7 shows the ratio of 239Pu fission rate in the fuel pinsof the driver fuel subassembly shown as “A” in Figure 1 to thatof the original radial reflector subassembly. In this calculation,
one subassembly was divided into 6 triangular meshes. Whenusing beryllium the fission rate at the mesh nearest the moderator (No. 5) is ~19% higher than the reference value for thereflector case. However, the maximum fission rate is only 7%
higher than the original maximum value at mesh No. 2.
Considering the radial fuel power distribution, this is not aproblem. The driver fuel subassemblies in the 4th and 5th rows
generally have adequate margin to the peak power limits during rated 140 MWt operation. This fuel power increase can becountered by replacing fresh fuel with burned fuel or rotating
the fuel subassembly. On the other hand, the fission rates whenusing ZrH1.65 moderator were decreased in all meshes due tothe large neutron flux depression.
Next, the influence on the B4C shielding subassemblies wasevaluated and the results are shown in Figure 8. The 10B(n, α)
reaction rate decreased in all cases due to neutron flux depression.

282 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005

AoyamaCore CenterYear

2005

2006

3cy~6cy

5.8E+15

×10−24 reaction/s

Fission Reactio n Rate of Pu-239

314 5 6SUS(Base)Be(90%)
ZrH1.65(30%)

5.6E+15

2009

2010

12cy~16cy

2011
17cy~

Basic Design
LicensingBe

Detail Design
Construction
Irradiation Test
Basic Design

HydrideMaterial

5.4E+15

Licensing
Detail Design
Construction5.2E+15

Irradiation Test

Figure 9. Schedule of LLFP transmutation test in JOYO.

5.0E+15
4.8E+15
4.6E+15123456Mesh Number at A in Figure 1

Figure 7. Pu fission rate at driver fuel A with and without moderator.

Core Center
4.5E+142314 5 6

4.0E+14
3.5E+14-2 4×10 reaction/s(n, α) Reaction Rate of B-10

2008

7cy~11cyReactor Operation

2

6.0E+152007

3.0E+14SUS BaseBe(90%)
ZrH1.65 30%

2.5E+142.0E+14
1.5E+141.0E+145.0E+13
0.0E+00

123456Mesh Number at No.2 in Figure 1Figure 8. 10B (n, α) reaction rate at shielding subassembly No. 2 withand without moderator.

Optimum spectral tailoring depends on the target LLFP tobe transmuted. Just the thermal portion of the neutron flux
achievable in JOYO can be comparable to many thermal experimental reactors. While the introduction of the moderated subassemblies into the reflector region affects the adjacent driver
fuel subassemblies, the effects are within the design margin or
can be easily mitigated by fuel management strategies.
Further investigations, such as detailed pin by pin model calculations using the MCNP code will be required to precisely
evaluate the effect of neutron moderators on fuel subassemblies.
Neutron self shielding effect on the transmutation ratio of thetarget nuclide will be calculated and the material concentrationin the test pin needs to be optimized. Thermal-hydraulic andstructural calculations need to be carried out for the moderator
subassembly design.
The planning schedule to install the neutron moderator into
JOYO is shown in Figure 9. Beryllium will be adopted firstbased on the previous experience in JOYO. The next step will
use hydride materials to achieve higher transmutation rates.
Acknowledgements. The authors would like to note thecontribution of Mr. T. Ito of Information Technology Japan Inc.
for the calculation using CITATION. We also would like toappreciate Mr. D. W. Wootan, the JNC international fellow, for
his cooperation and valuable comments.
References

5. Concluding Remarks
These studies demonstrated that the JOYO reactor has theflexibility to tailor the neutron spectrum in the reflector region
to accelerate LLFP transmutation experiments and also to meeta wide variety of irradiation test missions. The irradiation
space and excess neutrons inherent in the JOYO design allows
this spectral tailoring for LLFP transmutation and other purpose such as isotope production with minimal impact on othersimultaneous irradiation missions.

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