GENES4/ANP2003, Sep. 15-19, 2003, Kyoto, JAPAN
Paper 1102
MA/LLFP Transmutation Experiment Options in the Future Monju Core
Akihiro KITANO1, Hiroshi NISHI1*, Junichi ISHIBASHI1 and Mitsuaki YAMAOKA2
International Cooperation and Technology Development Center, Japan Nuclear Cycle Development Institute, Tsuruga, Fukui,
919-1279, Japan
2
Power and Industrial Systems Research and Development Center, Toshiba Co., Kawasaki, Kanagawa, 210-0862, Japan
1
Future experimental irradiation test options for the minor actinide (MA) and long-lived fission product
(LLFP) transmutation in Monju core have been studied to search for and to demonstrate the possible
contribution of Monju to the future commercialization of FBR technology. It was shown that MA
incineration rate for 148 EFPDs by 5 cycles operation was evaluated to be 26% to 32% for the core region
loading case and 13% for the radial blanket region loading case. As regards LLFP irradiation test, the
transmutation rate was evaluated to be 0.7%/year for without-moderator case and 1.2%/year to 1.4%/year for
with-moderator case. These results showed the capability of Monju core for profitable experimental data
acquisition, without any significant disturbance on the core characteristics with some slight design
modifications.
KEYWORDS: fast reactors, minor actinides, long-lived fission products, irradiation experiment,
transmutation
I.
Introduction
Deposit of high-level radioactive wastes (HLW) is
indispensable to nuclear fuel cycle.
Reduction of
environmental burden can be achieved by reducing the
radioactive hazard of the minor actinides (MAs: Np, Am,
Cm) and the long-lived fission products (LLFPs: 99Tc, 129I,
135
Cs and so on), contained in the HLW as a result of nuclear
power generation. It was known that the radioactive hazard
of the HLW would be decreased to that of the original
natural uranium ore after several hundreds years assuming
that all the MAs and LLFPs are separated from the HLW and
transmuted into short-lived radioactive nuclides or stable
nuclides in the nuclear reactors.
Transmutation of MA/LLFP can be efficiently and
effectively achieved by the fast neutron spectrum system
because of its high neutron flux level and its neutron surplus
available for transmutation when compared with the thermal
or other spectrum systems. Much larger fission-to-capture
cross-section ratio of MAs in the fast spectrum system
substantially makes it as the almost only effective way of
MA transmutation1-4). Negligibly small contribution of
thermal neutrons in the active core region in the fast
spectrum system also enables effective MA transmutation,
especially with large amount of core loading.
Fast reactors can offer wider range of neutron
spectrum and higher level of neutron flux than that of LWRs.
In the MA/LLFP separation and transmutation studies, fast
reactor has been and still is considered to be one of the most
feasible and promising candidates5).
MA/LLFP
transmutation experiment options in the future Monju core
are to be proposed based on the above mentioned
fundamental considerations in this study.
*
Corresponding author, Tel. +81-770-39-1031, FAX. +81-77039-9226, E-mail: [email protected]
are to be proposed based on the above mentioned
fundamental considerations in this study.
Investigation of the MA transmutation experiment has
been performed based on the preceding studies in this field.
Reactor design studies on MA transmutation in FBR cores
have revealed that it reduces burn-up reactivity swing while
increasing the sodium void reactivity and reducing the
Doppler coefficient, which needs some safety considerations.
Because of these safety considerations the maximum ratio of
MA mixture in the core fuel is said to be limited below 5%
and to be allowed up to 10% with some penalty on core
characteristics.
Taking into account the influence of the MA-mixed
test fuel assembly loading on the core characteristics, both
the nine test assemblies loaded in the core region case and
the eighteen assemblies loaded in the radial blanket region
case were studied. And the radial blanket region loading
case was evaluated to be preferable to avoid any significant
disturbance on the core characteristics.
As regards LLFP transmutation, 99Tc was selected as
the typical target nuclide to be irradiated6) because of its
large neutron absorption cross section, chemically inert form
in the reactor core as a metal and no need for isotopic
separation. LLFP loading affects the core characteristics as
well as MA loading. Fifty-four test assemblies, at the
maximum, loaded in the radial blanket region case was
studied. Zirconium-hydride (ZrH1.7 ; The hydrogen ratio is
an assumed value) was partially loaded in the test assembly
as the neutron spectrum moderator.
This maximum
number of assemblies was assumed in order to evaluate the
allowable number of loaded assemblies.
The results showed the capability of Monju core for
profitable experimental data acquisition, without any
significant disturbance on the core characteristics.
GENES4/ANP2003, Sep. 15-19, 2003, Kyoto, JAPAN
Paper 1102
Monju reference core layout
Core layout for MA transmutation experiment
(Homogeneous loading type)
Core layout for MA transmutation experiment
(Heterogeneous loading type)
Core layout for LLFP transmutation experiment
Inner core fuel assembly
Control rod (Back up)
Outer core fuel assembly
Irradiation test rig (MA)
Radial blanket fuel assembly
Irradiation test rig (LLFP)
Control rod (Primary)
Neutron source assembly
Fig. 1
Core layout of Monju
GENES4/ANP2003, Sep. 15-19, 2003, Kyoto, JAPAN
Paper 1102
II.
Prototype Fast Breeder Reactor Monju
Experimental irradiation of MA/LLFP transmutation
was assumed in the future Monju core in this study. Monju
is the prototype fast breeder reactor in our country with an
electric power of 280MWe (714MWt), a sodium-cooled
loop-type reactor.
The Monju core consists of 715
assemblies (198 core fuel assemblies, 172 radial blanket fuel
assemblies, 19 control rod assemblies, etc.) and is fueled
with UO2-PuO2 mixed oxide fuel. The core layout of
Monju is shown in Fig. 1 and the fundamental core
specifications are shown in Table 1. Figure 2 shows the
side view of Monju core. Monju has been shut down since
December 8, 1995 when the sodium leak accident occurred
in the secondary heat transport system.
The Role of Monju was clearly redefined in the
"Long-Term Program for Research, Development and
Utilization of Nuclear Energy”, revised by Atomic Energy
Commission in November 2000, after the accident. Monju
can be considered as one of the assets of the humankind,
which can demonstrate the prominent FBR characteristics in
the nearly commercialized scale. The confirmation of the
fundamental performances, such as breeding ratio, etc. has
been and continues to be its mission. This is essential for
the prototype reactor. Monju should be restarted at the
earliest stage possible, and the sodium handling technology
should be established for that purpose. The demonstration
of the reliability as a power station should be pursued
simultaneously.
The earliest demonstration of the world’s most
advanced technology will be the next mission. Adjustment
of the plutonium stockpile and incineration of trans-uranics,
etc., are to be demonstrated as pursued in the “Feasibility
Study on Commercialization of FBR Cycle Technology”,
conducted by JNC. The possibility of Monju, to contribute
to the future commercialization of FBR technology, should
be pursued and demonstrated.
Accordingly, the irradiation experiments of
Inner Core
930
Upper Axial Blanket
300
2400
1788
Lower Axial Blanket
Radial Blanket
Fig. 2
Monju core side view
350
Outer Core
Unit : mm
MA/LLFP transmutation are to be performed just after the
confirmation of the current core characteristics.
III. Analytical Calculations and Results
MA and LLFP transmutation irradiation experiments
were studied individually because the disturbances, to be
estimated, on the core characteristics of the MA and LLFP
loading are different depending on the irradiation
Table 1
Monju core specifications
Sodium-Cooled
Reactor type
Loop-Type
Thermal power
714MW
Electrical power
280MW
Operational cycle length
148 days/cycle
Core dimensions
Equivalent diameter
179cm
Height
93cm
Burn-up reactivity swing
2.5 %k/kk'
Core fuel burn-up (GWd/t)
Average
80
Assembly maximum
94
Maximum linear heat rate
360 W/cm
Fuel
No. of driver assemblies
198
Fuel type
UO2-PuO2
(Inner core/
Plutonium enrichment
Outer core)
Initial core
15/20 Pu fissile %
Equilibrium core
16/21 Pu fissile %
Fuel inventory
Core (U+Pu metal)
5.9ton
Blanket (U metal)
17.5ton
Cladding outer diameter
0.65cm
Cladding thickness
0.047cm
Cladding material
SUS316
No. of pins per assembly
169
Duct flat-to-flat
11.1cm
Duct pitch
11.6cm
Blanket
Material
UO2
Axial blanket
Top length
30cm
Bottom length
35cm
Radial blanket
No. of assemblies
172
Pellet diameter
1.04cm
Pin outer diameter
1.16cm
No. of pins per assembly
61
GENES4/ANP2003, Sep. 15-19, 2003, Kyoto, JAPAN
Paper 1102
experiment concepts.
Core
characteristics
were
evaluated
by
two-dimensional R-Z 7 energy groups diffusion calculations
taking burn-up effect into account. Seven energy group
cross sections were derived from the 70 energy group cross
section set JFS-3-J3.27) generated from the JENDL-3.2
library8) by collapsing. The energy group structure adopted
is shown in Table 2.
The cross sections of the rare earth metals, which
were considered to be inevitably mixed up with MAs up to
20%, were synthesized from the individual cross sections as
a lumped FP to preserve the total neutron absorption cross
section.
1. MA Transmutation Experiment
MAs can be incinerated more efficiently in fast
spectrum than thermal spectrum because MA nuclides have
Table 2
some threshold reaction and much larger fission-to-capture
cross-section ratio in the fast neutron spectrum. Following
two concepts of irradiation experiment were assumed for the
MA transmutation in this study.
The one is MOX fuel mixed with MAs, which is
called “Homogeneous loading type”, assuming the future
homogeneous whole core loading of MAs. Rare earth
metals were assumed to be included in MAs up to 20%,
which were considered to be difficult to separate from MAs
completely in the reprocessing procedure. The ratio of MA
in the MOX fuel was assumed to be 5 or 10 wt% because
MA mixture could cause a deterioration of material
properties such as thermal conductivity or melting point.
The maximum ratio of MA mixture in the core fuel is also
limited to avoid any significant disturbance on the core
characteristics.
7 energy groups structure
Energy Group
1
2
3
4
5
6
7
Table 3
Energy Range
10.0MeV-1.35MeV
1.35MeV-388keV
388keV-86.5keV
86.5keV-9.12keV
9.12keV-961eV
961eV-101eV
101eV-0.00001eV
Composition of MAs
MA
Nuclide
Composition
(%)
Np
Am
42.7
33.5
Cm
3.8
Rare earth*
20
Isotopes
Composition
(%)
237
Np
Am
242
Am
243
Am
242
Cm
243
Cm
244
Cm
245
Cm
246
Cm
100
75.4
0.2
24.4
0
1.5
93.7
4.3
0.5
241
*Decontamination factor: La/52, Ce/72, Pr/28, Nd/46,
Pm/20, Sm/20, Eu/10, Gd/10
Table 4
Effect of MA transmutation test rig loading on the core characteristics
Item
5%MA in MOX fuel
10%MA in MOX fuel
30%MA mixed with MgO
Number of test assemblies loaded
MA transmutation rate
(%/740EFPD)
Total amount of transmuted MA
(kg/740EFPD)
Reactivity change (%k/kk’)
Burn-up reactivity swing*
Coolant density coefficient*
Doppler coefficient*
9
9
18
28
32
13
0.33
0.76
0.93
-0.18
1.00
1.05
0.96
-0.29
0.99
1.10
0.93
+0.03
1.01
1.03
0.98
Power peaking factor*
1.02
* Normalized to the Monju reference core (1.0)
1.04
1.00
MA content (kg/assembly)
GENES4/ANP2003, Sep. 15-19, 2003, Kyoto, JAPAN
Paper 1102
MA content (kg/assembly)
Homogeneous loading type (5%MA)
MA content (kg/assembly)
Homogeneous loading type (10%MA)
in the material property aspects such as thermal conductivity
or melting point, compared with the homogeneous loading
type, while some disadvantage in the nuclear characteristics
aspects, such as coolant density coefficient or Doppler
reactivity feedback, should be mitigated.
The MA
assemblies of heterogeneous loading type were assumed to
be loaded in the radial blanket region to avoid any
significant disturbance on the core characteristics. The
core layouts of each case are shown in Fig. 1.
MA isotopic composition was estimated based on the
LWR spent fuel composition of 33GWd/t burn-up,
reprocessed after 5 years cooling-off interval from the
discharge. In both cases, nine isotopes of MAs, shown in
Table 3, were selected as the targeted nuclides to be
transmuted. The irradiation interval was assumed to be
148 EFPDs by 5 cycles as same as the current driver fuels.
The results of the core characteristics analysis are
shown in Table 4. In case of homogeneous loading type,
the 10% MA loading affected the core characteristics more
than the 5% of MA loading by double. The resulting MA
transmutation rate was evaluated to be 28% (5% MA loading
case) to 32% (10% MA loading case).
The Doppler coefficient was evaluated to be
decreased by 7% and the coolant density coefficient be
increased by 10% at the maximum. MAs are strong
absorbers and fissile materials especially in the fast neutron
spectrum, resulted in harder neutron spectrum. The control
rod worth was estimated to be reduced by 1% due to the
same reason. Special emphasize was put forward on the
power peaking increase by 2% (5% MA loading case) to 4%
(10% MA loading case).
On the other hand, as for the heterogeneous loading
type, the MA transmutation rate was evaluated to be 13%,
while the influence of the MA irradiation assembly loading
resulted in negligible core characteristics change, compared
with the homogeneous loading type. So, it seems to be
possible to load more than 18 assemblies without any
significant disturbance on the core characteristics.
Although the MA transmutation rate is not so large in this
case, the heterogeneous loading type in the radial blanket
region is preferable for large amount of MA loading.
The details of MA content change by irradiation for
each isotope are shown in Fig. 3.
2.
Heterogeneous loading type (30%MA)
Fig. 3
MA content change by irradiation
The other is MA diluted with MgO by 30%, which is
called “Heterogeneous loading type”, assuming the future
heterogeneous core loading of special MA incineration
assemblies. Higher MA mixture ratio can be achieved in
this case because this type of MA loading has an advantage
LLFP Transmutation Experiment
As for the LLFP transmutation experiment, 99Tc was
selected as the typical targeted nuclide, which needs no
isotopic separation because of its single isotopic
composition. 99Tc has relatively larger absorption cross
section of approximately 1 barn under fast neutron spectrum
and has stable chemical form in the reactor core as a metal,
which is favorable for in-core irradiation.
Experimental assemblies with both 99Tc pins and
moderator (ZrH1.7) pins were assumed to be irradiated in the
radial blanket region. The number of test rigs loaded was
assumed to be 54 at the maximum. The ratio of the
moderator pins was varied parametrically from 0% (without
GENES4/ANP2003, Sep. 15-19, 2003, Kyoto, JAPAN
Paper 1102
100% Tc-99 pins
78.7% Tc-99 pins
60.7% Tc-99 pins
99
Tc pin
ZrH1.7 pin
Fig. 4 Pin arrangement in the test rigs for 99Tc irradiation experiments
99
Tc absorption cross section
2.0
Total flux
Reference
99
Tc 100%
600
99
1.0
1.0
0.5
0.5
Power density (W/cc)
1.5
1.5
500
Total flux (×1015 n/cm2/sec)
Absorption cross section (barn)
2.0
0.0
0.0
0
0.2
0.4
0.6
0.8
1.0
Power peak
400
300
200
100
0
0
1.2
Volume ratio of 99Tc ; [99Tc / (99Tc + ZrH )]
20
40
60
80
100
120
Distance from center (cm)
Fig. 6 Power distribution in the core
Fig. 5 Correlation of total flux and absorption cross
for 99Tc transmutation experiment
section on the ratio of 99Tc
Table 5
Tc 60.7%
Effect of LLFP(Tc-99) transmutation test rig loading on the core characteristics
100% 99Tc pins
78.7% 99Tc pins
60.7% 99Tc pins
Reactivity change (%k/kk’)
Burn-up reactivity swing*
54
0.73
37.0
-3.3
1.26
54
1.20
47.9
-4.2
1.26
54
1.37
42.2
-4.5
1.26
Sodium void reactivity*
0.63
0.54
0.39
Item
Number of test assemblies loaded
LLFP transmutation rate (%/year)
Amount of transmuted LLFP (kg/ year)
* Normalized to the Monju reference core (1.0)
moderator) to 40%, as shown in Fig. 4, to investigate the
effect of the moderator on the transmutation rate and the
possible amount of loaded 99Tc. Core layout is shown in
Fig. 1.
The results of the core characteristics analysis are
shown in Table 5. The transmutation rate was evaluated to
be 0.7%/year for without-moderator case and 1.2%/year to
1.4%/year for with-moderator case. The reactivity was
decreased by 3.3%k/kk’ in case of 99Tc loading without
moderator, while the reactivity decrease with moderator was
GENES4/ANP2003, Sep. 15-19, 2003, Kyoto, JAPAN
Paper 1102
estimated to be 4.5%k/kk’. Sodium void reactivity was
reduced approximately by half compared with the reference
core in case with moderator. The disturbance on the core
characteristics seemed to be not negligible in this maximum
loading case.
The total amount of transmuted 99Tc per year
remained as the same level in both the with-moderator and
the without-moderator cases, in spite the transmutation rate
was improved in the with-moderator case. This comes
from the fact that the increase of the moderator pins limits
the number of 99Tc pins resulting in decrease of the total
amount of loaded 99Tc. Figure 5 shows the correlation
between the moderator ratio and the effective absorption
cross section. The higher moderator ratio leads to softer
neutron energy spectrum, and results in larger effective
absorption cross section.
On the other hand, in the with-moderator case, a steep
power skew at the adjacent fuel assemblies was observed
around the test rigs. Some design modification, such as
local adjustment of plutonium enrichment, etc. might be
necessary to mitigate this power skew, shown in Fig. 6.
As a result, the number of test assemblies was found
to be limited below one third or one sixth of the maximum
loading case, for example, to avoid any significant
disturbance on the core characteristics.
IV. Conclusion
1. MA Transmutation Experiment
In case of MA incineration experiment in the Monju
core, nine test assemblies with MOX fuel mixed with 5%
MA loaded in the core region seems to be feasible, taking
into account the power peaking increase. In case of radial
blanket region loading, over eighteen test assemblies with
MA diluted with MgO by 30% seems to be feasible without
any significant disturbance on the core characteristics.
Certain transmutation rate of over 10% to 30% can be
achieved in both cases. So the options can be chosen based
on the purpose of the experiment.
2.
LLFP Transmutation Experiment
In case of LLFP (99Tc) transmutation experiment in
the Monju core, the number of test assemblies, fifty four at
the maximum, loaded in the radial blanket region seems to
be limited below one third or one sixth of the maximum
loading case, for example, to avoid significant disturbance
on the core characteristics. Some design modifications
might be necessary to mitigate the steep power skew around
the test rigs, especially in the with-moderator case. The
irradiation experiment seems to be feasible under these
conditions. Transmutation rate of approximately 0.7%/year
to 1.4%/year can be obtained. This implies the possibility
of profitable experiment in the Monju core for LLFP
transmutation.
The capability of the Monju core for MA/LLFP
transmutation irradiation experiment has been confirmed,
which contributes to the future commercialization of FBR
technology. So, Monju should be restarted at the earliest
stage possible not only for this purpose but also to establish
the sodium handling technology and to demonstrate its
reliability as a power station.
References
1) T. Wakabayashi, et al., Nucl. Technol., 118, 14 (1997).
2) M. Yamaoka, M. Ishikawa, T. Wakabayashi, et al., Proc.
Int. Fast Reactors and Related Fuel Cycles, FR’91,
Kyoto, Vo. IV5.14-1 (1991).
3) M. Yamaoka, T. Wakabayashi, Proc. Int. Conf. Design
and Safety of Advanced Nuclear Power Plants, ANP’92,
Tokyo, Japan, I3.3-1 (1992).
4) K. Fujimura, et al., J. Nucl. Sci. Technol., 38[10], 879
(2001).
5) A. Mizutani., Trans. Am. Nucl. Soc., 81, 294 (1999).
6) M. Salvatores, I. Slessarev, A. Tchistiakov, et al., Nucl.
Sci. Eng., 130, 309 (1998).
7) H. Takano, Proc. 1994 Symp. on Nucl. Data, Tokai, Japan,
JAERI-Conf 95-008, 47 (1995).
8) T. Nakagawa, et al., J. Nucl. Sci. Technol., 32, 1259
(1995).