Journal of Nuclear and Radiochemical Sciences, Vol. 16, pp. 1-4, 2016

ArticlesThe 3rd Japan-China Academic Symposium on Nuclear Fuel Cycle, ASNFC 2015

Reducing the Burden of High Level Radioactive Waste with Transmutation— Proposal of Integral Molten Salt Fast Reactor (IMSFR)
Michio Yamawakia,* and Tadafumi KoyamabaResearch Institute of Nuclear Engineering, University of Fukui, 1-2-4 Kanawa-cho, Tsuruga, Fukui 914-0055,
JapanbCentral Research Institute of Electrical Power Industries, 2-11-1 Iwadokita, Komae, Tokyo 201-8511, Japan
Received September 7, 2016; Accepted October 12, 2016The transmutation methods to reduce the burden of high level radioactive waste had been compared to identify
the most suitable method for this purpose. The results showed that the molten salt reactor (MSR) was suggested
to be an effective transmutation system. Based on this result, a new concept of Integral Molten Salt Fast Reactor(IMSFR) as a highly efficient transmutation system for transuranium elements has been proposed in this study;
the system consists of a combination of an MSR fuelled with chloride molten salt fuel and the chloride-based
pyrochemical processing subsystem. In this system MSR fuel can be transferred between the reactor core and thepyrochemical processing subsystem without chemical form change.

After the Fukushima nuclear accident, the public acceptance of nuclear energy use has become more severe than ever.
The public’s concern stems not only from the worry over thesafety of nuclear power plants (NPPs) but also from that overthe management of high level radioactive waste (HLW). Inorder to alleviate the public’s concern over the management ofHLW, transmutation of long-lived radioactive nuclides can bean effective measure from the viewpoint of reducing the life,
heat and toxicity of HLW. The three systems of fast reactor(FR), accelerator-driven system (ADS) and molten salt reactor(MSR) are compared in terms of the efficiency of transmutation of HLW or minor actinide (MA) waste, the main component of HLW.
2. Effect of transmutation for managing HLW
Transmutation means to turn nuclei of long-lived trans-uranium (TRU) elements to short-lived nuclei by nuclear fission.
Figure 1 shows the decrease of potential radiotoxicity of eachgroup of radioactive materials constituting the HLW withelapse of time [1]. The radiotoxicity of HLW would decrease
down to the level of natural uranium after 400 y of storage incase of recycling of 99.5% of uranium and TRU, where recycling means separation and burning for transmutation. Thus,
transmutation is effective to drastically shorten the timelength necessary to keep HLW in controlled safe condition.
As for transmutation techniques, FR, ADS and MSR havebeen proposed as the main candidates [2,3].
2.1. FR. For FR cases, the low concentration of MA in
mixed oxide fuel (MOX) causes a low transmutation rate inFR. Generally less than 5% concentration of MA is assumed
in FR fuel mainly from the consideration of sodium void reactivity problem [4-6]. The sodium void reactivity is increased
with the amount of MA in MOX fuel, a serious problem fromthe safety point of view. As measures to alleviate this prob-*1Corresponding author. E-mail: myamaw@mug.biglobe.ne.jp,
TEL: +81-047-345-7038 Fax: +81-047-345-7038

109108107106105104103102

Potential Radiotoxicity (Sv/tHM)

1. Introduction

Actinides
Fission
Products
Natural U (7.5t)
MinorActinides
Activated
Products

10010

110210341010Time (yr)5106107108Figure 1. Potential radiotoxicity of radioactive nuclides generated in
the fuel cycle of LWRs (normalized to 1 ton of new fuel) [1].

lem, Na plenum and/or ZrHx-added internal blanket are underinvestigation in ASTRID project of France [7] in collaboration
with Japan Atomic Energy Agency (JAEA) and others [4].
The MA transmutation rate in FR has been estimated to attainabout 70% reduction of MA in case of irradiation of 3-4 wt%
MA-containing mixed oxide fuel there for nearly 150 years [4].
As a measure to increase MA transmutation rate, Yamawaki
and his co-authors proposed a concept of MA hydride fuelloaded FR [8], where hydrogen atoms moderate neutronenergy so as to increase the nuclear reaction rate contributing
to increase of MA transmutation rate. Thus-obtained transmutation rate was estimated to be higher than that predicted
for ADS as described in the next section [9].
2.2. ADS. The merit of ADS in regard of MA transmutation is that high neutron f luxes can be available thanks tospallation of neutron irradiation target to be brought about
through bombardment with a high energy proton beam.

© 2016 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/31/2016

2J. Nucl. Radiochem. Sci., Vol. 16, 2015

YamawakiAccording to Oigawa et al. [10-12], JAEA, transmutation of250 kg MA can be achieved with 300 day operation of one
800MWt ADS, corresponding to the amount of MA generated
in ten light water reactors operated for 1 year each.
However, to realize such an advanced system, new technologies such as a high performance proton accelerator andhighly irradiation-resistant material should be developed.
Thus, a lot of works need to be made before commercializationof ADS in the future.
2.3. MSR. For MSR cases, higher MA solubilities can beexpected in molten salts. TRU solubilit y in FLiNaK(LiF+NaF+KF mixture) is reported to be nearly 10%, whilethat in chloride molten salt (NaCl or NaCl+KCl) is muchhigher, with even 50% or higher values to be possible [13]. Inaddition, fission products can be separated from the molten
salt fuel on operation. Thus, MA transmutation rate can be
expected to be much higher compared to FR and even to ADS.
According to Mitachi [14], about 96% of TRU can be
reduced with two cycles of reactor irradiation for about 90
years in total using (FLiNaK+TRU)-fuelled MSR with uranium excluded. This performance is much better comparedwith that by FR as described above, i.e. about 70% reductionof MA in case of irradiation of 3-4 wt% MA containing mixedoxide fuel in FR for nearly 150 years.
Yamawaki et al. [15] proposed a concept of static-fuel-type
MSR (S-MSR) which is composed of a cylindrical core tankfilled with molten salt fuel and a vessel surrounding the coretank, filled with molten salt such as FLiNaK. This type MSR
is extremely safe from the viewpoint of severe accidents since
the molten salt fuel is confined in the core tank without circulation to the outside of the core.
3. Integral Molten Salt Fast Reactor (IMSFR)
Recently, Yamawaki proposed a new system, Integral
Molten Salt Fast Reactor (IMSFR), in December, 2015 [16].
The system combined an MSR fueled with chloride molten salt
fuel and a chloride-based pyrochemical processing subsystem.

Figure 2. Photo of discharged molten LiCl-KCl salt f low fromfreeze-seal valve.

As studied in the past at ORNL and other institutes, chloride isone of the candidate salt for molten salt fast reactor that has
high solubility for both actinides and fission products at operating temperature [17]. In addition, advanced pyrochemical
reprocessing technologies developed in the past three decades
in US, Japan and other countries for Integral Fast Reactor(IFR) [18,19] can be exploited.
Engineering technologies for scaled-up process equipment
such as counter current extractor and electrorefiner as well asbasic data sets for actinides and FP elements in chloride molten salt are currently available [20]. Figure 2 shows a scene ofan experiment to demonstrate the fluid dynamics of high temperature chloride molten salt for designing transport equipment [21]. Favorable compatibility with stainless steel at hightemperature is another advantage of chloride molten salt tofluoride molten salt although the former’s corrosion behavior
under irradiation has not been tested yet. Based on theseexperiences, fuel cycle flowchart for IMSFR is proposed as
Waste
Consolidation

LN-FP Removal
Zeolite column
Actinide Separation
Counter Current
Extraction

Salt

NM-FP

Composition
adjustment

Actinide Backextraction

CoreBi

MA from LWR

NM-FP/Bi
Separation

Salt-PuCl3-MACl3-UCl3

PuCl3
MACl3

Salt-UCl3

BlanketNM-FP/Bi
Separation

Bi

Actinide Backextraction
Actinide Separation
Counter Current
Extraction

Salt
LN-FP Removal
Zeolite column

UCl3
addition
U/Pu-MASeparation

NM-FPWaste
Consolidation

LN-FP : Less Noble FPs such as Cs, Sr, Ce, Nd
NM-FP : Noble Metal FPs such as Ru, Rh, Mo, Zr
Figure 3. Proposed pyrochemical fuel cycle flowchart for IMSFR.

Reducing the Burden of High Level Radioactive Waste with Transmutation

J. Nucl. Radiochem. Sci., Vol. 16, 2015

3TABLE 1: Characteristics of molten salt and reactor coolants
Material

Melting
Boiling
Volumetricpoint
point
heat capacity
(°C)
(°C)
(kJ m-3 °C)
NaCl-KCl-MgCl23851412(2511)*
NaCl-KCl-LiCl3881360(3184)**
FLiBe
459
1430
4540
Liquid Na
97.8883
1000
Liquid Pb328
1750
1700
Water01004040*) Estimated values for NaCl-KCl-MgCl2-30%(U,Pu)Cl3 [17]**) Estimated values for LiCl-RbCl [26]Figure 3. From this flowchart, actinides should be separated
from fission products with counter current extraction using
liquid metal such as Bi or Ga, while separation among
actinides (U/Pu-MA) should be carried out with electrorefining. MA elements recovered from LWR spent fuel or fueldebris of defected Fukushima reactors may be introduced into
IMSFR fuel cycle at composition adjustment step for transmutation at reactor core.
Toward the actual deployment of IMSFR, however, there
still exist many uncertainties in development. Composition ofbase molten salt, which is a key issue to design the whole system such as core/blanket configuration, reactor hydraulics,
reprocessing process chemistry and so on, has not been optimized yet. As a better fuel or fertile salt, selection of cation oranion of unfavorable nuclear properties such as an elastic
downscatter and large neutron absorption should be avoided tokeep hard neutron spectrum. According to the nuclear properties of cations by Ottewitte [22], Ca was categolized “good”
and, K, Mg, and Na were “intermediate”, while Li and Be were“bad”. As for anion, necessity of isotope separation due tolarge neutron absorption of 35Cl was first concerned [23], however, Nelson et al. [17] demonstrated the acceptability of natural chlorine based on their neutronics calculation. In addition,stability of liquid phase for operating temperature, fromaround 450 to 700 °C, is necessary for the salt. According to
the phase diagrams of chloride of these cations [24], ternary
systems such as NaCl-KCl-MgCl 2 and NaCl-KCl-LiCl wereselected as possible candidates. As for the better reactor
hydraulics, high thermal conductivity and low viscosity isfavorable. Table 1 summarizes the reported characteristics ofthe molten salts comparing with other reactor coolants [24,25]. The values in brackets show the reported rough estimatesfor similar chlorides [17, 26] because reliable measurements
were unavailable. This table also shows the possibility to usethese chlorides as coolant. For confirming the reprocessing
process, distribution characteristics of relevant elements
between molten salt and liquid metal or zeolite-A are needed
to be assessed because chemical stability of the elements in themolten salt is different from those in LiCl-KCl. According to
the measurement results reported by Koyama [27], distribution
coefficients between alkali metal salt (LiCl-KCl) and liquid
Cd were similar to those between alkali metal - alkaline earth
mixture salt (LiCl-KCl-CaCl 2-BaCl 2) and liquid Cd. Further
study is underway for evaluation of material balance in addition to the optimization of chloride salt composition.
4. Conclusions
Effective reactor systems, FR, ADS and MSR, for transmutation of minor actinides from high level radioactive waste
were compared with each other and MSR was estimated to be

Thermalconductivity(W m-1 °C)(0.87)*(1.3)**
1.062
16
0.56

Kinematic
Viscosity(m2 s-1)(1.3)*(0.69)**
2.9
0.25
0.13
0.13

able to offer a highly efficient transmutation system. Theauthors have newly proposed an Integral Molten Salt Fast
Reactor (IMSFR) as an enhanced efficiency transmutation
system of MA for spent fuels from LWR or fuel debris fromthe failed nuclear reactors. IMSFR was a combination of an
MSR fuelled with chloride molten salt fuel and chloride-based
pyrochemical processing subsystem. Then, the fuel cycle
f lowchart of IMSFR had been developed. Although there
were several uncertainties and issues to be solved to develop
the proposed system, further studies are necessary for realizing efficient reduction of radioactive waste burden.
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