High-frequency melting technology for radioactive metal waste
Nobutake Horiuchi, Takeshi Nishikawa, Naoto Sasaki, and Hirokazu Tanaka
Energy Project & Technology Center
Mitsubishi Materials Corporation
1002-14 Mukohyama, Naka-city, Ibaraki-pref.
Key words; Melting, radioactive metal scrap, decontamination, volume reduction, recycling

Abstract
Mitsubishi Materials Corporation has been developing the melting technology for metal waste by
high-frequency melting, for volume reduction, decontamination and recycling of metal scrap contaminated withradioactive materials from nuclear power plants and nuclear facilities. By the melting treatment, high volume
reduction of metal waste and decontamination of radionuclides such as cesium, strontium and α nuclides areexpected. In addition, the melting process is also suitable for the metal recycling, because the process canhomogenize the waste and cast it into any shape.
Through development, we confirmed the followings; - the volume of metal waste can be reduced from
1/5 to 1/3 by melting, - the radioactivity concentration of Cs of metal that actually contaminated due to anuclear accident can be set below the detection limit, - demonstration scale test with simulated substances candecontaminate Cs, Sr and U. In addition, we have been developing manufacturing technology to make themetal waste containers by centrifugal casting method, which was applied to advanced volume reduction
facilities of Japan Atomic Energy Agency (JAEA).

1. Introduction
Reducing the final disposal amount of radioactive
waste in decommissioning nuclear facilities is an
important issue. In order to reduce the amount ofwaste, decontamination and volume reduction areeffective. In addition, when reducing the amount ofwaste by recycling, it is necessary to measure theradioactivity concentration and establish a reusemethod.
Mitsubishi Materials Corporation (MMC) has beendeveloping a melting decontamination technology formetal waste by high-frequency melting. By using
meltingdecontamination
technology,
theradionuclide will be removed, and it will be possible
to collect metal materials and recycle it. Also, theamount of secondary waste to be final disposal can begreatly reduced compared to the amount of metal
waste before decontamination. Furthermore, since thecomposition of the metal becomes uniform, theradioactive concentration measurement becomeseasy.
Below we explain the mechanism of melt
decontamination, the experimental equipment owned
by Mitsubishi Materials, and the results ofdevelopment. In addition, we will also introduce thedevelopment of the ultra-high frequency furnace thatcan be used for the treatment of waste other thanmetals such as ceramics

2. The mechanism of melting decontamination
The melting decontamination technology is basedon the difference in free energy of formation of metaloxide or the difference in boiling point between metal
and radionuclide (Fig.1). Since the boiling point ofCs and its compounds is lower than the melting
temperature of iron (1,600 ° C.), when iron-based
metal waste is melted, Cs is volatilized and removed
from the metal. The part of Cs is supplemented byslag during the transition from the metal phase to thegas phase.
Sr and α nuclides (such as U, Pu) exist stable asoxide as compared with iron (Fig.2). In theelectromagnetic field of the high-frequency induction
furnace, the metal is move to the center of the furnaceby electromagnetic force, on the other hand, the oxide,
which is not affected by electromagnetic force,
moves relatively to the outside of the furnace. Inaddition, the metal has good homogeneity because
molten metal is stirred by electromagnetic force.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan3. Experimental apparatus and facilities
MMC owns three experimental furnaces (Table 1).
One is a basic test apparatus that measures materialbalance and mass transfer rate in a small test furnace(capacity: 1kg-Fe, U available). The other two are fordemonstration tests and the applicability of melt
decontamination on a real scale was confirmed.

slag
molten metal

Sr,U,Pu

Cs
high-frequency
induction furnace

Table 1 Specs of the experimental apparatus andfacilities
No.1

Because Cs has a low boiling point (678deg-C), it moves from metal to gas phase.
Some are caught by slag.
Sr and U, Pu are more stable as oxide than iron, so that they become a part of slag.

Standerd generation free energy (kJ/mol)

Fig.1 Image of nuclide behavior in high frequency
induction furnace

Heating
MethodCapacity(For Iron)
Input Power
Frequency

Cu+1/2O2=CuO
Fe+1/2O2=FeO

-400

No.3

High frequency induction heating
1kg

1ton

430kg

30kW
3kHz

600kW
500Hz

Furnace Size

φ60×150mm

φ490×900mm

250kW
1kHz
upperφ722
bottomφ400×600mm

RemarkU available

Fig.

Filg.4

0-200

No.2

Using
simulatedsubstances
Fig.5

U available
Fig.6

2Cs+1/2O2=Cs2O-600

2Al+3/2O2=Al2O3-800

Off gas tofilter

U+O2=UO2

Pu+O2=PuO2

Furnace lid-1,000
Sr+1/2O2=SrO

Glass cylinder-1,200
0500

1,000

1,500

2,000

Temperature(deg-C)

Outercrucible
Inner crucible

Fig.2 Free energy of formation of metal oxide*The Japan Society of Thermometry Calculated by thermodynamic
data calculation software “MALT2”

SlagMolten
metalOxide

Electromagneticforce

Center side of the furnace

Outer side of the funace

MetalInduction
coil

Fig.4 Overview of No. 1 basic test apparatus

Fig.3 Image of oxide behavior in high frequency
induction furnace.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japandecontamination effect of radioactive Cs and Sr bymelting.
Melting decontamination test using real metal
waste contaminated with radioactive Cs caused by
the accident was carried out using our No. 1 basic test
furnace. As a result, it was confirmed that real metal
waste can be decontaminated to less than thedetection limit (0.2Bq/kg) even when the metal iscontaminated with soil contaminated with Cs(Table2) [2].
To understand the behavior of Cs and Sr, melting
decontamination test using stable nuclides of Cs andSr were carried out (Fig.7). As a result, we confirmed
the followings. 1) It is possible to decontaminate Cs

4. Development Results
4-1 Volume reduction
Because of the wide variety of shapes of metal
waste, when metal waste is put into a disposal
container as it is, many useless voids are formed. On
the other hand, when the metal waste is
melt-processed, unnecessary voids can be eliminatedsince the metal is made into a uniform ingot.It wasconfirmed by the test that the volume of metal waste
can be reduced from about 1/5 to 1/3. Furthermore,

we confirmed that the amount of secondary waste
such as refractories, slag, filters, etc. generated by
the melting process is about 5% of the treatedmaterial [1].

Table 2. Results of melting decontamination test
using real metal waste contaminated with fall-out
radioactive Cs
BeforeRadio Activity[Bq/kg]
Run1
Run2
5.6
5.6
10,500－<0.2<0.2

Metal Scrap
SoilIngot

AfterSlag

Metal
0.008

Transfre rate of Cs(%)

Fig.6 No.3 engineering scale furnace (Capacity:430kg-Fe/batch)and Sr from the metal, 2) As the added amount ofthe slag material increases, the amount ofmigration of Cs and Sr to the slag increases, 3)
Since Cs and Sr concentrations in the ingot could
be confirmed with representative sample,
radioactivity concentration of metal after melting
process can also be measured by representative
sample measurement.
To confirm those phenomenon in thedemonstration scale, we carried out the test using
No.2 demonstration scale furnace (Fig.8). As aresult, it was confirmed that there is no difference
between basic test and demonstration test on
decontamination performance and uniformity byrepresentative sample (Table 3).

Transfre rate of Cs(%)

Fig.5 No.2 demonstration scale furnace (Capacity:1ton-Fe/batch)

0.006
0.004
0.002
Ditection Limit <0.002

00%

1%

2%

3%

60
40
2000%

Carbon steel

Carbon steel

Stainless steel

3%4%

Stainless steel

100
Transfre rate of Sr(%)

10080
60
40
2001%

2%

3%

Addition amout of Slag(wt%)
Carbon steel
Cast iron2%

Cast ironCast iron

0%1%

Addition amout of Slag(wt%)

Addition amout of Slag(wt%)

Transfre rate of Cs(%)

Due to the accident at Fukushima DaiichiNuclear Power Station, a large amount ofradioactive substance, Cs, Sr, etc. was released into
the atmosphere. For this reason, a large amount ofmetallic rubble contaminated with radioactive Csand Sr has been generated, and theirdecontamination and reduction has become asubject.
Therefore,
weconfirmed
the80

4%

Crusible and off-gas

4-2. Decontamination of Cs and Sr

100

Stainless steel

4%

80

60
4020Ditection Limit <0.01

00%

1%

2%

3%

4%

Addition amout of Slag(wt%)
MetalSlag

Crusible and off-gas

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

Fig.7 Behavior of Cs and Sr in the basic test.

Table 3. Concentration of Cs and Sr in thedemonstration scale test.
Concentration
[ppm]
Cs
Sr
BeforeSimulated Sample
501
500
Ingot 1
<0.02
<0.1
After
Ingot 2<0.02
<0.1
Representative sample
0.04
<0.1
Lower detection limit: Cs 0.02ppm, Sr 0.01ppm

Lower detection limit: U 0.5ppm

4-4 Recycle technology and deployment to actualfacility
In order to promote recycling, it is necessary tocreate a method of reusing metals. In the case ofrecycling metal contaminated by activation, it is
not possible to decontaminate radionuclides suchas Co - 60 by melt decontamination, so that thetreated metal needs limited reuse in nuclear
facilities.
Therefore, we have been developing technology tomanufacture metal waste containers by centrifugal
casting method, because this method has few
secondary waste, is easy to manufacture, and issuitable for remote operation. By using thecentrifugal casting method, it is possible tomanufacture a cylindrical container without a coremold. It was adopted for advanced volume reduction
facilities (AVRF) of JAEA. A part of AVRF, themelting furnace and related facilities were
designed and built by the MMC group.
If metal waste is made into ingots, they can beprocessed into various products later. Melting
technology may be effective as a means to reduce andrecycle radioactive metal waste generated at nuclear
facilities such as TEPCO’s Fukushima Daiichi NPP.

Fig.8 Implementation status of demonstration scale
test (casting)
4-3 Decontamination of U
In nuclear fuel cycle facilities such as nuclear fuelprocessing facilities, a large amount of metal waste
contaminated with U is generated. We confirmed theapplicability of melt decontamination to metals
contaminated with U.
To know the behavior of U, we carried out
engineering scale test using No.3 engineering scale
furnace (U available) [3], [4]. As a result, the followings
were confirmed (Table 4). 1) The uranium

concentration in the metal can be reduced toseveral ppm or less, in terms of natural U <0.1
Bq/g by melt decontamination. 2) U concentration
measurement is possible with representative
sample. 3) The metal after treatment has a uniformcomposition.
Table 4, Concentration of U in the engineering scaletest.

BeforeAfter

Simulated Sample
Ingot 1
Ingot 2
Ingot 3
Representative sample

Concentration ofU [ppm]
Run1
Run2
1,000
1,000
1.5
2.3
2.1
2.3
1.4
4.6
1.3
3.2

Fig.9 Metal Container made by centrifugal casting
method
5. Development of Ultra-high frequency induction
furnace.

In a high frequency induction furnace, since
only the metal is heated, it is not possible to makemiscellaneous solid waste that contained
nonmetals such as concrete and glass by direct
induction heating. Therefore, we developed anultra-high frequency furnace that can directly heat
melt of miscellaneous solid waste by induction
heating. The developed furnace uses an induction
frequency of about 100 kHz (Fig. 10).
As a result of melting several sample mixed
with carbon steel and glass using a ultra-highfrequency furnace, we confirmed that it is possible

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanto make solidified bodies with low porositycomposed of slag and metal, and the compositionsof the metal layer and the slag layer are
respectively uniform (Fig. 11) [5].

[2] 2015, MOE, FY2017 Demonstration Project for
Decontamination Technology.
[3] 2002, JNC, JNC technical report No.14[4] 2004, RWMC, Uranium waste decommission
technology investigation.
[5] 2008, MMC et al, Innovative and Variable
NuclearenergyTechnology(IVNET)
Development Project, Development of the Systemof the Volume Reduction Treatment System ofWaste by the Ultra-High Frequency Induction
Furnace.

Fig.10 Relation between total absorbed electricenergy and induction frequency

Metal / Slag =1/9,5/5
Fig.11 Example of a molten and solidified productproduced in an ultra-high frequency furnace
6. Conclusion
High frequency melting technology for radioactive
metal waste developed by MMC was summarized. It
was confirmed that Cs, Sr, U can be decontaminatedfrom metal, radioactive concentration measurement
can be easily performed by a representative sample.
We also confirmed the applicability of the ultra-highfrequency furnace. We are planning to continue todevelop the melting technology, with the aim ofcontributing to the decommissioning of nuclear
power plant including Fukushima Daiichi NPP.
7. Acknowledgments
The development of melting technology ofradioactive metal waste introduced here was carriedout under the guidance and cooperation of many
people, demonstration project of MOE, Basic

research programs of vitrification technology for
waste volume reduction of METI, JAEA, RWMC,effective use project of METI, and so on. I deeply
appreciate it here.
References[1] MMC, AESJ 2009 Fall Meeting, L19

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan