Twin torches generating arc transferred plasma with oxygen for
combustion of organics.
CHARVIN Patrice1, LEMONT Florent1, RUSSELLO Aldo1 and POIZOT Karine1
1
Commissariat à l’Energie Atomique (CEA) Site de Marcoule, BP 17171, 30207 Bagnols sur Cèze Cedex,
FRANCE
Abstract: Nuclear waste treatment and conditioning is a key point to allow
development of this, greenhouse gas free, energy source. Induction process used
for treatment of High Level Nuclear Wastes has been completed by arc
transferred plasma torches in the upper part of the reactor to accept a wide range
of wastes. Oxygen and wastes are introduced in the plasma to achieve the
combustion of the organic part of the waste. Ashes produced are integrated in a
glass molten bath in the lower part of the reactor. The glass molten bath is
heated by induction and casted in metallic canisters. High temperature involved
in the plasma zone improves combustion reactions and the post combustion of
exhaust gas may be achieved in the reactor if the sample is previously grinded
and introduced continuously.
Keywords: Arc transferred plasma, combustion, glass melting, nuclear waste
treatment
1. Introduction
Energy consumption increases such as greenhouse
gas in the atmosphere. Solutions are needed to
answer energy demand avoiding CO2 emissions.
Nuclear power plants can meet both requirements if
a proper management of radioactive wastes is found.
Vitrification of High Level radioactive wastes has
been carried out in France since the eighties to
obtain a glass-based monolithic waste with good
properties for radionuclide’s confining (leach
resistant).
Operations in nuclear facilities (industrial and
research) produce Medium Level radioactive wastes
without any treatment process associated nowadays.
These wastes are composed of an organic and a
mineral part (plastics, cellulose…). Organics are not
suitable in nuclear waste storage facilities because,
under irradiation, acids (HCl) and hydrogen are
some critical products of their decomposition. The
process devoted to the treatment of these wastes
must warrant the complete removal of organics in
the final waste.
Modifications have been implemented in the
vitrification process in order to remove the organic
fraction of the waste. Combustion which transforms
organics in gas (CO2, H2O) is the method employed
to separate organics from the mineral part and the
radionuclide. This reaction requires heat and oxygen
provided by plasma with oxygen.
This paper describes the reactor concept developed
at CEA for the treatment of organic waste and the
main benefits of combustion assisted by plasma.
Some results obtained with different wastes are
mentioned too.
2. Reactor concept
The concept proposed for the treatment of organic
wastes is composed of several elements developed
for other applications or wastes. The main objective
is to put together two systems of heating (plasma
and induction) in the same reactor to obtain a small
and compact process (Figure 1).
Waste
+
glass
Oxygen
Oxygen
Exhaust
gas exit
Water
jacket
walls
Glass
molten
bath
Induction
system
Figure 1. Incineration/vitrification reactor associating plasma
torches and induction heating.
2-1. Induction system
High frequency induction is an efficient method to
maintain a molten bath of glass above 1000°C
without any contact. This technology is commonly
used for vitrification of High Level Wastes. The last
evolution is the cold crucible with a system of water
jacket cooling down the reactor wall. Then, in the
contact of this cold wall, glass could not be molten
and a thick wall of solid glass is formed and protects
the metallic wall from corrosion by the molten bath.
2-2. Plasma twin torches
Electric arc is used in glass industry to melt and
prepare a batch of glass before pouring. In our
concept, electric arc is formed in the upper part of
the reactor in the gas phase. The electric arc heats
the gas around and generates plasma in the centre of
the reactor. Energy of the plasma is transferred to the
gas around and the molten bath of glass below
(Figure 2).
Figure 2. Photo of twin torches (left: cathode, right: anode) and
plasma zone.
Organic wastes are introduced in the center of the
reactor with a solid conveyor (screw). The waste
goes through plasma zone where it is heated quickly
at high temperature in presence of oxygen.
Combustion reactions occur in the plasma zone with
a good kinetic due to high temperature. Mineral
fraction and ashes containing radionuclide, stable at
high temperature, fall in the molten bath below the
torches and are integrated in the glass.
3. Twin torches
After the test of different configurations, the system
of twin torches introduced in the upper part of the
reactor has been validated. Electric arc can be in the
gas phase of partially transferred in the molten bath
if electrodes are close to the glass surface.
The Waste confinement and Vitrification service has
developed specific torches which suit nuclear
requirements and allow an efficient combustion of
organics.
Graphite rods play the role of electrode for cathode
and anode. These electrodes are consumed during
operation (thermal and chemical erosion). But,
electrodes are continuously moved forward in the
reactor to balance consumption of their extremity. A
system of nipples allows connection of new rods at
external extremity. The “life time” of graphite
electrodes is then infinite rather than metallic ones.
Graphite electrodes are protected by a water-cooled
metallic tube inserted in the reactor. This metallic
tube potential is floating with ceramic elements
guiding the graphite rod in the protecting tube. The
gap between the diameter of the electrode and the
metallic tube is used to introduce plasma gas (argon)
around the graphite rod (Figure 3). Thus, graphite
rod is surrounded by an inert gas and protected from
oxygen reducing its chemical consumption. Oxygen,
the reactive gas, goes through the protective tube
and exits at internal extremity where it is injected in
the plasma.
limiting formation of tar, if the initial waste is
grinded and introduced continuously
Both torches can be moved in angle and translation
in the reactor to change the plasma zone if
necessary. The distance between plasma zone and
the surface of the molten bath determine the heat
exchange between both phases. Surface temperature
of the molten bath can then be adjusted.
5. Range of possible waste
Water cooled
protective tube
Plasma torches have successfully processed Ion
Exchange Resins, sludge from water treatment
stations, bitumen. Complete combustion of organic
part has been demonstrated in several tests for all
types of wastes.
O2
Ceramic guide
Ar
- radionuclide and ashes are immediately integrated
in a glass (confining matrix)
Twin torches have been developed involving nuclear
requirements (resistance to radiation, reduction of
maintenance). Graphite rods with a regular feeding
of new rods to balance consumption can reduce
drastically maintenance compared to water-cooled
metallic electrodes.
The reactor is suitable for the treatment of various
wastes composed of organics. Numerous samples
have been introduced under plasma torches in order
to assess the efficiency of combustion with plasma
composed of oxygen.
Graphite rod
Figure 3. Graphite rod and its protecting tube.
6. Conclusion
4. Advantages of the concept
Combination of two systems of heating allows them
to be more dedicated at one task: combustion of
organics for plasma and heating the glass bath for
induction.
Plasma with oxygen above the glass molten bath
presents several advantages:
- molten bath surface is not cooled by the waste
introduced at ambient temperature
- high temperatures involved in plasma zone
improve kinetic of reactions, residence time is short,
thus reactor volume can be reduced or waste feeding
rate increased
- plasma can achieve combustion of organics and
post combustion of exhaust gas in the same reactor
A new concept of reactor has been developed to
propose a way of treatment for medium level nuclear
waste containing an organic fraction preventing a
direct storage. An inductor and plasma twin torches
working with oxygen have been associated to
achieve, in single reactor, combustion of organics,
post combustion of exhaust gas and vitrification of
ashes and radionuclide.
This reactor fulfils main requirements of nuclear
industry with small volumes, a reduction of process
steps (avoiding post combustion) and minimization
of maintenance.