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Treatment of radioactive solvent waste by catalytic oxidation

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Nuclear energy and safety
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ISSN 1018-5593
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European Commission
nuclear science
and technology
Treatment of radioactive solvent waste by
catalytic oxidation
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Report
EUR 16196 EN X
European Commission
Treatment of radioactive solvent waste by
catalytic oxidation
A. Luce, F. Troiani
ENEA - Eurex
Via Crescentino
1-13040 Saluggia
Contract No FI2W-CT91-0108
Final report
Work performed as part of the European Atomic Energy Community shared-cost programme (1990-94) on
management and disposal of radioactive waste
Task 2: Treatment ofe waste
PARI EU?G?. Siblicth.
r
Directorate-General XII N.C.o*v^£CC\
Science, Research and Development
ci 1995 EUR 16196 EN J
y<S{o\^A LEGAL NOTICE
Neither the European Commission nor any person acting on
behalf of the Commission is responsible for the use which might be made of the
following information
Cataloguing data can be found at the end of this publication
Luxembourg: Office for Official Publications of the European Communities, 1995
ISBN 92-827-0151-4
© ECSC-EC-EAEC, Brussels • Luxembourg, 1995
Reproduction is authorized, except for commercial purposes, provided the source is acknowledged
Printed in Luxembourg SUMMARY
A research activity to develop a suitable treatment and conditioning process for spent
solvent, coming from a reprocessing plant, is described.
The potentialities of the combination of distillation with wet-oxidation by hydrogen
peroxide have been investigated.
Scope of the distillation is the reduction of the waste volume, producing a non
contaminated distillate (mainly kerosene and alchyl-benzenes), to be conventionally
incinerated without containment, and a bottom residue (tri-butyl-phosphate and tri-capryl-
amine), containing all the activity, to be treated by wet-oxidation.
Scope of the wet-oxidation is the "transformation", at low temperature and pressure, of
the spent organic material into inorganic substances in aqueous solution. The oxidative
decomposition of TBP and TCA produces mainly carbon dioxide, water, phosphoric and
nitric acid.
The research programme has been carried out in the framework of the five years CEC
Programme (1990-1994) on "Management and Storage of Radioactive Waste".
All the experimental tasks are described and the main achievements are reported.
ACKNOWLEDGEMENT
The project has been carried out with the coordinated contribution of numerous colleagues
of the ENEA Nuclear Waste Management Department.
Particular thanks go to the technical staff of the Radiochemical Laboratory, for the
constant collaboration with setting-up the experiments: especially U. Bassano, M.
Ferrando and G. Gasso, deeply involved in the lab-scale tests and analytical process
controls.
Thanks also to Mr. G. Alonzo, Director of the EUREX plant, for the personnel allocation
to the experimental operations.
Special thanks to Mr. E. Casalino, head of CETRA Laboratory (ENEA Casaccia site),
and his staff, for the conduction of the cementation tests.
Finally we wish to thank the Commission of the European Community for the
opportunity to have a good interaction with the high level researchers and scientists of the
Task 2 working group, with the excellent coordination of Mr. L. Cecille and Mr M.
Hugon, well supported by Mr. J. Riesgo Villanueva.
Ill CONTENTS pag
1. INTRODUCTION 1
2. CHARACTERIZATION OF THE SOLVENT WASTE 5
3. DISTILLATION 7
3.1. Introduction
3.2. Experimental
3.3. Synthesis of the results
3.4. Mass and Activity Balance 8
4. WET OXIDATION 10
4.1. Introduction
4.2. Bench Scale Experiments1
4.2.1. Experimental
4.2.2. Synthesis of the Results
4.3. Hot Laboratory Scale Experiments 13
4.3.1. Experimental
4.3.2. Synthesis of the Results
4.4. Mass And Activity Balance5
4.4.1 Theoretical assessment
4.4.2 Balance Basic Parameters
4.4.3 Oxygen Balance 16
4.4.4 Water Balance
4.4.5 Activitye
5. CEMENTATION8
5.1. Experimental 1
5.2. Synthesis of the Results
6. PROCESS SCHEME 20
6.1. Scaling Up Basis
6.2. Steam Batch Distillation
6.3. Wet-Oxidation
6.4. Process Streams 21
V 6.4.1 Distillation 21
6.4.2 Wet-Oxidation
7. PLANT SCHEME AND LOCATION4
8. PRELIMINARY SAFETY CONSIDERATIONS 27
8.1. Main Hazards 2
8.1.1 Distillation
8.1.2 Wet-Oxidation
8.1.3 Other7
8.2. Safety Systems
9. CONCLUSIONS 28
10. REFERENCES 30
Appendix A - Distillation, Theoretical Assessment, Experimental and Results
Appendix B - Wet Oxidation, Bench Scale Cold Tests Description and Results
VI 1. INTRODUCTION
As a result of past reprocessing campaigns performed at the ENEA's EUREX
Reprocessing Pilot Plant (Saluggia, Italy), about 25 m3 of spent solvent were produced,
mixture of extractants (Tri-Butyl-Phosphate and Tri-Capryl-Amine) and diluents
(kerosene and several alchyl-benzene isomers). In the frame of Decommissioning and
Waste Management activities for the EUREX plant, conditioning hypotheses are under
evaluation for this kind of waste.
Incineration is currently the most generally used method for treatment of radioactive
organic waste, because of the good achievable volume reduction.
Nevertheless incineration is a very complex technology with high operating and
maintenance costs; the equipment is sophisticated and costly, particularly for the off-gas
treatment, which must assure the radioactivity containment, so producing relevant
quantity of secondary wastes such as scrubbing solutions, filters, etc.
Finally the public acceptability is not a secondary consideration to be taken into account
before the construction of an incineration plant
In particular, destruction of spent solvent (usually TBP/Kerosene) by incineration isn't
yet a mature technology due to the additional corrosion problems coming from the
phosphoric acid formation. This very corrosive acid is responsible for some problems in
selecting corrosion resistant materials or, alternatively, in finding means to convert the
acid into inert phosphate 1.
Taking into account the above considerations, alternative treatment options, mainly
chemical processes, have been considered in the past or are being investigated for spent
solvent. Some examples are briefly described below:
• The Eurowatt Process has been developed by the Belgian Eurochemic, consisting in the
extraction of TBP, degradation products and radionuclides from spent solvent mixtures
by anhydrous phosphoric acid, followed by the pyrolisis of the radioactive TBP-
phosphoric acid phase at about 200 °C, in order to decompose the organic compounds to
volatile hydrocarbons, to be burnt, and non-volatile inorganic phosphoric acid solutions,
containing all the activity, to be solidified 2.
• Immobilization in portland cement with the use of emulsifiers and other additives has
been realized by the Hanford Engineering Development Laboratory (USA), because
incineration was determined too costly for the small amount of liquid organic waste to be
yearly treated 3.
• Electrochemical oxidation is under development at Dounreay (UK) AEA Technology
site (Silver II Process). The process is based on the oxidation of the organic matter by
means of highly reactive species of silver ions (Ag2+) generated by electrical supply at the
anode of an electrochemical cell, at low temperature and pressure 4.
• Wet-Oxidation by using of hydrogen peroxide in presence of a catalyst (usually Cu, Fe
ions) has been studied by different operators (see 4.1 of this report). As for the
electrochemical oxidation the main advantages of this process are the soft reaction
conditions, e.g. atmospheric pressure and temperature up to 100 °C.
In this frame ENEA has planned a research activity in order to develop a suitable
treatment process for the above solvent waste, capable of reaching three major objectives:
• preliminary reduction of the radioactive organic waste volume by a factor at least equal
to 10;
• oxidative destruction of the reduced volume waste; • compatibility of the processed waste with conventional immobilization matrices like
cement.
For the specific purpose the potentialities of the combination of Distillation with Wet
Oxidation have been investigated, according to the management scheme of Figure 1.
Scope of the distillation is the reduction of the radioactive waste volume, producing a non
contaminated distillate (mainly kerosene and methyl-benzenes), to be conventionally
incinerated without containment, and a bottom residue (mainly tri-butyl-phosphate and tri-
capryl-amine), containing all the activity, to be treated by wet-oxidation.
Scope of the wet-oxidation is the chemical transformation of the organic matter to a form
(aqueous solution) suitable for cement encapsulation in order to allow the long term
storage.
The research programme, with the financial support of the European Commission
(Contract N°. FI2W-CT91-0108), has comprised the implementation of the following
tasks:
1) Laboratory Distillation Tests - Scope of the tests was the achievement of
distillation operational parameters and the determination of the decontamination factor of
the top product of the distillation column, in order to satisfy incineration criteria in
conventional plants.
2) Wet Oxidation Tests - Laboratory scale experiments with real waste have been
conducted using the bottom products of the distillation tests. Bench scale experiments
with simulated waste have been conducted to determine the process efficiency and the
procedure which enable a minimization of the final waste volume.
3) Cementation Tests - Cementation tests have been carried out on inactive simulated
aqueous waste coming from the bench-scale experiments, in order to determine the
compatibility of the wet oxidation product with conventional immobilization matrices.
4) Preliminary Design - A preliminary design of a full scale plant for processing the
EUREX solvent waste has concluded the research activity.
The realization of the above tasks was planned according to the time schedule shown in
Figure 2.

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