Partition of high-level radioactive wastes

Directorate-General For Research And Innovation European Commission

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Nuclear energy and safety

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• Publié par: DIRECTORATE-GENERAL-FOR-RESEARCH-AND-INNOVATION-EUROPEAN-COMMISSION
• Nombre de lectures: 10
• Langue: English
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ISSN 1018-5593
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European Commission
nuclear science
and technology
Partition of high-level radioactive wastes
Report
EUR 16958 EN European Commission
nuclear science
tecnnolOQy
Partition of high-level radioactive wastes
Z. Kolarik, R. Schuier, U. Müllich
Forschungszentrum Karlsruhe GmbH
Institut für Technische Chemie
P.O. Box 3640
D-76021 Karlsruhe
Contract No FI2W-CT90-0047
Final report
Work performed as part of the European Atomic Energy Community's
shared cost-programme (1990-94) on
'Management and storage of radioactive waste'
Task 2: Treatment ofe waste
Directorate-General
Science, Research and Development
1996 EUR 16958 EN A great deal of additional information on the European Union Is available on the Internet.
It can be accessed through the Europa server (http://europa.eu.int.)
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, 1996
ISBN 92-827-7987-4
© ECSC-EC-EAEC, Brussels · Luxembourg, 1996
Reproduction is authorized, except for commercial purposes, provided the source is acknowledged
Printed in Luxembourg SUMMARY
Partitioning of liquid high-level radioactive waste (HLW) was studied as an option of fa­
cilitating the final waste disposal. A flowsheet of a solvent extraction partitioning process
was proposed. Pu(IV), Np(VI), Zr(IV), Tc(VII), and U(VI) are extracted in the first cycle
of the process with tributyl phosphate. Selective back extraction yields two product frac­
tions, one of them containing Zr and Tc and the other containing Pu, Np, and U. Trans-
plutonides(III) are extracted in the second process cycle together with lanthanides(III) by
octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO). This cycle also
must involve separation of transplutonides(III) from lanthanides(III), preferably by selec­
tive extraction of the former element group.
A compilation of data related to the separation of actinides and fission products from the
HLW was performed in the first stage of the work. A review was written and conclusions
were drawn about the choice of promising methods applicable in a partitioning process.
Few experimental data had to be obtained for the first partitioning cycle. They concerned
oxidation of Np(V) to Np(VI), attainment of high decontamination factors for Np and Pu,
and back extraction of Pu(IV) in the presence of acidic butyl phosphates.
Work on the second partitioning cycle concerned search for an alternative modifier for the
CMPO extractant, and attainment of a high decontamination factor for Am. Main atten­
tion was paid to the investigation of extractants for selective extraction of transplu-
tonides(III) over lanthanides(III), performed with Am(III) and Eu(III) as representatives
of the two element groups. The required selectivity is reached in the extraction of thiocya-
nates by CMPO. Acidic sulfur donor extractants, namely dialkyl dithiophosphoric and di-
thiophosphinic acids in combination with phosphoryl synergists do not separate Am(III)
from Eu(III). Separation achievable with di(2-ethylhexyl)dithiophosphoric acid is limited to
systems in which micelle formation is possible. Most promising are neutral nitrogen donor
extractants. Schiff bases separate the Am(III)-Eu(III) pair well if they are derived from
1,2-benzenediamine, but they are chemically instable. Appreciable separation efficiency is
attained in the extraction of Am(III) and Eu(III) thiocyanates by some 2-substituted benz-
imidazoles, especially 6-methyl-2-(2-pyridinyl)benzimidazole. The separation potential of
benzimidazole derivatives was studied in relation with their molecular configuration, anion
accompanying Am(III) and Eu(III) ions in the organic phase, and the diluent used. Some
general conclusions were drawn. An acidic extractant, 2-(2-pyridinylazo)-l-naphthol, ex­
hibits moderate selectivity for Am(III) in spite of its mixed nitrogen/ogygen donor charac­
ter. Table of Contents
1. INTRODUCTION 1
2. PROPOSED PARTITIONING FLOWSHEET 2
3. SCOPE OF THE WORK 4
4. COMPILATION AND EVALUATION OF PUBLISHED DATA 5
5. OXIDATION OF Np(V) TO Np(VI) IN THE HAW SOLUTION 6
6. DECONTAMINATION FACTORS FOR NEPTUNIUM AND PLUTONIUM 9
7. BACK EXTRACTION OF Pu(IV) IN THE PRESENCE OF ACIDIC BUTYL
PHOSPHATES 11
8. ALTERNATIVE MODIFIERS FOR CMPO SOLVENT 14
9. DECONTAMINATION FACTORS FOR AMERICIUM(III)8
10. SEPARATION OF TRANSPLUTONIDES(III) FROM LANTHANIDES(III) . . 1
10.1 Extraction of Salts Other Than Nitrates by CMPO 1
10.2n by Dialkyl Dithiophosphoric and Dithiophosphinic Acids 30
10.3 Extraction by Nitrogen Electron Donors 4
11. GENERAL CONCLUSIONS 63
12. EXPERIMENTAL5
12.1 Chemicals, Isotopes, Solutions and Measurements 6
12.2 Procedures6
12.3 Syntheses of Extractants
13. ABBREVIATIONS, SYMBOLS AND DEFINITIONS 70
REFERENCES 72
V 1. INTRODUCTION
High-level radioactive waste (HLW), as produced mainly in the reprocessing of spent nu­
clear fuel, can be partitioned for two reasons: first, to obtain radionuclides as useful,
marketable products and, second, to reduce risks and costs of the final treatment and dis­
posal of the waste. To give examples of vendible products, the isotopes 137Cs and 241Am
are potential sources of gamma radiation, "Tc and 147Pm can serve as sources of soft beta
radiation, 90Sr and 238Pu are potential sources of heat which subsequently can be converted
to electric energy, and 237Np can be used as starting material for the production of 238Pu.
As for problems of waste disposal, long-lived isotopes of actinides and some fission pro­
ducts contribute considerably to the long-term radiotoxicity of radioactive wastes. Repres­
enting a small volume and weight fraction of the total wastes, isolated actinides and fission
products can be deposited separately at reduced risk and costs or, more favourably, trans­
muted to short-lived and less radiotoxic nuclides.
Recovery of radioisotopes as useful products was the first stimulus for the partitioning of
high-level radioactive wastes. Extensive R&D programmes were started with considerable
optimism as early as in the 1950s, but one or two decades later they were reduced or even
stopped when the marketability of the products failed to fulfill the initial expectations. In
the 1970s and 1980s, in the period of fading nuclear enthusiasm, the facilitation of waste
disposal became the only reason for eventual HLW partitioning. It was estimated with little
optimism at that time, but in recent years we have experienced a lively renaissance of the
partitioning idea. This tendency is mainly based on progress in studies of possible tran­
smutation reactions and in the development of concepts of burner reactors and powerful
accelerators. At present, the partition and transmutation (P+T) is one of seriously consid­
ered new ways in the strategy of the radioactive waste treatment. The potential, practica­
bility, and impact of P+T cannot be exactly predicted yet, and a benefit of it cannot be
expected in near future. However, it is generaly recognized that the P + T idea is worth not
only of further conceptual studies, but also of performing experimental R&D programmes.
It is a task of chemical research to elaborate a process for the separation of actinides and,
eventually, fission products from the HLW, and to demonstrate its feasibility. The process
must be conceived so that the nuclides of interest are removed from the HLW with a suffi­
cient effectiveness, and their purity as process products fulfils requirements of the subse­
quent transmutation. Production of additional radioactive wastes in the process must be
strictly limited, and the partitioned HLW stream must not be decontaminated with com­
ponents which would interfere with its subsequent treatment. Adequate safety and per­
formance of the process must be attained by the choice of chemicals and operations used. 2. PROPOSED PARTITIONING FLOWSHEET
We chose solvent extraction as the partitioning method. It has been applied since the 1950s
for the reprocessing of irradiated nuclear fuel, first in the Redox process and then, for se­
veral decades, in the Purex process. Extended experience has been gained in the operation
of the Purex process on the laboratory, pilot, and industrial scale, and the applicability of
solvent extraction in industrial nuclear processes has clearly been demonstrated.
A two-cycle flowsheet was foreseen for this project. The raffinate (HAW) from the first
extraction step of the Purex process was suggested to be taken as the feed solution without
any appreciable intermediate storage and without concentration by evaporation. In this way
it should be possible to avoid deposition of precipitates from the HAW before its parti­
tioning. A 30 vol.% solution of tributyl phosphate (TBP) in an aliphatic diluent was selected
as the solvent in the first cycle. Two main reasons can be given for this choice. First,