Sol - gel inorganic ion exchangers for conditioning of medium level radioactive waste

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Nuclear energy and safety

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• Publié par: EUROPEAN-COMMISSION
• Nombre de lectures: 56
• Langue: English
• Poids de l'ouvrage: 2 Mo

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Commission of the European Communities
nuclear science and technology
SOL - GEL INORGANIC ION
EXCHANGERS FOR CONDITIONING
OF MEDIUM LEVEL RADIOACTIVE WASTE
Report
EUR 11791 EN
Blow-up from microfiche original Commission of the European Communities
nuclear science and technology
SOL - GEL INORGANIC ION
EXCHANGERS FOR CONDITIONING
OF MEDIUM LEVEL RADIOACTIVE WASTE
P. GERONTOPOULOS*. G. ARCANGELI", R. FAVA*, D.M. TRAVERSO*
* AGIP S.p.A. - Via Sabbionara 611 - MEDICINA (BO) - ITALY
" ENEA/CRE - Via Anguillara Km 1+300 - CASACCIA (Roma) - ITALY
Contracts No. EUR 221-81-31 and 284-82-31 WAS I
FINAL REPORT
Work performed under the R&D programme on Management & Disposal
of Radioactive Waste of the European Communities
and the ENEA-AGIP Programme for the Development of Sol-Gel Technology.
(The experimental work was performed at the Institute for Transuranium Elements,
JRC Karlsruhe in the frame of the Contract AGN-CEC CR/01/80).
Directorate-General Science, Research and Development!
1988 h^EUR 11791 EN Published by Ih·
COMMISSION OF THE EUROPEAN COMMUNITIES
Directorate-General
Telecommunications, Information Industrie· and Innovation
Bâtiment Jean Monnet
LUXEMBOURG
LEGAL NOTICE
Neither the Commission of the European Communities nor any person acting on behalf
of then is responsible for the use which might be made of the following
information
ECSC—EEC—EAEC Brussels - Luxembourg. 1988 III
INDEX
SUMMARY V
1. SCOPE 1
2. STATE OF THE ART 2
3. PREPARATION OF THE ION EXCHANGER 4
4. ION EXCHANGE PROPERTIES OF HTO, TiP AND ZrP 7
5. CONVERSION OF SPENT HTO AND TiP ION EXCHANGE
PARTICLES TO HIGH DENSITY WASTE CERAMICS 17
6. EVAPORATIVE DEPOSITION ON XEROGEL PARTICLES
(EDXP) 19
7. PRACTICAL APPLICATION EXAMPLES 20
8. LEACH RESISTANCE TESTS
9. CONCLUSION 3
10. BIBLIOGRAPHIC REFERENCES5
FIGURESKEY WORDS
Concentration Factor, Decontamination Factor, Distribution Coefficient,
Evaporative Deposition on Xerogel Precursors (EDXP), Gel Supported
Precipitation (GSP), Hydrous Titanium Oxide (HTO), Hydrous Zirconium Oxide
(HZO), High Level Liquid Waste (HLLW), Ion Exchange (IX), Leach Resistance,
Medium Activity Waste (MAW), Synroc, Sol Gel, Titanium Phosphate (TiP),
Transuranic element (TRU), Xerogel, Zirconium Phosphate (ZrP).
SUMMARY
Decontamination of HLLW and MAW streams by inorganic ion exchange combined
with the conversion of the spent inorganic ion exchange material to waste
ceramics presents a considerable potential for utilisation in waste condi­
tioning and has been intensely investigated during the past decade in
various countries. Based on product quality performance criteria, ceramic
waste forms are found superior to other candidate waste immobilisation forms
but their acceptance for practical implementation is hampered because of the
complexity of the related fabrication technology. The work described in this
report shows the possibility of improving this situation by resorting to sol
gel techniques earlier developed for preparation of nuclear fuel ceramics.
The principal findings are :
- superior quality ion exchange xerogel titanates in the form of mechanical­
ly resistant, size controlled microspheres can be prepared using a simple
sol-gel technique;
- the titanate particles can be also used as precursors in Evaporative
Deposition on Xerogel Particles (EDXP) a new waste solidification process
based on physical impregnation of the xerogel material with the waste
liquid followed by evaporation;
- waste loaded ion exchange microspheres can be converted to leach resistant
ceramics by firing and/or cold pressing and sintering at 900°-1100°C;
- sol-gel inorganic ion exchange and EDXP may find useful application in
conditioning of various types of MAW streams. 1. SCOPE
Liquid medium level radioactive wastes (MAW) are produced in various
phases of the nuclear fuel cycle and differ widely in chemical composi­
tion and activity (10 - 10 Ci/m ). They include effluents from
nuclear fuel fabrication, reactor operation and spent fuel reprocessing
facilities. A detailed description of their origin and characteristics
has been given by CHAUVET and CARLEY-MACAULY (1).
The use of organic ion exchangers in decontaminating such waste streams
is a well established industrial practice (2). The utilisation of inor­
ganic ion exchangers in this type of processes is regarded with interest
because of their superior thermal, chemical and radiation resistance
properties. An additional advantage from using inorganic ion exchangers,
in particular titanates, derives from the possibility of their direct
conversion, once loaded with radwaste elements, to high density, leach
resistant, ceramic monoliths of excellent stability characteristics for
permanent waste disposal. In fact new impetus for the development of
inorganic ion exchangers for waste conditioning, results from the work
of LYNCH et al. (3), HULTGREN et al. (4) and RINGWOOD et al. (5) on the
immobilisation of liquid high level wastes (HLLW) in rutile and/or
SYNROC ceramics.
However large scale implementation of inorganic ion exchange in waste
decontamination and conditioning is obstructed from a series of techno­
logical difficulties and in the first place the morphology of the ex­
changer usually produced in the form of fine powders or friable powder
agglomerates. Such products are unsuitable for operation using fixed bed
column or counter current contactors, have a high potential for contami­
nation diffusion in off standard operating conditions, and their
conversion to waste ceramics is complex.
The basic assumption made in engaging into this study was that the above
mentioned difficulties could be solved or more easily tackled by
resorting to sol-gel preparation techniques earlier developed for
fabrication of ceramic nuclear fuel microspheres.
In a first phase of our work, the effort was directed toward the prepa­
ration and characterisation of Hydrous Titanium Oxide (HTO) microspheres
because of the well known affinity of this compound for a broad spectrum
of cations, the attractive characteristics of titania based ceramics for
permanent immobilisation of radioactive waste and the relatively low
price of the TiCl raw product. In a second phase the work was extended
to the preparation and evaluation of Titanium Phosphate (TiP) micro­
spheres to provide for uptake and conditioning of cesium not efficiently retained by HTO. A minor effort was spent in preparation tests of
Hydrous Zirconium Oxide(HZO), Zirconium Phosphate (ZrP), and mixed
HTO-Fe 0 xH 0 particles.
2 3 2
In treating high salinity MAW concentrates, generated in nuclear fuel
reprocessing, efficient decontamination of the waste stream based only
on ion exchange sorption of the radioelements present at trace concen­
tration levels, is not possible because of the rapid saturation of the
exchanger by the non radioactive elements present in great excess. For
the conditioning of this type of waste we propose the utilisation of
inorganic ion exchange on HTO and TiP in combination with an alkaline
precipitation step of the bulk of the non radioactive elements present
in solution. The sludge generated from the alkaline precipitation step
can be immobilised in titanate ceramics by Evaporative Deposition on
Xerogel Precursors (EDXP) a new technique developed in our laboratory
for the fabrication of SYNROC Ceramics (6).
2. STATE OF THE ART
2.1 Sol-gel processes for the fabrication of nuclear fuel
Sol-gel processes for the preparation of nuclear fuel microspheres
of heavy metal oxides, carbides and nitrides have been extensively
investigated since the middle sixties in various countries. They can
be subdivided into the following categories:
- dehydration processes: liquid droplets of an aqueous colloidal so­
lution of a metal compound, (a sol) are dispersed in an organic me
dium and converted to solid gel microspheres by water extraction.
- internal gelation processes: liquid droplets of an aqueous colloi^
dal solution of a metal compound, admixed with an ammonia "donor"
(e.g. hexamethylentetramine) are dispersed in an inert medium and
converted into solid gel microspheres by precipitation of the me­
tal hydroxide with ammonia set free by heating the inert medium.
- gel supported precipitation processes (GSP) : liquid droplets of
an aqueous solution of a metal compound mixed with an organic
high intrinsic viscosity polymer, a gel agent, are converted into
gel microspheres on contacting with an alkaline bath where
precipitation of the metal hydroxide occurs.