Objectif
BOROSILICATE GLASSES ARE PROPOSED AS WASTE FORM FOR THE FINAL DISPOSAL OF HIGH LEVEL WASTE (HLW) BY THE FEDERAL REPUBLIC OF GERMANY. BETWEEN 1980 AND 1985 EXTENSIVE STUDIES ON THE CORROSION MECHANISMS OF THE INACTIVE HLW GLASS TYPE SM 58 IN A SALT BRINE WERE PERFORMED BY THE CONTRACTOR (CONTRACT NOS. WAS-232-81-53 D (B) AND WAS-323-83-53 D (B)). IN 1988 THE LONG TIME CORROSION TESTS WITH THE HLW GLASS SM 58 WERE FINISHED (TEST DURATIONS UP TO 5 YEARS IN SOLUTION 1 (TABLE II) AT TEMPERATURES UP TO 200 CELSIUS DEGREES /1/); A SHORT SUMMARY OF THESE RESULTS IS INCLUDED IN THIS REPORT.
A THEORETICAL STUDY /2/ ON REPORTED CORROSION DATA (FOR DIFFERENT GLASSES AND DIFFERENT CORROSION CONDITIONS) SHOWED THAT MOST HLW GLASSES CORRODE ACCORDING TO A VERY SIMILAR PATTERN. THE MAIN MECHANIMS ARE:
- NETWORK DISSOLUTION,
- FORMATION OF REACTION PRODUCT LAYERS ON TOP OF THE GLASS, AND
- ION EXCHANGE.
THE FOLLOWING MODEL WAS DEVELOPED FROM THESE RESULTS: IN FRESH LEACHANTS THE CORROSION STARTS WITH AN INITIAL RATE R0 LINEARLY DEPENDING ON T. DUE TO THE ACCUMULATION OF REACTION PRODUCTS IN THE LEACHATE, THIS INITIAL RATE SLOWS DOWN AND REACHES FINALLY A LONG TERM RATE R00, WHICH IS TYPICAL OF CORROSION IN LEACHATES SATURATED WITH RESPECT TO SILICA. THE RATE R00 SHOULD DEPEND LINEARLY ON T.
AT THE START OF THE INVESTIGATIONS PRESENTED HERE, ONLY FEW DATA WERE AVAILABLE FOR THIS LONG TERM CORROSION OF HLW GLASS IN SALT SOLUTIONS. THEREFORE, THE CORROSION OF THE HLW GLASS TYPE SON 68 (OR R7T7)IN SALT BRINES WAS INVESTIGATED UNDER THE CONDITIONS OF SILICA SATURATION. SILICA SATURATION WAS ACHIEVED IN THE LABORATORY BY ENLARGING THE RATIO OF SAMPLE SURFACE AREA TO SOLUTION VOLUME (S/V). THE INTEREST WAS FOCUSED ON THE COMPOSITION OF THE SURFACE OF THE RESIDUAL GLASS AS WELL AS ON THE COMPOSITION OF THE LEACHATE AND ON THE INVESTIGATION OF THEIR INFLUENCES ON THE CORROSION PROCESS. THE CORROSION TESTS WERE PERFORMED IN COOPERATION WITH THE HAHN-MEITNER-INSTITUT (HMI), BERLIN.
A theoretical study, based on a broad set of corrosion data with different glasses and different corroding agents, showed that all high level waste (HLW) glasses corrode according to very similar patterns.
The main mechanisms can be summarised as follows:
the glass is submitted to a network dissolution process responsible for almost the whole corrosion process;
as long as an accumulation of reaction products in the solution is avoided, the network dissolution rate has an intital value r-0 which is a function of the individual glass composition and of the activities of the corresponding species in solution;
the dissolution rate slows down with increasing amounts of accumulated reaction products and finally reaches a long term rate r;
reaction products (eventually involving components of the attacking solution) form layers on top of the residual glass;
the reaction product layers in many cases act as transport barriers and reduce further corrosion attack;
the surface zone of the residual glass is altered by ion exchange processes.
A theorectical study based on a broad set of corrosion data with different glasses and different corroding agents shows that all high level radioactive wastes (HLW) glasses corrode according to very similar patterns. The main mechanisms can be summarized as follows:
(i) the glass is submitted to a network dissolution process responsible for almost the total corrosion progress;
(ii) as long as an accumulation of reaction products in the solution is avoided, the network dissolution rate has an initial value which is a function of the individual glass composition and of the activities of the corresponding species in solution;
(iii) the dissolution rate slows down with increasing amounts of accumulated reaction products and finally reaches a long term rate;
(iv) reaction products (eventually involving components of the attacking solution) form layers on top of the residual glass;
(v) the reaction products layers in many cases act as transport barriers and reduce a further corrosion attack;
(vi) the surface zone of the residual glass is altered by ion exchange processes.
B.1. CORROSION PRE-TESTS FOR THE OPTIMIZATION OF ANALYTICAL TECHNIQUES.
B.2. SUPPLEMENTARY CORROSION TESTS.
B.3. OPTIMIZATION OF ANALYTICAL TECHNIQUES (PHOTOELECTRON SPECTROSCOPY (XPS), ACID ETCHING).
B.4. OPTIMIZATION OF PREPARING METHODS.
B.5. PREPARATION AND MEASUREMENT OF STANDARD SAMPLES WITH XPS.
B.6. SURFACE ANALYSIS OF CORRODED GLASS SAMPLES.
B.7. PREPARATION AND SAMPLING OF SATURATED LEACHATES AT TEST TEMPERATURE WITH SUBSEQUENT CHEMICAL ANALYSIS.
B.8. EVALUATION OF TEST RESULTS.
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CORDIS classe les projets avec EuroSciVoc, une taxonomie multilingue des domaines scientifiques, grâce à un processus semi-automatique basé sur des techniques TLN. Voir: Le vocabulaire scientifique européen.
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