IN ORDER TO QUALITY CORROSION RESISTANT PACKAGING MATERIALS FOR DISPOSAL OF HIGH-LEVEL WASTE FORMS IN ROCK-SALT FORMATIONS, THE CORROSION BEHAVIOUR OF A NUMBER OF MATERIALS HAS BEEN INVESTIGATED IN SALT BRINES (POSTULATED ACCIDENT CONDITIONS) UNDER THE PRECEDING EC RESEARCH TASK. THE RESULTS OF THESE STUDIES HAVE SHOWN THAT UNALLOYED STEELS ARE A PROMISING PACKAGING MATERIAL BECAUSE THEY HAVE EXHIBITED SO FAR A LONG-TERM CORROSION BEHAVIOUR (GENERAL CORROSION, NO PITTING, CREVICE OR STESS-CORROSION CRACKING) THAT CAN BE CALCULATED; MOREOVER, IT CAN BE EXPECTED FROM THEIR CORROSION RATES OBTAINED SO FAR IN SALT BRINES THAT THE CONTAINER WALL-THICKNESS WILL BE ACCEPTABLE UNDER A CORROSION ALLOWANCE CONCEPT.
THESE ARE THE GOALS OF THE PROJECT :
- DEFINITION OF THE BETTER SUITED STEEL (REFERENCE STEEL) FOR FABRICATION OF A LONG-TERM RESISTANT HLW PACKAGING AND THE DETAILED DESCRIPTION OF THE TIME AND TEMPERATURE BEHAVIOUR OF ITS CORROSION IN DISPOSAL RELEVANT CORROSION MEDIA WITH AND WITHOUT GAMMA RADIATION.
- DETERMINATION OF THE CORROSION BEHAVIOUR OF HASTELLOY C4 SUBJECTED TO HIGH GAMMA DOSE RATES AND UNDER THE IN-SITU CONDITIONS PREVAILING IN THE ASSE SALT MINE. THESE ARE INVESTIGATIONS PERFORMED IN ORDER TO COMPLETE THE RESULTS AVAILABLE NOW.
To characterise the corrosion behaviour of unalloyed steels, titanium 99.8-palladium and Hastelloy C4, which have proved to be the most promising materials for long term resistant packagings for heat generating waste disposal in rock salt formations, in depth laboratory scale and in situ corrosion studies have been performed. Some in situ investigations of the iron based materials Ni-Resist D2 and D4, cast iron and silicon cast iron have also been carried out.
The results of detailed laboratory and in situ corrosion experiments were in good agreement and they confirmed the finding of previous investigations that unalloyed steels and titanium 99.8-palladium are promising materials for long term resistant packaging. The 3 steels (fine grained steel, low carbon steel, cast steel) investigated and titanium 99.8-palladium resisted pitting and crevice corrosion as well as stress corrosion cracking under all test conditions.
By contrast, Hastelloy C4 proved susceptible to pitting and crevice corrosion at gamma dose rates higher than 1 Gy per hour and in the presence of hydrogen sulphide (25 milligrams per litre) in magnesium chloride rich brine (Q-brine). The materials Ni-Resist D2 and D4, cast iron and silicon cast iron corroded at negligible rates in the in situ experiments performed in rock salt with limited amounts of sodium chloride brine. Nevertheless, these materials must be ruled out as container materials because they have proved to be susceptible to pitting and intergranular corrosion in previous laboratory studies conducted with Q-brine in excess.
In previous corrosion studies performed in salt brines, non-alloyed steels, titanium 99.8-palladium and Hastelloy C4, have proved to be the most promising materials for long term resistant packagings to be used in heat generating waste (vitrified high level waste, spent fuel) disposal in rock salt formations. To characterise the corrosion behaviour of these materials in more detail, further in depth laboratory scale and in situ corrosion studies have been performed. In addition to the above mentioned materials, some in situ investigations of the iron base materials nickel Resist D2 and D4, cast iron and silicon cast iron have been carried out.
2.1. SELECTIVE LABORATORY-SCALE CORROSION STUDIES ON THE THREE PRESELECTED STEELS, NAMELY FINE-GRAINED STEEL, LOW-CARBON STEEL, CAST STEEL, WITH A VIEW TO SELECTING A REFERENCE STEEL.
2.1.1. WITHOUT GAMMA IRRADIATION IN MOIST ROCK SALT (ROCK SALT/ROCK SALT BRINE AND ROCK SALT/Q-BRINE3) AT 90C AND 170C.
2.1.2. WITH GAMMA IRRADIATION (10 RAD/H) AT 90C IN Q-BRINE.
2.2. MORE DETAILED LABORATORY-SCALE CORROSION STUDIES ON REFERENCE STEEL SUBJECTED TO VARIOUS GAMMA DOSE RATES (102 TO 104 RAD/H) IN ALL THREE CORROSION MEDIA ENUMERATED UNDER ITEMS 2.1.1. AND 2.1.2.
2.3. POST-TEST EXAMINATION OF THE STEELS STORED IN SITU (MATERIAL SPECIMENS, WELDED TUBE SECTIONS) FOR CORROSION ATTACKS AT T = 35C AND 200C, ROCK PRESSURE AND HIGH GAMMA RADIATION (ABOUT 3X104 RAD/H).
2.4.1. LABORATORY-SCALE EXAMINATIONS OF THE INFLUENCE OF THE GAMMA DOSE RATE (102, 103, 104 RAD/H) ON CORROSION OF HASTELLOY C4 IN Q-BRINE AT 90C.
2.4.2. POST-TEST EXAMINATION OF THE MATERIAL SPECIMENS AND WELDED HASTELLOY C4 TUBE SECTIONS STORED IN ASSE FOR CORROSION ATTACKS. THE TESTING CONDITIONS ARE SIMILAR TO THOSE UNDER ITEMS 2.3.