Open Site Tests on Corrosion of Carbon Steel:Containers for Radioactive Waste Forms
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operational waste, containing
37
1 Cs,
34
1 Cs,
and
6°Co
as the dominant radioactive
constituents- Bituminized and vitrified waste samples with 30-50 wt.% waste loading were prepared. Combined effects of climatic factors on corrosion behaviour of carbon steel containers were estimated using gravimetric and chemical analyses. The observations suggest that uniform corrosion of containers prevails under open field conditions. The upper limits for the lifetime of containers were derived from calculations based on the model of atmospheric steel corrosion. Estimated lifetime values range from 300 to 600 years for carbon steel containers with the wall thickness of 2 mm containing vitrified waste, and from 450 to 500 years for containers with the wall thickness of 2.5 mm that were used for bituminized waste. However, following the most conservative method, pitting corrosion may cause container integrity failure after 60 to 90 years of exposure. INTRODUCTION The low and intermediate level waste disposal concept in Russia includes operation of regional near surface disposal facilities for the storage of waste packages. Since the middle of the 1980's we have observed and monitored the alteration of nuclear waste forms and packages under the ambient fluctuating conditions of an open testing area. This approach can be considered as a conservative one, as compared to the real burial conditions that are characterized by significantly reduced access of water and oxygen to the waste packages and by the nearly constant ambient temperature. 903
Mat. Res. Soc. Symp. Proc. Vol. 556 0 1999 Materials Research Society
EXPERIMENT
Intermediate level wastes used in the experiments originated from two types of NPP reactors and differed in composition. The principal radioactive constituents were
' Cs (40-90%), 13aCs (7-19%) and 6°Co (1-15%). The sum of 90Sr,
17
144
Ce,
106
Ru and
239
pu
was less than 1%. Nitrate salts of alkaline and alkaline-earth metals were the dominant non-radioactive waste constituents. Vitrified and bituminized wastes were prepared and poured into containers made from structural carbon steel
(Steel-3). Containers were
made in a form of a parallepiped and had the wall thickness of 2 and 2.5 mm and a volume of a few tens of litres. The upper part was open for contact of solidified waste and atmosphere. Waste packages were placed on trays made of stainless steel. The general view of the test unit is shown in Figure 1. The schematic appearance of waste packages and expected types of corrosion are shown in Figure 2.
.
Figure 1. Photograph of the test unit
Common borosilicate glass was used for waste vitrification. The compositions of waste glasses, as well as the mean values of climatic parameters, are given elsewhere [ I]. The climate of the disposal site is characterized by warm summer and temperate-cold winter with stable snow cover, and by clearly defined seasons (Table I).
904
Corrosion of waste package
general corrosion
Figure 2. Schematic appearance of the test sample and expected types o
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