Carbon Steel Canister Performance Assessment: Iron Transfer Study
- PDF / 61,500 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 76 Downloads / 151 Views
Carbon Steel Canister Performance Assessment: Iron Transfer Study AntonínVokál, Dmitrij Lukin and Dušan Vopálka1 Nuclear Research Institute Rez plc, Czech Republic 1 Department of Nuclear Chemistry, Czech Technical University in Prague, Czech Republic ABSTRACT Carbon steel has been chosen in the Czech disposal concept of spent fuel disposal in a granite host rock as a reference material for disposal canisters. On the basis of the results of performance assessment studies, it could be decided whether this material is suitable or whether a more corrosion resistant, and also more expensive, material should substitute it. A number of papers have convincingly shown that iron transfer constraints contribute to a significant decrease in corrosion rate, but no study has so far been devoted to modeling this process. In this paper the effects of initial corrosion rate, corrosion product solubility and porosity and other repository parameters on the transfer of iron to the host rock are modeled using a numerical transport computer code. It was found that the critical parameter for iron transfer is the solubility of corrosion products, considerably affecting the steady state corrosion rate. The initial corrosion rate of carbon steel and the sorption properties of bentonite primarily affect the time needed to achieve a steady state of corrosion. The results of the calculations strongly suggest that the constraints on iron transfer from the canister surface will govern the corrosion rate of carbon steel canisters, whose lifetime, owing to this effect, can stretch to millions of years.
INTRODUCTION Carbon steel covered by a thin layer of Ni alloy has been chosen in the Czech disposal concept of spent fuel disposal in granite host rock as a reference material for disposal canisters. On the basis of the results of performance assessment studies, it could be decided whether this material is suitable for Czech disposal concept or whether it should be replaced by a more corrosion resistant, but also more expensive material often requiring the development of sophisticated welding technologies. Carbon steel great merit is its low tendency to the localized corrosion since the formation of passive layers on a carbon steel surface requires a higher critical current density of cathodic reactions at the same pH than that on more corrosion resistant metals such as chromium, nickel or titanium. With decreasing cathodic current density due to reduced oxygen in the repository, it is thus difficult to get carbon steel to a passive state and therefore it will corrode in anaerobic repository conditions only through general corrosion. A large number of experiments have been devoted to measuring the carbon steel corrosion rate in anaerobic conditions similar to deep geological repository conditions [1–6]. All the results clearly suggest that the corrosion rate of carbon steels in the anaerobic environment of deep geological repositories will be very slow. Despite these findings, carbon steel canisters are often assumed to fail shortly after 1000 year
Data Loading...
