Gas Generation Behavior of Transuranic Waste Under Disposal Conditions
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ABSTRACT The generation of hydrogen-gas from metallic waste is an important issue for the safety analysis of geological disposal facilities for transuranic (TRU) radioactive waste in Japan. The objective of this study is to clarify the gas-generation behavior of stainless steel and carbon steel in non-oxidizing alkaline synthetic groundwater (pH 12.8 and 10.5) at 30 r simulating geological disposal environments. At pH 12.8, the observed gas-generation rate from stainless steel in the initial period of immersion was 1.0 x 102 Nml/m 2/y and 1.0 x 10 Nml/m 2/y after 200 days (N represents the standard state of gas at 0 'C and 1 atm). At pH 10.5, gas generation was not observed for 60 days in the initial period. At 60 days, the gasgeneration observed was 5.0 x 10 Nml/m2/y. After 250 days, the gas-generation rate approaches zero. At pH 12.8, the observed gas generation rate of carbon steel in the initial period of immersion was 1.5 x 102 Nml/m2/y and the gas generation rate began to decrease after 200 days. After 300 days, it was 25 Nml/m2 /y. At pH 10.5, the gas generation rate in the initial period was 5.0 x 102 Nml/m 2/y and was 1.0 x 10 Nml/m 2/y after 200 days.
INTRODUCTION The transuranic (TRU) radioactive wastes, including metals, are planned to be buried in a repository deep underground in Japan. In the wastes, there will be a large quantity of stainless steel and carbon steel, including steel drums and contaminated steel from the decommission and demolition of nuclear-reprocessing facilities. These metals corrode and generate hydrogen-gas under non-oxidizing highly alkaline conditions expected in the repository. Hydrogen-gas-generation is an important issue from the viewpoint of accumulation of inner pressure in the disposal facilities which could disrupt the backfill environment and thus increase water flow through the near field and increase the flux of radionuclides to the surrounding. Therefore, it is necessary to evaluate both the gas-generation rate and gas permeability through the engineered barriers to analyze the safety of geological disposal facilities. The corrosion behavior of carbon steel has been reported by NAGRA [1], SKB [2] and others [3,4]. The corrosion behavior for stainless steel has not been measured sufficiently. Because this metal has a low corrosion rate, it is difficult to determine the corrosion rate by measuring the weight loss of a specimen and electrochemical methods. We studied the effect of pH on the gas generation behavior of stainless steels. We measured the hydrogen-gasgeneration volume for 500 days. We obtained the raw data on hydrogen-gas-generation from stainless steel and carbon steel under conditions simulating geological disposal relevant to the safety analysis for disposing transuranic radioactive waste in Japan. 1199 Mat. Res. Soc. Symp. Proc. Vol. 556 ©1999 Materials Research Society
CORROSION TEST METHOD Specimens Stainless steel wires (type 304) with the size of 0.5 mm in diameter and 100 mm in length were used. The stainless steel was composed of Fe (71.6 wt%),
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