ESTIMATION OF LONG-TERM CONCRETE STRENGTH IN MASSIVE SHIELDING WALL SUBJECTED TO GRADIENT ELEVATED TEMPERATURE IN BWR TYPE NUCLEAR POWER PLANT

摘要

This paper presented a new system to estimate concrete strength in massive walls subjected to gradient elevated temperature in nuclear power plants. Equations to estimate water distribution in massive wall heated at liner side assuming moisture migration due to pressure, dispersion and evaporation were proposed. Estimation of concrete strength affected by moisture migration were made by substituting moisture content in concrete for strength determined with the correlation between weight loss and residual strength after sustained elevated temperature exposure obtained in the author's previous experimental results. Prediction models for concrete strength in shielding wall heated at 65 °C and 90°C on liner side and cooled at 20°C on open side were proposed, as well as prediction models of strength and modulus of elasticity based on the experiment using normal portland cement: NP made in 1979 for 1970s' concrete and NP and fly-ash cement: FB made in 2011 for 2010s' concrete. Simulations were made based on analyzed distribution of temperature, pressure and weight loss at 65 °C and 90°C heating for 1 to 60 years. The simulation results indicated that strength was lower at liner side and open side corresponding to moisture loss and that strength was higher in central zone with increased moisture content. For concrete with NP made in 1979, concrete strength heated at 65°C showed 10 % increase at liner side, 20 % increase at central zone and practically no change at open side after 60 years, as well as those heated at 90°C showed practically no change at liner side and open side after 60 years. Simulation results with cement made in 2011 showed different tendency at 90°C heating. NP showed about 8 % strength reduction at liner side. FB remained 5 % strength increase at liner side and open side. Differences in strength distribution between heating temperature of 65 °C and 90°C, between NP made in 1979 and 2011 and the effects of cement type were evident.