This study aims to assess the degradation of below-grade concrete of nuclear power plants (NPP) in Japan, considering possible acid and sulfate attacks. A survey on the underground environments of several NPPs and residential buildings across the country was conducted, and their associated concrete performance was evaluated where concrete samples from core drillings were available. Moreover, acid and sulfate exposure tests on mortar specimens lasting for up to four years were carried out in the laboratory to simulate actual field situations. The effects of exposure conditions, such as solution concentrations, temperature, and immersion conditions, were examined. The surveyed environments were classified into non-aggressive or slightly aggressive environments. The concrete core samples investigated showed insignificant degradation and satisfactory strength after 40 years of exposure. The laboratory test results showed that the accelerated tests using highly concentrated solutions could exacerbate the extent of decalcification and even alter the degradation mechanism for magnesium sulfates. Therefore, a close-to-reality concentration is preferred for reproducing field situations. The carbonation/neutralization depth was used as an indicator to estimate the degradation extent. The measured values in the laboratory using low-concentration solutions correlated well with the field results, suggesting that the below-grade concrete’s degradation in the NPPs investigated may be less than 10 mm after 60 years of exposure.