This article (from 2016, three years later than TFA) says it’s estimated to be ‘slightly higher than the ambient temperature’ - https://www.atlasobscura.com/art

This article (from 2016, three years later than TFA) says it’s estimated to be ‘slightly higher than the ambient temperature’ - https://www.atlasobscura.com/articles/the-famous-photo-of-chernobyls-most-dangerous-radioactive-material-was-a-selfie https://www.atlasobscura.com/articles/the-famous-photo-of-ch... While it reaching groundwater was definitely a concern in the 80’s, I don’t think it’s actually moved significantly or is expected to in the future.