For Chornobyl, I found estimate at Wikipedia: «An early estimate for total nuclear fuel material released to the environment was 3±1.5%; this was later revised to 3.5±0.5%. This corresponds to the atmospheric emission of 6 tonnes (5.9 long tons; 6.6 short tons) of fragmented fuel.[127]»

https://en.wikipedia.org/wiki/Chernobyl_disaster#Relative_is...