Normal ocean water has potassium and rubidium radionuclides that run to 11Bq/L.
At the higher level of 5MBq/L, they would need to dilute it into 3000km^3 of seawater in order to reach double background radiation levels, or a patch of the 6km deep Pacific abyssal plain that is 22km x 22km.
At the higher level of 5MBq/L, they would need to dilute it into 3000km^3 of seawater in order to reach double background radiation levels, or a patch of the 6km deep Pacific abyssal plain that is 22km x 22km.