https://www.abc.net.au/news/2017-11-27/the-dome-runit-island...
"Some of the debris buried beneath the dome includes plutonium-239, a fissile isotope used in nuclear warheads which is one of the most toxic substances on earth.
It has a radioactive half-life of 24,100 years."
The chemical toxicity is a different matter though.
As an alpha emitter, plutonium-239 is not particularly dangerous as an external radiation source, but if it is ingested or breathed in as dust it is very dangerous and carcinogenic. It has been estimated that a pound (454 grams) of plutonium inhaled as plutonium oxide dust could give cancer to two million people. However, ingested plutonium is by far less dangerous as only a tiny fraction is absorbed in gastrointestinal tract. 800 mg would be unlikely to cause a major health risk as far as radiation is concerned. As a heavy metal, plutonium is also toxic.