Why don’t we drop medical waste and nuclear waste into active volcanoes, the "ultimate high-temperature incinerators"?

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The answer: Because the hazardous parts would come right back out.

We
do incinerate more than 90 percent of the medical waste in the United
States, both to reduce its total volume and to kill off infectious
agents. But burning up blood-soaked bandages, discarded needles, and
stray organs like tonsils and appendices creates many dangerous
byproducts, including dioxins and carbon monoxide, as well as fly ash
laced with heavy metals. While an onsite incinerator at a hospital
would be equipped with scrubbers and filters to capture these
byproducts for landfill, any gases or ash produced in a volcano would
be emitted straight into the atmosphere.

It’s also not clear that
volcanic incineration would be hot enough to sterilize garbage.
According to the Environmental Protection Agency, medical waste should
be burned at more than 1,600 degrees Fahrenheit, with some states
demanding temperatures of 1,800 or 2,000 degrees to reduce volatile
organic emissions. Molten lava can be hot enough to do the trick,
measuring about 1,800 or 1,900 degrees on average. But the exact
temperature varies widely depending on the volcano. The unusual black
lava at Central Africa’s Mount Nyiragongo, for example, can be as cool as 800 degrees.

It’s
an even worse idea to toss nuclear waste into a volcano. Combustion
won’t have any effect on spent nuclear fuel, nor will it reduce the
radioactivity of low-level waste like contaminated clothing and equipment. The only reason to incinerate miscellaneous radioactive garbage would be to reduce its overall volume,
so it’s easier to sequester. As with the incineration of medical waste,
this produces dangerous emissions that would pop right out of a volcano.

Even
without medical or radioactive waste, volcanoes already release
dangerous gases into the environment. In Cameroon, carbon-dioxide
pollution from a volcanic crater lake asphyxiated several thousand valley-dwelling people when it displaced the available oxygen. Active volcanoes in Hawaii have caused problems with acid rain and "vog," a combination of volcanic gases and fog.

It would also be dangerous
and impractical to transport large quantities of garbage to the top of
an active volcano. At best, you might drop small items through
"skylight" openings on the tops of lava domes.

Via Slate