About 237,000 tonnes of arsenic trioxide dust is stored in giant chambers under the Giant Mine site just outside Yellowknife. (CBC)

Workers at the Giant Mine site near Yellowknife have begun fine-tuning a method to freeze nearly a quarter of a million tonnes of highly toxic arsenic trioxide dust.

Crews are testing the method on one of 15 giant subterranean chambers that altogether contain 237,000 tonnes of arsenic trioxide, a byproduct of more than 50 years of gold production.

Project manager Martin Gavin said workers will drill holes in the rock around the chamber, then pump cooling brine into the holes to freeze the rock and — if all goes to plan — prevent the poisonous dust from leaking out.

"So essentially we're taking the arsenic chamber and we're turning it into a giant fridge — the biggest beer fridge in Canada, except we're not putting beer in it," Gavin said.

"What we do is we freeze the rock around that, and it's a very simple system. It's a brine system, the same as what's used at the Yellowknife Curling Club or any hockey rink."

The Gold Mine produced more than seven million ounces of gold produced between 1948 and 1999, creating the arsenic trioxide dust as a byproduct, according to the website of the Giant Mine Remediation Project.

The underground chamber being tested this week is about 100 metres long by 100 metres wide, and about about 76 metres deep.

Gavin said the holes will be drilled and the freezing process will begin by the fall.