The Department of Energy and its contractor for the Waste Isolation Pilot Plant (WIPP) near Carlsbad, N.M., finished a crucial modification to the deep-underground facility’s ventilation system, according to an official letter dated May 2, bringing the parties a step closer to reopening the facility in mid-December.

In the official correspondence to Santa Fe, DOE’s Carlsbad Field Office and Nuclear Waste Partnership (NWP), the AECOM-headed conglomerate that operates WIPP, said they finished installing two large fans and associated equipment — known officially as the interim ventilation system — required to keep the air underground clear enough to resume waste storage at the facility.

In the the letter to John Kieling, head of the state Environment Department’s  Hazardous Waste Bureau, DOE Carlsbad Field Office Manager Todd Shrader and NWP President and Project Manager Philip Breidenbach said they had obtained “a New Mexico registered Professional Engineer certification that the Waste Isolation Pilot Plant (WIPP) facility has been modified in compliance with the Hazardous Waste Facility Permit.”

WIPP has been shuttered since February 2014 following a fire and subsequent, unrelated underground radiation release.

The interim ventilation system will increase underground air circulation at WIPP to just over 110,000 cubic feet per minute from the current 60,000 cubic feet per minute. The system is one of two stopgap upgrades planned so DOE can safely store waste and mine out more space at WIPP, once it reopens. However, even with both systems active, DOE will only be able to do one of those things at a time.

A permanent ventilation upgrade that will free the agency to expand the mine while also placing waste shipments from DOE sites and performing routine maintenance that requires the use of diesel-fueled equipment will cost $270 million to $400 million, and not be ready until 2021 at the earliest, a DOE official said during an industry conference in March.