The Integrated Waste Treatment Unit (IWTU) at the US Department of Energy (DOE) Office of Environmental Management (EM) Idaho National Laboratory (INL) Site has begun converting radioactive sodium-bearing liquid waste stored in underground tanks to a more stable, granular solid. The waste was generated during decontamination activities following used nuclear fuel reprocessing that ended in 1992.

Workers constructed the IWTU from 2007 to 2011 to convert 900,000 gallons of radioactive liquid waste to a granular solid resembling coarse sand using steam-reforming technology. In 2012 three demonstration runs identified a number of chemical and equipment challenges. As a result, from 2012 to 2023, IWTU underwent testing with non-radioactive simulated waste, or simulant. EM also completed some 50 major modifications to the facility’s primary reaction vessel, off-gas treatment vessel, process filters and canister fill cells to address these challenges.

Initially, the IWTU will treat a blend that is 10% sodium-bearing waste (SBW) and 90% simulant. The blend will later contain 50% waste and 50% simulant before it eventually becomes 100% sodium-bearing waste based on the plant’s operating conditions. Following treatment, the granulated waste will be stored in stainless-steel canisters within concrete vaults at the IWTU. Ultimately, the waste will be disposed of at a national geological repository.

In the run-up to starting radiological operations, the IWTU Operations crew coordinated with New Waste Calcining Facility (NWCF) Operations personnel to pump SBW from NWCF tanks to the IWTU waste feed tank where it was blended with simulant to achieve the desired blend ratio.

The SBW waste to be treated was generated from operations at Idaho Nuclear Technology & Engineering Center (INTEC). The liquid was stored in three stainless steel 300,000-gallon storage tanks that are part the high-level waste Tank Farm. This includes 11 underground stainless-steel storage tanks used to store the radioactive liquid waste generated during the reprocessing of used nuclear fuel and plant decontamination work. No leakage has occurred from the tanks. The tanks are encased in concrete vaults which have sumps and leak detection. Seven tanks have been cleaned to Resource Conservation and Recovery Act (RCRA) standards and have been grouted in place for final closure. The remaining four tanks (three full and one spare) will be cleaned and grouted once the SBW has been removed.

IWTU, located east of INTEC, uses a steam-reforming technology to convert the liquid to a solid and will package it in stainless steel canisters to be stored in concrete vaults at the site. Any emissions generated during the treatment campaign are filtered through high-efficiency particulate air (HEPA) and Granulated Activated Charcoal filters and sampled to ensure regulatory requirements are met. NWCF converted high-level radioactive liquid waste from INTEC tanks until May 2000 in a process called calcining. The infrastructure from that former operation – such as waste transfer lines and holding tanks – will be used by IWTU.

Image: A view of the exterior of the Integrated Waste Treatment Unit at the Idaho National Laboratory Site (courtesy of US DOE)