Hanford’s melters

Two 300-ton glass melters tower over workers at the Hanford Waste Treatment and Immobilization Plant (WTP), in southeastern Washington state, in the northwestern USA. Now installed, the massive grey steel boxes that are the melters are impressive in size, but otherwise understated. However, their role in the plant is vital, and their installation is an important milestone.

The largest of their kind in the world, the melters – similar in form to the melters found at Sellafield in the UK – are central to the vitrification process that will be used to stabilise Hanford’s liquid radioactive waste into a solid glass form that is safe and impervious to the environment.

The US Department of Energy (DOE) plans to begin vitrification as early as 2022 and will be taking a phased approach to startup.

The WTP’s story starts during World War II and the Cold War, when nine nuclear reactors at the Hanford site were used to produce plutonium for the US nuclear weapons programme. Chemical separation and purification processes produced huge amounts of liquid waste. The reactors stopped in 1987, but Hanford is now home to 56 million gallons of radioactive and chemical waste ranging in consistency from liquid to thick sludge. The waste sits in ageing, underground carbon steel tanks, just a few miles from thriving communities and the Columbia River. Many of those tanks are single-shell, decades past their design life, and some have already leaked chemical and radioactive waste into the earth beneath.

To address the problem, the Department of Energy contracted engineering and construction firm Bechtel to design and build the WTP. When completed, it will be the world’s largest and most complex radioactive and chemical waste treatment plant.

The WTP will use vitrification, the process of turning radioactive waste into a stable glass form, as used at other waste vitrification plants at West Valley in New York state, Savannah River in South Carolina, and Sellafield in northwest England. However, the scale of the task at Hanford is significantly larger and the waste more complex. Hanford’s waste contains a mix of radioactive constituents and more than 1800 different chemicals.

“Hanford may be unique in its size, but vitrification is a proven technology. We’ve brought people from successfully operating vitrification plants around the world to use their experience and lessons learned to ensure we design and build a safe plant at Hanford that meets the challenges,” said Ian Milgate, WTP technical director. Milgate previously worked as the vitrification engineering manager at the Sellafield site.

The WTP spans 65 acres and comprises major nuclear facilities, including the low-activity waste (LAW) facility, high-level waste (HLW) facility, analytical laboratory, and pretreatment facility, as well as operations and maintenance buildings, utilities, and office space.

Twenty support facilities and 56 systems will allow the WTP to operate like its own small city. It will have its own infrastructure, including a steam plant, a power station and a water treatment facility. It also has an emergency diesel generator in case of power failure.

Direct feed low-activity waste

The newly completed glass melters are in the LAW facility, which is key to the approach to vitrifying Hanford’s waste that was finalised in December 2016. WTP was originally envisioned to treat Hanford’s low-activity and high-level waste simultaneously. However, to begin treating waste as soon as possible, the US DOE developed a phased approach to treat low-activity waste first by December 2021.

The phased approach is called direct-feed low-activity waste or DFLAW. It takes low-activity waste stored in the Hanford tank farms, removes solids and caesium, and sends it directly to the LAW facility, bypassing the large WTP pretreatment facility still under construction. Success with DFLAW will be the foundation for completing the remaining HLW and pretreatment facilities as DOE and Bechtel apply the expertise and lessons learned, and plant operators gain experience on the lower-activity waste.

DOE and Bechtel are concentrating their efforts on completing the LAW facility and analytical laboratory, and the necessary support facilities. An effluent management capability is being built to handle the liquid secondary waste generated by the LAW facility melters and off-gas treatment system. Modifications to the support facilities will be made to handle the reduced capacity needs.

Because a phased approach allows LAW facility operations to begin before the pretreatment and HLW facilities are complete, and because the volume of low- activity waste is much higher than the volume of high- level waste in Hanford’s tank farms, this approach has the potential to shorten the overall duration of the tank waste treatment mission.

The melters

The LAW facility melters will heat the waste and glass- forming materials to 1150°C (2100°F) before the mixture is poured into stainless steel containers where it will harden into a highly durable, leach-resistant borosilicate glass.

The melters are composed of a melter assembly (melter base and lid), a gas barrier lid, a shield lid, several layers of refractory brick interior, and other components that will feed, stir, and monitor the glass mixture. Eighteen pneumercators, strategically placed on the melter lid, are designed to improve melter throughput by agitating and significantly increasing melt-rate kinetics between blended feed material and molten glass.

Completed, the melters measure 20ft by 30ft and are 16ft high, requiring workers to climb a flight of stairs to reach their tops. The melters are mounted on rails so removal and replacement is easy when they have reached their five- year design life. Spent melters will be disposed of in a safe repository.

When the WTP is operational, liquid radioactive waste and glass-forming materials will be blended in feed makeup tanks using mechanical agitators and fed to the melter by air displacement slurry pumps. The melter will then heat the conductive feed, turning it into viscous glass. The glass mixture will be poured into stainless steel containers 4ft in diameter and 7ft high, where it will cool and harden into the safe and stable glass form.

During operations, the LAW facility melters will produce 15t of glass daily, enough to fill five containers. Off-gas produced from the vitrification process will be removed. The melters are connected to off-gas equipment designed to remove solids, mercury, NOx, organics and salt.

“Completing the LAW facility melters is a significant milestone that demonstrates we are committed and on track to completing DFLAW and treating Hanford’s low- activity waste by the end of 2021,” said Bill Hamel, WTP assistant manager and federal project director.

Two smaller, 90t melters are planned for use in the HLW facility.

WTP status

Physical construction of the LAW facility is expected to be complete by June 2018. The analytical laboratory is 98 percent complete, from an engineering, procurement and construction perspective, and the collection of support facilities is largely complete.

“Installing the melters was a tremendous accomplishment because of their size and weight, as well as the spatial constraints and tight tolerances in the LAW facility,” said Felice Presti, Bechtel area project manager for DFLAW.

DOE and Bechtel have already begun the multi-year startup and commissioning process, starting with the support facilities and systems.

The infrastructure must be in place before anything can operate, and 39 of the 56 systems have been turned over for startup testing. Of those, 11 have completed testing and are entering commissioning. These include the steam plant, chiller compressor facility, electrical substation and switchgear building, and the water treatment facility.