PNNL

Maintaining optimum temperatures while the glass waste cools is a key function of the heating and cooling system at the Vit Plant.

Based on results from a recent PNNL study, the Department of Energy (DOE) now has strong evidence that the heating and cooling system at the Hanford Tank Waste Treatment and Immobilization Plant otherwise known as the Vit Plant, will safely cool the radioactive waste glass produced at its Low-Activity Waste (LAW) and High-Level Waste (HLW) Vitrification Facilities. What’s more, DOE may be able to boost glass production rates, which in turn could shorten the mission life of the multi-billion dollar plant.

DOE is designing and building the Vit Plant to immobilize millions of gallons of radioactive waste stored at the Hanford Site in southeastern Washington state into a durable glass form for safe, long-term storage and disposal.

The waste in the LAW and HLW facilities will be mixed with glass-forming materials, heated to a molten state, and then poured into stainless steel canisters (for LAW) or containers (for HLW) to cool. External experts called in by DOE to review the facility design expressed concerns that, based on the system’s size and past estimates of the thermal properties of the glass, the heating and cooling system could be challenged to maintain safe temperatures as the glass cools.

The study, performed by a PNNL team led by Carmen Rodriguez, aimed to replace these estimates with actual measured values by heating simulated glass waste samples and measuring the rate of heat release.

A Hard Look at Hot Glass

As a starting point, the team used standard methods for measuring the thermal properties of commercial glass. These quickly proved inadequate. During heating, the glass samples spread up the sides of the "crucibles"—tiny platinum dishes that hold the samples during analysis. And at high temperatures the waste glass formed crystals, which is not typical of commercial glass. Both of these factors prevented accurate measurement.

"Waste glass has unique characteristics that make the methods used for commercial glass inappropriate" said PNNL scientist John Vienna, part of the PNNL study team. "People have tried and failed to accurately measure these properties with waste glasses over the last four or five decades. The challenge to us was to figure out why they failed and to develop techniques to properly measure them."

Rodriguez then made several changes to her process. A key finding was the crystallization only happened during heating, so instead they measured the sample as it cooled—with the focus of the study being heat released by glass during cooling. She also replaced the crucibles with customized ones with flatter, thicker bottoms, preventing the samples from becoming deformed. Rodriguez used an automatic sample changer to ensure that the samples were positioned in exactly the same place for each measurement. Finally, she reduced the maximum temperature during heating to prevent crystallization.

The results, validated against an independent study from a world-leading expert in Japan, show that the heating and cooling system can definitely take the heat, and then some. In fact, the system may be as much as 40 percent larger than needed, according to DOE Office of River Protection (ORP) projections based on the PNNL study. Given the added “bandwidth”, the Vit Plant may be able to produce glass faster than originally thought, which could shorten the mission life and significantly reduce costs.

“This closes out a potential question as to whether the LAW Facility can operate as designed,” said Albert Kruger, manager of the ORP Glass Program, which funded the research. “So that has a major financial impact.”

PNNL Research Team: Carmen Rodriguez, Jaehun Chun, Jarrod Crum, Nathan Canfield, Ewa Ronnebro, and John Vienna.