At the Hanford Site, waste retrieval has been completed in 17 of 149 large concrete underground single-shell tanks. The tanks were constructed of carbon steel and reinforced concrete between 1943 and 1964 to store a radioactive mix of sludge and saltcake waste from past nuclear processing activities.
Management and disposition of this waste is the responsibility of DOE's Office of River Protection, assisted by Washington River Protection Solutions (WRPS), operations contractor for the Hanford “tank farms.”
Because of the potential to reduce the overall time and cost required to retrieve the waste and prepare the tanks for closure, WRPS is considering options for installing new access holes in the tank domes for future retrieval efforts.
Working with Becht Engineering Co., Inc., in Richland, Wash., PNNL completed a structural analysis of a concrete single-shell tank (SST) dome with new access holes for deploying waste retrieval equipment.
Their analysis is contained in the paper “Finite Element Structural Analysis Evaluating New Retrieval Strategies in the Hanford Waste Tanks.” It confirms the continued structural integrity of the SSTs with new dome penetrations and retrieval equipment loads on the soil above the tank dome. Specifically, the analysis concluded:
- It may be less expensive to bore new penetrations and install new risers in the tank domes than to remove existing contaminated hardware in existing risers.
- The new large access holes could be up to 6 feet in diameter.
Modeling loads, making holes
The paper describes the team’s analysis procedure, as well as important considerations for installing new risers. Using detailed finite element computer models, they analyzed the tank response to static thermal and operating loads, as well as dynamic seismic loads.
- Thermal and operating loads. This model evaluates the degraded condition of the reinforced concrete tanks by including temperature dependent concrete stiffness, strength, and cracking. The model also includes elastic rebar, pressure dependent soil yielding, and contact between the soil and the concrete tank.
- Seismic loads: The seismic model includes contact interfaces between the tank and the surrounding soil, between the tank waste and the inner surface of the tank wall, and within the soil above the tank dome.
They evaluated the results from these models to analyze global concrete section demands versus section capacities, as well as rebar and concrete stresses near the new riser hole. All loading scenarios were found to be acceptably low compared to the load capacities of the tank dome.
“Removing long-length equipment from Hanford’s SSTs is one of the most difficult and time-consuming activities associated with tank waste retrieval,” said Keith Carpenter, WRPS engineer. “The analytical work completed by PNNL is another great step forward in our pursuit of installing new risers in the SSTs and minimizing the amount of long-length equipment that must be removed.”
While the paper presents a single example of the analyses considered, a variety of loading conditions were evaluated as well as additional configurations with four dome penetrations. The study team includes Ken Johnson, Naveen Karri, and John Deibler, Pacific Northwest National Laboratory; and F. George Abatt, Ken Stoops, Larry Julyk, and Brian Larsen, Becht Engineering Co., Inc.
This work builds on a comprehensive structural Analysis of Record (AOR) completed by PNNL in 2015 of the four SST designs at the Hanford Site. PNNL maintains computer models for both single-shell and double-shell tanks to assist WRPS in evaluating changes to the tank structures or operating loads.
Published: March 5, 2019