New Approach Would Use Increased Number of Smaller Plant Vessels
Mike Nartker
WC Monitor
4/04/2014
In a new effort to finally resolve the long-standing technical concerns that have significantly delayed the completion of the Hanford Waste Treatment Plant’s Pretreatment Facility, the Department of Energy is considering a simplified design approach that would employ smaller-sized plant vessels based on one standardized design. Late this week, DOE released its proposed testing strategy to evaluate the new approach, outlining an anticipated three-year effort expected to cost $147-180 million, though the Department said that figure was likely to decrease as plans are more fully developed. Langdon Holton, DOE’s senior technical authority for the WTP, told WC Monitor this week that the new proposed approach would “allow us to more confidentially resolve a number of issues associated with the vessel design and demonstrate that the vessels meet their nuclear safety functions for mixing to support hydrogen gas release and help on resolution of outstanding issues on the potential for criticality.” Holton added, “We feel good about this from a technical perspective in terms of helping us to resolve the issues in the Pretreatment Facility.”
Number of Vessels Could Double
DOE’s new proposed approach is intended to help put to bed concerns over the effectiveness of the pulse jet mixing systems to be used in the Pretreatment Facility. Adequate mixing is necessary to prevent solids in Hanford’s tank waste from settling to the bottom of WTP vessels and thereby posing criticality and hydrogen generation concerns. Mixing concerns at the Pretreatment Facility now center on eight of the facility’s 34 total vessels—those expected to handle waste with higher solids concentrations. The eight vessels are based on five differing designs, according to DOE, and have capacities ranging from 30,000 to 160,000 gallons, with diameters ranging from 14-to-38 feet in diameter. DOE had previously planned to resolve mixing concerns with the vessels through a full-scale testing program the Department said this week would have taken six-to-eight years to complete at a “very large” cost.
Under DOE’s proposed approach, the eight vessels of concern would be replaced with smaller ones, expected to measure 14-to-16 feet in diameter with capacities of approximately 20,000 to 25,000 gallons, based on one standardized design. The number of new replacement vessels that would be installed has yet to be determined, but Holton said it could be between 14 and 16. The proposed approach is expected to “add confidence that the vessel design will effectively resolve a hydrogen event by ensuring more complete mixing thereby releasing any trapped hydrogen gas” and “add confidence that the vessel design will effectively resolve any nuclear criticality issue by ensuring solids are well mixed and do not accumulate in the vessel,” according to DOE’s new test plan.
The proposed approach is expected to maintain the WTP’s planned waste treatment rates, Holton said, noting that another anticipated benefit of the proposed approach is that it would provide the Pretreatment Facility with spare vessels if necessary. “In our assessment of treatment capability, we do not have to use all of those vessels, but we will use the space in the black cells to install vessels. And those vessels not used for treatment will become spares. So in the case there would be a premature failure of one of the primary vessels, we will now have a back-up,” he said. “So that gives us a benefit that we didn’t have with the existing design, where if one of the large vessels failed, it could potentially cause a major impact to the overall treatment rate.” The new design approach is also expected to expected to avoid “structural design” concerns posed by the larger tanks, Holton said. “The smaller design is easier to design and will be easier to qualify from a structural perspective,” he said.
New Approach Not Expected to Increase Complexity
Holton said that DOE’s new proposed approach would result in some redesign and rework at the Pretreatment Facility, where construction has been halted since 2012 due to unresolved technical issues. He also said, though, that the new approach is not expected to increased the complexity of the Pretreatment Facility. “There is redesign and rework, but we don’t see that we will be adding what I call new air capacity to the facility,” Holton said. As an example, he cited one currently planned vessel for the facility that would have 18 pulse jet mixers, noting that the replacing that vessel with three smaller ones with less pulse jet mixers in each would not have an impact on piping needs. “As we looked at the concept, we looked at the current air demand and the current piping associated with supporting those demands, so in effect we will be living within those bounds,” Holton said. “More importantly, the smaller vessels will allow us an opportunity to reduce the air sparging requirements for the vessels.” Holton said, “This design alternative gives me an opportunity to simplify the piping in the design of the black cells, but as well as the support piping in the air service racks above the cell.”
New Testing Plan to Get Underway in July
The first phase of DOE’s new proposed testing approach is set to get underway this July to certify the pulse jet mixer control system “under a range of fluid conditions,” Holton said, adding that the first phase is expected to take six-to-nine months to complete. “We’re talking about essentially starting with water and then going to very high concentrated solids. That test program then allows us to provide the necessary information to confirm the control system design for essentially all of the vessels in the plant,” he said.
In the second phase of the program, currently scheduled to be performed in late FY 2014-2015, DOE would evaluate a small set of design concepts through small-scale testing to come up with a standardized vessel design to replace the currently planned high solid vessels. “This testing would identify the preferred PJM configurations to test at full-scale,” the test plan says. The third phase, scheduled for FY 2015-2016, would then involve full scale-testing of the chosen standardized vessel design. “This portion of the program will include evaluation of the design to meet safety and precessing mixing requirements. This test phase will be used to generate PJM mixing and control data that will be used to verify the design.”
