EM’s Chief Engineer John Marra chaired an independent panel of experts that recently recommended DOE stabilize a second waste storage tunnel at the Plutonium Uranium Extraction Plant (PUREX) on the Hanford Site using engineered grout. EM Update's Q&A with Marra is below.
How was the panel formed?
One of the main things we did is look for panel members with a diverse experience base, not only within the Department of Energy here at Hanford, but also across the complex and internationally. We utilized a broad set of people, some of whom had extensive familiarity with Hanford and the PUREX tunnels, such as David Kosson. He is the lead investigator for the CRESP (Consortium for Risk Evaluation with Stakeholder Participation) Consortium that has done a lot of risk assessment work at Hanford. Beyond that, we reached out to people who had suitable industrial experience and could offer a different perspective, notably on worker safety. We reached out to Christine Lee, who is the Environmental, Safety, Health, Quality and Security manager for CH at West Valley. We reached out to Kurt Kehler, who is another CH2 person, but he is at the Chalk River Nuclear Laboratory in Canada, and his expertise is in D&D. So, he is familiar with the types of actions that would be taken if we ultimately ended up disposing. Beyond that, we reached out internationally to John Ballantyne who is also with CH, but is their chief of nuclear structures in the U.K. He is a civil structural-type expert on nuclear structures, all the way from nuclear reactors to other support-type buildings. So, we looked for a broad experience base - not just expertise, but people who had direct involvement with PUREX and people who had no involvement. (Panelist biographies are in the panel’s.)
Is it common for panel members to get together and undergo a process like this, and has this group gotten together before?
We never met face to face. Part of that was because we were doing our work at the start of the academic year, and the academicians on our panel didn’t have a lot of time. So, we met for five consecutive Fridays via web conference for a couple of hours at a time. In between Fridays, we’d kick back-and-forth emails with questions, and things like that. It worked really well. A few of the panel members have worked with each other in the past, but the rest of us, myself included, were relatively new.
Looking at the stoplight chart in the back of the report, can you explain if there were actual numbers assigned?
Yes, there were actual numbers. We went through an evaluation. I call it a parametric evaluation. We identified first the criteria in three main areas, safety (not just the worker safety for implementation, but also protection of the public and the environment going forward) in the near term and the long term. We looked at implementation and what kind of experience we had with the solution. How long would it take? How much would it cost? Finally, we looked at a couple of criteria associated with future options. Did the interim stabilization option facilitate future disposal in place or removal? We didn’t want to do something in the near term that would make the job harder in the long term.
As a panel, we discussed each of those criteria in detail, and we rated each individual criteria from one to five, with one being the worst and five being the best. What I did was create spreadsheets and each panel member assessed the criteria individually, and we did that intentionally to avoid what is referred to as “group think" or one person dominating the discussion. I then took those individual inputs and determined the average score for each criteria, but also looked at the distribution and variance between scores. We used that as a way of targeting our discussions going forward. If one panel member rated a certain criteria very low and the other panel members rated it as high, it was something I flagged for further discussion. As panel members, we went through each one of those criteria, one by one, and reconciled any differences. We used that to come up with a consensus opinion.
Did that happen often? Was there a lot of reconciling that had to be done?
There were quite a few that had some variance, but the reconciling that had to be done was mainly because someone didn’t have experience. It was mainly misinterpretation of guidance or different understandings, but there were no major disconnects between panel members. Another thing we did was weigh the criteria differently, recognizing that not all criteria are created equally. For instance, we would rate something like worker safety as a “3" and cost was a “1." We didn’t want to make decisions that were cost expedient but would compromise worker safety or protection of the public. The thing that was surprising was when we looked at the results of the analysis, the grouting option was the preferred option in both the raw scores, unweighted, as well as the weighted. That happens from time to time. I’ve done a lot of these evaluations, but it is kind of rare when that happens. It tells you that the option is clearly the preferred option, because it comes out on top, with or without weighing criteria.
In the report, you didn’t go into any specifics about kinds of grout that could be used. Should you have?
There were a couple of panel members who were grout experts, but we intentionally tried not to engineer the solution, but to provide input to the Department and Richland Operations Office so they could use it. We did talk quite a bit about what was going on with PUREX Tunnel 1, because that grouting operation was just getting started when we were doing the evaluation. So, we had that type of information, but we didn’t get into details about the engineering of the grout placement or grout composition, recognizing that not all grout is the same. Certainly, looking at it now, with the successful completion of Tunnel 1, a similar type of grout would be the place to start.
In reading the report, it doesn’t seem like you focused on the fact that Tunnel 2 design is so different. Will that affect the challenge of placing grout.
We did talk about that. We didn’t get into the engineering but, obviously, Tunnel 2 is much bigger, not only in length but in cross section, and it will be a significant evolution to emplace grout. We did talk about the fact that there will be some additional engineering needed. You can’t just move from Tunnel 1 to Tunnel 2 without looking at the differences in configuration.
Is the design-to-capacity ratio for Tunnel 2 really concerning for the panel members? How much time can be allotted for engineering?
One of the things we looked at very early on is the structural evaluation. For Tunnel 1, the wall timbers had a design-to-capacity ratio at 1.5, and any design-to-capacity ratio greater than 1 is concerning. For Tunnel 2, several of the components were just slightly above 1. So, one of the things we did talk about was, can we reduce some of the conservatism in that analysis or change the configuration, i.e. remove some of the structural load by taking some of the backfill off that would reduce the load on Tunnel 2 so the design-to-capacity ratio would come down below 1? We determined very quickly the risk associated with doing that, not just to the workers, but to damaging the tunnel, as well as the fact that it really didn’t solve the problem and kind of just kicked the can down the road. So, we looked at that structural evaluation, and while there are design-to-capacity ratios that are above 1, in my estimation, there is no imminent risk. The Department is continuing to monitor Tunnel 2 with extra surveillance and looking at potential shifts, so it is one of those things where there is certainly time to do the engineering, but it is not something where you want to wait forever. Our recommendation was to make the decision and move expeditiously.
Was there anything, now that the report has been written and you can look back, that you wished you would have included?
Not really. What I’ll tell you, having done this for 30 years, this was one of the cleanest evaluations I’ve ever been associated with. A lot of good work was done on PUREX Tunnel 1, and we took that into account, but we didn’t let it heavily influence our decision. I can’t stress enough that both the raw score and the weighted score favored the grout option. It is a clear indication that grout is the preferred alternative. It is really satisfying when it lines up that way.
Source: U.S. Dept. of Energy, Office of Environmental Management
