Q&A with a Neutron Thrower

Bernie Riemer, a keynote presenter at Science in the Age of Experience, has worked at Oak Ridge National Lab (ORNL) for more than 30 years on projects from fusion energy to neutron sources—and has used Dassault Systèmes’ applications along the way to overcome a variety of esoteric engineering challenges. Now focusing on the generation of pulsed neutron beams from a liquid mercury target—and the accompanying thermal, stress and dynamics issues—his team is harnessing the full Power of the Dassault Systèmes’ SIMULIA portfolio (Abaqus, Isight, fe-safe and Tosca). We met with him before his Science presentation to find out a bit more about how he became an engineer and what he likes best about his unique and unusual field.

You can watch his presentation, or read an interview conducted by SIMULIA Community News to find out a bit more about how he became an engineer and what he likes best about his unique and unusual field.

Dassault Systèmes: Tell us a bit about your upbringing.

Riemer: I grew up in suburban Long Island in NY during the space age in the 60s so the lunar program was very exciting for me to watch as a youngster. My father worked for the company that built the lunar module and he got to meet astronauts and get their autographs—I still have one from Alan Shephard. All of that probably had something to do with my interest in engineering.


What did you do before ORNL?

My first job was with Sikorsky helicopters and then I worked on fixed-wing aircraft with Grumman, including the F14 Tomcat of “Top Gun” fame. Grumman had a division of advanced energy systems and they were involved in some weird stuff. Nothing to do with planes, but along the lines of nuclear fusion energy. They approached me about filling a short-term opening on a subcontract they had here at ORNL in Tennessee. This turned out to be a life-changing event for me. Fusion was really exciting in very different ways, with interesting technical challenges and a lot of international involvement.


Bernie Riemer

How did the international involvement affect you?

These programs entail a lot of collaboration across the world and in fact it took me to Japan for a bilateral exchange with the Japanese from ’87-’88. That was one of the best years of my life; my wife and I had a fantastic time there. When I got back from Japan I continued to work on fusion for some years—including the very early days of the ITER project that’s still underway in France. One of the problems I worked on was the dynamic response of the vacuum vessel for the Tokamak (the ITER fusion reactor) for which I used Abaqus/Explicit. It was a very efficient way to deal with this kind of large, complicated problem. I was also working with the general engineering group at ORNL on a variety of stress, dynamics, buckling, seismic, thermal, and thermal stress challenges for different projects and programs.


So you were an early user of Abaqus FEA software?

Yes, computers were not as powerful as they are today so a modal approach for the ITER problem was not practical. Explicit was a great way to solve that problem and that helped me later on when I started on the mercury target. When they asked me to take a crack at that one I remembered what I had done on the ITER project and I thought, explicit dynamics is a good way to approach this problem. And we still use that tool today for our analyses.

I also got involved in welding simulations where we did a three-way coupled simulation of resistant spot welds—electrical, thermal, mechanical. We could predict the microstructure of the steel throughout the weld and Abaqus software was key to those developments.


What exactly is involved in your mercury target project, the Spallation Neutron Source (SNS)?

The SNS is a next-generation spallation neutron source that is currently the most powerful facility in the world providing pulsed neutron beams for scientific research. An accelerator-based system delivers high-energy proton pulses to a one-ton, 316- steel target vessel, through which 20 tons of mercury flows per minute, to produce the neutron beams. By contrast, a nuclear reactor provides a steady flow of neutrons, but the SNS’ pulsed neutron beams at the right temperatures are really good tools for probing the structure of matter at the atomic and molecular levels. These special capabilities attract users from all over the world to our site in Tennessee.


What kind of design, operation and maintenance challenges does such a setup involve?

The accelerator sends 60 proton beam pulses a second so fatigue is a big issue in the lifetime of our target. Each pulse has more than 20 kilojoules of energy to be stopped, which cause large pressure waves in the mercury that dynamically stress the target vessel. While the mercury can be reused, the vessel itself only lasts 4-6 months before it needs replacing. This is partially due to embrittlement from radiation, but it’s turned out in practice that other phenomena have limited the lifespan more severely. One of them is related to fatigue, in particular with welds. The other is cavitation damage from the action of the pressure wave on the mercury. This makes a big difference on the vessel stress simulation. For that we use Explicit with the equation-of-state model combined with a tensile failure feature to make an approximation of how the mercury behaves macroscopically once it cavitates. Both fatigue and cavitation/erosion have led to target leaking and interruption of services for our users, so we are working very hard to minimize that.

We started operation in 2006 and we’re now on our 16th mercury target. Many of them operated at more than 1 Megawatt of accelerator beam power, which is pretty good, but we’d like to be at 1.4 Megawatts, steadily and reliably, and we’re not there yet. So we’ve been using SIMULIA tools for incremental design changes that make things like welds more robust, and we continue to refine our models and use our analysis results to show us what designs are better than others, and get some sense of how much better they are than others.

How have you been using other SIMULIA products besides Abaqus?

We started employing Isight a year or so ago and plan to make much stronger use of it in the coming year. It helps us do parametric studies more efficiently. We’re challenged now by the department to increase our performance to 2 Megawatts of operation, so our fatigue stresses are going to go up, our cavitation intensity is going to get worse. We’re going to use Isight to drive design exploration studies for the pulse problem to improve fatigue life margins in a semi-automated way and to show us what geometries would have best prospects for improved fatigue life. Then we’ll combine that with fe-safe to give us the fatigue life prediction and come up with a solution space which says, “change this angle,” or “make this thicker repair,” etc. We’re looking to explore this further by developing an Isight workflow which uses Abaqus+fe-safe+Tosca to look at fatigue life shape changes that will guide refinements to a particular design.


What future uses do you see for pulsed neutron research that might attract even more users to your facility?

A growing area for neutrons is in biology and molecular chemistry. If people want to develop a new medicine or a new polymer for a tire or a new high-temperature superconductor, if you want to understand that on the atomic or molecular level, neutrons offer very good capabilities for doing that. I’d like to learn a little bit more about the capabilities of BIOVIA to model behavior on the atomic level when I come to Science in May. We’re also in discussion about a project to test aircraft electronics’ resistance against cosmic radiation—which we can simulate with the SNS. Neutrons have been used for determining internal stresses deep within components, and we have excellent capabilities for that at the SNS and at our companion research reactor. The growth of additive manufacturing is leading to new users wishing to assess residual stresses in AM components with neutrons.


So when you’re not producing neutron pulses what do you do for fun?

I like to road and mountain cycle, and in fact I’ll go for a ride when I get home tonight. My wife and I both like to cycle and travel so we do charity rides around East Tennessee and we also spent several days on a bike tour in Spain last year. All three of our children are in STEM-related careers; our son is an ME in the electrical equipment industry, our older daughter is in the aerospace industry working with physics and math on stuff I can’t explain. And our other daughter is a veterinarian. So they’ve all done well, we’re very happy that they’re all on their own and my wife and I feel that’s the biggest accomplishment of our lives.


What would you have done if you hadn’t become an engineer?

Looking back I wish I had taken baseball a little more seriously. I had a pretty good arm. If I’d done a little more practice and showed a little more devotion to pitching, it would have been interesting to see if I could have achieved a professional level. I still love to go watch a live baseball game.


So you didn’t get to throw baseballs for a living—but you do now get to throw neutrons!

Yes I do!


For More Information, visit ORNL.



Irina is Dassault Systèmes' Industry Marketing Director for the Energy & Materials industry.

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