September-October 2017 NPJ

Proven Technology Reactor By Eben Mulder, X Energy. Eben Mulder As chief nuclear officer at X-energy, Dr. Mulder maintains the architectural framework for the X-energy nuclear program in guiding design and implementation. He is also responsible for the R&D and technology roadmap associated with reactor development and contributes to business development strategy. Prior to joining X-energy, Eben served as chief scientific officer for South Africa’s PBMR pebble bed project and as consultant to ESKOM, South Africa’s state utility. He has consulted for organizations including the US Department of Energy, Vattenfall in Sweden, and the International Atomic Energy Agency in Austria. He also worked in Germany on the AVR, an experimental pebble bed reactor designed and operated to develop the pebble fuel elements. Eben received B.Sc. and M.Sc. degrees in Mathematics from the University of Port Elizabeth, a B.Sc. degree in Applied Mathematics from the University of Pretoria and a Doctorate in Nuclear Engineering from the Technical University of Aachen. He is the author of more than 100 works on nuclear topics and serves as Extraordinary Associate Professor within the Faculty of Engineering, North-West University. An interview by Newal Agnihotri, Editor of Nuclear Plant Journal on August 8, 2017 at the American Nuclear Society Utility Working Conference in Amelia Island, Florida. An Intrinsically Safe Generation IV Reactor 1. What lessons have we learnt from the construction of reactors similar to Xe- 100? The three pebble bed reactors that I have been closely associated with that were operated, were the AVR (AVR = Working Group Experimental Reactor) in Germany, the thorium high tempera- ture gas-cooled reactor (THTR) in Ger- many and the HTR-10 in China. Both the AVR and HTR-10 reactors were experi- mental reactors, whereas the THTR was constructed as a demonstration reactor. As they were about to construct the reac- tor building for the THTR, the regulators insisted on increas- ing the reinforcement in the concrete to the point where the build- ing actually sagged or sank into the earth by two inches. This example was repre- sentative of an un- derlying problem that no set of design rules existed, that was mu- tually agreed before start of construction. I think these exam- ples embody important lessons learned for us. On the engineering side, an important principle in Xe-100 is to avoid implementing any new technologies, deploy proven technologies as far as possible. There were numerous examples in demonstration plants, such as the THTR and Ft. St. Vrain where this practice has led to long time delays and related ensuing issues. As mentioned during our discussion, we compiled series of lectures on lessons learned that we heed and observe as closely as possible, during the design of the Xe-100. We have deliberately avoided any of those issues as far as possible. We have also learned a great deal from the pebble bed experience, where we have been personally involved with the PBMR design in SouthAfrica. This design featured a Brayton cycle for the power conversion unit (PCU). Even though the Brayton cycle is well proven, we also built a test model, the PBMM (Pebble Bed Micro Module) in South Africa to demonstrate that we could startup and control the plant. We not only demonstrated with that experiment that we could accurately predict bootstrapping, but also that we could construct the plant as scheduled and on budget. There were absolutely no issues in doing that. The important lesson we have learned on the PBMR, however, was that trying to license something like a first-of-a-kind (FOAK) PCU together with what was considered to be a FOAK reactor on the nuclear island, would be especially tough to achieve in a developing country like South Africa. I would say, stick as far as possible to proven technology. What we intend to do is base everything on technologies that were proven on the AVR, proven on the THTR, and that we now have gotten information on from the HTR-10 from China, including lessons learned there. Since we have decided to follow ASME design codes, our reactor inlet/ outlet temperatures have been kept to a point where we would not exceed any of the code limits. TheASME code supports our design selection of inlet 250° C, outlet 750° C. These temperatures in other words, would not challenge any of the metallic component limits, in terms of the ASME code definition. 2. Provide an overview of reactor internals. Our philosophy was not to opt for the largest reactor pressure vessel (RPV) available worldwide. It was rather to go for a road transportable RPV, allowing us to transport this to any place without the need for waterways. That limited the flange to a flange diameter of the RPV to 4.88 m (16 feet). The RPV would house internal reflector blocks, which is about a meter (3.28 feet) thick, maintaining the core cavity diameter, of 2.4 m. (7.87 feet) The core cavity is the volute that would house the fuel pebbles. 3. Describe the fuel design of Xe-100? The fuel spheres or pebbles are 60mm in diameter and each one contains around 18,000 TRISO (Triple ISOmetric) coated particles. These 1mm diameter TRISO coated particles each contain a 0.425mm kernel of uranium oxicarbide fuel. Each (Continued on page 24) Nuclear Plant Journal, September-October 2017 NuclearPlantJournal.com 23