1 - Computational Reactor Physics

Bret Kugelmass: Where did you grow up?

Dave Kropaczek: Dave Kropaczek grew up in New Jersey and studied engineering science at Newark College of Engineering, now called the New Jersey Institute of Technology. Within engineering science, Kropaczek minored in nuclear engineering after a visit to the Tokamak reactor at Princeton while in high school. He went on to receive his Master’s and PhD at North Carolina State University. During his industry-sponsored Master’s degree, Kropaczek spent nine months in school and nine months at a sponsor site, Westinghouse in Pittsburgh. Kropaczek went back to Westinghouse after his PhD and was exposed to two-thirds of the reactor fleets in terms of solving engineering problems. Everything has a process, but there is still a lot of creativity allowed. The fuel within the core is reconfigured every cycle, as the highest exposed or highest burnt fuel is replaced with new fuel and it is all shuffled and reconfigured to get the maximum energy out. The utility is the end customer and has to produce a certain amount of energy over time and multiple cycles and outages are planned years in advance. Westinghouse is also a nuclear fuel vendor. As Kropaczek was in his Master’s level nuclear classes, he became more interested in nuclear methods, leading him to become a computational reactor physicist.

2 - Boiling Water Reactor Core Design

Bret Kugelmass: How does temperature affect cross-sections?

Dave Kropaczek: Everything has a temperature that impacts the number densities of water. If the number density changes, the moderation or slowing down of neutrons to thermal energy takes place, which is a feedback effect. There is negative feedback when water is boiling and the reactor is in accident mode. Light water reactors are designed to have a negative reactivity. The reactor is designed for a thermal spectrum, so if the neutrons can’t be slowed down, the uranium is kept below a certain enrichment limit. The decay heat must go somewhere, and since the fuel rod has no water to cool it, it will eventually fail and fuel will melt. In a loss of cooling accident (LOCA), active systems must come on and get water into the reactor. The coolable geometry must be maintained as part of a transient analysis. Dave Kropaczek also worked for General Electric in global nuclear fuels, a joint venture of General Electric, Toshiba, and Hitachi. Boiling water reactors have different issues of thermohydraulic stability. Kropaczek worked on optimization codes to design assemblies in reactor cores to get the best economic value. Boron is used as a dilution process, which is eliminated in boiling water reactors. The control mechanism is the void itself and the void feedback is a great control because it decreases a moderation of neutrons which tends to slow down the reactions.

3 - Studsvik Scandpower

Bret Kugelmass: Was Global Nuclear Fuels looking at supplying fuels to boiling water reactors that don’t exist yet?

Dave Kropaczek: The ABWR (American Boiling Water Reactor) and ESBWR (Economic Simplified Boiling Water Reactor) are both General Electric (GE) developed power plants, as GE was at the forefront of boiling water technology. Fuel vendors have a set of nuclear analysis codes. Dave Kropaczek went to work at Studsvik Scandpower as a co-developer, a company that evolved from the National Laboratory in Sweden. Technology was developed at Studsvik that was brought to the U.S. and commercialized. The commercial model for Studsvik Scandpower software was that it was independent of the fuel vendors, giving the utilities an independent analysis capability. Different fuel products from different fuel vendors can be analyzed by things like flow behavior in a bundle or fuel rod diameters. The configuration of fuel bundles determines power output and what the control will look like. The applications of Scandpower codes include licensing, design, or other areas across the spectrum, but there must be permission to deliver products outside the U.S. Kropaczek came in to Scandpower in a technical role, but jumped into a strategic leadership role also responsible for profit and understanding international sales. The U.S. base for Studsvik was in Wilmington, North Carolina.

4 - The CASL Energy Innovation Hub

Bret Kugelmass: What prompted the teaching role you took at North Carolina State University?

Dave Kropaczek: Dave Kropaczek learned a lot and enjoyed the CEO role, but wanted to be more in the technical realm of things. He took a teaching role at North Carolina State University and was also the chief scientist of CASL (Consortium for Advanced Simulation of Light Water Reactors). Kropaczek taught one class a year and also advised senior design classes and individual graduate projects. CASL was formed in 2010 and was the first energy innovation hub focused on nuclear energy. It was initially a five year program that looked at whether advanced modeling simulation could be used to attack problems of interest in the nuclear industry. The U.S. nuclear fleet is very safe and has high capacity factors. CASL was a partnership between the National Labs, industry partners, and academia. The challenge problems for CASL have remained fairly constant, with the number one challenge problem still being crud in nuclear reactors. Crud is the deposits of nickel and iron compounds on the surface of the fuel rod in the presence of sub-cooled oil. It is related to the chemistry of the reactor coolant and the corrosion of the piping systems. In pressurized water reactors (PWR’s), the soluble boron in the coolant come out of the solution and become a solid in the crud.

5 - Local and Global Impacts of Crud

Bret Kugelmass: What happens when crud collects the boron?

Dave Kropaczek: When boron collects on crud on the fuel rods, the operating power of the nuclear power plant starts shifting down to the bottom of the core. If there is a significant event in which a temporary shutdown needs to happen, the boron comes out of the crud layer back into the solution causing it to disappear. It can never be seen in the reactor, but the impacts can be observed during operation. When the reactor is started back up, the boron is gone and the power is no longer suppressed, causing the power to shoot to the top of the core and the axial offset swing flips, called crud-induced power shift. Crud-induced localized corrosion can impact the heat transfer around the fuel rod, degrading the heat transfer where the cladding can heat up and fail. Crud has local and global effects in the reactor. To flatten the power in the core, more burnt fuel has to be removed and more fresh fuel has to be installed, causing the fuel to not get to the high level of burn up as it needs to for economic reasons. Pellet clad interaction is another challenge issue studied at CASL. The fuel pellet can expand rapidly inside the cladding and the clad can creep down onto the fuel pellet. If the expansion is rapid, the clad can be stressed and fail. Water is very hard to model in terms of fundamental physics, so a departure from nuclear boiling challenge focuses on computational fluid dynamics of the boiling of water on the surface of fuel rods and potential dry outs.

6 - Departure of Nucleate Boiling

Bret Kugelmass: How do you see the departure of nucleate boiling (DNB) inside a reactor?

Dave Kropaczek: Partial dry out of a fuel rod, departure of nucleate boiling (DNB) looks a different color and may not have failed, but instead been in a poorly degraded heat transfer state in which it starts to morph into something that’s not supposed to be there. Large scale facilities have heater rods that simulate the departure of nucleate boiling. The products are tested in the facilities and correlations are developed for critical power or DNB, based on the functions of mass flux or various thermohydraulic parameters. The correlation is developed for the product and applied to the code. In the DNB challenge problem, testing could be replaced or supplemented with a fundamental, first principles method. CASL is working closely with NuScale’s small modular reactor (SMR) design, looking specifically at modeling crud in the reactor to potentially address issues with the natural circulation in the design.

7 - Mission of CASL

Bret Kugelmass: What do you want to see happen at CASL before and after the end of its second extension?

Dave Kropaczek: Dave Kropaczek’s primary goal at CASL is to complete the challenge problems and provide the solutions to industry. To deploy technology, there are multiple requirements on software such as insurance and usability. CASL is now looking at deployment in a commercial setting and how advanced modeling simulation tools fit in the process. The team is working with industry to find the high value, high impact problems that CASL can aid in solving. CASL’s mission has moved from fundamental research and development early on to development and deployment now, with a focus on getting codes out to industry. CASL has a sister program for advanced reactors called NEAMS (Nuclear Energy Advanced Modeling & Simulation). The vision is to have a new program that looks at whether advanced modeling and simulation could be used in areas that haven’t been imagined yet, combined with data analytics. There is a plan to get a post-CASL user community into a self-sustaining entity. Kropaczek’s focus is delivering on what was promised at CASL. Plants are well-maintained now and there is tremendous economic pressure. A new nuclear renaissance is needed. Small modular reactors (SMR’s) lower the capital cost, allowing competition and modularity. The safety systems in the new AP-1000 are way beyond the existing fleet, which are already completely safe. If there were a large demand for SMR’s, the cost would come down.