The adventure started at the University of Illinois at Urbana-Champaign, where I was allowed to decide the subject of my Ph.D. study. I decided to test a new idea of coupling neutronics and thermal-hydraulics after taking courses in neutronics, thermal-hydraulics, and programming. I named the new idea Integrated Tight Coupling, or ITC, which modified the equations instead of solving the same equations with different approaches as the traditional loose coupling and tight coupling do. Implementing ITC is similar to loose coupling, but primary results suggest that ITC allows a coarser mesh and fewer iterations [01]. During my Ph.D. period, I completed an internship at Idaho National Laboratory. I added and validated new features and models to a code that predicts hydrogen distribution inside a containment. Results of the 12-week work are published in Annals of Nuclear Energy [02].

 References

[01] Hsingtzu Wu, Rizwan-uddin, “A Tightly Coupled Scheme for Neutronics and Thermal-hydraulics using Open-source Software,” Annals of Nuclear Energy. Vol. 87, January 2016, pages 16-22. https://doi.org/10.1016/j.anucene.2015.08.004

[02] Hsingtzu Wu, Haihua Zhao, “Validation of hydrogen gas stratification and mixing models,” Annals of Nuclear Energy. Vol. 85, November 2015, pages 137-144. https://doi.org/10.1016/j.anucene.2015.05.003

After graduation, I wanted to learn more about the Fukushima accident and do something for it, so I joined the Japan Atomic Energy Agency (JAEA) as a postdoctoral scholar. I collaborated with the researchers to gain insights into fuel behavior during LOCA experimentally and computationally. I scrutinized experimental data and found that the mechanical behavior of cladding was affected by its thermal diffusivity, thermal expansion, and oxygen dissolved in the metallic layer, which had not been considered in the analysis. Therefore, I derived equations using published data, and the updated code gave better predictions [03].

In addition, I designed experiments to reveal the order of cracks formed in cladding during LOCA quench conditions using different thermal stresses, which was achieved economically by different thermal capacities inside the cladding. High thermal stress resulted in cracks in the oxide, while low thermal stress did not. It suggests that the cracks form in the αZr(O) layer first, then in the oxide, and finally in the prior-β layer when the fracture of cladding occurs [04]. I visited Helmholtz-Zentrum Dresden-Rossendorf (HZDR) after the end of the work contract in Japan. I successfully developed an interface to couple DYN3D (the in-house neutronic code of HZDR) and OpenFOAM. I verified it with a 3-D reactor model with the help of my supervisors in three months [05]. 

Then, I was attracted by the aggressive nuclear power program in China. I thought that people around the world would suffer if something went wrong with a nuclear power plant in China, and therefore, it would be meaningful to work on nuclear safety in China. I led an independent research group that focused on nuclear safety. The study was guided by the idea that the development of nuclear power would be mainly affected by two factors. One is the price of nuclear energy, and the other is the public perception of nuclear safety. Therefore, we explored three economical ways to enhance nuclear safety. 

First, I studied the concept of a passive residual heat removal heat exchanger (HX) and proposed changing the configuration of C-shaped HX tubes to spiral-shaped ones. The CFD computation suggests that the new design enhances heat transfer and increases cooling capacity [06]. In addition, I noticed that none had examined the effect of tube defects on the performance of the HX, so we did it. The computational results suggest that it is correlated with the size of the surface area of the deficient tubes instead of their locations [07]. 

Second, I was inspired by moisture spikes on the lid of a Staub cookware that were designed to create a rainforest effect and return juices evenly to the food. I proposed placing moisture spikes on the dome of the containment to enhance water circulation during an emergency. We tested two designs—spherical and conical spikes—and the computational results suggest that the former has a better heat transfer performance [08]. A spherical spike improves the heat transfer rate by 11.7%. The spiral-shaped heat exchanger tubes and moisture spikes have the potential to be implemented in any nuclear power plant without much cost.

In addition to nuclear power, I proposed new ideas in areas including irradiation and renewable energy. After visiting a gamma irradiator, I came up with a new configuration of a shuffle-dwell gamma irradiator based on the fact that radiation emits in an isotropic fashion. Compared to the current configuration, the proposed one has the potential to increase energy utilization and throughput, which leads to higher profitability [11]. I attended a seminar and learned that forecasting models of renewable energy were evaluated statistically. I proposed evaluating these models from the perspectives of the power grid's stability and usable energy availability [12].