Professor of Nuclear Engineering, Joint Faculty Appointment with ORNL
North Carolina State University
Nov 30, 2019
Impacts of Reactor Design Modifications (0:19)
0:19-12:50 (Nam Dinh shares his path to nuclear engineering and the international education and research that kicked off his career)
Q: Where did you grow up?
A: Nam Dinh grew up in Vietnam, a country that previously had a nuclear program, but abandoned it due to economic reasons. When the program was still in place, Vietnam held a competition for university entrance. Nam scored very highly and the government sent him on a university scholarship to study nuclear engineering. Nuclear is very exciting for young people, which made it attractive, and people recognize that nuclear requires a lot of mathematical and scientific skills. In 1982, Nam went to study nuclear engineering at the Moscow Power Engineering Institute. He was in the country when Chernobyl happened, a complex time because the country was not very open as a society, leading Nam to focus on nuclear safety. Nam spent six years in Moscow to complete his undergraduate and Master’s degree. He received his PhD and Doctor of Science degrees in Moscow as well.
Nam then went to Sweden’s Royal Institute of Technology for six years; he returned recently and was named Chair Professor of Nuclear Safety. The focus during his first time in Sweden was on severe accidents. Sweden had a different severe accident strategy. When there is a core melt, it will heat up like at Three Mile Island. A majority of Sweden’s reactors are boiling water reactors (BWR). “China syndrome” is a term that describes decay heat eating up the concrete due to a lack of cooling mechanism. Core melt could easily go three meters out or down; the concrete layer is usually one meter thick, so in this case the meltdown could decompose the concrete and get into the groundwater and soil. In the 1960’s and 1970’s, the concept of a severe accident was very modest and the reactors were not designed for such. Sweden decided to put water in the cavity to try to mitigate the core and concrete interaction. However, the addition of water brought up another threat - a steam explosion - which could damage the containment. The Hitachi Economic Simplified Boiling Water Reactor (ESBWR) has a system in which the whole reactor vessel is submerged in the water to cool the vessel and prevent rupture. Without cooling, the system will increase temperature. The gas will put a load on the system and if there is any crack, the fission product can come out. In Europe, Sweden and Switzerland, a system was developed for containment venting. This intentional venting is used with a filter to catch the fission products and prevent a rupture in a severe accident. Understanding aerosols and other containment behavior was much later than the initial design of the reactor.
Nuclear Safety Analysis of Severe Accidents (12:50)
12:50- 26:04 (Why nuclear reactors were not originally designed with failure scenarios in mind and how that has changed)
Q: Why couldn’t the people who originally invented nuclear reactors predict the behavior in the case of a failure?
A: Behavior in a nuclear reactor involves a combination of materials, such as radium dioxide, silicon dioxide, metallic silicon, iron, and fission products. At that time, in the 1940’s and 1950’s, the study of radiochemistry and nuclear material had just started, so the knowledge base was very limited. This knowledge was limited to operation conditions at best, not accident conditions. In nuclear safety, you need to know the limit of your knowledge. The concept of the “worst case scenario” could get very huge, increasing the safety margin and affecting the economics. The engineering aspect is always balancing with uncertainty. Safety is not observable, but must be imagined. However, unsafe conditions, like an accident, are observable. Safety is not the sole issue for nuclear engineering, but the challenge is balancing safety and economics.
According to the current regulator requirements, no one died from Fukushima. However, there were still societal impacts on human health and the land. From that point of view, it was a bad accident. The whole issue is how to manage the very low probability and high consequences risk. The economic consequences of Fukushima were very large and many plants were shut down. Radiation cannot be seen and the risk is perceived very differently. The consequences of shutting down the plants and disrupting production boiled down to the international challenge for nuclear. The question is not as much an engineering issue, but is about much a deeper scientific and societal challenge. If there is a beautiful technology which can make people very happy, but would kill 100,000 people every year, people would not approve of it. However, they do approve of this in the form of the car industry. The question becomes how to make people voluntarily internalize the risk and see the benefit from the risk. In Nam’s research, validation becomes a central question of safety analysis because safety is not observable, so most analysis is done by calculation. This makes the cost of safety very high because engineers have to develop a model through prediction which must validate certain data. There is an intellectual gap between how to model and validate things on such a small scale. In safety analysis, this comes down to uncertainty calculation and safety management.
Role of Advanced Technology in Reactor Design (26:04)
26:04-36:06 (Nam reviews how current research projects are changing the way technology and simulation impact the design and licensing of nuclear reactors)
Q: Should there be an option for companies that bring designs to the NRC to be assessed on a single factor, like proximity to people, instead of assessment based on millions of calculations?
A: A pathway for regulation is available through licensing by testing. Technology is developed, tested, and refined. EBR-II, the Experimental Breeder Reactor, in Idaho was developed for passive safety. This concept is attractive, but is challenging for the vendor. A review process is needed and the NRC convinced of the framework and the process. A single vendor doesn’t have time to do that; they just want the product to be accepted without being involved in the process.
In a reactor design, you must make a safety case by putting evidence together to present. Probabilistic risk assessment is used to do this, but this process takes a long time. Nam Dinh is developing a computerized, formalized safety case as part of his research, similar to artificial intelligence. The computer would represent the safety case in place of a lawyer and can learn a high volume of precedence. If a reasoning can be achieved, the computerized, formalized system can be updated. Engineers may make inconsistent assumptions and a very experienced engineer is usually required to catch these inconsistencies. This computerized, formalized safety case system will make the design and licensing process streamlined and predictable. Many times, unexpected questions may take years to respond. A virtual reactor would allow the system to be tested and able to play with different variations. It then comes down to a question of validation.
In the 1950’s and 1960’s, when the nuclear industry just started, top notch people were teaching nuclear classes at universities. The computer at Los Alamos was developed for supporting nuclear safety analysis and design. New advancing technology, computer science, and modeling simulation bring a new way to look at the same thing. In Nam’s research, he studies boiling heat transfer. This simple concept still has a lot of mystery around it. He measures the temperature and heat flux for an engineering approach and also uses an infrared camera to look into nucleation phenomena. A new advanced simulation can follow each individual bubble and how they interact. Advanced simulation can now go deep into the physics, improving validation, and produces a lot of data very quickly. This is the era of big data and it must be transferred and digested into knowledge by mining the data. Intellectual value and knowledge must be brought into the formation of new technology.