Supercritical Conditions

Last week, 22 students from 18 different countries gathered at ICTP to learn about the next generation of nuclear reactors from international experts.  One promising idea is the use of supercritical water as a coolant for reactor cores, and this was the focus of 'The Joint ICTP/IAEA Course on Science and Technology of Supercritical Water-Cooled Reactors'.

Supercritical water exists at very high temperatures and pressures, such that there is no longer a distinct boundary between the liquid and gas phases. Supercritical water-cooled reactors could have an efficiency as high as 45%, compared to around 33% for conventional water-cooled reactors. The course discussed the both the advantages and the challenges involved in developing these systems.

Igor Pioro noted in one lecture that "the vast majority of modern power nuclear reactors are water-cooled units," and that moving from subcritical conditions to supercritical conditions is a natural next step for the technology. It is analogous to developments made in the thermal power industry more than 50 years ago.  Pioro is a professor at the University of Ontario Institute of Technology in Canada.

Supercritical water-cooled technologies build off conventional technologies, which is advantageous, because it will limit the cost, time, and risk associated with research and development.

Before this new type of reactor can be built, engineers need a better understanding of the thermal-hydraulics of supercritical water, and the behaviour of materials and chemicals under high temperature and pressure.

Katsumi Yamada of the IAEA, one of the course directors, felt it was important for students to learn more about conventional nuclear reactors in addition to discussing developing technologies.  "The methodology is the same and the safety philosophy is the same," he said.

Safety was a large focus throughout the course, and one lecture was devoted to the recent disaster in Fukushima, Japan.  After presenting a timeline of the earthquake and related events that led to the safety system failures at the Daiichi and Daini power plants, Yamada discussed in detail which safety features had worked and which needed improvement.  He explained that the nuclear reactions had stopped like they were supposed to, but that the system meant to remove heat from the reactor core had failed when the backup power supply failed and the barriers designed to confine radioactive material had not been sufficient.  Those are observations that will play a role in designing the safety features on new reactors, regardless of what technology is used.

Yamada gave a less technical version of that presentation to a centre-wide colloquium at the conclusion of the course.

This programme was the first course Yamada has organized, and he said it was a learning experience for him, as well.  He was impressed by how active the participants were, especially in team discussions.  It was a lesson for him in how important interactive activities are to the learning process, even at a very technical level.  Yamada said he plans to incorporate more activities into future training sessions. "Students need some time to speak out," he said.