Nuclear fusion is a process that happens every second in the Sun, providing light and heat that the Earth lives on. For human who is dreaming to bring the sun power to the earth through controlled fusion reaction will face tons of challenges of which some are physical problem the other are technology or engineering. One of the great challenges is to study in-vessel loss of coolant accident (LOCA) for helium-cooled blanket.

A research group from the Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences (CAS) conducted a simulation to explore LOCA for helium-cooled blanket and what they have found may bring some inspiration.

In fusion reactors, plasma operates inside with extremely high energy reacts inevitably with the First Wall of the vacuum vessel. Sputtering and stripping of the First Wall material take place during the reaction then generating metal dusts like tungsten, beryllium and other micron metals. This kind of dusts, after absorbing tritium and activation of high-energy neutron, turns out to be highly radioactive.

When in-vessel LOCA of a helium-cooled blanket happens in the fusion reactor, the high-pressure helium gas ingresses into the vacuum vessel and generates a highly underexpanded jet, which impacts the radioactive dusts depositing at the bottom of vacuum vessel. Resuspension and migration of the radioactive dust come after. These movements can adversely affect the operation of the reactor and even lead to the release of radioactive materials.

The team, in this study, simulated how the flow field in the vacuum vessel changed and how the dust migrated at certain time in the in-vessel LOCA. And based on their simulation, they took the exploration further to compare the difference of flow field between in-vessel LOCA and the loss of vacuum accident (LOVA).

The simulation showed Mach disk structure that formed near the break after helium entered the vacuum vessel. This change got the pressure in the vacuum vessel rise rapidly to relief limit of the vacuum vessel pressure suppression system.

The in-vessel LOCA went on. Mach disks were moving toward the equatorial break of the vacuum vessel with its shape continued to shrink and the number of structures gradually increased.

Compared with LOVA, the in-vessel LOCA had a higher friction velocity leading to a greater probability of dust resuspension and radioactive risk.

"We really hope our work might be some help for exploration of LOCA study," said CHEN Zhibin, who led the team.

This work was supported by the National Key Research and Development Program of China and the National Natural Science Foundation of China.

Velocity distribution of the underexpanded jet in the LOCA. (Image by ZHANG Shichao) 

Evolution of friction velocity. (Image by ZHANG Shichao)