That means cooling water is not getting to the reactor core, causing a build up of steam inside the containment vessel.

Then you have the reactor vessel and piping which is the primary boundary which has an eight to ten inch thick reactor vessel and three to four inch thick steel piping to contain the cooling water.

Reactor vessel cooling system is one of the important auxiliary systems in the primary system of China experimental fast reactor (CEFR), which can cool the reactor vessel in various conditions.