What is the name of the dangerous limit where a layer of steam suddenly coats a fuel rod, preventing the liquid coolant from touching it and causing the rod to overheat?

The dangerous limit where a layer of steam suddenly coats a fuel rod, preventing liquid coolant from touching it and causing the rod to overheat, is primarily known as Critical Heat Flux (CHF). The event or phenomenon itself, which marks the transition to this dangerous state, is called Departure from Nucleate Boiling (DNB).

A fuel rod contains nuclear fuel that generates intense heat through nuclear fission. A coolant, typically liquid water in many reactor designs, circulates around these rods to absorb and remove this heat. Under normal operating conditions, heat is transferred efficiently from the hot fuel rod surface to the liquid coolant through nucleate boiling. In nucleate boiling, small steam bubbles form on the rod surface, detach, and rise, allowing fresh liquid coolant to continuously re-wet the surface and maintain excellent heat removal.

Critical Heat Flux (CHF) refers to the maximum heat transfer rate that can be achieved from the fuel rod surface to the liquid coolant while still maintaining this efficient nucleate boiling. If the heat flux generated by the fuel rod exceeds this critical limit, the liquid coolant can no longer rapidly re-wet the surface. This leads to Departure from Nucleate Boiling (DNB), which is the abrupt breakdown of nucleate boiling.

During DNB, a stable, continuous layer of steam, known as a vapor film, suddenly forms and blankets the fuel rod surface. This state is called film boiling. Since steam is a much poorer conductor of heat than liquid water, this vapor film acts as an insulating barrier, drastically reducing the efficiency of heat transfer from the fuel rod to the coolant. Consequently, the heat generated within the fuel rod cannot be adequately removed, causing a rapid and significant increase in the temperature of the fuel rod and its outer casing, known as the cladding. Such overheating can lead to damage, melting, or failure of the cladding, potentially releasing radioactive materials into the primary coolant system.