The Departure from Nucleate Boiling (DNB) – which occurrence is characterized with the DNB Ratio (DNBR) – is a phenomenon in which a local vapor layer forms between the coolant and the fuel cladding, impeding the heat transfer from the latter to coolant. This results in a sharp increase in fuel cladding temperature, possibly damaging it and provoking the release of radioactivity in the primary circuit. The core is protected in the 1300 MWe and 1450 MWe reactors of the French NPP fleet by the protection system SPIN, which in particular safeguards the core against DNB thanks to the low DNBR protection channel.