Study and optimization of low Departure from Nucleate Boiling Ratio protection channel intervention during an Uncontrolled Rod Cluster Control Assembly Bank Withdrawal at Power

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. The configuration of the low DNBR protection channel incorporates some conservatisms that are currently questioned by Framatome, among which the use of a reference axial power distribution – a cosine-shaped profile peaked at 1.55 times the nominal power – for the Uncontrolled Rod Cluster Control Assembly (RCCA) Bank(s) Withdrawal at Power transient and the presence of a set of correcting functions that aligns the DNBR computed by the SPIN as closely as possible with the actual physical DNBR. The transient considered for the simulation is an Uncontrolled RCCA Bank(s) Withdrawal at Power – a symmetric reactivity-initiated accident – initiated at full power and with minimal feedback mechanisms. It is simulated within MANTA, a thermal hydraulic code simulating the vessel, in conjunction with the SPIN to model the reactor’s response. The respect of the safety criterion on DNBR is verified using FLICA III-F, a sub-channel thermal hydraulic code, which is able to calculate the actual physical DNBR. On one hand, this study aims to analyze the influence of the initial power distribution in the core on the evolution and minimum value of DNBR during the transient. Thanks to a sensitivity analysis to the initial power distribution, it is shown that the reference axial power distribution is not the most conservative for the reference method and that a parameter needed for the method skews the reality of the physics, with an increase of minimal DNBR for more penalizing axial power distributions. On the other hand, this study leads to the development of an algorithm based on the SPIN able to calculate the physical DNBR, thus reducing the reliance on the correcting functions. The results given by the new algorithm are satisfying, suggesting a possible substitution of FLICA III-F in the reference method for the computation of the physical DNBR.