Valuation of changes in the cladding and fuel temperatures after an extreme transient case of control rod banks withdrawal in a BWR

Resumo

Instabilities in BWR (boiling water reactor) are possible to occur when an operating condition becomes unstable after some change in system parameters. As a consequence, state variables identifying the reactor working conditions are observed to oscillate in different ways depending on the modalities of the departure from the stable operating point. In this work, the RELAP5/MOD3.3 thermal-hydraulic system code and the PARCS/2.4 3D neutron kinetic code were adopted to simulate coupled instability phenomena in the Peach Bottom BWR. In the transient investigated, the control rod banks (CRB) are continuously removed from the core starting from 20 s (steady state) up to 40 s. The reactor was brought to unstable behaviour. The power remained with a behavior approximately constant up to about 90 s but it begins to oscillate with amplitudes reaching more than 80% of total power. The cladding temperature increases drastically in one extreme of a selected fuel assembly (axial level 3) after the rod banks are removed. This phenomenon is directly connected with the change in axial power distribution, which is drastically affected by the rod banks withdrawal. Since after rod withdrawal, the coolant density is much higher at the bottom core inlet, the expected bottom-peaked power profile is observed. The fuel temperature rises drastically, at the level 3. Temperatures of about 1200 K were observed. These values are below the melting point of the fuel (≈ 3073 K) during this transient. In the calculation, the scram intervention was not considered, because the main interest was to assess the core parameters evolution during an extreme event.