Nuclear power reactors in countries with an important share of nuclear power electricity generation are increasingly required to operate under flexible conditions (“Load Follow” or more generally “Flexible Power Operations”, FPO). The underlying reason is the increasing share of renewable energy production which receives grid priority. NPPs may today be required to change power rapidly (e.g. 5% min-1), deeply (e.g. 75% power reduction) for short (a few hours) or for longer periods (several days).
One of the principal factors limiting such flexible operation, are the so-called Pellet-Clad Mechanical Interaction (PCMI) constraints. PCMI phenomena (see further) are far from being completely understood and as a result, large margins are adopted by the regulator. There is thus an important incentive to better understand and better quantify the limiting factors towards more flexible operation of NPPs, in particular the PCMI phenomena.
For several year, Tractebel relies on the fuel performance code FRAPCON/FRAPTRAN to investigate the thermo-mechanical behaviour of the fuel under normal operations and accident scenarios. Although substantial changes were recently made to the code, several of the model still need further development, specifically in the context of increased demand for flexible reactor operation. Tractebel intends to consolidate expertise and experience in a TE version of the code, including improvement in the PCI/PCMI, LOCA and RIA models.
The PhD proposal fits in a roadmap towards full PCMI compliance for flexible operation in the sense that the correct operational and safety margins are applied. The main fuel performance aspects which should be better quantified and addressed for this purpose are related to an improved understanding and modelling of fuel and fuel-cladding behavior at the mesoscopic scale. By this, it is understood that fuel fragmentation, axial and radial relocation and interaction with the cladding are taken into account, that axial gas transport is better quantified.