Ageing of Nuclear Fuel Cladding in Advanced Gas-Cooled reactors

Within the United Kingdom a significant contributor to electricity generation for the national grid is nuclear power. Due to improvements in technology many current nuclear reactors have received life-time extensions, or are applying for them in the future. In order to ensure the safe running of these reactors and to ensure that all waste produced is processed and stored safely, it is important that all aspects of the process are understood.
In the reactor the uranium dioxide fuel is held inside stainless steel fuel rods. The behavior of this steel must be well understood, especially on removal from the reactor, as the fuel is air cooled, transferred to containers, transported and submerged in a pond to cool, and shield radiation. It is important that the physical properties of the steel can withstand this transportation and that it will not corrode and rupture in the cooling ponds.
The following body of work considers the effects of time in reactor of the 20% chromium, 25% nickel, niobium-stabilised stainless steel used to clad the uranium dioxide pellets and form fuel rods. Samples of the steel were held at 350°C, 550°C, 750°C and 850°C for 1000 hours and subsequently examined using scanning electron microscopy, scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy to determine changes in the microstructure and precipitates present. Thermodynamic equilibrium software Thermo-Calc was also used to predict phases present and predict the effects of increasing carbon content on its microstructure.
This work observed development of G-phase in the sample held at 750°C for 1000 hours and coarser niobium carbide particles than the as-received steel in all treatments. This contrasts with observations in literature which saw more dramatic transformations, either in the form of greater variety of precipitate phases, or a much higher rate of transformation of niobium carbide to G-phase.