Cladding Performance

Advanced clads are being developed to exhibit better corrosion, mechanical properties and reduced growth under normal operating conditions. The models are under review for application to different cladding materials. As noted above, the clads may also experience different loads from newer fuels, e. g. MOX fuels compared with more traditional uranium oxide fuel.

16.3.1 Transient Fuel Rod Codes

Transient codes have been developed that include not only the physical models of the steady-state codes but also include additional modelling for transient thermal behaviour, heat capacity and heat transfer, transient mechanical properties such as long-term creep, cladding plastic stress-strain phenomena and ballooning at high temperatures. Other effects such as the effects of annealing, oxidation and hydriding, and changes of phase will also be modelled. Examples of transient fuel rod codes are the EPRI codes, FALCON/FREY, FRAPTRAN and the French IRSN code SCANAIR (IRSN: Scientific and Technical Report, 2002).

The main purpose of the transient fuel codes is for analysing the fuel rod response for RIAs and LOCAs. The main issues in modelling are related to the time-scales of different transient phenomena in relation to the time-scales of these transients. For example, fission product release may occur on both short-term and long-term time-scales. The time-scales of non-transient swelling and axial growth are much longer than the above accident transient time-scales. Different codes include different modelling assumptions, e. g. in addition to modelling pre-failure fuel behaviour, some of the codes include rod failure models.

At higher burn-ups, for example greater than 40-50 MWd kgU1, the Rim zone in the fuel requires special modelling attention. This is to make sure that the degradation in fuel thermal conductivity caused by structural changes in the fuel in this region is correctly modelled. Further differences in modelling requirements may also exist for MOX fuel and for advanced clads compared with more traditional UO2 fuel and clads.

Regarding other reactor types, HTR technology was under development in the 1980s but is now believed to be a realistic alternative to LWR. Ceramic fuel technology has been

established but further research is required to ensure that fuel performance is sufficiently reliable at high temperature (Hesketh, 2001). Current research is focussing on the fuel manufacturing process but methods will need to be developed to demonstrate that the fuel will be reliable to its design discharge burn-up.

Modelling codes for liquid metal fast reactors have been developed in various national programmes (IAEA-TECDOC-1083, 1999). The principal codes are TRAFIC (UK), GERMINAL (France), SATURN-TRANSIENT (Germany), LIFE (US), CEDAR (Japan) and KONDOR (Russia). These codes have a reasonable validation for moderate levels of burn-up (less than 12-15 at.%). The codes predict fuel pin thermal and mechanical behaviour for oxide fuels in steady-state and transient conditions. Some of these codes, e. g. TRAFIC also describe the behaviour of fuel pins after failure.

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