Various components of nuclear power plants are submitted to very sharp multiaxial thermomechanical loadings, due for instance to the incomplete mixing of flows at different temperatures. As an example, thermal fatigue damage has been detected in auxiliary loops of the primary cooling circuits of Pressurized Water Reactors. In particular, crack networks were observed in in-service pipes submitted to thermomechanical loading resulting from cyclic temperature gradients across the wall-thickness of components in 304 L type austenitic stainless steel. The thermal fatigue behaviour of AISI 304 L type steel has been studied using a specific thermal fatigue test, called Splash, developed in order to reproduce experimentally such thermomechanical biaxial loading in the thickness of parallelepipedic specimens. All tests have been performed at a maximum temperature of 320°C, but with different minimum temperatures. First, the morphological characteristics of the growing networks were analysed, in surface and in depth. Crack initiation is multiple and occurs on sliding lines or at material defects. Crack network stabilization is observed after 400 000 cycles at a temperature of 150°C. The maximum depth is 2.5 mm. Secondly, the stability of the thermal-fatigue cracknetworks previously obtained was investigated under additional isothermal mechanical loading (four-point bend tests). Selection mechanisms of a dominating crack are observed, showing a great influence of shielding effects, branching and tortuous path. Comparison of the dominating crack behaviour with one having a single crack initiated at a notch tip reveals a significant delay effect.