[en] In the frame of nuclear safety studies about corium and water interactions, we address spreading and cooling stage of corium fragments in a liquid pool. Considering the complexity of encountered flow regimes and mechanical and thermal interactions coupling, modelling validation is based on a thermal-hydraulic computer code (MC3D). A bibliographical study shows that classical modelling of three phase flow is based on constitutive laws already established in the case of two phase flow. The present study states a complete analysis of BILLEAU experiments and defines a characterisation method for a sphere cloud. Some complementary QUEOS experiments are also described. Mechanical interaction terms such as added mass, lift and turbulent dispersion have been presented in the frame of a three phase flow and their influence has been tested in numerical simulations of BILLEAU tests. The effect of film vapour overheat, as well as particle diameter evolution have been studied. Moreover a radiative heat transfer modelling developed in Karlsruhe research centre (FZK) has been analysed and completed. Numerical simulations achieved for this study show that mechanical and thermal behaviour of the system are actually coupled. Taking into account lift and turbulent dispersion terms as well as heat transfer modifications all wed better results. This study also presents some considerations about flow regimes identification as a preliminary for studies about numerical diffusion that was already estimated in the present state of the computer code MC3D. (author)

[en] The BILLEAU experimental program studies two dimensional flow of solid spheres in a plant test section. New tests with cold ZrO2 and glass spheres and hot ZrO2 spheres heated up to 2200 K are presented as well as calculations of two cold tests and one hot test with the MC3D multidimensional multiphase thermal-hydraulic premixing code for Fuel Coolant Interaction (FCI) studies. 4 refs., 13 figs., 3 tabs

No abstract available

[en] This paper presents the << Triphasic>> application of the multiphasic 3D computer code MC3D. This application is devoted to the premixing phase of a Fuel Coolant Interaction (FCI) when large amounts of molten corium flow into water and interact with it. A description of the new features of the model is given (a more complete description of the full model is given in annex). Calculations of Billeau experiments (cold or hot spheres dropped into water) and of a FARO test (<< true>> corium dropped into 5 MPa saturated water) are presented. (author)

[en] This article presents the different programs led in laboratories concerning the mitigation of the hydrogen risk in nuclear power plants. 3 aspects are considered: the generation of hydrogen during a reactor major accident, the distribution of this gas inside the reactor containment building and the different combustion modes of hydrogen. Studies show that it is difficult to prevent at any time and place the formation of a combustible mixture despite the presence of hydrogen recombiners. Studies have led to the setting of criteria concerning flame acceleration and detonation-explosion transitions, it has been shown that a mixture whose expansion parameter stays below a limit value can not lead to a flame acceleration over 400 m/s. (A.C.)

[en] Large companies like EDF or Framatome-ANP have to face a big turnover of staff because a lot of employees are expected to retire by 2015. They have drawn special training policies in order to avoid the loss of practical knowledge particularly in the field of maintenance. Technological progress, bringing more flexibility and reactivity to the power demand, are constant challenges for nuclear industry. In this series of short articles major improvements are reviewed. A new cladding material: the M5 alloy (zirconium + niobium + oxygen) can undergo up to 78 GWj/tU burnup and presents high resistance to corrosion, low generation of hydrogen, good behaviour in power ramps and in accidental situations. Progress in instrumentation and the broad use of digit technology have also led to improvements in reactor monitoring systems. Studies concerning the hydrogen risks back the decision to implement in each reactor unit a hydrogen catalytic re-combiner whose role will be to reduce the concentration of gaseous hydrogen through the production of water. (A.C.)