Last update in July 2011
Assess understanding of steam explosion phenomena and the computing potential of numerical tools in this field
Context and objectives
SERENA - Steam Explosion REsolution for Nuclear Applications – is an international experimental project coordinated by the OECD to which the IRSN contributes its MC3D software in partnership with the CEA.
SERENA was launched in 2001 to assess the capabilities of current computer codes to predict steam explosion-induced loads in water reactor situations (as a result of contact between fuel or corium melt and water) and to identify how they could be improved to offset this risk.
The first phase of this project ran from 2001 to 2006. Specialist scientists from 10 countries working in the field of nuclear fuel-coolant interaction resulting in steam explosions collaborated during this phase. Results obtained from different codes supplied by the project partners, including the IRSN MC3D code in particular, were compared with existing experimental data and with each other on water reactor designs for both the premix and explosion phases.
This comparison demonstrated the existence of significant differences between the models used from one code to another. In the light of these results, specialists nevertheless considered that in-vessel fuel-coolant interaction (FCI) would probably not compromise the integrity of nuclear reactor containment. On the other hand, certain results demonstrated that this could not be ruled out in the event of ex-vessel FCI, making it impossible to quantify safety margins.
These findings highlighted a lack of knowledge regarding the FCI phenomenon. The results clearly showed that uncertainties remain:
concerning the premix end state and its link with explosion triggering, and especially the effect of steam quantity and distribution.
concerning the impact of corium melt properties (matter, oxidation) on the initial conditions (premix) and explosion propagation and more specifically, concerning the fact that prototypic corium melts would seem to generate fairly moderate explosions. The limited number of geometric configurations and corium compositions tested up to the present time make it impossible to generalise conclusions or to justify using specific parameters or models.
Therefore a 4-year second phase of the SERENA project was launched in 2008, with the aim of:
providing experimental data to explain the behaviour of prototypic corium melts during interaction,
providing innovative experimental data to validate models for prototypic materials, including spatial distribution of fuel and void during premixing and at the time of explosion,
providing experimental data for steam explosion in more reactor-like situations to verify the codes’ geometrical extrapolation capabilities.
This work will also be used by the IRSN in developing Level-2 PSAs.
The project’s Phase 2 uses two facilities for testing steam explosion with prototypic corium melt and water: TROI run by KAERI (Korea Atomic Energy Research Institute in South Korea) and KROTOS transferred to the French atomic energy commission (CEA) in Cadarache following several years’ operation at the European Community joint research centre at Ispra, Italy.
Thanks to their technological capabilities and top-level instrumentation, TROI and KROTOS will provide better quality data than that used during the project's first phase. More specifically, these facilities will make for better observation of corium melt behaviour (high-energy X-ray in KROTOS and tomography in TROI), better control of corium injection characteristics (thus improving modelling) and enable physical and chemical analysis of post-test waste.
KROTOS’ dimensions make it suitable for investigating intrinsic FCI characteristics in one-dimensional geometry. It can be used to characterise mix behaviour (fuel melt and void distribution) during interaction acceleration and propagation phenomena with the possibility of checking code results in real-time.
With its larger test facility, TROI is better suited to testing FCI behaviour in more reactor-like conditions given its multi-dimensional fuel melt/water interaction geometry.
The program comprises six series of two complementary tests in each facility.
The effect of fuel material properties will be analysed using four different compositions representative of conditions in a reactor.
Tests will be made under typical ex-vessel conditions, i.e. 0,2 MPa pressure and undercooling at 50 K.
The first test series (1 at KROTOS and 1 at TROI) based on current knowledge will serve to check if violent explosions are possible with corium. Subsequent test conditions will be defined to study the effects of materials and geometry separately, based on the experience acquired through these first two test.