Preliminary Considerations from the 2nd Phase of Experiments at the SIET/SWAM Facility

Severe accident codes study the thermo-hydraulics of the suppression chamber with a limited numbers of nodes, generally solving mass and energy equations and assuming perfect mixing conditions. In a long station black out the effect of the sparger’s design might create local phenomena (e.g. stratification, hot-spots) which are hardly predicted by the current practices, resulting in mispredictions of the containment pressure evolution. In order to understand the effect of the sparger geometry, steam mass flux, water sub-cooling and air concentration the SWAM facility (Steam Water Air Mixing) at the SIET laboratory was employed performing around twenty different experiments, in conditions close to what is expected during the Fukushima Daiichi accident. The test facility (poll and pipes) is built with polycarbonate (transparent material) to ease the acquisition of the standard and high-speed cameras. Vertically distributed thermocouples and high-frequency pressure measurements are employed to obtain quantitative values for phenomena investigation and future CFD validations. It was shown that experiments with pure steam and relatively large diameter holes induce chugging that enhances mixing in the pool. Once chugging ceases, because of the reduced sub-cooling, a hot water layer is created in the upper part of the pool. The presence of air in the pipe induces large stratification from the condition of large subcooling because of the limited mixing introduced in the region below the pipe mouth

Thermal-hydraulics of internally heated molten salts and application to the Molten Salt Fast Reactor

The Molten Salt Reactors (MSR) are an innovative kind of nuclear reactors and are presently considered in the framework of the Generation IV International Forum (GIF-IV) for their promising performances in terms of low resource utilization, waste minimization and enhanced safety. A unique feature of MSRs is that molten fluoride salts play the distinctive role of both fuel (heat source) and coolant. The presence of an internal heat generation perturbs the temperature field and consequences are to be expected on the heat transfer characteristics of the molten salts. In this paper, the problem of heat transfer for internally heated fluids in a straight circular channel is first faced on a theoretical ground. The effect of internal heat generation is demonstrated to be described by a corrective factor applied to traditional correlations for the Nusselt number. It is shown that the corrective factor can be fully characterized by making explicit the dependency on Reynolds and Prandtl numbers. On this basis, a preliminary correlation is proposed for the case of molten fluoride salts by interpolating the results provided by an analytic approach previously developed at the Politecnico di Milano. The experimental facility and the related measuring procedure for testing the proposed correlation are then presented. Finally, the developed correlation is used to carry out a parametric investigation on the effect of internal heat generation on the main out-of-core components of the Molten Salt Fast Reactor (MSFR), the reference circulating-fuel MSR design in the GIF-IV. The volumetric power determines higher temperatures at the channel wall, but the effect is significant only in case of large diameters and/or low velocities

External heat transfer capability of a submerged SMR containment: The Flexblue case

Flexblue® is a 160 MWe, transportable and subsea-based nuclear power unit operating up to 100 m depth several kilometers away from the shore. The concept is based on existing technologies and experience from the oil&gas, civil nuclear and shipbuilding industries. In a post-Fukushima world, its safety features are particularly relevant. The immersion provides inherent protection against most external aggressions including tsunamis, extreme weather conditions and malevolent actions. The vicinity and the availability of an infinite, permanent heat sink â the ocean â enhances the performance of the safety systems which, when designed to operate passively, considerably extend the grace period given to operators in case of accident. The present work investigates seawater natural convection fluid dynamics and heat transfer features, induced by the heating of Flexblue® reactor containment, to evaluate the capabilities of the system to reject the decay power to the exterior in case of an accident. A preliminary lumped parameters approach has been adopted, revealing that the large diameter of the hull (14 m) is such that ranges of validity of empirical correlations for natural convection heat transfer are always exceeded and conditions for their correct application are not satisfied. Hence, a 2D, unsteady CFD analysis has been performed to simulate the natural convection flow in the ocean, thus obtaining predictions for heat flux distribution, hull superficial temperature profile and heat transfer coefficient. Both CFD sensitivity and parametric analyses have been carried out, even if within a 2D approach, to limit the computational burden. The results showed that the heat transfer process is globally satisfactory to ensure the safe cooling of the reactor. A 3D approach and an experimental campaign aimed at validating the CFD results have been planned

Modeling of Multi-Physics Phenomena in Fast Reactors Design/Safety and Experimental Validation

The paper provides a cursory look at current approaches in numerical modeling and simulation of typical multi-physics phenomena of concern relevant to the sodium-cooled fast reactor design and safety. Emphasis is placed on the methods that are in practice and their verification and validation programs, including for those of fluid-structure thermal interactions due to thermal striping, thermodynamics of sodium-water chemical reactions, multi-component and multi-phase flows in the fuel degradation and core meltdown phases. Several of numerical simulations of these multi-physics phenomena are shown with verification and validation programs that employ not only separate-effect small-scale experiments of clean geometry but also for large-scale integral tests or mock-up experiments. The last part of this paper will be spent on discussions on more quantitative validation basis with identification of errors and/or uncertainties based on the Bayesian rule

Pioneering role of IRIS in the resurgence of Small Modular Reactors

This paper presents an overview of the first 10 years of the IRIS project, summarizing its main technical achievements and evaluating its impact on the resurgence of small modular reactors (SMRs). SMRs have been recurrently studied in the past, from early days of nuclear power, but have never gained sufficient traction to reach commercialization. This situation persisted also in the 1990s; the focus was on large reactors based on the presumed common wisdom of this being the only way to make the nuclear power plants competitive. IRIS is one of several small reactor concepts that originated in the late 1990s. However, the specific role and significance of IRIS is that it systematically pursued resolving technology gaps, addressing safety, licensing, and deployment issues and performing credible economics analyses, which ultimately made it possible—together with other SMR projects—to cross the “skepticism threshold” and led the making of a convincing case—domestically and internationally—for the role and viability of smaller reactors. Technologically, IRIS is associated with a number of novel design features that it either introduced or pursued more systematically than its predecessors and ultimately brought them to a new technical level. Some of these are discussed in this paper, such as the IRIS Safetyby- Design, security by design, the innovative thermodynamic coupling of its vessel and containment, systematic probabilistic risk assessment–guided design, approach to seismic design, approach to reduce the emergency planning zone to the site boundary, active involvement of academia, and so on. Many individuals and organizations contributed to that work, too many to list individually, and this paper attempts to pay tribute at least to their collective work

Modelling of Multi-Physics Phenomena in Fast Reactor Design: Safety and Experimental Validation

The paper provides a cursory look at current approaches in numerical modelling and simulation of typical multi-physics phenomena of concern relevant to the sodium cooled fast reactor design and safety. Emphasis is placed on the methods that are in practice and their verification and validation programmes, including for those of fluid–structure thermal interactions due to thermal striping, thermodynamics of sodium–water chemical reactions, multi-component and multi-phase flows in the fuel degradation and core meltdown phases. Several of the numerical simulations of these phenomena are shown with verification and validation programs that employ not only separate effect small scale experiments of clean geometry but also for large scale integral tests or mock-up experiments. The last part of this paper will be spent on discussions on a more quantitative validation basis with identification of errors and/or uncertainties based on the Bayesian rule