After the accident in the Fukushima Daiichi nuclear power plant, the interest of adding Filtered Containment Venting Systems (FCVS) on existing nuclear power plants to prevent radioactive releases to the environment during a severe accident has increased. Wet scrubbers are one possible design element which can be part of an FCVS system. The efficiency of this scrubber type is thereby depending, among others, on the thermal-hydraulic characteristics inside the scrubber. The flow structure is mainly established by the design of the gas inlet nozzle. The venturi geometry is one of the nozzle types that can be found in nowadays FCVS. It acts in two different steps on the removal process of the contaminants in the gas stream. Downstream the suction opening in the throat of the venturi, droplets are formed by atomization of the liquid film. The droplets are contributing to the capture of aerosols and volatile gases from the mixture coming from the containment. Studies state that the majority of the contaminants is scrubbed within this misty flow regime. At the top of the venturi, the gas stream is injected into the pool. The pressure drop at the nozzle exit leads to the formation of smaller bubbles, thus increasing the interfacial area concentration in the pool. In this work, the flow inside a full-scale venturi scrubber has been optically analyzed using shadowgraphy with a high-speed camera. The venturi nozzle was installed in the TRISTAN facility at PSI which was originally designed to investigate the flow dynamics of a tube rupture inside a full-length scale steam generator tube bundle. The data analysis was focused on evaluating the droplet size distribution and the Sauter mean diameter under different gas flow rates and operation modes. The scrubber was operated in two different ways, submerged and unsubmerged. The aim was to include the effect on the droplet sizes of using the nozzle in a submerged operation mode.

The two national technical universities in Switzerland, viz. the Swiss Federal Institutes of Technology at Lausanne (EPFL) and at Zurich (ETHZ) have a rich and long tradition in nuclear education. Student research in nuclear engineering, particularly at the doctoral level, has usually been conducted in collaboration with the Paul Scherrer Institute (PSI) at Villigen, the national research centre where most of the country’s fission-related R&D is carried out. A significant part of this R&D is carried out in close collaboration with the Swiss Nuclear Utilities (swiss nuclear ). The four above, key national players in nuclear teaching and research in Switzerland — EPFL, ETHZ, PSI and swiss nuclear — have recently pooled resources in implementing a new Master of Science degree in Nuclear Engineering (NE). The present paper describes the main features and experience acquired to date in the running of this, first-ever, common degree offered jointly by the two Swiss Federal Institutes of Technology. The program, although naturally addressing Switzerland’s needs, is clearly to be viewed in an international context, e.g. that of the Bologna Agreement. This is reflected in the composition of the first two batches, with about 70% of the students having obtained their Bachelor degrees from universities outside Switzerland. Starting September 2010, the curriculum of the EPFL-ETHZ NE Master will be upgraded, from its current 90 ECTS credit points (3 semesters) to a 120 ECTS (4 semesters) program. An overview is provided of the current 90-ECTS curriculum, as also a sketch of the changes foreseen in going to 120 ECTS.

The work was aimed at the experimental investigation and numerical simulation of coolant mixing in the downcomer and the lower plenum of pressurized water reactors (PWR). For the investigation of the relevant mixing phenomena, the Rossendorf test facility ROCOM has been designed. ROCOM is a 1:5 scaled Plexiglas model of a German PWR allowing conductivity measurements by wire mesh sensors and velocity measurements by LDA technique. The CFD calculations were carried out with the CFD-code CFX-4. For the design of the facility, calculations were performed to analyze the scaling of the model. It was found, that the scaling of 1:5 to the prototype meets both: physical and economical demands. Flow measurements and the corresponding CFD calculations in the ROCOM downcomer under steady state conditions showed a Re number independency at nominal flow rates. The flow field is dominated by recirculation areas below the inlet nozzles. Transient flow measurements with high performance LDA-technique showed in agreement with CFX-4 results, that in the case of the start up of a pump after a laminar stage large vortices dominate the flow. In the case of stationary mixing, the maximum value of the averaged mixing scalar at the core inlet was found in the sector below the inlet nozzle, where the tracer was injected. At the start-up case of one pump due to a strong impulse driven flow at the inlet nozzle the horizontal part of the flow dominates in the downcomer. The injection is distributed into two main jets, the maximum of the tracer concentration at the core inlet appears at the opposite part of the loop where the tracer was injected. For turbulent flows the CFD-Code CFX-4 was validated and can be used in reactor safety analysis. Due to the good agreement between measured results and the corresponding CFD-calculation efficient modules for the coupling of thermal hydraulic computer codes with three-dimensional neutron-kinetic models using the results of this work can be developed. A better description of the mixing processes inside the RPV is the basis of a more realistic safety assessment.

A generic investigation of the influence of density differences between the primary loop inventory and the ECC water on the mixing in the downcomer was made at the ROCOM Mixing Test Facility at Forschungszentrum Rossendorf (FZR)/Germany. ROCOM is designed for experimental coolant mixing studies over a wide variety of possible scenarios. It is equipped with advanced instrumentation, which delivers high-resolution information characterizing either temperature or boron concentration fields in the investigated pressurized water reactor. For the validation of the Trio_U code an experiment with 5% constant flow rate in one loop (magnitude of natural circulation) and 10% density difference between ECC and loop water was taken. Trio_U is a CFD code developed by the CEA France, aimed to supply an efficient computational tool to simulate transient thermal-hydraulic single-phase turbulent flows encountered in the nuclear systems as well as in the industrial processes. For this study a LES approach was used for mesh sizes according to between 300000–2 million control volumes. The results of the experiment as well as of the numerical calculations show, that a streak formation of the water with higher density is observed. At the upper sensor, the ECC water covers a small azimuthal sector. The density difference partly suppresses the propagation of the ECC water in circumferential direction. The ECC water falls down in an almost straight streamline and reaches the lower downcomer sensor position directly below the affected inlet nozzle. Only later, coolant containing ECC water appears at the opposite side of the downcomer. The study showed, that density effects play an important role during natural convection with ECC injection in pressurized water reactors. Furthermore it was important to point out, that Trio_U is able to cope the main flow and mixing phenomena.

Partial depletion of the primary circuit during a hypothetical small break loss of coolant accident can lead to the interruption of one-phase flow natural circulation. In this case, the decay heat is removed from the core in the reflux-condenser mode. For the scenario of a hot leg side leak and hot leg safety injection thermal hydraulics analyses using the system code ATHLET showed, that weakly borated condensate can accumulate in particular in the pump loop seal of those two loops, which do not receive safety injection. According to these ATHLET-calculations, one-phase flow is maintained in the remaining two loops at high residual heat conditions because of the entrainment of safety injection coolant into the steam generators. After refilling of the primary circuit, natural circulation in the two stagnant loops simultaneously re-establishes and the de-borated slugs are shifted towards the reactor pressure vessel (RPV). Mixing in the downcomer and the lower plenum is an important phenomenon mitigating the reactivity insertion into the core in this postulated scenario. Therefore, mixing of the de-borated slugs with the ambient coolant in the RPV was investigated at the four loop 1:5 scaled ROCOM mixing test facility. Based on the ATHLET-calculations, a volume flow rate of 5% of the nominal rate was set in the loops running in one-phase flow. The volume flow rate in the two restarting loops increases from zero to 6%. In these two loops, de-borated slugs of 7.2 m 3 were assumed corresponding to the volume of the whole loop seal. An experimental parameter study was carried out with different duration of the flow ramp and variation of the density difference between de-borated slug and ambient coolant due to differences in boron concentration and temperature. The variation of the density difference significantly changes the mixing behavior. With no density difference, the weakly borated coolant almost perpendicularly flows down in the downcomer and a maximum of 64% of the initial perturbation is detected in the core entry section below the loops where the slugs were formed. Increasing the density difference, a stratification is observed in the downcomer. The less dense slugs flow around the core barrel at the top of the downcomer. At the opposite side the lower borated coolant is entrained by the colder safety injection water and transported to the core. This entrainment effect leads to the admixture of boron from the safety injection to the under-borated slugs. Consequently, the maximum under-boration at the core entry is lower. For the maximum investigated density difference of 2%, a value of 31% only of the initial under-boration was measured at the core entrance.