Workpackages

Latest News

Upcoming Events

WP5: Fuel Dispersion Study

Work package number: 5
Start date or starting event: M1
Work package title: Fuel Dispersion Study
Activity Type: RTD
Participant number: 5 - 2 - 3 - 7 - 8
Participant short name: CRS4 - ENEA - KIT - UNIPI - VKI
Person-months per participant: 18.7 - 8.5 - 14 - 6.8 - 7.65

Objectives
CFD modelling

  • Realize two quite complete numerical CFD models of the MYRRHA-FASTEF facility primary coolant loop according to the latest available design and its relevant variants during the time extension of the project. The first step will be the grid generation of the primary coolant loop using CAD files of the MYRRHA reactor. The grid generation must be flexible enough to take into account the evolution of the design during the study. The first computational model (CFD) must then be foreseen for the simulation of long term events. It will be single phase with one-way coupling Lagrangian particles. The second model will be two-phase (Eulerian-Eulerian or VOF) for the simulation of short term events.
  • Apply the first CFD model to the simulation of the fuel dispersion in the coolant in case of long term small rate release due to one or several pin failures. The fuel dispersion will depend on the release position and on the chemical compound released. The solid products dispersion will depend on its specific weight and size. The gaseous products dispersion will depend on the bubble size and on the product solubility. The modelling of the thermal effects and of the turbulence will be one of the challenges of the simulation.
  • Apply the second CFD model to the simulation of the fuel dispersion in the coolant in case of gas release due to one or several pin failure. The gas is produced by nuclear reaction inside the fuel pin and normally trapped herein during normal operation. The impact of the gas release on the main flow will be taken into account.

SIMMER modelling

  • Assessment of behaviour of fuel redistribution in heavy liquid metal nuclear systems under fuel failure conditions.
  • Realize two quite complete numerical SIMMER-III models of the MYRRHA-FASTEF facility primary coolant loop according to the latest available design and its relevant variants during the time extension of the project. One model must be foreseen for the simulation of long term events. The other one must be foreseen for the simulation of relatively short term events and must be dynamically coupled with a neutronic code to capture a gas release impact of voiding on the neutronic core conditions, including the core sub-criticality status and heat release repartition.
  • Apply the first SIMMER-III model to the simulation of the fuel dispersion in the coolant in case of long term small rate release due to one or several pin failure.
  • Apply the first SIMMER-III model to the simulation of the fuel dispersion in the coolant in case of large core melting.
  • Apply the second SIMMER-III model to the simulation of the fuel dispersion in the coolant in case of short term consistent material release due to one or several pin failure. The material would be the fission gas produced and accumulated inside the fuel pin and the simulation coupled to neutronic calculation.
  • Realize a SIMMER-IV numerical model of the MYRRHA-FASTEF facility primary coolant loop according to the latest available design and its relevant variants during the time extension of the project. The model will be more precise to capture the geometry, including the core sub-criticality status and heat release repartition.. No coupling with neutronic is foreseen.
  • Apply the SIMMER-IV model to the simulation of the fuel dispersion in the coolant in case of long term small rate release due to one or several pin failure.

Description of work (CRS4, ENEA, KIT, UniPi, VKI)
Task 5.1: CFD simulation and modelling of fuel products dispersion in the MYRRHA-FASTEF primary loop coolant ( CRS4, VKI)
The task is developed in two directions corresponding to the two models described in the objectives. These two models share some common aspects. They must be built in close collaboration between CRS4 and VKI and carefully cross-checked. The main shared feature is the construction of a steady-state model comprehensive of all the necessary physical modelling. This steady-state model will be frozen in the first application to concentrate on the particles Lagrangian dispersion. It will serve as initial condition for the second application.

From the CFD point of view, all the necessary features already exist. An exception may be the free-surface between LBE and cover gas having a density ratio of ten thousands while the limit of stability in CFD codes is generally only slightly above the water/air (at sea level) ratio. The cover gas density will be artificially increased to bypass this issue without sensible effect on the modelling. Also, for the gas release simulation, we expect to have only general indications.

The difficulty of the work package lies in the consistent merge of all necessary features inside a complex geometry and operating condition, subject to the computational power limitation.

VKI will focus its activity on the single phase approach with Lagrangian tracking. The mean flow field inside the primary loop of the MYRRHA reactor will be computed for the forced convection regime under nominal condition. These nominal conditions include the complete 3D, steady simulation including thermal effects. Afterwards, solid particles, of different sizes and of different densities (simulating the partial core melting) will be released inside the primary loop at different positions and with different initial conditions. The Lagrangian tracking (without coupling with the liquid phase) of the particles will allow the follow-up of the elements inside the primary loop and provide useful information on the trajectories of these particles. The Lagrangian tracking will only be valid for small objects.

This CFD approach will provide the trajectories into the primary loop of the different particles (fuel) and will localize the region of fuel accumulation. VKI has a large experience in CFD and particularly in multi-phase flows [5.1,5.2,5.3] so has to fulfil this part of the work with good confidence.

CRS4 will focus its activity on the Eulerian two-phase flow approach based on the Volume of Fluid (VOF) paradigm to capture the upper free-surface dynamics. An extension to the full Euler-Euler paradigm may be investigated in a second time if necessary to better capture the fission gas release dynamics.

To correctly describe the transport of fuel/cladding products in the MYRRHA-FASTEF primary loop flow, we need to dispose of a very accurate flow velocity field. This flow field is driven by the pumping system. It is controlled by the hydraulic resistances, the bypass flows and the level of the free surfaces. A strong influence also comes from possible thermal stratifications in different parts of the pool, therefore also from the heat transfer through the structures. The combined effect of stratification, by-pass flow and free-surface reaction is likely to result in a non-trivial flow path, mainly in the upper hot plenum. The first objective is to obtain a comprehensive description of the entire pool loop.

We will be faced with the limit imposed by the computational power available, so some mitigation strategies will have to be developed. There are two Heat-Exchanger(HE)/Pump casing, each one consisting of two HE and one pump. These casings will require specific CFD modelling. Also, the HE requires a large computational power for a reasonable description. It is priori foreseen that in the global CFD loop, only one casing will be accurately discretised, and in this casing, only one HE will be discretised at the tube level. Separate models will have to be derived to make this approach consistent. There are also two or four casing foreseen for the repository of non-active fuel assemblies, each one with its own by-pass flow required to evacuate the (essentially) decay heat. Here again, a specific modelling must be performed, but at most one of these casing will have a more precise numerical description.

While it is out of range to have a description of the FAs internals, because of the non-uniform heat release in the core, each FA (or small group of FA) must have its separate description. Some refinement on the FA where the pin failure occurs can be evaluated. The spallation beam heat release must also be consistently modelled.

The global model must be organized in such a way that the single casing discretisation can be chosen on a case by case basis.

Once the model is reasonably satisfying, we will investigate the rapid release of fission gas due to the successive failure of a pin cladding and a pin fuel. The scenario leading to this event must be carefully described. This description is part of the next task and should be available for month 12. If it involves a partial blocking of the FA, then the flow field will be modified and a corresponding initial condition must be retrieved. The released gas will be followed in its path under the combined effect on the carrier fluid and its own drift velocity. The proportion of gas directly escaping through the hot plenum free surface will be evaluated. The behaviour of the residual gas fraction, entering the heat exchangers, will also be analysed until it becomes negligible. These features are likely to depend strongly on the position of the faulty FA and on the temporal signal of gas release rate.

The modelling and simulation will be performed with Starccm+, currently version 6.02. There are about two version updates a year and we will have to regularly adapt to the latest one available.

While the foreseen simulations are very ambitious from the technical point of view, CRS4 has gained in the very recent last years some confidence that is can be successfully realized. A single phase model of the XT-ADS version or MYRRHA has already been elaborated during the FP6 IP-EUROTRANS [1]. Nominal condition steady-state and an accidental shut-down have been simulated [2], however with a much simpler geometry than the one foreseen in this project. In the framework of the FP6 THINS project, CRS4 is currently working on free-surface flows and has already ascertained the possibility to model thermal free-surface flows within Starccm+, limited however to the Boussinesq approximation.

T5.2: Simulation and modelling of fuel dispersion in the coolant with SIMMER-III (KIT, ENEA, UNIPI)
The work is articulated in two main phases.
During the first phase, KIT will assess the incidental and accidental conditions leading to fuel pin failure and successive release of fuel, fission gas and finally partial core melt. The first part is propaedeutic for all partners and KIT will lead the interaction with the other WP so as to define sound incidental and accidental scenarios.

At the same time, UNIPI, ENEA and KIT will build the SIMMER-III reference model. This reference model must have the largest possible common basis for all three organisations. UNIPI will coordinate the elaboration of the SIMMER-III model to avoid duplication of work and optimise the model quality.

In the second phase, ENEA, with the support of UNIPI, will perform 2D SIMMER-III thermo-fluid-dynamic analysis (no neutronic coupling) of fuel dispersion and redistribution in the primary circuit of LBE-cooled MYRRHA-FASTEF reactor, starting from pre-defined fuel rod failure conditions. Several parametric calculations are envisaged to evaluate the influence of important parameters such as: the fuel break-up and particle size, the amount of released fuel, forced/natural circulation in the primary system, etc.

The need and the possibility to extend the SIMMER-III analysis to hypothetical severe accident scenarios involving large core melting will be investigated.

We will extend our modelling also to the 3D SIMMER-IV thermo-fluid-dynamic analysis (no neutronic coupling) of fuel dispersion and redistribution in the primary circuit of LBE-cooled MYRRHA-FASTEF reactor for the more representative fuel rod failure conditions.

The 3D version of the SIMMER code (SIMMER-IV) is obviously not able to give the same detailed information about the fluid-dynamic aspects that can be obtained by the CFD codes. It remains a system code and its use can be justified if we recognize that we have important multidimensional effects that can’t be reproduced using an axial-symmetric geometry (SIMMER-III). The use of the SIMMER-IV version code can be suitable, again, to simulate the complex phenomena involved in our problem to obtain boundary conditions that must be passed to CFD codes. Its use will be restricted to the more representative fuel failure conditions because of the very high computational power it requires.

In the second phase, KIT will focus his activity on SIMMER-III coupled neutronic/thermal-hydraulic calculations of fuel and clad redistribution after the postulated pin failures under Pb/Bi flow conditions. The work is articulated in three main parts:

  • Simulation of fission gas release and impact of voiding on neutronic core conditions, depending on pin damage configuration and size.
  • Assessment of release of fuel pellets, chunks, particles and impact on fuel motion behaviour in Pb/Bi and neutronic feedback on core sub-criticality status.
  • Assessment of granulated fuel and clad motion behaviour within the primary system.

During the entire project, UNIPI will take care of the congruence of ENEA and KIT models with the design evolution.

Deliverables
D5.1 Report on the operability of the SIMMER-III and SIMMER-IV models for the MYRRHA- FASTEF reactor (UniPi, KIT, ENEA) (M12)
D5.2 Single-phase CFD model of the MYRRHA-FASTEF primary coolant loop including all relevant thermal aspects (VKI) (M18)
D5.3 Two-phase CFD model of the MYRRHA-FASTEF primary coolant loop including all relevant thermal aspects (CRS4) (M18)

D5.4 Report on the 3D SIMMER-IV analysis for the more representative fuel failure conditions (ENEA, UniPi) (M24)
D5.5 Report on assessment on fuel dispersion after pin failure under blockage conditions (KIT) (M30)
D5.6 Characterisation of long term dispersion of fuel in the coolant with CFD modelling (VKI) (M36).
D5.7 Characterisation of the fission gas and other species release dispersion in the coolant with CFD modelling (CRS4) (M36)
D5.8 Report on the 2D SIMMER-III analysis for the reference case and parametric study (ENEA, UniPi) (M36)