Work Package 2
Safety of the reactor coolant system
to stress-test the resistance of the engineering barriers
In a nutshell

ork Package 2 experimentally investigates, and validates simulation models and codes for, asymmetric circulation conditions in a pool and heat exchange in deformed grid-spaced fuel rod bundles.

Work scope
Task 2.1
Asymmetric pool thermal-hydraulics

Test campaigns will be performed on the SCK CEN’s E-SCAPE facility, on transient cases that are of particular interest and challenging for the validation of numerical models and codes, and for this defined based on the lessons learned from the Euratom co-funded MYRTE project and in close collaboration with the numerical analysts.

Specific focus will be on asymmetric transients (e.g. single pump failure, single heat exchanger failure) in forced and natural circulation conditions as these result in three-dimensional velocity and temperature patterns that will give rise to asymmetric loads on structures and are very challenging for stand-alone system thermal-hydraulic codes.

Numerical analysis of the experimented cases will be performed at first refining computational fluid dynamics (CFD) models, leveraging those already qualified in MYRTE, and building on the multi-scale approach developed in the Euratom SESAME project. Three different codes will be used: STAR-CCM+ (NRG), Fluent (SCK CEN) and OpenFOAM (VKI), along with different approaches with respect to RANS modelling of turbulent heat transfer to assess the sensitivity of the results – both momentum and thermal fluctuations – to the turbulence modeling and to the initial condition.

The effect of pins deformation on the temperature field in a fuel bundle with grid-spacers will be experimentally studied in a new water loop to be realized at VKI, where a deformed 7-pin grid-spaced rod bundle representative of ALFRED will be installed. The use of the particle image velocimetry (PIV) technique will also allow to retrieve accurate flow velocity distribution measurements (mean and turbulent quantities) in critical regions of the fuel bundle, thereby assessing also the hydraulics of deformed bundles.

All these data will be used for comparison with numerical simulations by the task partners, against different codes. Reduced resolution modelling experience will be used by NRG on a CFD framework, allowing parametric studies in a relatively efficient way, and the final validation of the models. A different approach will be followed by CIRTEN in parallel, exploiting the CFD-based simulation tool based on OpenFOAM leveraging previous studies of the flow and heat transfer in bundles with nominal geometry. A third, completely different approach will be also followed by CIRTEN, extending and validating the capabilities of the ENEA’s sub-channel code ANTEO+ to distorted bundle geometries.

A final benchmarking phase between the different approaches will be performed with validation aims, to highlight the advantages and limitations of each method, thereby defining their scope and, possibly, the associated confidence in predicting experimental conditions.

Task 2.2
Heat exchange in deformed grid-spaced bundles