ork Package 4 investigates potential mechanisms impairing the integrity of the cladding and reactor coolant bounday, as the first two engineering barriers for defence-in-depth, and the effectiveness of in-service inspection methods to secure their resistance.
Task 4.1 experimentally and numerically investigates the phenomenon of flow-induced vibrations (FIV) in the grid-spaced fuel bundle representative of the ALFRED fuel assembly.
A portion of the full-scale fuel assembly will be installed in the ENEA’s HELENA forced circulation loop, and vibrations of the rods induced by several flow rates, as high as 100 kg/s, for a parametric investigation of the effect of forcing actions by the coolant. Accurate measurements of the displacements of the rods will then provide relevant information for strengthening the design of the assembly, and for the validation of numerical approaches for the simulation of FIV.
Based on the development performed within the Dutch national R&D program and in the Euratom MYRTE and SESAME projects specifically concerning fluid-structure interaction modeling, NRG will use the experimental results to validate the STAR-CCM+ CFD code and the novel Pressure Fluctuation Model (PFM) which allows to sustain the vibrations in an unsteady RANS framework implemented in the open source code platform OpenFOAM for the fluid flow and Deal.II for the structural deformations. Additionally, further code development will be done to include structural contacts (e.g. to model a pin touching the grid) in the URANS simulations either within the STAR-CCM+ code platform or in the OpenFOAM-Deal.II code platform.
Task 4.2 will investigate experimentally and numerically the effects of sloshing in a heavy-liquid-metal-cooled system.
Preliminary experimental tests will be performed on a simplified geometry, filled first with water and then with Mercury, to compare the behavior of the free surface in order to validate the use of non-dimensional numbers and similarity rules to extrapolate the results from water to a heavy liquid metal scenario. Afterwards, the experimental campaign will be based on an existing model of MYRRHA at a scale 1/25 and the use of the SHAKESPEARE seismic table of the VKI seismic test laboratory. Different (up to five) seismic scenarios will be defined at the beginning of this task, that will be generic enough to be of interests for different HLM reactors.
The experimental results will also serve as database for the validation of numerical simulation, that will be performed mostly by CRS4, building and operating the CFD model of the experiments. Before the validation phase, CRS4 and VKI, using STAR-CCM+ and Open-FOAM respectively to enforce the independency of the numerical methodology from the platform used, will assess the impact of factors such as superficial tension and viscosity on second order features important to understand possible resonance behavior.
Task 4.3 focuses on the calibration and testing in representative conditions of a prototypic ultrasonic (US) device for the inspection of the integrity of the vessel in an HLM-cooled reactor.
As a first step, the work will be focused on inspection techniques of the vessel using a single element angle type sensor looking at both longitudinal and transversal cracks. KTU will perform US inspection on a set of calibration pieces in conditions already included in the code (room temperature, air or water) and in conditions representing the application in an HLM system, to extend calibration to real-case conditions by assessing the influence of wetting.
Successively, samples of the reactor vessel material will be artificially cracked while submerged in HLM to simulate representative faults. KTU will then use its US device to test its capability at correctly detecting the cracks while the samples are immersed in HLM at the outage temperature at which inspections are planned. After the test, the sample cracks will be characterized, if necessary by a destructive method at SCK CEN.