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WP2: Coolability of a wire spaced fuel bundle

Work package number 2
Start date or starting event: Month 1
Work package title: Coolability of a wire spaced fuel bundle
Activity Type: RTD
Participant number: 2 - 3 - 4
Participant short name: ENEA - KIT - NRG
Person-months per participant: 11.25 - 34 - 5

In this work package the goal is the investigation of the coolability of a wire spaced fuel assembly by the experimental analysis and numerical modelling of thermal-hydraulic behaviour of a wire wrapped rod bundle simulator. In order to meet the requirements of MYRRHA two different sized prototypical rod bundles will be investigated in a heavy liquid metal flow using LBE (lead bismuth eutectic) in the regime of forced, mixed and natural convection with a maximal attainable heat flux of 1 MW/m² by electrical heating.
Measured pressure drop and temperature profiles will give important input for the MYRRHA design. Nevertheless, for a deeper understanding numerical support and CFD-modelling to support the experiments is indispensable.

Description of work (ENEA, KIT, NRG)
Task 2.1 Fuel assembly thermal-hydraulic tests (ENEA, KIT)
This task will be devoted to the experimental campaign on THEADES (KIT), NACIE (ENEA) and HELENA (ENEA) HLM loops. The experiment in the THEADES loop will study forced convection and it will be performed on a 19 pin wire wrapped rod bundle with a diameter of 8.2 mm, pitch to diameter (p/d) ratio of 1.28 and a heat flux up to 1 MW/m2. Results will be in terms of temperature measurements in the pin cladding, bulk temperatures in the subchannels, pressure drop measurements and overall temperature drop.

Information from THEADES experiment will give the base for the subsequent experiment on free and mixed convection on the NACIE loop (ENEA). A wire-spaced 19 pins hexagonal fuel bundle will be assembled in a hexagonal lattice, with pin diameter 6.55 mm, active length of 600 mm, p/d ratio of 1.28. The pins provide an azimuthally uniform heat flux up to 1 MW/m2. These values are relevant for the MYRRHA design. Free and mixed convection tests will be performed on the NACIE loop. The transient tests will simulate the transition from forced to free convection as a consequence of a loss of flow accident, and therefore the initial condition will result from the THEADES experimental tests. The following quantities will be measured during the transient: the cladding temperature through wall-embedded TCs; the integral mass flow rate through a proper flow meter; pressure losses in the FA; bulk temperature in the sub-channels; heat transfer coefficient.

The reason for using 8.2 mm pins in THEADES and 6.5 mm pins in NACIE lies in the short term availability of 8.2 mm rods from former experiments. In this way the whole campaign will save a lot of time and money without compromising on physical relevance, as the results of forced convection experiments can be transferred to the final MYRRHA-design using thermal hydraulic similarity laws and simulation tools validated using the 8.2 mm pin experimental results.

As Long Term step, the NACIE bundle will be dismantled and mounted in the horizontal HELENA loop, working with lead. The forced convection experiment above described for THEADES will be repeated on the new bundle and the coherence of the scaling laws used as Initial Conditions in the NACIE tests will be verified in HELENA. A post-test analysis will be accomplished on the whole experimental campaign.

Task 2.2 Numerical fuel assembly thermal-hydraulic analyses (NRG)
This task will provide detailed CFD analyses using appropriate turbulence modelling in support of the experiments. From these detailed analyses, input parameters will be derived for a coarse grid model of the NACIE facility to perform parameter studies at KIT. Detailed post-test CFD analyses on a section of the experimental FAs will be performed for the THEADES and NACIE experiments. These detailed analyses will provide input for LRGR and coarse grid analyses of the complete experimental FA to be performed by NRG and KIT. These analyses will support the interpretation of the experimental results, as limited measurement data will be available due to the complex experimental environment (LBE). The validated approaches used in support of the experiments will be applied to the real 127 pin MYRRHA FA. Firstly, a detailed CFD analysis of a section of a FA will be performed. This will provide input for LRGR and coarse grid analysis of a complete MYRRHA FA or even the complete MYRRHA core. These analyses will serve to deduct heat transfer correlations to be used in less resolved numerical approaches.

Task 2.3 Numerical support to design and experiments on wire-wrap fuel assembly (KIT)
This task will be devoted to pre-test parametric and post-test numerical simulations on the rod bundle flow. Pre-test numerical simulations provide the input for the selection of parameters like axial pitch and help to choose an experimental test-program including sensor locations, error estimation and optimization of flow conditioners. Detailed interpretation of experimental data is performed based on post-test simulations. Simulations are performed with the coarse grid CFD technique with sub-grid data derived from representative simulations with fully resolved CFD (provided for NACIE-experiments by NRG and for KALLA experiments by KIT).

D2.1 The fuel rod bundle design for the NACIE facility (ENEA) (M06)
D2.2 8.2 mm rod bundle design and instrumentation report (KIT) (M06)
D2.3 NACIE: a test facility for free and mixed convection heat transfer measurements in liquid metals for the MYRRHA reactor (ENEA) (M18)
D2.4 Experimental results on free convection in heavy liquid metals using the NACIE facility (ENEA) (M24)
D2.5 8.2 mm rod bundle experiments report (KIT) (M24)
D2.6 Experimental results on forced convection in heavy liquid metals using the HELENA facility (ENEA) (M36)
D2.7 Post-test analysis on free and forced convection measurements in HLM using NACIE and HELENA facilities (ENEA) (M36)
D2.8 Report on pre- and post-test analyses of the NACIE and THEADES experi¬mental loops and assessment of the 127 pin MYRRHA FA (NRG) (M36)
D2.9 Report on pre-test analysis (KIT) (M24)
D2.10 Report on post-test analysis (KIT) (M36)