Nuclear Energy for New Europe 2009

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No: 208

Conference: Nuclear Energy for New Europe 2009

Title: CFD Modelling of the Thermal Hydraulics inside a Spent Fuel Element at the ILL Research Reactor

Theme: Thermal Hydraulics

Author(s): Johannes Baggemann, St. Kelm, Herve´ Guyon, J. Lehmkuhl, Hans-Josef Allelein

Contact : Johannes Baggemann

E-mail: j.baggemann@fz-juelich.de

Address: Forschungszentrum Juelich GmbH Institute for Energy Research – Safety Research and Reactor Technology (IEF-6)
52425 Juelich

Country: Germany

The Institute Laue Laugvine (ILL) in southern France operates a strong neutron source for scientific use. This heavy water moderated pool type reactor has a single, compact fuel assembly. After being unloaded from the reactor pool, such a fuel element has a high residual
power of about 160 kW and thus has to be cooled sufficiently in the fuel storage pool. For this purpose, a passive cooling concept has been developed at ILL: The fuel element is placed at the storage pool in an “exchange flask”. The residual heat is removed from the exchange flask by internal natural circulation to the pool. The compact fuel assembly is about one meter high and consists of 280 evolvent shaped plates which are arranged between two concentrical aluminium cylinders. The exchange flask consists primarily of an aluminium cylinder and a tube bundle heat exchanger. The heat exchanger is mounted on one side of the exchange flask with two supply lines, one above and one below the fuel element. The fuel element hangs inside the aluminium cylinder between these supply lines so that a natural circulation through the element and heat exchanger is established in normal operation, the residual heat is transferred to the fuel pool by the heat exchanger and cold water is fed to the lower plenum of the flask again. The aim of a numerical study performed at Juelich in cooperation with Aachen University of Applied Sciences is to investigate the safety margins of this storage and cooling
concept. For a subsequent accident analysis, it is important to gain an insight into each cooling channel of the fuel assembly and determine local effects, thus the fuel assembly has to be geometrically resolved. The model development is performed in 3 steps. In first
subchannel studies, the effects of grid resolution, wall treatment, turbulence and buoyancy model have been investigated on a single channel. As a second step, a porous medium has been implemented in order to get a first insight into the flow in the exchange flask and
address the influence of the boundary conditions, the complete exchange flask is considered but the plates and the water channels are modelled by a porous medium. Based on the results of these scoping tests, a final half symmetry model was built, resolving the subchannels of the fuel element and the flow in the exchange flask.

No:	208
Conference:	Nuclear Energy for New Europe 2009
Title:	CFD Modelling of the Thermal Hydraulics inside a Spent Fuel Element at the ILL Research Reactor
Theme:	Thermal Hydraulics
Author(s):	Johannes Baggemann, St. Kelm, Herve´ Guyon, J. Lehmkuhl, Hans-Josef Allelein
Contact :	Johannes Baggemann
E-mail:	j.baggemann@fz-juelich.de
Address:	Forschungszentrum Juelich GmbH Institute for Energy Research – Safety Research and Reactor Technology (IEF-6) 52425 Juelich
Country:	Germany

The Institute Laue Laugvine (ILL) in southern France operates a strong neutron source for scientific use. This heavy water moderated pool type reactor has a single, compact fuel assembly. After being unloaded from the reactor pool, such a fuel element has a high residual power of about 160 kW and thus has to be cooled sufficiently in the fuel storage pool. For this purpose, a passive cooling concept has been developed at ILL: The fuel element is placed at the storage pool in an “exchange flask”. The residual heat is removed from the exchange flask by internal natural circulation to the pool. The compact fuel assembly is about one meter high and consists of 280 evolvent shaped plates which are arranged between two concentrical aluminium cylinders. The exchange flask consists primarily of an aluminium cylinder and a tube bundle heat exchanger. The heat exchanger is mounted on one side of the exchange flask with two supply lines, one above and one below the fuel element. The fuel element hangs inside the aluminium cylinder between these supply lines so that a natural circulation through the element and heat exchanger is established in normal operation, the residual heat is transferred to the fuel pool by the heat exchanger and cold water is fed to the lower plenum of the flask again. The aim of a numerical study performed at Juelich in cooperation with Aachen University of Applied Sciences is to investigate the safety margins of this storage and cooling concept. For a subsequent accident analysis, it is important to gain an insight into each cooling channel of the fuel assembly and determine local effects, thus the fuel assembly has to be geometrically resolved. The model development is performed in 3 steps. In first subchannel studies, the effects of grid resolution, wall treatment, turbulence and buoyancy model have been investigated on a single channel. As a second step, a porous medium has been implemented in order to get a first insight into the flow in the exchange flask and address the influence of the boundary conditions, the complete exchange flask is considered but the plates and the water channels are modelled by a porous medium. Based on the results of these scoping tests, a final half symmetry model was built, resolving the subchannels of the fuel element and the flow in the exchange flask.