• ITER; VVPSS; hydrogen deflagration; reverse flow

The objective of the graduation thesis is to analyse from thermal-hydraulic and structural point of view the reverse flow due to a hydrogen combustion/deflagration accident in the Vacuum Vessel Pressure Suppression System of ITER nuclear fusion reactor.
The ITER Vacuum Vessel (VV) acts as a first safety containment barrier for the radioactive materal releases. Cooling water circulates through the shielding blanket to remove the heat produced by the nuclear fusion reaction. Loss of Coolant Accident (LOCA) is a postulated design basis accident. In this event, pressurized water is discharged into the Vacuum Vessel from the cooling water system. The VV pressure increases due to the evaporation water. The steam is also heated by the hot internal vessel structures.
The Vacuum Vessel Pressure Suppression System (VVPSS) is designed to mitigate the over-pressurization in the VV and to maintain the integrity of this primary confinement barrier. Cold water, stored in four tanks of the VVPSS, is used to condense the steam released from the VV. These tanks are provided by submerged spargers to discharge the steam into the water and to enable direct contact condensation.
LOCA produces hydrogen by radiolysis and thermolysis , desorption from structures and reaction between water and the beryllium first wall. This hydrogen could be accumulate in the tanks but the dangerous mixture could not produce a flammable mixture due to small quantity of oxygen.
Loss of Vacuum Accident (LOVA) is due to a breach in the primary confinement barrier allowing air to enter the VV. Hydrogen is also released from the VV in the event of LOVA. The almost pure hydrogen atmosphere of the VV is gradually diluted by the inlet of air and the mixture becomes flammable as the hydrogen concentration reaches the upper flammable limit.
A third accident category is a simultaneous occurrence of LOCA and LOVA. In this event an explosive mixture of hydrogen and air may produce within the headspace of the tanks of VVPSS. In order to prevent the build-up of large quantities of flammable mixtures, the tanks are fitted with igniters to initiate hydrogen combustion/deflagration and help to prevent creation of detonable mixtures.
These hydrogen combustion/deflagration could produce a water reverse flux from the sparger to Vacuum Vessel relief line.
The aim of this thesis is therefore to investigate the effect of the reverse flow on the sparger structure inside the tanks determining the pressure history, the water/air mass flow rate inside the sparger and the water hammer effect on the check valve in the Vacuum Vessel relief line.
The forces generating by reverse flow on the sparger system are illustrated in chapter 5. Pressure histories, water/air mass flow rates of the reverse flow in the different parts of the sparger are calculated by the RELAP5 code. Several simulations with different scenarios of hydrogen accidents have been performed in order to determine the most severe conditions in terms of pressure acting on the sparger internal surfaces. The calculated pressure transients have been used as an input loads for Mechanical analyses.
The structural simulations performed by the FEM code ANSYS Mechanical are illustrated in chapter 6. A 3D FEM model of the sparger and its supports have been implemented and a modal analysis has been performed in order to find the natural frequencies of the sparger. Transient analyses have been performed to determine the stress state of the sparger structure loaded by reverse flow effects.
The effect of the reverse flow load which impacts against the valve in the Vacuum Vessel relief line is reported in Chapter 7. A model of the Vacuum Vessel relief line and the sparger has been implemented in AFT Impulse code. Several simulations have been performed to determine the force acting on the check valve. A sensitivity analysis of the valve closure time has been carried out in order to minimize the water hammer effect.