Nuclear ventilation design office

Ventilation is a support function of equipment that ensures normal operation or must remain available under accidental conditions. In new nuclear projects as well as in the developments of the French nuclear fleet, its role in defence in depth has become increasingly important.

Nuclear ventilation must today ensure optimal operation in all seasons, including under extreme outdoor conditions, but also in exceptional situations, such as a loss of voltage. The availability of backup equipment depends on it: poor control of temperature or humidity can jeopardise the qualification of safety classified materials.

The constraints on ventilation systems are therefore strong: they must be functional both at low load and at high load. Poor sizing can lead to cycling phenomena, result in oversized and energy-consuming equipment, or, in the worst case, prevent the fulfilment of their function.

These systems are generally sized based on steady-state operating points, but must also ensure specific operations during transient phases. The modelling assumptions are then very different: some physical phenomena that are negligible in steady state can become critical in transient.

Having a model capable of quickly simulating these two types of situations, while taking into account the relevant physical phenomena in each case, allows for better quantification of margins and reliability of sizing and architectural choices.

For these studies, we use the modelling language Modelica, suitable for representing complex thermo-fluid systems and coupling their various components. SIL3X also contributes to the OpenModelica consortium and develops its own model libraries to meet the specific needs of nuclear ventilation studies.

Cette maîtrise de la modélisation nous permet d’aller au-delà de l’utilisation de modèles standards. Nous développons notamment des outils sur mesure pour les besoins de dimensionnement et de justification, dont TAeZo, une bibliothèque qualifiée au sens du guide n°28 de l’ASN et dédiée à la simulation des systèmes de ventilation.

Our qualified tool for HVAC studies 

Complex ventilation systems can present several operating points, particularly when certain branches of the network are isolated to confine sensitive areas or adapt the configuration of the installation. These configuration changes can significantly alter flow rates, pressure losses, and the operating conditions of the fans. It then becomes necessary to assess their impact on sizing and the available margins for each scenario.

To do this, we carry out aerodynamic modelling of the networks incorporating the geometric characteristics of the circuits, temperatures, and equipment characteristics. Supplier data for the fans — flow/pressure curves, efficiency, operating range — as well as control devices, such as frequency converters, can be integrated into the models.

These models allow for the rapid simulation of the different configurations and sizing scenarios of the project, to verify the network balance and to quantify the available margins. They can also be used to define a pre-setting of the networks before the start-up tests and to assess the necessary leak rates to maintain the vacuum cascades.

In certain accidental situations, such as the vaporisation of a pool containing nuclear fuel, the ventilation systems and evacuation devices must be sized to absorb and evacuate the masses and energies involved. An envelope approach may consist of instantaneously considering the total available energy. While this allows for an increase in the phenomenon, this assumption can lead to very significant stresses and a penalising sizing of outlets or protection devices, such as burst membranes.

Dynamic modelling, on the other hand, allows for the consideration of the different thermal phenomena that govern the kinetics of the phenomenon: heat transfers, temperature evolution, progressive vaporisation, and exchanges with surrounding structures and equipment. The energy and mass released are then calculated during the transient rather than being considered as instantaneously available.

We conduct these studies to quantify the actual loads transmitted to the systems and assess their impact on sizing. This approach allows for a better distinction between physical margins and conservatisms related to calculation assumptions, and to optimise the sizing of outlets and protection devices while maintaining the necessary margins for safety.

Some installations have intrinsically heterogeneous configurations, for which a system approach no longer allows for sufficiently fine representation of physical phenomena. This is particularly the case for hoods, transformers, or rooms with hydrogen-related risks. Flows, heat exchanges, and mixing phenomena can then strongly depend on the geometry and spatial distribution of sources.

CFD studies allow for the representation of these phenomena in three dimensions to verify design criteria and validate sizing choices. They particularly enable the identification of critical areas, characterisation of temperature or concentration heterogeneities, and quantification of available margins.

Nous utilisons le code EDF Code_Saturne, développé par la R&D d’EDF, pour réaliser ces simulations et justifier les conceptions. Notre expertise ne se limite toutefois pas à l’utilisation du code : nous développons également des fonctionnalités au sein de Code_Saturne afin d’adapter les modèles aux problématiques rencontrées et de réduire les temps de calcul. Cette maîtrise du code et de ses modèles nous permet de répondre rapidement à des besoins de justification par la CFD.