You are here : Home > The DSBT > Study of a natural convection experiment for the safety of the new generation of nuclear reactors

Study of a natural convection experiment for the safety of the new generation of nuclear reactors

Page Web française. This internship proposal has been filled, nevertheless if you are interested in this subject and think you have a corresponding profile, do not hesitate to contact us for a spontaneous application.
Published on 14 September 2020
the safety of nuclear reactors is directly linked to their proper cooling. Thermohydraulics is therefore a major safety issue within the CEA. Proper cooling of the reactor cores is ensured by forced convection of pressurized water (PWR system). In the event of an incident, the fall of the control rods abruptly stops the fission reactions. However, some power is still generated in the core, which must be evacuated. In order to further increase the safety of the installation, it would be advantageous if the cooling of the core at standstill be provided by an entirely passive system. This might be achieved thanks to natural convection, as long as we make sure that this process will be efficient enough, to evacuate the residual energy released by the reactor core. This is one of the goals of the new SMR Research and Development program (acronym for "Small Modular Reactors": Figure 1).
Nuclear reactors installed in a pool filled with water for cooling by natural convection.

This new program aims to design reactors smaller in size than existing reactors (typically the power targeted is 200 MW). However, the heat transfer associated with natural convection is a complex phenomenon, as it can become turbulent, and hence difficult to simulate with CFD. To characterize natural convection, we use to introduce the Rayleigh number Ra:

Where α is the coefficient of thermal expansion of the cooling fluid, g the acceleration of gravity, ΔT the temperature difference between the fluid and the core, L the dimension of the experiment, υ the kinematic viscosity, κ the thermal diffusivity of the fluid. Ra describes the ratio between heat transport through natural convection and through diffusion. While the numerical codes are able to model the heat transfer for modest Rayleigh numbers, for our case, where the Rayleigh number ranges well beyond 1012, the flow is strongly turbulent, and hence difficult to model. In addition, there are few data for correlations for Rayleigh numbers beyond 1012. This is why experiments are mandatory. However, an experiment representative of an SMR and using water as a coolant would have the same size as an SMR, and therefore would be decametric. By replacing water with a fluid with somewhat different properties (with a factor (α / υ κ) much greater than water), it would be possible to design an experiment of dimension L much smaller, and therefore less expensive in terms of investment and operation costs. Cryogenic helium seems to be a good candidate. In fact, it has allowed, in an academic context, to study heat transfer at very high Rayleigh numbers (up to 1015), enabling to identify a turbulent regime which had been predicted before only theoretically [1].

Internship subject:
The trainee will identify the existing correlations for the heat transfer through natural convection relevant for the case of SMRs, and their limit of validity. Reference [2] is certainly a tool with which to start this work under optimum conditions. On this basis, and with the accidental situation scenarios, the trainee will determine the Rayleigh numbers involved for the different parts of an SMR (upper plate, external diameter, etc.).
The work then will consist in determining which conditions in cryogenic helium (hot and cold temperatures, height, diameter, power to evacuate, etc.) will allow to achieve similar Rayleigh numbers. For this, the student will have access to the Hepak and Coolprop libraries, which provide the properties of helium as a function of temperature and pressure.
Then, and depending on the skills and/or preferences of the trainee, one of the two following studies will be carried out. (i) A bibliographical study on the knowledge of the properties of helium not far from its critical point would be very useful. Indeed, it will make it possible to specify which additional measurements will be necessary in order to increase our knowledge on the properties of helium. (ii) a preliminary draft of a cryogenic experiment may be proposed, as a first step towards the design and construction of a cryogenic experiment in Rayleigh similitude of an SMR.

[1] Chavanne, X., F. Chillà, B. Castaing, B. Hébral, B. Chabaud and J. Chaussy, « Observation of the ultimate regime in Rayleigh‐Bénard convection », Physical Review Letters 79(19): 3648‐3651 (1997)
[2] Incropera, DeWitt, « Fundamentals of Heat and Mass Transfer » John Wiley & Sons, 2002.

Required skills:
Fluid Mechanics; heat transfer.

Alain Girard,
Jean‐Marc Poncet ,