Evaluation of Natural Circulation sustainability for the MYRRAH project

The objective of this thesis was to investigate how fast natural circulation can be established and sustained for different core initial conditions. The main conservation equations were derived for energy and momentum, and the main parameters were taken into account when evaluating which sensitivity studies were best to analyze natural circulation (NC). Noting that the core was potentially the main driver for NC, parametric studies were done outside of the core and the passive heat-exchanger (PHX) in order to see if the loop had any effect on the development of NC. A Matlab 1-D code was developed in order to compare it to the RELAP sensitivity results. In order to improve and iterate on the code, the code results were benchmarked against existing data for lead-bismuth eutectic loops studying natural circulation. Temperature profiles as well as the loop steady-state fluid velocities were compared and analyzed. The first sensitivities were done by changing the hydraulic diameter of the loop outside of the core and PHX region. Limited effects on the development of natural circulation and peak cladding temperature (PCT) were observed. For hydraulic diameters (hd) larger than 0.4m, the evolution of mass flow rate (MFR) and PCT is similar and for hd smaller than 0.4m the development of natural circulation was slower and PCT reached up to 350 Co from a starting temperature of 200 Co. For increasing the loop total length, and thus the mass of the system, a similar trend was observed. Up to a certain horizontal piping length of 6.5 m the evolution of natural circulation and PCT is similar. For lengths larger than 10 meters, a slower development is observed and a higher PCT is reached. This phenomenon is explained by the increase in kinetic energy taken in by the system in the core region in order to establish natural circulation. Since it becomes more difficult to drive flow through smaller piping sections, due to the inertia of the LBE, a smaller hydraulic diameter in the loop will require more energy from the system in order to initiate natural circulation. The same occurs with core length, where the increase in mass requires more kinetic energy in the core region to drive natural circulation. This slower development is what leads to higher PCT. The impact of the system velocity is much greater than the effect of mass since this variable is squared as compared to mass in the kinetic energy equation. Another sensitivity study is done with RELAP for a fuel pin that has a temperature profile similar to one at normal operating conditions. The latent heat of the fuel pin would transfer quickly to the bulk LBE in the core region. PCT reaches almost 800 Co for a core power of 1 MW and 5 MW at around 5-10 seconds before natural circulation can be established. NC is fully developed at around 15-20 seconds. This shows that for ’hot pin’ conditions, the NC does not develop fast enough in order to remove the heat from the core. The PCT in this case is not dependent on the power but on the amount of bulk LBE found in the core that can absorb the heat in this small period of time. Since the starting temperature of the LBE was 200 Co, it is suspected that higher initial LBE temperatures might lead to the PCT being over 800 Co which can ultimately lead to cladding degradatio