HEATREM: design and performance evaluations of an experimental natural circulation loop

This master thesis focuses on the conceptual design of an experimental facility to study the passive heat removal by natural circulation, with focus on the decay heat removal from a nuclear reactor. This study includes the description of the natural circulation phenomena, a literature analysis on the passive heat removal systems already in use in nuclear reactors and the experimental natural circulation facilities already built and operated worldwide, the developments of a simplified steady-state theoretical model to be applied for the preliminary design of an experimental facility named HEATREM (HEAT REMoval) in the Energy Department of Politecnico of Turin, and finally the development of a nodalization of the facility and prediction of its performance by the ATHLET system code (Analysis of THermal-hydraulics of LEaks and Transients). First of all, a system, consisting of only one loop, was simulated by a lumped parameter steady state model; the model and its implementation in Matlab is described by equations and flow charts. Parametric studies were carried out with this model to investigate the effects of the geometry and operating parameters as pressure, compact heat exchanger pressure losses, loop thermal power, pipe diameter and heat source-sink height difference on natural circulation flow rate and heat source inlet and outlet temperature. Afterwards, a second loop has been introduced: a compact heat exchanger has been chosen to allow the heat transfer from the primary to the secondary loop. This double-loop system was first modeled by a steady-state simplified model. The results highlighted the capacity for both systems to operate in natural circulation conditions remaining in single phase. The whole system performances have been studied and the effects of the variations of pipes diameter, thermal load and compact heat exchanger pressure losses on the natural circulation flow rate and heat exchanger inlet and outlet temperatures for both primary and secondary loop have been analyzed. Based on the results of this preliminary study, the geometry and operating conditions of lumped parameter model loop have been chosen and used in the development of a model by ATHLET. The simulation of the lumped parameter system by ATHLET allowed to better describe the phenomena and the transient behavior during the start-up phase of the facility and the steady-state at the end of its operation.