Analytical and Experimental Investigation of Natural Circulation Transients in a Large Scale Thermohydraulic Facility

Sammanfattning

This thesis simulates the High-Pressure WAter Test (HWAT) facility at KTH operating at different pressures in its Primary Loop (PL) and Secondary Loop (SL), and Power Output (Q). This aims to identify and map the domain in which a natural-circulation cooling system remains stable and, in doing so, identify potential limitations and risks associated with applying such a system to a small modular reactor that adheres to industry standards. Ultimately, this thesis will seek to establish a robust statistical basis for determining whether natural-circulation cooling can serve as a primary or secondary driving mechanism. This task will be performed using a previously developed GOTHIC Code model of the HWAT. Various search methods were used to optimise simulation time, including random search and binary search. Key results indicate that a stable region of natural circulation exists at lower powers and under specific balances between Primary Loop Pressure (PLP) and Secondary Loop Pressure (SLP). The stable region was mapped in the interval [6,136] bar PLP and [10,110] bar secondary loop pressure. The system’s tolerance for higher powers appears to be dependent on PLP, whilst varying little with PLP. The stable region’s outline was mapped with an upper interval of uncertainty of 2.5 kW. Time plots of Mass Flow Rate (MFR) and Temperature (T) were taken to analyse and categorise different behaviours leading up to oscillations and early terminations of the simulations. Conclusions drawn from this work are as follows. A stable region at lower powers is indicated, sustaining natural circulation for extended periods. Furthermore, a general outline of the region was mapped within an uncertainty of 2.5 kW. Failures at upholding natural circulation were generally categorised by small oscillations in mass flow rate that diverge and cause spikes of temperature. In regard to industrial applications, natural circulation appears so far unfit to serve as the primary driving mechanism for reactor coolant at higher powers. However, there appear to be distinct benefits to incorporating natural circulation into reactor design, primarily to ensure that cooling functions are maintained in the event of a pump failure.