[en] The authors are currently studying the feasibility of pool type reactors, for heating commercial buildings. To investigate thermal hydraulic aspects of reactor design, including the stability under extreme conditions, an electrically heated natural circulation loop was designed and constructed. Experimental tests with this loop show that it is stable under all single phase conditions, an expected result. In tests with two phases present in the circuit, different types of behaviour were observed depending on the experimental conditions. These were either stable flow, non-divergent oscillating flow or divergent oscillating flow. All of these type of behaviour were successfully simulated using the newly developed SPORTS stability computer code