Abstract: The main subject of study has been the influence of subcooling, of a sine-shaped heatflux, and of a combination of both on the steady-state and dynamic characteristics of a natural-circulation, pressurised, boiling water system. In natural-circulation boiling systems hydrodynamic instahilities may occur at constant power. They appear to arise from a dependenee of the vapour volume production rateuponthe flow-rate as a result of energy conservation and simultaneously the flow-rate depends upon the resident vapour volume in the system as a result of momenturn conservation and continuity. The steam pressures were taken 15.5 and 30 atm, corresponding to saturation temperatures of 200°C and 234°C respectively. Although the experimental results disclose the fact that the transition from stable to unstable behaviour is not accompanied by a discontinuous change of all physical variables, preferenee has been given to a classification of the experiments in steady-state and dynamic measurements. It was preferred to incorporate the transfer fUnctionmeasurements in the dynamic part. Chapter 2 describes the experiences with void measurements by applying the impedance technique in addition to data conceming the loop dimensions and the measuring equipnent. The calibration of gauges has been referred to Maxwell' s theory. Chapter 3 deals with the measurements under steady-state conditions of the inlet velocity, the axial void distribution and pressure drops at different condit i ons of channel power, subcooling at the inlet, fluxshape and pressure. Data reduction was applied to calculate local values of the slipfactor and of two-phase friction. The slipfactor has been represented by adopting the correlations derived by Bankoff and Zuber. The values of two-phase friction were established according to ~~rtinelli-Nelson. Chapter 4 covers the results of the analysis of steady-state noise and of oscillations under unstable conditions in terms of amplitudes and 13 phase differences of ~Pinlet and the axially distributed voids. The conditions were chosen equal to those of chapter 3. The void propagation veloeities have been campared with the expression derived by Neal fram the energy equation. A limited number of transfer function measurements between the channel power as the perturbed quantity, and the dependent variables Apinlet and the various axial voids, supplemented the experiments. The void propagation velocity as a function of frequency has been compared with the expression of Zuber, basedon the theory of kinematic waves. Chapter 5 describes a theoretica! study based on a solution of the familiar laws of conservat ion, · wi th the addition of sui table expres. . sions for the slipfactor, two-phase friction multiplier and the heatdistribution parameter in the region of subcocled boiling. No special attention has been paid to the boiling boundary, the external system and estimated second order effects. For the solution procedure, a profitable application was made of the CSMP-program, developed by IBM, which facilitated the prograrnming of the integration process. The results of the computations were surprising owing to the close agreement with the experimental results with respect to the threshold-powers, the frequency, the destabilising effect of moderate subcooling and the influence of a non-uniform heatflux. Chapter 6 sunmarises the main conclusions to be drawn from the present study. The value of two-phase two-component measurements with the aim of transposition to boiling-water conditions is doubted. The equations of Neal and Zuber for the void propagation velocity are discussed. The influence of a non-uniform heatflux on the system stability·is reviewed and its consequence is stated for dynamic boiling water experiments and for the anticipated stability of steamboilers in general and of boiling water reactors in particular. Where it was possible and profitable chapters and parts of it have been completed with conclusive remarks. Note: A srnall part of the experiments to be reported here is similar to the experiments described by Spigt (S 2). Camparisen between both sets of results is impossible as void-fraction is concerned (see eh. 2.2.). Other results can 14 deviate some per cent. owing to the use of different heatilJ,g elements.