The reactivity-initiated accident (RIA) accident has stirred wide interest for the need of designing advanced and high tolerance fuels for next generation nuclear power plants. For pressurized water reactors (PWRs), the accidentally induced reactivity pulse adds transient energy input to the fuel. As a result, boiling may happen locally. Boiling heat transfer usually is efficient and sufficient to absorb the heat until boiling crisis occurs. When the input heat flux reaches the Critical Heat Flux (CHF), the boiling behavior may shift to film boiling which cannot remove the heat addition effectively. Core melting may be a direct outcome. Unlike steady-state CHF – which has been widely studied – the power transient CHF is still not well characterized.
Using an exponential function with certain time period to simulate the RIA pulse, current work aims to study the influence of the exponential period and other systematic parameters to the power transient CHF problem.
For decades, quasi-steady approaches were used to extend the steady state CHF models into transient CHF problem under the assumption that they are based on the same driving mechanism. However, a recent study shown that under vary rapid power increases (an exponential ramp with a period on the order of milliseconds), the Heterogeneous Spontaneous Nucleation (HSN) may be a new trigger mechanism. This phenomenon describes an instant generation of massive bubbles on the surface and a vapor film is formed directly without reaching Departure from Nuclear Boiling (DNB). While for a heating ramp on the order of seconds, the transient behavior is essentially the same as steady-state CHF and the conventional hydraulic instability (HI) happens to be the driving mechanism. While there must be a transition between these two mechanisms as the heating ramp period varies, when it transitions and why remains unclear.
This study experimentally examines the power transient CHF phenomenon under a wide range of heating periods and degrees of subcooling. The CHF was measured and observed through a photographic study. The HSN induced direct to film transition phenomenon was confirmed to occur at transients on the order of milliseconds. The conventional HI induced CHF was also observed in saturated boiling tests on the order of seconds. However, for the subcooled boiling test, the conventional HI phenomenon was not observed. Instead, the irreversible dry spot mechanism is the only observed mechanism. For subcooled transient boiling, the surface temperature overshoot phenomenon is constantly reported by previous researchers and was observed by current study as well. It is discovered in this study that the overshoot phenomenon is strongly related to the non-condensable gas trapped in surfaced cavities and a pre-boiling process is effective in eliminating the phenomenon.
The transition between the HSN mechanism induced CHF and the non-HSN induced CHF is also observed and characterized in this experiment for both saturated and subcooled boiling cases. An analytical approach is proposed to predict the occurrence of the transition between mechanisms, by comparing the bubble departure size and the average distance between nucleation sites. The model successfully predicts the occurrence of mechanism transition for 25K and 50K subcooling cases. Although discrepancy between the proposed model and test results is found for saturation boiling case and 70K subcooled boiling case, qualitative discussion is reasoned and recommendation for future work is provided.