A natural circulation (NC) pressure tube type boiling water reactor may be susceptible to type I and type II density wave flow instability [1] at low and high power, respectively. In general, inlet flow resistance has a stabilizing effect on the type I instability under a forced-circulation system [2] due to an increase in the single-phase pressure drop. However, in the case of NC, inlet friction may not always stabilize the two-phase instabilities as the NC flow rate decreases with an increase in inlet friction, culminating in a relatively high power-to-flow ratio, which is the typical cause of density wave instability. To what extent this inlet friction can impact the occurrence of type I instability is still not fully understood and needs to be investigated for safe operation of the reactor during low-power condition. In view of this, experiments have been conducted in advanced heavy water reactor (AHWR) thermal hydraulic test facility (ATTF) test facility installing orifices of loss coefficient value of 300 at the inlet of both the channels. It was observed that the inclusion of orifice at the inlet of the channels provides an overall stability to the system, and type I oscillations dies out.
