A BWR is an interesting basis for any discussion on instability because so many different modes of instability have been identified. In practice, any instability would actually be a combination of two or more of the following:

a. Simple voidage coupling. This has already been referred to as oscillatory instability in Section 1.5.3.1. Reactivity, power, and voidage rise and, in combination with suitable delays, a reactivity decrease follows; if the delays are unfavorable, the reactivity may increase. A typical frequency would be in the range of 1 to 5 Hz.

b. Hydraulic instability,+ This is nonnuclear and oscillatory with the following sequence of interactions. Following a disturbance to the inlet velocity (say an increase), the boiling boundary rises and the pump head thus required increases. Again, if delays in the circuits are unfavorable, then the resulting decrease in inlet velocity due to pumping inefficiency would come too late. Oscillations would occur with a frequency of about 1 Hz.

c. Pressure variations. A static instability, independent of time delays, was thought to exist. A pressure rise resulting in an inlet enthalpy decrease and thus a boiling boundary rise, would result in a reactivity, a power and thus a voidage increase, and then a pressure rise. The frequency would be of the order of 0.1 Hz. However, this effect is no longer important relative to the others.

d. Ship’s motion. This would perhaps result in instability with a natural circulation BWR because the circulation head would be increased with an upward motion of the ship and vice versa. A 50% oscillation in apparent gravity could result in 100% oscillations in power and 10% in temperatures with a frequency of 0.1 Hz.

e. Parallel channel instability.+ Such an instability would be exhibited between two or more channels having the same pressure difference across them and with the same common inlet and outlet headers so that flow can be divided between them in different fractions. The mass flow can oscillate between them, resulting in different two-phase pressure drops and voidages. It can only occur for exit qualities greater than 20-30%.

Sodium-cooled reactor systems are not subject to these types of instability and, indeed, a sodium-cooled system does appear to have excellent stability. However during accident conditions, unstable conditions might suddenly result. Section 4.4 will refer to sodium chugging after assembly failure, which is a form of static instability.

Stability studies are required to demonstrate that the particular system will be stable under normal and abnormal conditions. Such studies give information on such design features as the necessity for channel inlet gagging (in a BWR to make the total channel pressure drop less voidage dependent), pressure relief systems, or the avoidance of critical time constants in the circuit. Pump characteristics would also be tested for their satisfactory behavior following flow disturbances.

* See Davies and Potter (13).