Being a LWR most of the basic reactivity characteristics of the BWR are similar to those of the PWR (negative void coefficient, etc.). However, the fact that it is a direct cycle boiling system does mean that the overall response characteristics are subtly different. This is best illustrated by considering the response to a step increase in turbine demand.

10.12 Reactor water cleanup system (Source: USNRC).

The initial response to an increase in steam demand is a fall in the steam line pressure and a consequent increase in the steam generation rate. In a PWR this will increase the evaporation in the steam generator leading to a fall in level and increased cooling of the primary coolant. This will reduce the temperature of the coolant entering the core, which will increase the core power, a response which will be reinforced by the control system. The control system will increase the steam generator feed flow and reactor power to match the demand change, but the natural characteristics of the system aid this.

In a BWR the increased evaporation will be in the core, which will increase voidage and thus reduce reactivity and hence core power. Thus the control system must compensate for this power drop as well respond to the increased demand. This can be achieved by increasing the core flow as well as by moving control assemblies. Increasing the flow will raise the boiling boundary increasing moderation and hence power.

Because the coolant boils in the core the power production and reactivity characteristics are not uniform, because the moderator feedback changes with core height. The majority of the power is produced in the lower parts of the core where moderation is more effective. This is the reason why BWR control assemblies enter from the bottom of the core. The control and shutdown assemblies are positioned where small movements have relatively large effects so that they will be immediately effective as they start to insert. A normal gravity driven system with the control assemblies positioned above the core would have poor control characteristics since they would be relatively ineffective until they reached regions of higher water density. Thus the control assemblies are bottom entry to be more effective. These are discussed in the next section.