In this issue we take a final look at the Delayed-Critical Reactor Rate Diagram before moving on to other topics. The D-C Rate Diagram is as valid for transients in the Power Range as for transients in the Intermediate Range below the point-of-adding-heat (POAH). That is to say, the same relationship exists between reactivity and reactor rate over the entire Delayed Critical region, whether it be above or below the POAH. Granted, reactor behavior in the Power Range appears to differ from that below the POAH. However, this is not due to any alteration in the relationship between reactivity and reactor rate, but rather due to the influence of reactivity coefficients above the POAH. Below the POAH temperatures and voids tend to remain relatively constant.
Below the POAH, reactivity change occurs only when the operator moves control rods or alters the boron poison concentration. Thus, when off-critical with essentially constant reactivity, power changes exponentially with time; the reactor rate is constant (stable). Above the POAH temperatures (or voids) are only constant when power production matches power demand. Any mismatch between production and demand causes ongoing thermal changes and reactivity coefficients introduce ongoing reactivity change (rho-dot). Hence, when off-critical in the Power Range, this ongoing reactivity change with time always creates a transient reactor rate. Negative reactivity coefficients tend to move reactor power toward thermal balance and in the direction of restoring criticality. The reactivity coefficients preclude exponential power change in the Power Range. The only stable rate when above the POAH is at criticality. In addition, thermal changes initiate and terminate in a gradual manner and are relatively slow. The net result is that constant reactivity is common below the POAH, whether critical or off-critical but above the POAH constant reactivity occurs only when power production and demand are in balance (at criticality).
Figure 21.1 shows the track of reactor rate on the PWR D-C Rate Diagram for a steam flow increase from 20% to 100% rated flow, as generated on THE REACTOR TRAINER.
Reactor power lags behind the steam increase. Reactor rate increases very gradually from 0 DPM to a maximum of +0.14 DPM and begins to decrease even as the steam increase continues. The rate tracks only slightly above and below the stable rate curve. The rate loop on the diagram starts from criticality and rate first moves above the stable rate reference to form the loop in a clockwise direction. Full power is reached at about +0.05 DPM, where the rate curve displays a distinct change in slope. On termination of steam increase, the reactor power increase decelerates until reaching criticality, 0 DPM, at 100% power. We will look at this rate response in real time in a later NUKEFACT.
Figure 21.2 shows the track of reactor rate on the BWR D-C Rate Diagram for a core flow increase from 50% to 60% rated flow. Reactor power and steam flow increase from 20% to 40% of rated.
Steam flow lags behind the increasing reactor power. Reactor period moves upscale from infinite seconds to +300 seconds, before the power increase begins to decelerate, even as the core flow increase continues. On termination of the core flow increase, the power increase continues to decelerate until reaching criticality, infinite seconds, at 40% power. Here, also, the rate forms a loop by moving in a clockwise direction, first above the stable rate reference and then below the stable rate reference. We will look at this rate response in real time in a later NUKEFACT.
In summary, operational reactor transients in the Power Range tend to exhibit slow reactor rates, i.e. small startup rates and large periods, which reflect slowly evolving power change.