Page 184 - Dynamics and Control of Nuclear Reactors

13.9 The power flow map and startup 181

FIG. 13.8
Closed-loop frequency response amplitude for various values of K.
Fig. 13.9 shows the feedback paths that determine stability.
Instabilities of different nature can occur. These include single channel instabil-
ity, in-phase core-wide instability (the whole core responds in unison) and out-of-
phase core instability (different regions respond out-of-phase with one another).
The processes involved in coupled thermal-hydraulic and neutronics are very
complex and not amenable to a simple analysis. Very detailed computer codes are
necessary and several analysis codes have been developed.
Many publications have addressed the BWR stability problem, its analysis and
its mitigation. The magnitude of the effort to deal with BWR instability illustrates
the importance of the problem. Details may be found in the literature (see Refs.
[2, 4, 6–10]).
The typical conditions for instability are power levels of 35–60% and core flow
rates of 30–45%. The strategy for avoiding instability is to avoid operation in the
range of reactor power levels and core flow rates where instability occurs. Specifi-
cally, the reactor power is kept below the instability threshold at low core flows by
using control rods to adjust power.

13.9 The power flow map and startup
A reactor with a negative power coefficient (such as a BWR) experiences a specific
new steady state power following a reactivity change (see Chapter 7). Since BWR
externally controlled reactivity depends on control rod positions and recirculation
flow there are many different paths to change reactor power. Control rods are used
for reactivity adjustment to keep power low at low flows to avoid the instability