Reactivity is not directly measurable, and therefore most power reactors procedures do not refer to it, and most technical specifications do not limit it. Instead, they specify a limiting rate of neutron power rise (measured by excore detectors), commonly called a startup rate (especially in the case of PWRs).
The reactor startup rate is defined as the number of factors often that power changes in one minute. Therefore the units of SUR are powers of ten per minute or decades per minute (dpm). The following equation gives the relationship between reactor power and startup rate:
n(t) = n(0).10SUR.t
SUR = reactor startup rate [dpm – decades per minute]
t = time during reactor transient [minute]
The higher the value of SUR, the more rapid the change in reactor power. The startup rate may be positive or negative. If SUR is positive, reactor power is increasing. If SUR is negative, reactor power is decreasing. The following equations give the relationship between reactor period and startup rations:
Suppose keff = 1.0005 in a reactor with a generation time ld = 0.01s. Calculate the reactor period – τe, doubling time – DT, and the startup rate (SUR) for this state.
ρ = 1.0005 – 1 / 1.0005 = 50 pcm
τe = ld / k-1 = 0.1 / 0.0005 = 200 s
DT = τe . ln2 = 139 s
SUR = 26.06 / 200 = 0.13 dpm