Method for monitoring boron dilution during a reactor outage
View Patent ↗A method for monitoring changes in the boron concentration in the coolant of a reactor during a nuclear plant outage that applies temperature compensation to the source range detector output. The method then monitors the compensated output signal to identify changes in the detector count rate above a preselected value.
1. A method for determining a change in boron concentrations in a reactor coolant system as a result of a reactivity change during a nuclear plant outage comprising the steps of:
monitoring an output signal representative of a count rate of a source range neutron detector positioned outside of a reactor vessel within proximity of a core of a reactor, as a function of time, during a plant outage;
monitoring a temperature of a coolant within the reactor coolant system as a function of time;
generating a compensation signal which is a function of the monitored temperature, that when combined with the count rate output signal compensates the count rate output signal for substantially any change in the count rate output signal resulting from a change in the temperature of the coolant;
applying the compensation signal to the count rate output signal to obtain a compensated count rate output signal; and
identifying a preselected increase in the compensated count rate output signal as an indicia of a change in boron concentration.
2. The method of claim 1 wherein the compensation signal is defined as a Downcomer Temperature Attenuation Factor (DTAF) given by the expression:
DTAF( T 1 )= e −(p(T i )−p(T R ))R ;
Where the value of R is a function of distance between the source range detector and the reactor vessel and the effective macroscopic neutron removal cross section between the source range detector and the fuel assemblies on the core periphery.
3. The method of claim 2 wherein a deviation in the compensated count rate output signal (ΔC c (t)) from a selected reference ΔC (ΔC R ) is an indication that a reactivity change is happening or has happened and the value of ΔC c (t) is given by the expression:
ΔC c (t)DTAF(t)−C R .
4. The method of claim 3 wherein the application of error propagation techniques to determine the expected random fluctuations in ΔC c from one monitored count rate output signal set to the next allows the expected range of random fluctuation in ΔC c (ΔCE) to be expressed substantially as:
Δ
C
E
(
t
)
∈
0
±
2
σ
CR
[
1
+
]
1
DTAF
(
t
)
;
Where the value of σ CR is the measured mean deviation of significant population of source range count rate measurements obtained in an interval around time t; the value of C(t) is the mean value of the data used to determine σ CR ; and the number of count rate measurements used to determine σ CR is an operator addressable constant that is a function of the desired maximum value of σ CR needed to obtain a desired reactivity change detection sensitivity.
5. The method of claim 4 including the step of determining if ΔC(t) is outside an expected range of ΔC provided by the equation
Δ
C
E
(
t
)
∈
0
±
2
σ
CR
[
1
+
]
1
DTAF
(
t
)
;
and
identifying that a reactivity change is occurring or has occurred.
6. The method of claim 5 wherein a selected number of consecutive samples of ΔC(t) are determined with a given fraction of the samples being outside the expected range of ΔC before identifying that a reactivity change is occurring.
7. The method of claim 6 wherein the selected number of consecutive samples is approximately ten.