Patent Number: 061817597
Section: claims

1. A method for determining the closeness to criticality of a nuclear reactor during start-up, comprising the steps of: completing a control rod withdrawal step, thereby generating a change in an output signal of a neutron detector;  measuring the output signal after the completion of the control rod withdrawal step and during a transient portion of the output signal;  calculating the effective neutron multiplication factor (K.sub.eff) based upon the measured output signal and elapsed time between the output signal measurements; and  determining the closeness to criticality of the nuclear reactor based upon the calculated value of the effective neutron multiplication factor K.sub.eff,  wherein the step of calculating the effective neutron multiplication factor (K.sub.eff) based upon the recorded output signals and elapsed time between at least two of the recorded signals, comprises the step of:  .beta.=effective 1-group delayed neutron fraction,  .lambda..sub.eff =effective 1-group neutron precursor decay constant, ##EQU6##  t=a time after completion of the control rod withdrawal step,  n(t.sub.s)=the value of the output signal measured at a time t.sub.s following completion of the control rod withdrawal step,  n(t)=the value of the output signal measured at a time t following completion of the control rod withdrawal step, and  n(.infin.)=the value of the output signal measured after neutron flux stops increasing.  determining the time required for the output signal to achieve a specific fraction of an equilibrium signal level at the calculated value of the effective neutron multiplication factor (K.sub.eff).  measuring the relative change in the output signal occurring in a predetermined time after the control rod withdrawal step. 2. The method of claim 1, wherein the step of determining the closeness to criticality of the nuclear reactor based upon the calculated value of the effective neutron multiplication factor (K.sub.eff), comprises the step of: 3. The method of claim 1, wherein the step of determining the closeness to criticality of the nuclear reactor based upon the calculated value of the effective neutron multiplication factor (K.sub.eff), comprises the step of: