Patent Number: 
Section: claims

1. A method of determining the nearness to criticality of a nuclear reactor having a control rod configuration in a core region of a nuclear reactor vessel and a coolant moderator circulating therethrough, comprising the steps of:Monitoring a first source range detector signal (CMC) during a transient portion of the detector signal to obtain a neutron radiation level of the core when the coolant moderator is at a density corresponding to a first temperature;Raising the temperature of the core to a second temperature;Monitoring a second source range detector signal (CMH) during a transient portion of the detector signal to obtain a neutron radiation level of the core when the coolant moderator is at a density corresponding to the second temperature, the monitored first source range detector signal and the monitored second source range detector signal each having a background non-neutron signal component (N);Determining the background non-neutron signal component based upon the monitored first and second source range detector signals in accordance with the relationship  N  =                              {                                                                      K                  eff                  H                                -                                  K                  eff                  C                                                            1                -                                  K                  eff                  H                                                      +                                          R                ⁡                                  (                                      T                    H                                    )                                                            R                ⁡                                  (                                      T                    C                                    )                                                      +                                          F                H                                            F                C                                      -            1                    }                ⁢                  C          MC                    -              C        MH                            {                                                            K                eff                H                            -                              K                eff                C                                                    1              -                              K                eff                H                                              +                                    R              ⁡                              (                                  T                  H                                )                                                    R              ⁡                              (                                  T                  C                                )                                              +                                    F              H                                      F              C                                -          1                }            -      1      Removing the background non-neutron signal component from the monitored neutron radiation level obtained at the second temperature to obtain background adjusted neutron radiation level; andDetermining the nearness to criticality based upon the background adjusted neutron radiation level. 2. The method of claim 1 wherein the nuclear reactor is a pressurized light water reactor. 3. The method of claim 2 wherein the moderator is borated water. 4. The method of claim 1 wherein the control rod configuration and a concentration of the moderator remain unchanged between the monitoring of the first source range detector signal and the monitoring of the second source range detector signal. 5. The method of claim 4 wherein in the step of determining the background non-neutron signal component a term 1/ICRREH is approximated from a ratio of the monitored values of CMH and CMC to arrive at an initial value of N. 6. The method of claim 5 including the step of subtracting the initial value of N from the values of CMH and CMC and calculating the residual value of N. 7. The method of claim 6 including the steps of:Raising the temperature of the core to a third temperature;Monitoring a third source range detector signal during a transient portion of the detector signal to obtain a neutron radiation level of the core when the coolant moderator is at a density corresponding to the third temperature;Determining N from the second source range detector signal and the third source range detector signal using an approximation for 1/ICRREH;Calculating a new residual value of N;Iterating the foregoing process until a final residual value of N is determined which is less than the resolution limit of a measurement of the source range signal; andDetermining a final value of N from the sum of all the values of N used to produce the final residual value of N.