Apparatus for and method of monitoring for breached fuel elements

Claim 1

In the operation of a nuclear power reactor having a core, cladded fuel in the core, a coolant flowing through the core over the cladded fuel, and heat exchanger means to cool the coolant and obtain useful energy thereby, an improved safety control to detect if any fuel cladding has a breach that allows delayed-neutron emitters to enter the coolant, the combination comprising means for conveying the coolant from the core through a loop flow circuit and back to the core, at least three separate delayed-neutron detectors and means for mounting each detector proximate the flow circuit operable to count the delayed neutrons released by emitters in the coolant when passing the detector, the first detector being located downstream from the core a distance corresponding to coolant flow time therebetween of between fifteen and forty seconds and the second and third detectors being spaced apart and downstream from the first and second detectors, respectively, each corresponding to coolant flow time therebetween of between 1 and 3 seconds, the delayed-neutron activity at the detectors being a function of the delay time after reaction in the fuel until the coolant carrying the delayed-neutron emitter passes the respective detector, where T TOTAL =T h +T t +T d , means for calibrating the detectors during calibration operation of the reactor to determine T d and at least one of T h and T t with respect to measured delayed-neutron counts on the detectors for the calibration fuel, and means for monitoring the detectors during reactor operation and for making repeated comparisons for the best fit line by the method of regression approximation of the delayed-neutron counts measured at the respective detectors and the delayed-neutron counts approximated for each detector according to the equations: ##EQU5## where A dn i =Total delayed neutron activity at each detector attributable to isotope i, in counts/second/cm 3 of coolant; A dn TOTAL =Total count of delayed neutrons at any detector, in counts/cm 3 of coolant; λ P i , λ D i =Decay coefficient of parent and daughter isotopes of species i, in seconds -1 ; S P i , S D i =Fission production rates of parent and daughter isotopes of species i, in fissions/second; P n i =Probability of emission of a delayed neutron for each daughter isotope, dimensionless; and F=Flow rate of primary coolant through the reactor core, in cm 3 /second; T h =Isotopic holdup time of delayed-neutron emitter in moving from reaction in the fuel to the coolant, in seconds; T t =Transit time of the delayed-neutron emitter in the coolant in flowing from breach to the inlet of the detector loop flow circuit, in seconds; and T d =Transit time of the delayed-neutron emitter in the coolant in flowing from the inlet of the detector loop to the detector, in seconds.