Experimental Verification And Analysis Of The Probability Of Continuous Fission Chain Induced By CFBR-II Reactor Neutrons

The probability of initiating persistent fission chain(referred to as neutron initiation probability)is one of important characteristic parameters for the field of studying the neutron stochastic transportation and fission chain random process in prompt super critical system,which applies to reactor safety,accident prediction and nuclear engineering.The initiation experiments in burst reactor can directly be used to validate the initiation theory and the initiation probability value in the regime of burst reactor operation,as well as verify the algorithm and parameters used in the stochastic simulation program.The function expression of initiation probability in static and slightly super prompt critical,and the theoretic calculation expression of initiation probability for pulsed neutron source are deduced based on the neutron initiation transportation equation.With the neutron balance principle and the point model,the form of initiation probability is obtained and the internal relation between initiation probability and neutron fluctuation is discussed.The theoretic foundation of burst waiting time distribution in the weak source is described and the equation for interpreting the waiting time distribution is established.The effect of the two order approximation and the applicability of the point model are discussed.The results of the theoretical research lay the foundation of the theory and methodology for the following experiments and simulation.The neutron kinetic parameters,delayed neutron parameters,pulse rod reactivity curve,equivalent strength of spontaneous fission source(454 s-1),reactivity discrepancy and reproducibility for burst experiment are obtained.The measuring technologies with their principle are represented with their uncertainties,deviation,and response delay.The algorithm,physical model and flow chart of analog simulation program are introduced and validated with the spherical HEU data,Godiva-Ⅱ and Caliban experiment data from reference.Simulation results indicate that:the simulation results of Godiva-II,the initiation time is larger than the experiments by 30%,but for Caliban the initiation time is two times larger than the experiments;the delayed neutron strength increases sharply before burst initiation and the average neutron initiation probability,including the intrinsic and delayed neutron source,is consistent with the theoretical results.The burst experimental conditions for CFBR-II in the intrinsic source are established,and the data of reactivity and initiation time are obtained through experiments.Simulation results indicate that:the ramp approximation of reactivity insertion and fluctuation of reactivity insertion time have negligible effect on the initiation time;the simulation results agree with the experimental results,and the mean initiation probability of simulation(2.81 E-4)is consistent with the theoretical results(2.85E-4);the analytical equation of initiation time distribution based on the changing of intrinsic source also agree with the experiment results.The basic principle and methodology of validating initiation probability with external pulsed neutrons in CFBR-II reactor are established,and the configuration of experimental layout as well as the parameters are determined,and the preliminary,first validation and re-validation experiments are carried out.The contribution of the scattered neutron is investigated which makes the equivalent injection source a quite larger by 24.6%,and the uncertainty of the experiment are induced.The results of initiation probability for the first validation and re-validation experiment are 0.344(1 ± 27.6%)and 0.766(1 ± 14.7%)respectively,which agrees well with the theoretical calculation results(0.334 and 0.765,both with uncertainty of 16.2%)when the contribution of scattered neutrons is taking into account.The experimental results directly validate the neutron initiation probability in the burst state of CFBR-Ⅱ reactor.Another experimental method for the initiation probability validation with external pulsed neutron in sub-critical is proposed,and configurations for the layout and the technical conditions are established.The 134 burst experiments are carried out through which probability of initiation before totally insertion(0.804)is obtained,and the step power measurement data show that the contribution of scattered neutron from environments increases the strength of actual injection neutron by 23%.Simulation results indicate that:the effect on experimental results from the fluctuations of both the reactivity in zero time and insertion time is less than 5%;the simulation result(0.739)agrees with the experiment result(0.804);the initiation time distribution equation obtained with the curve of delayed neutron intensity is consistent to the results of experiments and simulation.