The Research Of Temperature Field Reconstruction In Reactor Core Of Nuclear Power Plant

In PWR nuclear power plants, the heat released by fuel assemblies can be brought out from the reactor by the coolant of the primary circuit system, the size of the coolant outlet temperature reflects heat transfer characteristics between the fuel element and the coolant. Temperature of the fuel element exceeds a predetermined value is one of the major threats of the safe operation of nuclear reactors during the operation of the unit. Although little over-temperature cannot cause serious nuclear accidents, but can cause thermal damage of fuel element cladding materials and can cause loss of fuel element's working life, and if the problem cannot be solved, it will cause serious nuclear leakage accidents. Existing "point" temperature measuring method cannot observe the temperature field distribution of reactor core because of finite temperature measuring points. Even if temperature measuring points are arranged by design parameters such as thermal channels, thermal channels and thermal points can drift with the changes of burn-up, xenon oscillation, control rod positions during actual operation, therefore the maximum temperature of reactor core cannot be monitored. Currently the researches of temperature field reconstruction of conventional power plant has been carried out and got some results, but it cannot be applied to temperature field reconstruction of reactor core. To solve this problem, a domestic nuclear power plant reactor core as research subjects, the calculation model of the core temperature field is constructed with the steady state heat transfer heat conservation equation and modern means of error analysis, and three-dimensional temperature distribution of core coolant and fuel elements can be calculated, details are as follows.(1) In connection with the condition of dispersed arrangement of measuring points of Pressurized Water Reactor coolant outlet temperature, core coolant outlet temperature field can be calculated with scattered data processing method on the basis of analysis on measuring point distribution. First of all, existing temperature measurement points are calculated with a variety of scattered data processing methods, and the two dimensional temperature distribution of coolant outlet is obtained. Secondly, on the basis of comparison of the calculated and measured values, the accuracy and precision of results with various methods can be judged by using error theory to analyze results. Finally, by analyzing data obtained distribution of the coolant outlet temperature, this method overcomes the difficult that suitable scattered data processing methods cannot be found directly on the basis of finite temperature measuring points and less known data.(2) Taking pressurized water reactor core as research object and on the basis of available power model of it, a calculation model for temperature field of nuclear reactor core is established, with heat energy conservation equation in steady-state heat transfer condition combined and numerical calculation methods like scattered data interpolation, fitting, iteration utilized. In view of the fact that the horizontal flow of coolant in the core is much smaller than the vertical flow, the mass, momentum and energy exchange between channels is negligible, so when choosing the flow channel of the coolant within the reactor core, the single channel model is adopted with consideration given both to calculation accuracy and velocity. For convenient data calculation and analyses, nodal method is employed in space as the solution of the model, which specifically means one fuel assembly is taken as one nodal point in radial direction and divided into 16 nodal points in axial direction. In other words, the continuous physical variable in space coordinates system is replaced by a set of limited discrete data and then a discrete solution is got, which avoids no analytic solution caused by the complexities of the geometrical shape and boundary condition of the reactor core. The heat transfer procedure is cleared; the model is simplified and a reasonable method to calculate the temperature of fuel elements is approached if the heat conduction and heat transfer model of reactor core is divided into the following 4 parts, the heat conduction within fuel elements, the heat conduction within the gaps between fuel elements and claddings, the heat conduction within cladding pipes and the heat convection between the outside surfaces of claddings and coolant.(3) Fourier function, Gaussian function, Thin Plate Spline, Radial Basis Function Neural Network and so on, altogether 10 methods are applied to calculate the temperature field of reactor core, among which reasonable methods are selected by comparison between the reconstructed results. First, the outlet temperature of coolant is fitted; the total power of reactor core and the flow of each flow channel are calculated; the temperature of reactor core coolant is positively and reversely derived. After analysis and comparison of the data above, it is indicated that the results calculated by 9 methods including Fourier function, Gaussian function, Thin Plate Spline, Radial Basis Function Neural Network have consistent with the basic law of procedure parameter. Then by application of error theory, the mean values E and standard deviations ? of the absolute error between calculated and measured outlet temperature of coolant are obtained. By comparison the E and ? of selected 9 methods after first step, the conclusion is reached that the reconstructed results of 8 methods such as Fourier function, Gaussian function and Thin Plate Spline satisfy the engineering requirements and verify the feasibility and veracity of methods themselves. The minimum E and ? are both achieved by TPS, which means that TPS is the optimal method to this reconstructed model. On the base of TPS, the two-dimension temperature profiles of inside and outside surface of claddings, the center of fuel pellets and different sections of fuel pellets are obtained, which provide theoretical basis to the real-time monitoring of inner temperature of reactor core.(4)The software of a core temperature reconstruction is prepared running in the matlab environment, the software can implement some features such as parameters input, storage, output and data fitting, iteration and graph. And the use of 9 different burn-up of steady conditions of the software has been tested, and temperature distribution of core coolant and fuel elements can be calculated, this result is consistent with the measured values of the field which verifies the correctness of the core temperature field reconstruction model and the feasibility of reconstruction software.