Investigation of transient processes in a BN-600 reactor using a quasisteady-state approximation

Abstract

An investigation was made of transient processes caused by rearranging the control and protection system devices: releasing one AZ-P device switching the reactor to reduced power in a regime w.ith the cooling loop disconnected, a disconnected-loop regime with AR1 and AR2 devices introduced into the active zone without releasing the AZ-P device, and determination of the efficiency of the control and protection system devices. The investigation was performed using a quasisteady-state approximation in which the spatial and temporal distributions of the neutron flux were divided into a form function (the spatial characteristic) and an amplitude function (the temporal characteristic). The form function consisted of solving the steady-state two-group diffusion equation in three-dimensional hexagonal geometry [1]. The amplitude function was determined with the usual point-kinetics equations. Feedback effects were taken into account by means of programs for calculating the temperature fields in the reactor and a library of constants organized according to the principle of utilizing previously prepared blocked multigroup microconstants [1]. The principal components of the temperature reactivity'effect included in the problem were the Doppler effect and the effect of the coolant being heated up by reducing the flow rate for a constant input temperature. The form functions were calculated for each time interval in accordance with a program for scaling the macroscopic cross sections of the control and protection system devices as a function of their immersion depth. The kinetic equations were solved by three methods: by a finite-difference method using an implicit scheme and time steps of 10 -5 and 10 -6 sec, by a numerical integration method with a time step of 10 -5 sec, and by a method which neglects the lifetime of the prompt neutrons [2]. The power change in the last case was expressed by means of a linear response function. The difference in the results of the reactor power change in the regime with a disconnected cooling loop, as calculated using the three methods, was - 1%. The deviation of the calculated results from the corresponding experimental power was as high as -4%. Figure 1 gives the calculated results and the experimental curve of the power reduction for the disconnected-loop regime at a power of - 1% of Nno m. The comparison of the calculated results was performed as the first stage of debugging the method of calculation. By the time the calculations were made two tests of the modernized automatic power regulator had been carried out with the cooling loop disconnected at a power of - 1% of Nno m. The available data were used to make a preliminary analysis of the operation of the automatic power regulator in the disconnected-loop regime at a power of 100% of Nno m taking temperature feedback effects into account. An analysis was made of the disconnected-cooling-loop regime without releasing the AZ-P device at a power of 100% of Nno m with the reactor being switched to reduced power by two automatic power regulators (AR1 and AR2). The displacement velocity of the AR1 and AR2 devices in this case was 70 mm/sec. It was shown that in this case the approximately 5 % dip in the reactor power usually observed when disconnecting the loop and releasing the AZ-P device was not present. Algorithms were considered for disconnecting the loop and simultaneously introducing two AR1 and AR2 devices into the active zone in terms of the signal of the reduction of the rotational frequency of one GTsN-1 main circulation pumps by 220 and 160 min -t from the specified value. The results of the calculation are given in Fig. 2.