Research On Crack Problems Of The Main Shaft In A Reactor Coolant Pump Under Cyclic Thermal Load

The Nuclear Reactor Coolant Circulating Pump, referred to as the Nuclear Main Pump, is the main boundary subjected to the pressure in the Reactor Coolant System of the nuclear plant, and is the second safe barrier of radioactivity in the nuclear plant. As the continuous source in the nuclear plant, the Nuclear Main Pump operating safely and stably is critical to the transport of the reactor coolant as well as the cooling of the reactor core. Under the service condition, the temperature fluctuation of the cool coolant, which is used to cool the stator and rotor of the canned motor, is able to cause its volume fluctuation resulting the mixing of the hot and cool coolant in the mixing area of the main shaft, which is responsible for the thermal fatigue cracks on the main shaft surface. Therefore, research on the evolution and arrest of the surface cracks on the main shaft and the shielding effect of crack networks is of great significance to the engineering applications.In the paper, the sophisticated thermal boundary condition is simplified as the sinusoidal load, the effect of the fluid boundary layer is characterized by the convective heat transfer coefficient between the coolant and the shaft, and the three dimensional issue is simplified as the plain strain problem considering the specific operating condition investigated.Under the help of the ANSYS platform, the transient heat transfer analysis of the main shaft without cracks is performed, and the transient temperature distribution, the radial distribution of temperature of the main pump and its influential factors are studied, followed by the transient thermal stress analysis, and the transient thermal stress of shaft, the circumference thermal stress distribution along the radial direction as well as its impact factors are investigated. FEM of the main shafts with different depths of cracks are established, the transient thermal stress intensity factor(SIF) of the main shaft is studied through the APDL function in the ANSYS software. The effects of the thermal load’s frequency and amplitude and the convective heat transfer coefficient on the change of the maximum SIF of the main shaft with respect to its crack depths are investigated. The results show that,(1)Under certain sinusoidal cyclic thermal load, a decay on the temperature, thermal stress and the thermal stress intensity factor of the main shaft is observed compared with the cyclic thermal load, and there is a transient evolution before they finally reach the steady state. The higher the thermal frequency is, the more cycles they need to go through before reaching the steady state.(2)Increasing the frequency, decreasing the amplitude and reducing the convective heat transfer coefficient within certain range helps to reduce the maximum thermal stress as well as its fluctuation range on the surface of the main shaft, and restrain the innovation of the surface cracks consequently.(3)The frequency of the cyclic thermal load is the decisive factor of the arrest depth of the surface crack, and the arrest depth goes up significantly as the frequency drops. Increasing or decreasing the convective heat transfer coefficient leads to increasing or reducing the maximum SIF of the main shaft but do not affect the curve shape of the maximum SIF of the main shaft with respect to its crack depths. The amplitude has marginal influence on the arrest depth of the main shaft when the frequency is high, compared with noticeable effect when the frequency is low. However, as long as the amplitude is lower than a critical value, deep cracks do not appear on the main shaft surface even if the frequency is low.FEM of the main shaft with crack network including two and three radial cracks are built. Apart from the relationship between the shielding effect and the number of the radial surface cracks, the impacts of the angle between the main crack and the secondary cracks as well as the depth ratio between them on the shielding residual percent of the main crack in the crack networks, and the effect factors, including the features of the thermal load and the convective heat transfer coefficient, of the shielding residual percent of the main crack are studied.This thesis probes that the root reason of the deep surface cracks of the thermal mixing area in the nuclear main pump is the frequency of the thermal load, providing essential data for precautions to avoiding the deep surface cracks.