Effect of Pressure Tube Radial Creep on Thermal Hydraulic Performance of Pressure Tube Type Nuclear Reactors

摘要

In pressure tube type nuclear reactors, the pressure tube is the pressure retaining component in the core and is subject to high pressure and temperature in addition to neutron flux. This causes the pressure tube to creep in radial as well as axial directions. Axial creep is generally taken care of by allowing axial movement of the pressure tube assembly. This paper discusses the effects of radial creep on thermal hydraulics in horizontal pressure tubes. Broadly, radial creep causes redistribution of coolant flow within the subchannels of a rod bundle. The redistribution with a significant flow bypass affects the thermal hydraulic conditions in the interior subchannels thereby influencing Critical Heat Flux (CHF) significantly. Often these variations are undesirable, leading to reduction in thermal margins. Thus there exists a permissible value of radial creep based on thermal margin considerations. The exact effects of pressure tube radial creep on thermal hydraulics, however, depend on various factors viz., local peaking factors, mode of coolant circulation in nuclear reactor (I.e., pumped circulation or natural circulation) etc. The paper also discusses the effects of various factors on the thermal hydraulic performance of pressure tube type nuclear reactors under crept conditions.