Modelling of pressure distributions over fuel-strings for fuel irradiation experiments in the NRU reactor

A pressure-drop model has been developed for the U-1 and U-2 loops of the NRU reactor. It provides local pressure distributions between instrument locations at the inlet and outlet of each loop. Piping components and fuel-string assemblies are modeled for single-phase and two-phase flows. The overall pressure drop comprises frictional and form losses, as well as changes due to acceleration and gravity. Detailed pressure profiles have been established along the U-1 and U-2 loops at two powers. The pressure loss over the fuel string is typically larger than losses over the inlet, connecting, and outlet piping components. A linear pressure profile has been observed over the fuel string at low power, indicating single-phase flow in the channel. The pressure profile becomes non-linear at high power, signifying the presence of two-phase flow at the downstream end. Different combinations of fuel type in the string appear to have little impact on the pressure profile. This is mainly due to the similarity of pressure-drop characteristics of the fuel types used in fuel irradiation experiments.