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Core-Wide (In-Phase) Stability of Supercritical Water-Cooled Reactors - I: Sensitivity to Design and Operating Conditions

Jiyun Zhao, Pradip Saha, Mujid S. Kazimi

Nuclear Technology / Volume 161 / Number 2 / February 2008 / Pages 108-123

Technical Paper / Thermal Hydraulics /

Using a three-region supercritical water flow model, the core-wide in-phase stability of the U.S. reference supercritical water-cooled reactor (SCWR) design is investigated. The reactor core is simulated as three channels according to the radial power distribution. A method based on modes (reactivity modes) expansion of neutronic kinetic equations is applied. A constant pressure drop boundary condition between the feedwater pump and the turbine control valve is assumed. Cases with and without water rods heating are studied.

It is found that the stability of the U.S. reference SCWR design is sensitive to the flow restrictions in the hot fluid or the steam line. As long as the restriction in the steam line is small, the design will be stable. A pressure loss coefficient of 0.25 is assumed for the exit valve on the steam line in this analysis. With this value, the SCWR is stable with a large margin. It is concluded that the presence of water rods heating will reduce the stability margin and increase the flow rate sensitivity while maintaining the power sensitivity level.

The decay ratios for the three density wave oscillation modes, i.e., single hot channel, coupled neutronic out-of-phase and in-phase, are compared at steady-state conditions. It is found that the single hot channel oscillation mode is the most limiting one in the absence of the water rods heating, while the in-phase oscillation mode is most limiting in the presence of water rods heating.

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