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Core-Wide (In-Phase) Stability of Supercritical Water-Cooled Reactors - II: Comparison with Boiling Water Reactors

Jiyun Zhao, Pradip Saha, Mujid S. Kazimi

Nuclear Technology

Volume 161 / Number 2 / February 2008 / Pages 124-139


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To compare the stability features of a supercritical water-cooled reactor (SCWR) design with that of a typical boiling water reactor (BWR), a stability analysis model for a typical BWR has been developed in addition to an already-developed model for the SCWR as presented in a companion paper. The homogenous equilibrium two-phase flow model, which is adequate at high pressures, is applied to the BWR stability analysis. The reactor core is simulated by three channels according to the radial power distribution and the inlet orifice coefficients. Similar to the SCWR model, the neutronic kinetic equation is expanded based on modes (reactivity modes). The model is evaluated based on the Peach Bottom Atomic Power Station stability test data, and the results agree well with the experiment.

The SCWR is found to be less sensitive to the coolant density neutronic reactivity coefficient than the typical BWR, since most of the neutronic moderation function is provided by the water rods, where the density variation is either zero (if the water rods are insulated) or small (if the water rods are not insulated). The BWR is found to be less sensitive to changes in power level than the SCWR but has the same sensitivity level to the flow rate as the SCWR.

A stability envelope that combines the single-channel and in-phase stability modes is developed. The decay ratios for the SCWR together with those for the typical BWR and the new Economic Simplified Boiling Water Reactor at nominal operational conditions are shown in the map. The stability sensitivity to operating conditions is also shown in the map, by increasing the power to 120% of nominal value and decreasing the flow rate to 80% of nominal value. It is found that the SCWR is more sensitive to the single-channel stability compared to the core-wide in-phase stability for all cases.

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