The Reactivity Worth of Protactinium-233 Inferred from Measurements

In a thorium-fueled reactor, the conversion of ²³²Th to ²³³U involves the intermediate nuclide ²³³Pa. This isotope has a significant reactivity effect in any thorium reactor, especially one with an important epithermal flux component. The light water breeder reactor, operating in the Shippingport atomic power station, is a ²³³U-Th reactor with about half the power produced at energies above thermal. The reactivity effect of full-power equilibrium ²³³Pa has been inferred from critical position measurements and control element reactivity worths to be ∼2.5% Δρ in this reactor, confirming calculational predictions.