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.