Analysis of Instrument Tube Ruptures in the Zion-1 Pressurized Water Reactor

Abstract

In Westinghouse four-loop pressurized water reactors (PWRs), many long small-diameter instrument tubes are employed to route flux monitoring instrumentation lines from the lower plenum of the reactor vessel to a flux mapping seal table. A recent safety concern is that a seismic event could hypothetically rupture instrument tubes at the seal table, effectively causing a lower plenum small-break loss-of-coolant accident (SBLOCA). Continued cooling of the reactor core during a SBLOCA requires depressurization of the primary coolant system such that emergency core cooling (ECC) injection flow balances the break flow. For a lower plenum SBLOCA, the break remains liquid-covered, thus retarding primary coolant system depressurization. As a result, for continued cooling of the core, the break must be sufficiently small such that ECC flow balances break flow at elevated pressures. This study investigates instrument tube ruptures at the seal table location. Separate effects analyses investigate instrument tube pressure and heat loss, instrument lines remaining within the tubes, and tube nodalization effects. Systems effects analyses evaluate the significance of the safety concern through a best-estimate, single-failure analysis for the Zion-1 PWR.