Containment electrical penetrations current carrying capability. A finite difference heat transfer approach

IEEE Std. 317-1983, supplemented by IEEE Std. 741-1990 provides the criteria regarding the current carrying requirements of conductors inside the Containment Electrical Penetrations for normal operation and for design basis accident scenarios. Appendix A of IEEE Std. 317 provides guidelines for determining the ampacity of conductors inside the penetrations. The heat transfer characteristics of heat generated inside the penetration canister, and modeling of the heat transfer from within the canister of the penetration to the concrete is complex, and if modeled to its physical configuration, gives a lower allowable heat dissipation from the canister than the 30 watts per foot indicated in Appendix A of IEEE Std. 317. The heat generated inside the canister of a penetration is transferred, through conduction, convection and radiation to the containment wall. It is a function of the arrangement of the penetration sleeves, distance between adjacent penetrations, the size of the annulus between the sleeve and the canister, and the relative ambient temperatures onside and outside the containment structure. In addition, the short term temperature withstand capability of the cable insulation Inside the penetration canisters, and that of the concrete need to be addressed, where analysis of the power circuits (e.g. containment recirculation fans and pumps) are required to be operable for an extended duration, subsequent to a loss of coolant accident (LOCA). This paper presents results of the thermal loading, and heat transfer analyses performed by three dimensional modeling of the medium and low voltage power penetrations utilizing “HEATINC6”-a finite difference heat transfer software developed by Oak Ridge National Laboratories for Nuclear applications. Results presented cover computations associated with the allowable Watts/ft budget, the I-t curves, and the current carrying capability of penetration conductors during normal and accident conditions for full load, locked rotor, and short circuit currents for typical cases