Scoping calculations for TMI-2 lower head instrument tube failure

Tube ejection and tube rupture were both examined as possible failure mechanisms of the TMI-2 instrument tubes. Failures by creep or from exceeding the ultimate strength of the tube material were considered for both mechanisms. Metallurgical examinations indicated that welds did not melt; therefore, margins to failure for the weld were calculated. Tube weld failure results show large margins to failure, making a tube ejection analysis unnecessary. Temperature and pressure conditions in ex-vessel tubes were required for the tube rupture analysis. Evaluation of melt flow through the tubes revealed that melt did not penetrate outside the lower head. Thus, ex-vessel tube failure analyses used coolant temperatures, along with an appropriate pressure condition. A force equilibrium calculation showed that very large margins to failure exist for tube rupture. 4 figs., 13 refs