TMI-2 Vessel Investigation Project integration report

Abstract

The Three Mile Island Unit 2 (TMI-2) Vessel Investigation Project (VIP) was an international effort that was sponsored by the Nuclear Energy Agency of the Organization for Economic Cooperation and Development. The primaiy objectives of the VIP were to extract and examine samples from the lower head and to evaluate the potential modes of failure and the margin of structural integrity that remained in the TMI-2 reactor vessel during the accident. This report presents a summary of the major findings and conclusions that were developed from research during the VIP. Results from the various elements of the project are integrated to form a cohesive understanding of the vessel's condition after the accident.

TMI-2 - A Case Study for PWR Instrumentation Performance during a Severe Accident

“…During attempts to unclog a pipe leading from the full-flow demineralizers downstream of the condenser, the accident was initiated by a shutdown of secondary feedwater flow due to a trip of the condensate booster pumps followed by a trip of the feedwater pumps. Best estimates for plant data and events during the accident, as obtained from various post-accident evaluation programs, 6 are depicted in Figure 2 , the steam generator boiled dry; and the resultant reduction of primary-to-secondary heat exchange caused the primary coolant to heat up, surge into the pressurizer, and increase the primary system pressure. The Pilot Operated Relief Valve (PORV) opened to relieve pressure when the RCS pressure reached 15.7 MPa.…”

“…The event differed from a typical small break LOCA in that the pressurizer liquid level remained high. This was incorrectly interpreted by the reactor operators 4,6 as indicating that the RCS was nearly full of water, when in fact, the RCS was continually losing its water inventory. Emergency core cooling was reduced by operators to address their concerns about a full RCS.…”

“…The steam inventory in the primary system piping increased to such an extent that RCS pumps were tripped by the operators to prevent permanent damage from pump cavitation after 100 minutes. 4,6 At the time that pump operation ceased (see Figure 2-3), increases in Source Range Monitor (SRM) count rate and coolant system temperature and pressure, suggest that the reactor vessel liquid level had decreased. Studies correlating the response of the SRMs with the core liquid level suggest that core uncovery began between 114 and 120 minutes and that the liquid level had dropped to the core midplane by approximately 140 minutes.…”

Corium Behavior in the Lower Plenum of the Reactor Vessel Under Ivr-Ervc Condition: Technical Issues

“…In the TMI-2 accident, the hot spot in the lower head vessel was generated by corium attack in spite of water existence in the lower plenum of the reactor vessel [37].…”

“…However, the molten core material did enter the instrumentation tube central chamber and penetrate significant distances outside the reactor vessel. The water inside the annulus between the ICI nozzle and the thimble tube was important for the integrity maintenance of the reactor vessel penetrations, which did not result in any failure of penetration in the TMI-2 accident [37].…”