Three Mile Island Unit-2 core status summary: a basis for tool development for reactor disassembly and defueling

Abstract

The accident at Three Mile Island Unit-2 (TMI-2) on March 28, 1979 caused extensive damage to the core. A variety of analyses were performed using three general approaches to determine the extent of core damage. First, thermal-hydraulic events were reconstructed using available data, thermal-hydraulic principles, and computer analyses. Second, determinations of tne hydrogen generated yielded estimates of tne amount of zircaloy oxidized and embrittled. Third, the type and quantity of fission products released during the accident were used to estimate the location of core damage and the fuel temperatures which were achieved. Uncertainties exist in each type of determination due to the equivocal nature of the data. Thus, the purpose of this paper is to review and summarize the core damage assessments which have been made, identify the minimum and maximum bounds of damage, and establish a "reference" description for the current status of the core. The different degrees of damage present in the reference core will be considered during development of contingency tooling and procedures for inspection, sample acquisition, and defueling of the core. From reconstruction of the thermal-hydraulic events, it was concluded that the core remained covered up to 100 minutes into the accident and that most of the damage occurred during the period from 100 to 210 minutes when the core is thought to have been uncovered. Damage to the core is a strong function of the time-dependent steam-water mixture level in the core which was greatly affected by the net makeup flow during this period. Cladding reached temperatures between 1030 K (1395°F) and 1150 K (1610°F) and failed by ballooning between 137 and 142 minutes. The cladding continued to increase in temperature, becoming oxidized and embrittled. The fuel reached peak temperatures varying between 2000 K (3140°F) and 2900 K (4760°F). At temperatures greater than 2175 K (3455°F), uranium dioxide fuel pellets in contact with molten cladding could nave been dissolved by the zircaloy, forming a liquid phase of zirconium-uranium-oxygen termed "liquified fuel." The temperatures achieved are also high enough to melt 1 \/

TMI-2 Reactor Building source term measurements: surfaces and basement water and sediment

“…Based on TRAC computer code calculations, temperatures of the fuel rods rose to peak values of between 2130 and 2330°C before high pressure injection was resumed. 24 When the control rods began to fail, presumably between 1399 and 1454°C, Cd vapor and liquefied alloy would have escaped from the rods. The boiling temperatures of Ag and In at atmospheric pressure are 2212 and 2000°C, respectively, but the high pressure in the RCS would have suppressed the boiling of the two elements when temperatures in the core reached these levels.…”