Metallographic Assessment of the Thermal Exposure to the Three Mile Island Unit 2 Reactor Lower Head
Mr. George Vander Voort
The accident at Three Mile Island on March 28, 1979 was the worst nuclear accident in US history.
One of the tasks of the International TMI-2 Vessel Investigation Project was to assess the integrity of the vessel.
By January 1990, it was possible to electrochemically machine coupons from the lower head using a specially designed tool.
This is a progress report on the quantification of changes in both the degree of carbide precipitation and delta-ferrite content and shape in the vessel cladding as a function of temperature and time to refine the estimates of the maximum temperatures experienced.
The lower head of the reaction container was constructed of 5" thick A533B Steel with a 5mm thick #308 Stainless steel claddng. The TMI-2 committee had extracted, decontaminated, and evaluated 15 samples from the lower head using general etching and evaluation procedures. Dr. Vander Voort extended this work by selectively etching the stainless and steel portions, and using programmable electron beam microscopy together with advanced image analysis techniques to refine estimated of temperatures achieved during the accident as well as rates of heat travel during the potential melt down.
Specifically, heating to critical in the steel converted the original tempered bainite structure into "untempered" bainite. In the stainless, temperature could be mapped by the degree of delta ferrite which had spherodized, the amount of an accumulated carbide band (at the steel/SS interface) that was taken into solution, and the size of grains resulting from subsequent grain growth. The path of delta ferrite conversion proved very sensitive to temperature differences in the range of 1050 - 1100C. The volume fraction of carbides was a good indicator for temperature 800 - 1000 C.
The maximum temperature seen by the reactor lower head was about 1100C. This agrees with the findings of the TMI-2 committee. In some samples, the refined metallography found that temperatures higher than originally determined were experienced.