Metallographic assessment of the thermal exposure to the Three Mile Island Unit 2 reactor lower head

Abstract

The accident at Three Mile Island Unit No. 2 (TMI-2) on March 28, 1979 was the worst nuclear accident in US history. One of the tasks of the international TMI-2 Vessel Investigation Project (VIP) was to assess the integrity of the vessel. By January 1990, it was possible electrochemically to machine coupons from the lower head by using a specially designed tool. These specimens were taken from regions believed to have experienced the highest temperatures, based on the location of the core debris. The specimens contained the ER308L stainless-steel cladding and the A533 grade-B plate material to a depth of about midwall. The microstructures of these specimens were compared to that of specimens cut from the lower head of the Midland, MI, reactor vessel, made from the same grade and thickness but never placed in service. These specimens were subjected to known thermal treatments between 800 and 1100 °C for periods of 1 to 100 minutes. Because the initial comparison work was qualitative in nature, the writer volunteered to quantify microstructural parameters in the control specimens and in those from TMI-2. Furthermore, selective etchants were utilized to better discriminate desired microstructural features, particularly in the cladding. Hardness traverses were conducted using Vickers microindentations, which have revealed more information than the original bulk hardness traverses. Microprobe analysis that has been conducted on a few specimens has revealed new details about the cladding, interface, and heat-affected zone. 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 cladding as a function of temperature and time to refine the estimates of the maximum temperatures experienced.