During and after the March 1979 accident at Three Mile Island Unit 2, highly contaminated water was released to the reactor building (RB) basement, which submerged basement structures to a depth of 2.59 m. When the water was removed from the RB, the radiation fields in the upper portions of the RB did not decrease as expected. Basement radiation source terms were identified and characterized with strings of personnel thermoluminescent dosimeters (TLDs). The TLD data were then used to model the radiation sources using the ISOSHLD computer code to demonstrate the significance to personnel exposures during subsequent recovery operations within the RB.