SUBSURFACE RADIONUCLIDE INVESTIGATION OF A NUCLEAR TEST

This paper reports on an environmental investigation into the vertical distribution of radionuclides from a nuclear test. Dalhart is the nam e of an underground nuclear test that was executed at the Nevada Test Site at a depth of 2100 ft on October 13, 1988. The test occurred belo w the static water level of 1667 ft and created multiple radioactive i sotopes or fission products. These radioactive isotopes penetrated the surrounding formations and chimney region above the test and were ret ained there. A 19-degrees 9-7/8-inch diameter slant hole was drilled t o sample the geologic material in the chimney region above the Dalhart test for the purpose of assessing the distribution of radioactivity i n and around the shot site. A 30-ft core recovered from a vertical dep th of 1628 ft in the collapsed zone or chimney region and above the or iginal static water level was found to be free of radionuclides. Drill ing was completed to a vertical depth of 2156 ft with the present stat ic water level at a vertical depth of 1644 ft. Gamma-ray spectroscopy log measurements, made within the drill pipe while drilling fluid was pumped through this pipe, indicate that radioactive material produced by the test was present from the vertical depth interval of 1746-2156 ft. Side-wall samples acquired from the vertical depth interval of 172 1-2089 ft and analyzed in the field contained radionuclides such as Cs -137, Sb-125, Ru-106, plus the natural radioactive background of potas sium, uranium, and thorium. These samples were sent to Los Alamos to d etermine the complete radionuclide content at each depth. These analys es were used with the gamma-ray spectroscopy logging data to determine the subsurface vertical radionuclide distribution at the Dalhart site .