Complex sentence: The previous two tests had indicated that the produced natural gas would be too radioactive for safe use; the Rio Blanco test found that the three blast cavities had not connected as hoped, and the resulting gas still contained unacceptable levels of radionuclides.
Split into simpler sentences: The previous two tests had indicated that the produced natural gas would be too radioactive for safe use. But the Rio Blanco test found that the three blast cavities had not connected as hoped, and the resulting gas still contained unacceptable levels of radionuclides.