 The early 1970s included the opening of Disney World in Orlando, Florida and an important political shift between the U.S. and China when President Nixon visited China. This visit improved relations between the two countries and ended 25 years of separation between the U.S. and China. Godfrey Hounsfield revolutionized medical imaging by inventing the X-ray computer tomography scanner, also known as a CT scanner. Hounsfield later won the Nobel Prize for his invention in 1979. A committee of the National Research Council developed estimates of cancer risk after exposure to low doses of radiation. The Biological Effects and Ionizing Radiation Committee, or BEER Committee, used cancer mortality data from groups who were exposed to high doses of radiation such as Japanese bomb survivors to form a linear, no-threshold model of cancer risk for low doses. That same year, the Atomic Energy Commission introduced the safety principle of as low as reasonably achievable, more commonly known as the ALLERA principle, striving to minimize radiation doses. These actions by the National Research Council and the Atomic Energy Commission provided a foundation for the discipline of health physics to be devoted to the protection of workers and the public against small doses of ionizing radiation. In 1974, the Atomic Energy Commission was split into the Energy Research and Development Administration and the Nuclear Regulatory Commission. The NRC would regulate the commercial nuclear power industry and other uses of nuclear materials such as in nuclear medicine to ensure the safe use of radioactive materials for beneficial civilian purposes while protecting people and the environment. ERDA would manage the energy research and development, nuclear weapons, and naval reactors program. By the end of the decade, nearly 70 commercial nuclear power reactors had been licensed by the NRC to provide electricity across the United States. This commercial nuclear power fleet operated without any major incidents for several years. However, on March 28, 1979, an event occurred at the Three Mile Island Unit 2 reactor near Middletown, Pennsylvania that resulted in a partial meltdown of the reactor core. This was the most serious accident in U.S. commercial nuclear power plant operating history, although its small radioactive releases had no detectable health effects on plant workers or the public. Its aftermath brought about sweeping changes to the operation and regulation of nuclear power, including major changes to emergency response planning, reactor operator training, and radiation protection.