Norman Carl Rasmussen, a pioneer of probabilistic risk assessment for nuclear power plants, died on 18 July 2003 in Concord, Massachusetts, of complications from Parkinson’s disease.
Born in Harrisburg, Pennsylvania, on 12 November 1927, Norm served in the US Navy from 1945 to 1946. He graduated from Gettysburg College in 1950 with a BA in physics, entered graduate school in the department of physics at MIT, and earned his PhD there in 1956. His doctoral research was in the field of low-energy nuclear physics.
Norm remained at MIT as an instructor in physics until 1958, when he was appointed assistant professor in the newly formed MIT department of nuclear engineering. He was promoted to full professor in 1965, served as head of the department from 1975 to 1981, and was named McAfee Professor of Nuclear Engineering in 1983.
His early research was in the field of radiation detection, including activation analysis, low-level counting techniques, and gamma-ray spec-troscopy. He carried out extensive work on the development of lithium-drifted germanium gamma-ray detectors and their application to problems of gamma-ray spectroscopy.
During the period 1972 to 1975, Norm served the US Atomic Energy Commission and the US Nuclear Regulatory Commission (NRC) as director of a study to evaluate the risks from nuclear power plant accidents. The final, multivolume report resulting from that study was issued in 1975 and is now known as the Reactor Safety Study, WASH-1400, or the Rasmussen Report. The report was a pioneering effort in which probabilistic methods were used for the first time to quantify the level of risk to the public from nuclear power plants. It identified the most likely accident sequences, and thus provided a very useful knowledge base to the nuclear industry and regulators for risk management.
After President Dwight D. Eisenhower delivered his “Atoms for Peace” speech at the United Nations in 1953, the development of nuclear power reactors started in earnest. The prevailing thinking at the time was that accident frequencies could not be evaluated, yet the plants should be designed and regulated so that these frequencies would be very low; the predominant belief was that the accident consequences would be catastrophic. The regulatory system, based on those beliefs, evolved over a number of years. Its cornerstone was the concept of defense in depth, which is defined in a 1999 white paper by the NRC as the “safety philosophy that employs successive compensatory measures to prevent accidents or mitigate damage if a malfunction, accident, or naturally caused event occurs at a nuclear facility.” The most severe design-basis accident was considered to be the hypothetical double-ended guillotine break of the largest pipe in the plant (large loss-of-coolant accident).
The Rasmussen Report changed the prevailing thinking in fundamental ways. Taking a systems approach to reactor safety, the report identified accident sequences that were previously unknown, pointed out the significance of human error, and identified the small loss-of-coolant accident as a significant contributor to risk. It also showed that the frequency of severe core damage was not as low as previously believed—the best estimate was about five core damage events every 100 000 reactor years, with an upper bound of three events per 10 000 reactor years. In addition, it was determined that the accident consequences were significantly smaller than previously believed.
Opponents of nuclear power attacked the report vigorously. Norm did a remarkable job defending its methods and results. He gave several hundred lectures around the world and media interviews and press conferences. The significance of the report’s methods and findings was soon recognized. In 1982, President Ronald Reagan appointed Norm as a member of the National Science Board, on which he served for six years. He received the US Department of Energy’s highest scientific award, the Enrico Fermi Award, in 1985.
The methods of the Rasmussen Report have stood the test of time. Probabilistic risk assessment is now recognized as a discipline and is used internationally for nuclear power plants and other major technological facilities. The report has allowed the NRC to issue quantitative health objectives and to pioneer the development of risk-informed regulations, with the goals of maintaining safety, increasing public confidence in the regulatory system, and removing the unnecessary regulatory burden that results from excessive application of defense in depth.
Norm was an avid skier and a dedicated birdwatcher. He was extremely quick witted, with a marvelous sense of humor. According to Kent Hansen of MIT, who knew Norm since his graduate student days, Norm was once testifying before the US Senate about the Reactor Safety Study. Senator John Pastore of Rhode Island was chairing the session. The quorum bell rang; the senator interrupted Norm to say that the senators would have to leave in a few minutes and asked Norm how much longer he needed. Norm replied, “Senator, that depends on how smart you are.” The staffers and members of the NRC were aghast. Pastore roared with laughter and said perhaps they’d better adjourn now and save some time for Norm for another appearance.
Norm was a major figure in the field of reactor safety. Those of us who knew him consider ourselves fortunate to have worked with him, enjoyed his wit and great sense of humor, and benefited from his wisdom.
