Arnold "Arnie" Gundersen (born 4 January 1949, Elizabeth, New Jersey) claims to be a former nuclear industry executive, and engineer with over 30 years of experience who became a nuclear whistleblower in 1990. His curriculum vitae  describes him as Critical Facility Reactor Operator from 1971-1972.
Gundersen questioned the safety of the Westinghouse AP1000, a proposed third-generation nuclear reactor and has expressed concerns about the operation of the Vermont Yankee Nuclear Power Plant. He served as an expert witness in the investigation of the Three Mile Island accident and has provided commentary on the Fukushima Daiichi nuclear disaster.
Gundersen is a graduate of the Rensselaer Polytechnic Institute (1971), with a B.A. in nuclear engineering, holds a master's degree in nuclear engineering, and gained an Atomic Energy Commission Fellowship (1972). Gundersen has more than 40 years of nuclear power engineering experience. Gundersen holds a nuclear safety patent, was a licensed reactor operator, and is a former nuclear industry senior vice president. During his nuclear power industry career, Gundersen also managed and coordinated projects at 70 nuclear power plants in the US. 
From 1972 to 1976 Gundersen worked at the Northeast Utilities Service Corporation as a nuclear engineer; and from 1976 to 1979 at New York State Electric & Gas as an engineering supervisor. From 1979 to 1990 Gundersen was employed at Nuclear Energy Services, a Danbury, Connecticut-based consulting firm. Gundersen served as an expert witness in the investigation of the 1979 Three Mile Island accident. He co-authored the DOE Decommissioning Handbook, First Edition (1981-2).
In 1990 Gundersen was a senior vice president at Nuclear Energy Services when he discovered radioactive material in an accounting safe. Three weeks after notifying the company president of what he believed to be radiation safety violations, Gundersen was fired. According to the New York Times, for three years, Gundersen was "awakened by harassing phone calls in the middle of the night" and "became concerned about his family's safety". Gundersen believes he was blacklisted, harassed and fired for doing what he thought was right.
Gundersen is chief engineer of Fairewinds Associates, an energy consulting company that handles money as a 501c3 non-profit organization.
In April 2010, Gundersen released a report (commissioned by several anti-nuclear groups) which explored a hazard associated with the possible rusting through of the AP1000 containment structure steel liner. In the AP1000 design, the liner and the concrete are separated, and if the steel rusts through, "there is no backup containment behind it" says Gundersen. If the dome rusted through the design would expel radioactive contaminants and the plant "could deliver a dose of radiation to the public that is 10 times higher than the N.R.C. limit" according to Gundersen. Westinghouse has disputed Gundersen’s assessment. Gundersen has testified before the Nuclear Regulatory Commission’s Advisory Committee on Reactor Safeguards saying that "if a hole appeared, the chimney effect would disperse radioactive material far and wide".
Gundersen has also expressed concerns about the operation of the Vermont Yankee Nuclear Power Plant, saying a leak of radioactive tritium there could be "followed by releases of other, more dangerous materials if the plant keeps operating".
Gundersen has said that the U.S. nuclear industry and regulators need to reexamine disaster planning and worst-case scenarios, especially in reactors such as Vermont Yankee, which have the same design as the crippled nuclear plant at the center of the 2011 Japanese Fukushima nuclear emergency. He says that Vermont Yankee and similar plants are vulnerable to a similar cascade of events as in Japan.
As part of Fairewinds Energy Education, Gundersen has hosted numerous videos and provided updates about the Fukushima Daiichi nuclear disaster. He was also a regular guest on media outlets such as Democracy Now and CNN discussing the issue. In an interview with Al Jazeera, Gundersen referred to Fukushima as "the biggest industrial catastrophe in the history of mankind".
In September 2013 Gundersen accused the Japanese government of lying in order to secure its position as host of the 2020 Olympic Games when it claimed that the disaster was under control and that there existed no health concerns. The plant, Gundersen said, "is leaking into the Pacific Ocean extensively" and that thyroid cancers, deformed fish and radioactive animals were being discovered. Gundersen suggested that the contamination could be contained by building a two metre thick Zeolite wall around the reactors, which would absorb the cesium.
- Anti-nuclear movement in the United States
- Fukushima Daiichi nuclear disaster
- Nuclear accidents in the United States
- Nuclear whistleblowers
- Nuclear safety in the United States
- pbadupws.nrc.gov, CURRICULUM VITAE RESUME: Arnold Gundersen, Energy Advisor, February 2009
- Julie Miller (February 12, 1995). "Paying The Price For Blowing The Whistle". The New York Times.
- Robynne Boyd, Scientific American, 29 July 2010, Safety Concerns Delay Approval of the First U.S. Nuclear Reactor in Decades
- David Case (March 14, 2011). "Nuclear expert: "50-50 chance of a catastrophic radiation" from Japan". Global Post.
- Shows featuring Arnie Gundersen | Democracy Now!
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- Nuclear Crisis at Fukushima Could Spew Out More Than 15,000 Times as Much Radiation as Hiroshima Bombing
- Endless Fukushima catastrophe: 2020 Olympics under contamination threat — RT Op-Edge
- Fairewinds, About Us
- Katherine Boughton, The Litchfield County Times, 10 December 1999, The Whistleblower: Arnold Gundersen of Goshen
- Matthew L. Wald. Critics Challenge Safety of New Reactor Design New York Times, April 22, 2010.
- Wald, Matthew L. (January 31, 2011). "Disputed Reactor Design Moves Forward". The New York Times.
- John Dillon. Nuclear Expert Says Yankee Should Shut Down VPR News, February 11, 2010.
- John Dillon (March 15, 2011). "Nuclear expert: U.S. should review worst case scenarios". Reuters.
- Fairewinds Videos | Fairewinds Energy Education
- Fukushima Daiichi | Fairewinds Energy Education
- "Japan's radiation twice as bad". CNN. 7 June 2011.
- Fukushima: It's much worse than you think - Features - Al Jazeera English
- Nuclear Engineer: Japan's PM "Lying to the Japanese People" About Safety of Fukushima