For most of its lifespan, Millstone Power Station has been a point of contention in Connecticut, generating protests, legal challenges, and several citizen campaigns to shutter the facility over the last 45 years. The concerns center on the public health and security risks the nuclear plant poses, as well as its environmental impacts on the Long Island Sound.

Millstone opponents argue there is an increased cancer risk associated with living near a nuclear facility. "Living near Millstone is hazardous to children, infants, and unborn babies," the Connecticut Coalition Against Millstone, a grassroots anti-nuclear group, writes on its website.^[1] "Millstone continuously releases toxic and radioactive waste byproducts into the Long Island Sound… and continuously releases radiation into the air."

Some statistics show that New London County, in which Millstone is situated, has one of the highest cancer rates in Connecticut.^[2] However, a 1991 study by the National Cancer Institute, and a 2006 study by the Illinois Department of Public Health, found no correlation between residence near a nuclear power plant and elevated cancer rates. An updated, five-year federal study into cancer risk near Millstone and five other U.S. nuclear plants was canceled by federal regulators last year for running over schedule and over budget.^[3]

In recent years, there have been several small leaks from Millstone’s reactor coolant system, though no injuries have occurred, and the Nuclear Regulatory Commission has found no radioactivity released in these leaks. The plant does, however, periodically release atmospheric radiation, though Dominion responds this is in levels deemed safe for humans by federal standards. There have also been several minor safety violations detected by the NRC during quarterly inspections. According to a 2005 NRC environmental assessment,^[4] "Tests of soil and ground-water samples have not detected a residual radioactive contamination. Testing has found some limited nonradiological chemical constituents."

Opponents of Millstone and nuclear energy also argue that the issue of where to store radioactive waste — either in the short- or long-term — and the energy costs associated with storage cancel out any greenhouse gas savings generated while the plant is producing electricity.

Another point of debate is Millstone's impact on Long Island Sound. The facility pulls about 2.2 billion gallons of water per day from Long Island Sound — 3 percent of the mean tidal flow estimated for the Niantic Bay — to cool its nuclear reactors, later releasing it back into the estuary. According to federal regulations, Millstone can discharge water up to 32 degrees Fahrenheit warmer than the surrounding Sound. Fishermen say the heated water — known as the plant's "thermal plume," which stretches 8,000 feet out from the discharge pipes — attracts flounder, striped bass, and other species. But scientists have also found that warm zones near other nuclear facilities in the U.S., such as Indian Point along New York's Hudson River, can alter oxygen or other nutrient levels enough to threaten closer-to-shore marine life, as well as change fish migration patterns or reproduction.^[5] Populations of several commercially important species, including lobster and winter flounder, have steeply declined in Long Island Sound over the past two decades, but scientists are unsure whether overfishing, habitat degradation, disease, or warm water discharge from Millstone is to blame.

Biologists at New York's Stony Brook University are currently conducting a study to examine the thermal plume's impact on the Sound, which has been gradually warming over recent decades. Part of the aim of the study is to determine whether Millstone's discharge, as well as thermal plumes from two conventional fuel generating stations in Bridgeport, are causing water temperatures to rise, or whether the warming is being driven by climate change.

Security is another area of significant concern for opponents of nuclear energy, one that tends to increase following meltdowns like Chernobyl in Ukraine, Three Mile Island in Pennsylvania, and most recently, Fukushima in Japan. Natural disasters like earthquakes, hurricanes, or flooding can damage a plant structurally, setting off a nuclear meltdown. Nuclear facilities are also seen as prime targets for cyber or terrorist attacks. A nuclear meltdown can have “devastating consequences,” killing, sickening, and displacing thousands of people in nearby communities and causing "extensive long-term environmental damage," as the Union of Concerned Scientists puts it.[6]