There are a number of things to consider-  if the attack is on the nuclear reactor or if it is on the spent fuel pool next to it. The size of the nuclear power plant, and how old it is are also factors. 

Most nuclear reactors are not designed for extreme events, like aerial bombing, missile strikes, or being crashed into by an aeroplane. These activities could break the containment building and destroy the reactor- causing a meltdown of the reactor core. However, most reactors are protected by thick concrete and steel containment structures- but the necessary surrounding areas, which may include spent fuel pools, cooling equipment, fire suppression equipment and more, may not be. 

Some researchers suggest that the release of Cesium-137 from spent fuel pool fires would be potentially much larger than from reactor accidents, such as occurred during the Chernobyl and Fukushima accidents. Those accidents forced the evacuation of more than 100,000 in Ukraine and 160,000 in Japan. Exposure to Cesium- 137 can cause burns, acute radiation sickness and death. Spent fuel pools are often densely packed, which greatly increases the chances for a hydrogen explosion if they lose cooling and catch fire.  One of the  world’s first nuclear power station accidents at Windscale (now Sellafield) in the UK occurred when one of two graphite reactors caught fire in 1957. Iodine–131 and Polonium-210 were released and fallout spread across the UK and Europe. Iodine-131 can cause thyroid cancer and Polonium-210 is highly toxic to human life.

Nuclear power plants under construction have been attacked a number of times throughout history. For example,  power plants under construction in Iran were repeatedly bombed from the air by Iraq in the period 1984-1987. Yugoslav Air Force fighters made a threatening overpass of the Krsko nuclear plant in Slovenia -- which was operating at the time -- a few days after Slovenia declared independence in 1991. So-called research reactors in Iraq were destroyed by aerial bombing by Israel in 1981 and by the United States in 1991. Bombs damaged reactors under construction in Spain in 1977 and in South Africa in 1982. Anti-tank missiles struck a nuclear plant under construction in France in 1982.

The blast and heat impacts of  even a small nuclear weapon (e.g. 10kT) would blow apart a nuclear reactor, and any containment buildings in the vicinity. The radioactive components within the reactor and stored fuel rods would be highly fragmented, and in some cases vaporised, and would contribute additional radioactive particles to the smoke and debris transported downwind from the blast centre. It is estimated that 100% of the Cesium-137 contained in reactor cores and spent fuel pools would be released into the environment following a nuclear attack on a nuclear power plant. The combined impact could release tens of millions of curies of Cesium-137, and it is estimated that 2,000 square kilometres are rendered uninhabitable by every million curies released. 

The cesium-137 would be released to the atmosphere in small particles that would travel downwind and be deposited on the ground and other surfaces. The deposited particles would emit intense gamma radiation, leading to external, whole-body radiation doses to exposed persons. Cesium-137 would also contaminate water and foodstuffs, resulting in internal radioactive contamination and significantly increasing the likelihood of cancers and other health problems.

The short answer is we do not know, however Israel’s Prime Minister, Benjamin Netanyahu, has threatened to attack Iran’s nuclear facilities in the past. Since Iran launched a conventional missile attack on Israel on 1 October 2024, there has been considerable media speculation Israel could retaliate by attacking Iranian nuclear facilities, but Tel Aviv has been under pressure from its main ally,  the United States, not to do so, although some American commentators and former administration officials, as well as former President Donald Trump, have encouraged Israel to do so. 

Iran has several research sites, uranium mines, a research reactor and several uranium enrichment facilities. Its programme began in the 1950s with support from the U.S under the “Atoms for Peace” program. Iran has been a party to the Nuclear Non-Proliferation Treaty (NPT) since 1970 and so it is legally bound not to develop or acquire nuclear weapons.

As Iran’s nuclear facilities contain enriched uranium and other elements that could be spread through the air, if struck by explosive weapons this would cause radioactive contamination and a health risk to people in the area. The level of risk would depend on how many people were in the area at the time and the size of the weapons used. 

Iran’s nuclear facilities have been attacked before. In the 1980s, plants under construction in Iran were repeatedly bombed by Iraq.

The International Atomic Energy Agency (IAEA) has warned of the risk that nuclear power stations at Zaporizhzhia in Ukraine and Kursk in Russia could be damaged by fighting in the area near the plants. 

The IAEA says the safety and security of nuclear power plants is one of its central and fundamental concerns because of the risk of accidental hits on the plant or the loss of external sources of power to operate the plants, including cooling their reactors.

In the event of an incident the risk is that radioactive elements would be released into the air and potentially spread over a wide area. The Chernobyl accident in 1986 released Iodine, Strontium and Cesium that contaminated 150,000 square kilometres in Ukraine, Belarus and Russia. Some fallout reached as far north as Sweden, Finland and other parts of the northern hemisphere.