Nuclear fallout is radioactive material that remains after a nuclear explosion or accident. During explosions, this material is first found in the radioactive cloud created by the blast. Over time, the wind and weather carry the cloud, and the radioactive material falls to the ground as fallout. The amount and spread of fallout depend on several factors, such as the power of the weapon, the type of fuel used, how high the weapon is exploded, and weather conditions.

Fission weapons and many thermonuclear weapons use large amounts of fissionable materials like uranium or plutonium. These weapons produce fallout mainly from fission products and leftover fuel. Cleaner thermonuclear weapons create fallout mostly through neutron activation, which changes materials into radioactive forms. Salted bombs, which are not widely used, are designed to spread specific radioactive materials chosen for their properties.

Nuclear accidents, such as those involving nuclear reactors or waste, can also cause fallout. These accidents often spread fission products into the air or water. Fallout can harm human health in both short and long periods and may contaminate areas far from the original explosion site.

Atmospheric and underwater nuclear testing, which spread fallout widely, stopped in 1963 with the Partial Nuclear Test Ban Treaty signed by the United States, Soviet Union, and United Kingdom. Underground testing, which sometimes causes fallout if gases escape, mostly stopped in 1996 with the Comprehensive Nuclear-Test-Ban Treaty. The "bomb pulse," an increase in global carbon-14 caused by nuclear testing, is expected to affect humans for up to 8,000 years, potentially harming a small number of people.

Types of fallout

Fallout is usually divided into two main types, based on how high the explosion happens. If the explosion occurs at a high enough altitude that the fireball does not mix much with ground material, radioactive particles from the explosion stay in the air longer than if the explosion happens on or near the ground. This extra time in the air allows the most dangerous radioactive materials, which break down quickly, to decay before they fall to the ground. This reduces the overall danger of the fallout that reaches the surface. It also spreads the radioactive cloud over a larger area, making the radioactive material less concentrated in any one place. This type of fallout is called "global fallout," because it raises background radiation levels slightly over large areas. It is different from "local fallout," which creates a concentrated area of radioactive material downwind of the explosion within minutes or hours.

Weather conditions can affect these differences. For example, "rainout" can happen when rain falls from a nuclear cloud, causing radioactive material to fall to the ground quickly. An underwater explosion also creates a different type of local fallout compared to an explosion on land. Some explosions, even if they occur below a certain altitude, may still avoid creating local fallout. For example, the 1961 Tsar Bomba test had its fireball lifted by a shockwave, preventing it from mixing with the ground.

When an explosion happens at or above a certain altitude (called an air burst), the heat from the fireball turns fission products, un-fissioned nuclear material, and weapon parts into tiny particles. These particles, which are between 10 nanometers and 20 micrometers in size, rise into the stratosphere. They can stay there for months or years and may settle anywhere in the world. These particles increase the risk of cancer. One report estimated that widespread nuclear testing in the 1950s, which peaked in 1963, could lead to about 430,000 additional cancer deaths by the year 2000 and up to 2.4 million more over thousands of years. The extra radiation exposure from these tests is estimated to be an average of 3.5 mSv, compared to natural background radiation levels of about 2.4 mSv per year.

Radioactive fallout has been found worldwide. For example, people were exposed to iodine-131 from nuclear testing. Fallout settles on plants, including fruits and vegetables. People may have been exposed to fallout starting in 1951, depending on whether they were outside, the weather, and whether they drank milk or ate contaminated food. Exposure can happen quickly or over a long time. Quick exposure happens when fallout is carried into the troposphere and falls with rain in the first month. Long-term exposure comes from tiny particles in the stratosphere that take years to settle. By the time these particles reach Earth, their radioactivity is much lower. After a year, many radioactive materials move from the northern to the southern stratosphere. Quick exposure lasts between 1 and 30 days, while long-term exposure happens after that.

Both quick and long-term fallout occurred after the 1986 Chernobyl accident, which contaminated over 20,000 square kilometers in Ukraine and Belarus. The reactor used uranium, and graphite surrounded it. A hydrogen explosion destroyed the reactor and released radioactive material. About 31 people died within weeks, including two workers at the site. Residents were evacuated within 36 hours, but others reported symptoms like vomiting and headaches. Officials closed an 18-mile area around the site. Long-term effects included at least 6,000 cases of thyroid cancer, mainly in children. Fallout spread across Europe, heavily affecting Northern Scandinavia, where reindeer herds were contaminated, and France, where salad greens became unavailable. Some farms in Wales and northern England monitored radioactivity in their flocks until 2012.

When explosions happen on the ground, in shallow water, or below a certain altitude, heat vaporizes large amounts of earth or water, which mixes with the radioactive cloud. This material becomes radioactive when it combines with fission products or is activated by neutrons.

The table below shows how common isotopes can create fallout. Some radioactive materials pollute large areas of land and water, causing genetic changes in animals and humans.

Explosions on the ground create large amounts of particles, ranging from less than 100 nanometers to several millimeters in size. Larger particles fall from the fireball within an hour, and more than half of the total debris lands on the ground within 24 hours as local fallout. The chemical properties of the materials in the fallout determine how quickly they settle. Less volatile materials fall first.

Severe local fallout can spread far beyond the blast area, especially after large ground explosions. The path of fallout depends on weather conditions after the explosion. Stronger winds carry fallout farther but spread it more thinly, reducing the width of the fallout area for any given radiation level. The total amount of radiation deposited remains the same, so the number of people affected by fallout is usually not influenced by wind speed. However, thunderstorms can cause fallout to fall faster as rain mixes with the radioactive cloud or washes it down.

When people stay in areas with radioactive contamination, they receive immediate radiation exposure from the outside and may later inhale or eat radioactive material, such as iodine-131, which can collect in the thyroid.

Factors affecting fallout

The location of an explosion has two main factors: height and the type of surface where it happens. A nuclear weapon that explodes in the air, called an air burst, creates less fallout than a similar explosion near the ground. If a nuclear explosion happens close enough to the ground that the fireball touches it, soil and other materials are pulled into the cloud. Neutrons make these materials radioactive before they fall back to Earth. An air burst leaves behind only a small amount of highly radioactive heavy metals from the device itself.

When a nuclear explosion happens over water, the particles created are usually lighter and smaller. This causes less fallout in one area but spreads it over a larger region. These particles mostly contain sea salt and water. They can cause rain to form quickly, which may lead to areas with high radiation levels nearby. Fallout from a seawater explosion is hard to clean if it soaks into surfaces like concrete or steel. Washing with water and detergent removes less than half of the radioactive material. To fully clean surfaces, methods like sandblasting or using acid are needed. During the Crossroads underwater test, it was found that water must be washed off ships immediately using systems like fire sprinklers to prevent contamination.

Sometimes, parts of the ocean floor can become covered in fallout. After the Castle Bravo test, white dust made of radioactive calcium oxide from broken coral fell for hours. This caused radiation burns and exposure to people on nearby islands and the crew of a fishing boat. Scientists named this fallout "Bikini snow."

For explosions under the ground, a special event called a "base surge" can happen. A base surge is a cloud that spreads outward from the bottom of the rising smoke. It forms when the air has too many dust or water droplets. In underwater explosions, the visible surge looks like a cloud of liquid droplets that move like a single fluid. After the water evaporates, an invisible cloud of small radioactive particles may remain.

For underground explosions on land, the base surge is made of small solid particles, but it still moves like a fluid. Soil helps create base surges during underground explosions. Although a base surge usually contains only about 10% of the total debris from an underground explosion, it can cause higher radiation doses than fallout because it reaches the ground faster, before the radioactive material has had time to decay.

Weather conditions strongly affect how fallout spreads, especially local fallout. Winds can carry fallout over large areas. For example, after the Castle Bravo test, which used a 15-megaton thermonuclear bomb on March 1, 1954, a long, cigar-shaped area of the Pacific Ocean, about 500 kilometers long and up to 100 kilometers wide, became highly contaminated. Scientists had three different maps of the fallout pattern because they only measured radiation levels on a few distant islands. Two of the maps showed high radiation levels on Rongelap Atoll because fallout particles about 50–100 micrometers in size carried a lot of radioactivity.

After the Castle Bravo test, scientists learned that fallout landing on the ocean spreads through the top layer of water (above 100 meters deep). To estimate radiation levels on land, scientists multiply the ocean radiation levels measured two days after the explosion by about 530. In other tests from 1954, like Yankee and Nectar, hot spots of high radiation were mapped using underwater probes. Similar hot spots were found in 1956 tests like Zuni and Tewa. However, the U.S. "DELFIC" computer program uses natural soil particle sizes instead of data from wind patterns, which results in simpler fallout patterns without the downwind hot spots.

Snow and rain, especially when falling from high altitudes, can speed up the spread of local fallout. In some weather conditions, such as a rain shower that starts above the radioactive cloud, areas close to the ground downwind of a nuclear explosion may become heavily contaminated.

Effects

A wide range of biological changes can happen after animals are exposed to radiation. These changes range from quick death after very high doses of whole-body radiation to normal lives for some time before delayed effects appear in some people who received low doses.

The unit used to measure radiation exposure is called the röntgen, which is based on how much air is ionized. Instruments like Geiger counters and ionization chambers measure exposure. However, the effects of radiation depend on how much energy is absorbed by the body, not just the exposure measured in air. A dose of 1 joule per kilogram is called 1 gray (Gy). For gamma rays with 1 MeV energy, an exposure of 1 röntgen in air results in about 0.01 gray (1 centigray, cGy) in water or skin. Bone marrow receives about 0.67 cGy when air exposure is 1 röntgen, while skin receives 1 cGy. Some LD50 values, which describe the radiation dose that kills 50% of a group, are based on bone marrow doses, which are about 67% of the air dose.

The LD50 is a common measure used to compare the effects of different radiation exposures. It is usually defined for a specific time period and focuses on short-term death. For small animals, this time is often 30 days or less, while for larger animals and humans, it is up to 60 days. The LD50 assumes no other injuries or medical care were involved.

In the 1950s, the LD50 for gamma rays was set at 3.5 Gy. However, under harsh wartime conditions (like poor nutrition or limited medical care), the LD50 dropped to 2.5 Gy. Few people survive doses above 6 Gy. One person at Chernobyl survived more than 10 Gy, but many there were not exposed evenly across their bodies. If radiation is unevenly distributed, the overall body dose is lower, reducing the risk of death. For example, a hand dose of 100 Gy with an average body dose of 4 Gy is less likely to be fatal than a 4 Gy dose over the entire body. A hand dose of 10 Gy or more could lead to losing the hand. A British radiographer who received a lifetime hand dose of 100 Gy lost his hand due to radiation dermatitis. Most people become ill after exposure to 1 Gy or more. Fetuses are especially vulnerable and may miscarry, especially in the first trimester.

Nuclear fallout radiation levels drop quickly after release. Activity decreases by 50% in the first hour and 80% in the first day. Early decontamination, like removing contaminated clothing, is more effective than later cleaning. Most areas are safe for travel and decontamination after three to five weeks.

One hour after a nuclear explosion, radiation levels in the crater region can reach 30 Gy per hour. Normal civilian radiation exposure is 30 to 100 microgray per year.

For nuclear explosions with yields up to 10 kilotons, prompt radiation is the main cause of battlefield casualties. A dose of 30 Gy causes immediate performance loss and renders people ineffective within hours, though death may occur five to six days later. Doses below 1.5 Gy do not cause incapacitation. Doses above 1.5 Gy lead to disability and possible death.

A dose of 5.3 to 8.3 Gy is considered lethal but not immediately disabling. Cognitive performance declines within two to three hours, and people remain disabled for at least two days. After recovery, they can perform simple tasks for six days before relapsing for about four weeks. Symptoms of radiation poisoning then appear, leading to death around six weeks after exposure.

Delayed radiation effects can occur months or years after exposure and affect many body tissues and organs. These effects include cancer, cataracts, chronic skin damage, reduced fertility, and genetic changes.

The only confirmed birth defect linked to nuclear attacks on Hiroshima and Nagasaki is microcephaly, a condition where the baby’s head is smaller than normal. Fewer than 50 children born to mothers exposed during the bombings had microcephaly. No increase in other birth defects was found in children born later to survivors.

The Baby Tooth Survey, started by doctors Eric and Louise Reiss, aimed to study strontium-90, a radioactive substance linked to cancer. Strontium-90 is absorbed into bones and teeth through water and dairy products because it behaves like calcium. The survey collected baby teeth from children in St. Louis, Missouri, and found that strontium-90 levels in children born in the 1950s and later were much higher than before atomic testing. These results influenced President John F. Kennedy to sign the Partial Nuclear Test Ban Treaty, ending above-ground nuclear testing.

The Baby Tooth Survey used media to raise public awareness about nuclear testing risks. While it did not prove that strontium-90 levels were life-threatening, it helped end above-ground testing, which reduced atmospheric fallout.

Fallout protection

During the Cold War, the governments of the United States, the USSR, Great Britain, and China worked to teach their citizens how to survive a nuclear attack. They provided instructions on how to reduce short-term exposure to fallout, which is the radioactive material that falls to the ground after a nuclear explosion. This effort was known as Civil Defense.

Fallout protection focuses on shielding people from radiation. Radiation from fallout includes alpha, beta, and gamma types. Ordinary clothing can block alpha and beta radiation, but most fallout protection methods aim to reduce exposure to gamma radiation. For radiation shielding, materials have a "halving thickness," which is the amount of material needed to cut gamma radiation exposure in half. Examples include 1 cm of lead, 6 cm of concrete, 9 cm of packed earth, or 150 meters of air. When multiple layers of material are used, the shielding effect increases. A practical fallout shield uses ten halving-thicknesses of a material, such as 90 cm of packed earth, which reduces gamma ray exposure by about 1024 times. A shelter built with these materials for fallout protection is called a fallout shelter.

As nuclear energy use grows, concerns about nuclear warfare and the risk of radioactive materials being used by harmful groups have increased. Scientists are researching ways to protect human organs from high-energy radiation. Acute radiation syndrome (ARS) is the most immediate danger when people are exposed to ionizing radiation at levels greater than about 0.1 Gy per hour. Low-energy radiation, like alpha and beta, has limited penetrating power and usually does not harm internal organs unless it enters the body through ingestion, inhalation, or contact with the skin. High-energy gamma and neutron radiation, however, can easily pass through the skin and thin shields, damaging stem cells in bone marrow. While full-body shielding in a fallout shelter is the best protection, it requires staying in a thick bunker for a long time. For emergencies, mobile protection equipment is needed for medical and security personnel to perform tasks like containment and evacuation. Shielding the entire body would be too heavy to move, so scientists are studying partial body protection, such as shielding the pelvic region where bone marrow is concentrated. This strategy is inspired by medical procedures like hematopoietic stem cell transplantation. More details on bone marrow shielding can be found in the Health Physics Radiation Safety Journal or the 2015 OECD/NEA report on radiation protection.

Radiation danger from fallout decreases quickly over time because radioactive materials decay exponentially. A book by Cresson H. Kearny explains that radiation levels drop by a factor of ten for every seven-fold increase in time after an explosion. For example, it takes about seven times longer for radiation to decrease from 1000 roentgens per hour (1000 R/hr) to 10 R/hr (48 hours) than to decrease from 1000 R/hr to 100 R/hr (7 hours). This is a general rule based on observations, not an exact formula.

In the 1960s, the U.S. government, through the Office of Civil Defense in the Department of Defense, created guides to help people survive nuclear fallout. These guides were often in booklet form and included instructions for building fallout shelters for families, hospitals, or schools. They also provided advice on creating improvised shelters and steps to improve survival chances if unprepared.

The main idea in these guides was that materials like concrete, soil, and sand are needed to block fallout radiation. A large amount of these materials is required for protection, so safety clothing cannot block fallout radiation. However, protective clothing can prevent fallout particles from touching the body, though radiation from these particles can still pass through clothing. Clothing thick enough to block radiation would be too heavy to use.

The guides recommended that fallout shelters have enough supplies to keep people alive for up to two weeks. Community shelters were preferred over single-family shelters because they had more resources. If a family had a basement, it was best to build a shelter in a corner, as the center of a basement receives more radiation. Walls made of cinder blocks filled with sand or soil and a roof of concrete were recommended. Shelters should include water, food, tools, and waste management systems.

If no shelter was available, people were advised to go underground. If a basement was accessible, food, water, and a waste container should be placed in a corner, with furniture piled around the person for protection. If underground access was not possible, tall apartment buildings at least ten miles from the blast site were suggested as temporary shelters. People in these buildings should stay near the center and avoid the top and bottom floors.

Schools were often chosen as fallout shelters because they housed about one-quarter of the U.S. population during school sessions. Their widespread locations across the country made them useful for protecting large numbers of people.

Nuclear reactor accident

Nuclear fallout can also happen because of nuclear accidents, even though a nuclear reactor does not explode like a nuclear weapon. The type of radioactive material in fallout from nuclear bombs is very different from the fallout caused by serious accidents at power reactors, such as Chernobyl or Fukushima.

The main differences are how easily elements evaporate and their half-life. The boiling point of an element (or its compounds) determines how much of that element is released during a power reactor accident. The ability of an element to form a solid affects how quickly it settles on the ground after being released into the atmosphere by a nuclear detonation or accident.

A half-life is the time it takes for radiation from a substance to decrease to half its original strength. Fallout from nuclear bombs contains many short-lived isotopes, such as Zr. These isotopes are created regularly in power reactors, but because accidents happen over long periods, most of these short-lived isotopes decay before they can be released.

Nuclear fallout can come from many sources, including nuclear reactors. Because of this, steps must be taken to control the risk of nuclear fallout at reactors. In the 1950s and 1960s, the United States Atomic Energy Commission (AEC) started creating safety rules to prevent nuclear fallout from civilian reactors. Since the effects of nuclear fallout are more widespread and last longer than other types of energy accidents, the AEC wanted to take action before problems occurred.

One way to prevent accidents was the Price-Anderson Act, passed by Congress in 1957. This law ensured government help if the cost of a nuclear accident exceeded $60 million, which was covered by private insurance. The goal was to protect large companies that operate nuclear reactors. Without this protection, the nuclear industry might have stopped growing, and safety measures against fallout could have weakened.

At the time, engineers had limited experience with nuclear technology and struggled to calculate the risks of radiation leaks. They had to imagine many unlikely accidents and their possible fallout. The AEC’s rules focused on preventing the Maximum Credible Accident (MCA), which is a large release of radioactive material after a major reactor meltdown caused by a failure in the cooling system.

To prevent the MCA, new safety measures were introduced. Static safety systems, which do not need power or human input, were used to reduce human error. For example, containment buildings are effective at stopping radiation leaks without needing to be powered. Active systems, which require sensors and power, can also help but are less reliable. A sensor might fail, leading to a local nuclear fallout.

Because there were no clear standards or calculations for risks, the AEC and industry disagreed on the best safety methods. This disagreement led to the creation of the Nuclear Regulatory Commission (NRC) in 1974. The NRC focused on using research to create rules. Much of its work aimed to shift safety systems from a deterministic approach—trying to predict all possible problems—to a probabilistic approach, which uses math to assess the risks of radiation leaks. This method is based on radiative transfer theory from physics, which explains how radiation moves through space and barriers.

Today, the NRC remains the main group responsible for regulating nuclear power plants.

Determining extent of nuclear fallout

The International Nuclear and Radiological Event Scale (INES) is the main way to classify how nuclear or radiological events might affect health and the environment, and to share this information with the public. Created in 1990 by the International Atomic Energy Agency and the Nuclear Energy Agency of the Organization for Economic Co-operation and Development, the INES scale groups nuclear accidents based on how much fallout they might cause:

• Defence-in-Depth: This is the lowest level of nuclear accidents. These events do not directly harm people or the environment but are important for improving safety in the future.
• Radiological Barriers and Control: These events do not harm people or the environment but involve damage within major facilities.
• People and the Environment: This category includes more serious accidents. These events might cause radiation to spread to people near the accident site. This also includes unexpected and large releases of radioactive material.

The INES scale has seven levels that classify nuclear events, from small issues that need to be recorded for safety improvements to major accidents that require immediate action.

The 1986 nuclear reactor explosion at Chernobyl was classified as a Level 7 accident, the highest on the INES scale, because it caused major environmental and health effects and released a large amount of radioactive material from the reactor core. This was the only commercial nuclear accident to cause radiation-related deaths. The explosion and fires released about 5,200 PBq, or at least 5% of the reactor core, into the air. The explosion killed two workers, and 28 others later died from radiation poisoning. Young children and teenagers in highly contaminated areas had a higher risk of thyroid cancer, though a United Nations study found no major public health effects beyond this. The accident also caused environmental harm, including contamination of urban areas and crops like green leafy vegetables.

The nuclear meltdown at Three Mile Island in 1979 was classified as a Level 5 accident because of severe damage to the reactor core and a radiation leak. This was the most serious accident in U.S. commercial nuclear history, but its effects were different from Chernobyl. A study by the Nuclear Regulatory Commission found that the nearly 2 million people near the plant received an average radiation dose only slightly higher than normal background levels. Unlike Chernobyl, thyroid cancer in people near Three Mile Island was less severe.

The Fukushima incident was first classified as a Level 5 accident in 2011 after a tsunami disabled power and cooling systems at three reactors, causing significant damage. Later, the accident was upgraded to Level 7 when all three reactors were considered together. Radiation exposure led to evacuations for people up to 30 km from the plant, but tracking exposure was difficult because 23 of 24 monitoring stations were also damaged by the tsunami. Removing contaminated water from the plant and nearby areas became a major challenge. During cleanup, thousands of cubic meters of slightly contaminated water were released into the sea to make space for more contaminated water. However, the impact on the surrounding population was minimal. A study by the Institut de Radioprotection et de Sûreté Nucléaire found that over 62% of people in Fukushima prefecture received less than 1 mSv of radiation in four months after the accident. Screening for thyroid cancer in children in Fukushima and other parts of the country showed no significant differences in risk.

International nuclear safety standards

The International Atomic Energy Agency (IAEA) was created in 1974 to establish international standards for nuclear reactor safety. However, without an enforcement group, these guidelines were not always followed properly or ignored. In 1986, the Chernobyl disaster showed that nuclear safety needed more attention. During the Cold War, the Nuclear Regulatory Commission (NRC) worked to improve safety at Soviet nuclear reactors. As IAEA Director General Hans Blix said, "Radiation does not respect national borders." The NRC shared the safety rules used in the United States with the Soviets, including strong rules, safe operations, and good reactor designs. The Soviets, however, focused more on keeping reactors operational than on safety. Eventually, the approach to safety changed from focusing on certain safety measures to considering possible risks. In 1989, the World Association of Nuclear Operators (WANO) was created to work with the IAEA to ensure the same three safety principles worldwide. In 1991, WANO concluded (using a risk-based approach) that nuclear reactors in former communist countries could not be trusted and should be closed. Similar to a "Nuclear Marshall Plan," efforts were made during the 1990s and 2000s to ensure safety standards for all nuclear reactors globally.