November 10, 2026
I. Introduction: Incidents and Isotopes
Less than seven years passed from the experimental discovery of nuclear fission of heavy nuclei in a German laboratory in December 1938 to the deployment of the first atomic bombs over Hiroshima and Nagasaki in August 1945. Physicists understood some of the impacts of nuclear radiation on human health before 1945, but they had no experience with acute radiation sickness resulting from massive full-body exposures or with the long-term disease effects of long-lived radioisotopes inside the human body.
Before the long-term impacts of those first atomic bombs were well understood, the U.S. and other emerging nuclear-armed states were carrying out tests of more advanced nuclear weapons, usually involving hydrogen bombs fueled by thermonuclear fusion of light nuclei, but often triggered by a fission explosion to raise fuel temperature enough to ignite the fusion fuel. We have explained the basic science behind nuclear fission and fusion in a previous post. The U.S. carried out its first thermonuclear bomb test (see Fig. I.1) in 1952 over the Eniwetak Atoll in the Marshall Islands, roughly halfway in the Pacific Ocean between Hawaii and Australia. It conducted a total of 67 thermonuclear tests over Eniwetak and the Bikini Atoll in the Marshall Islands between 1952 and 1958. Those first-generation thermonuclear warheads caused explosions with up to thousands of times the yield of the Hiroshima and Nagasaki bombs.
All told, eight nations carried out an estimated total of 2,056 nuclear weapon tests at more than 60 sites around the world during the second half of the 20th century. Slightly more than one-quarter of these – 528 to be exact – were atmospheric tests, from which radioactive fallout was dispersed over large areas, contaminating air, water, and soil. The total yield of the atmospheric tests was equivalent to about 29,000 Hiroshima bombs. Growing evidence of severe impacts on human health from the atmospheric tests led to their being outlawed in the Partial Test Ban Treaty of 1963, signed by the U.S., Soviet Union, and the UK. Despite the treaty, “France, which carried out its first nuclear test in 1960, and China, which tested first in 1964, continued to test in the atmosphere until 1974 and 1980, respectively.” The cumulative yield of all atmospheric tests by 1980 was equivalent to 440 million tons of TNT, with widespread fallout effects.
Meanwhile, underground nuclear weapon tests continued until the late 1990s. The 1996 Comprehensive Test Ban Treaty (CTBT) established a de facto moratorium on all nuclear weapons tests – violated only by North Korea in 2017 — even though it has never officially gone into effect. For the CTBT to be in force, 44 named countries must ratify the treaty, but 9 of them have either never ratified it (including the U.S. and China) or have revoked their ratification, as Russia did in 2023.
In parallel with the nuclear weapons tests, nuclear fission was harnessed to produce commercial electric power beginning in 1954 in the Soviet Union. Figure I.2 shows the cumulative worldwide operating years of nuclear power reactors from 1960 through 2019. The vast majority of fission power plants around the world have operated safely, reliably, and continuously for decades, currently providing about 10% of the world’s electricity needs. However, there have been a handful of serious nuclear reactor accidents that have provided another source of significant public releases of radiation. The three best-known of these – Three Mile Island in 1979, Chernobyl in 1986, and Fukushima in 2011 – are indicated in Fig. I.2. In addition to these there were two significant, but less widely reported, reactor accidents in 1957, one near Kyshtym in Chelyabinsk, Russia and one at the Windscale reactor in Cumbria, England. We will consider the impacts of all five of these incidents in Section IV of this post.
When uranium or plutonium nuclei undergo fission under neutron bombardment, they produce a wide range of isotopes of elements near the middle of the periodic table, along with multiple neutrons. Among the fission products included in nuclear fallout are radioactive isotopes which, when they subsequently decay, emit particles – mostly, electrons (so-called beta particles) and gamma rays (photons) – carrying enough energy to remove electrons from atoms and break chemical bonds, damaging living cells and DNA. Not all of the fuel in a bomb may be burned, so the fallout may also contain particles carrying some of the residual uranium or plutonium isotopes themselves. These radioisotopes decay most often by emitting helium-4 nuclei – so-called alpha particles – which can do serious damage over very short distances. The neutrons released in an atmospheric test of a fission or fusion bomb can interact with nitrogen nuclei abundant in Earth’s atmosphere to produce carbon-14, which is another dangerous radioactive isotope. Table I.1 lists the radioisotopes in fallout that carry the biggest risks to the health of living organisms and the environment, along with their half-lives, decay modes, and characteristic impacts.
Table I.1. The radioisotopes in nuclear fallout that carry the greatest risks to human health and the environment.
| Radioisotope | Production | Half-life | Main Decay Mode | Health Impact | Environmental Impact |
| Iodine-131 | Fission product | 8.0 days | β-decay | Concentrates in thyroid | Airborne fallout |
| Cesium-137 | Fission product | 30.1 years | β-decay | Deposits broadly in soft tissue | Soil uptake plus surface water transport |
| Strontium-90 | Fission product | 28.9 years | β-decay | Accumulates in bones and teeth | Soil uptake plus surface water transport |
| Plutonium-239, 240 | Unfissioned material particles | 24,100/6,560 years | α-decay | Localized radiation if inhaled or ingested | Airborne fallout, deposits on land, water, roofs, vegetation |
| Carbon-14 | Neutron bombardment of nitrogen in atmosphere | 5,700 years | β-decay | Increases radioactive content of body’s carbon | Airborne fallout, inhaled and absorbed by vegetation as CO2 |
In the immediate aftermath of a major nuclear radiation release, there is usually intense concern over the presence of iodine-131. Although this radioisotope has a half-life of only 8 days, it tends to be absorbed in human bodies by the thyroid gland, where it can do damage that leads eventually to thyroid cancer. Cesium-137 and strontium-90 each have half-lives around 30 years. In human bodies they tend to accumulate in soft tissue for cesium-137 and in bones and teeth for strontium-90, which behaves chemically like calcium and is especially likely to show up in children due to their growing bones and teeth. Indeed, one of the findings that contributed to adoption of the 1963 Partial Test Ban Treaty was study of strontium-90 accumulation in baby teeth, showing that levels in U.S. children born in 1963 were 50 times higher than those in children born in 1950. Both the cesium and strontium isotopes, with their roughly 30-year half-lives, furthermore contaminate soil and are transported by surface water for multiple decades following a radiation release.
Plutonium-239 and 240 can be deposited as part of residual nuclear weapon fuel that has not undergone fission. When these long-lived radioisotopes are ingested or inhaled they tend to get deposited and retained on bone surfaces. The alpha-particles from their decay lose energy very quickly and travel no more than about 50 microns (or 1/20 of a millimeter) in soft human tissue. Plutonium exposures are associated with increases in cancers affecting bone growth. Enhanced carbon-14 abundances in the atmosphere are concerning because all life forms on Earth are carbon-based; 12% of the atoms in a human body are carbon. When carbon-14 combines with oxygen atoms in the atmosphere it can form radioactive carbon dioxide, which tends to remain in the atmosphere for many decades. When those carbon dioxide molecules are inhaled by humans or absorbed by vegetation, the carbon-14 can form a very long-lasting (half-life = 5,700 years) radioactive part of the anatomy, producing cancers or other health impacts many years, and possibly many generations, after the weapon tests that produced it. When all of these radioisotopes are dispersed through the air, waterways, soil, and the food supply, they can impact people far from the original site of the radiation release.
In this post we will explore the studies that have been done to track the long-term health and environmental impacts of radiation released in the first atomic bombs (Section II), in atmospheric nuclear weapon tests (Section III), and in major nuclear reactor accidents (Section IV).
II. Hiroshima and Nagasaki
The atomic bombs dropped over Hiroshima and Nagasaki in August 1945 released intense radiation over a short period of time. Many people in those cities were exposed to whole-body radiation doses exceeding 0.75 Gray, where one Gray (symbol Gy) is defined as the absorption of one Joule of radiation energy per kilogram of matter. A dose of 0.75 Gy is roughly equivalent to receiving 18,000 full-body chest X-rays. That dosage level induces acute radiation syndrome, more commonly known as “radiation sickness.” Radiation sickness typically causes extreme nausea and vomiting within hours of receiving the dose and often leads to death over the following days, weeks, or months. According to reports from Hiroshima and Nagasaki, more than 210,000 people exposed to the atomic bombs had died by the end of 1945.
Surprisingly, there was no long-term mandatory evacuation ordered for either city after the bombing, as one might have expected due to concerns about irradiation of the surroundings. About 150,000 people fled Hiroshima in the immediate aftermath, but many soon returned: “Hiroshima’s population, down to roughly 83,000 soon after the bombing, swelled to 169,000 by February of 1946. But only some 6,500 lived in the city’s center, i.e., within close space 1 kilometer of ground zero. For several years more, population growth was concentrated at the same remove from the city’s devastated center. The increase was largely due to the return of evacuated persons, civilians repatriated from overseas colonies, and demobilized military personnel.” However, “When food scarcity became especially acute in the summer of 1946, the city imposed compulsory evacuation of 50,000 people to surrounding farm villages, and arranged for relief rice supplies to be provided.” In Nagasaki, there was also an immediate exodus after the bombing, but residents began returning in 1946, although reconstruction proceeded quite slowly.
People who received lower doses of radiation from the atomic bombs often suffered long-term health damage, which has been tracked since 1950 by the Radiation Effects Research Foundation (RERF), known before 1975 as the U.S.-funded Atomic Bomb Casualty Commission. As indicated in Fig. II.1, RERF has kept records on all-cause mortality and (starting in 1958) cancer incidence rates on 120,000 survivors of the atomic bombs, 3,600 individuals who were exposed in utero within pregnant survivors, and 77,000 offspring of survivors to assess the heritability of radiation damage. Subsets within each of these groups have been tracked in longitudinal studies to assess radiation-related non-cancer outcomes over the long term.
The RERF study provides the best information we have on long-term health impacts of significant nuclear radiation doses, but over a wide range of doses. The cohort involved in the Life Span Study (LSS) of survivors was divided into four groups depending on their distance from the hypocenter of the atomic blast: less than 2000 meters; 2000-2499 meters; 2500-9999 meters; and greater than 10,000 meters. The distribution of estimated doses received among the cohort who joined the study before 1968 is shown in Fig. II.2. 80% of the LSS survivors received less than 0.1 Gy, while 2% actually received a dose greater than or equal to 1.0 Gy, i.e., above the threshold at which acute radiation syndrome normally kicks in. As of the end of 2020, 21% of the initial LSS cohort were still alive, with an average age of 83 among the living. 67% of those still alive had been exposed at 10 years of age or younger, ages at which sensitivity to radiation is known to be enhanced by ongoing growth and maturation.
In summarizing results from the LSS study, Kamiya, et al. note that “the first radiation-related excess of death from malignant disease observed in atomic bomb survivors was leukemia, which appeared ~2 years after exposure, peaked 6–8 years after the bombing, and rapidly declined thereafter. However, excess risk of leukemia death has persisted even beyond 50 years after exposure. Among leukemia subtypes, acute myeloid leukemia (AML), chronic myeloid leukemia (CML), and acute lymphoblastic leukemia (ALL) show significant increases in radiation-related risk.”
In contrast to leukemia, Kamiya, et al. note that excess deaths due to radiation-induced solid cancers began to be observed about 10 years after the bombing. The enhancements deduced from the LSS study from 1958 through 2009 in the relative risk of various solid cancers, averaged for both sexes and standardized to a 1 Gy radiation dose with dose-response models, are summarized in Fig. II.3. The bottom line is that a 1 Gy radiation dose raises the risk of all solid cancers by about 50%, relative to the general public, with notably enhanced risk for colon cancer, lung cancer, breast cancer, unitary tract cancers, brain cancer, thyroid cancer, and non-melanoma skin cancer. The number of cases of each solid cancer observed among the LSS cohort is given in the figure.
Genetic analyses have been performed on the pathological tumor tissues extracted from members of the LSS cohort, in order to see if particular radiation-induced gene mutations have been responsible for the enhanced risk. Several such genes show up prominently in the samples. In cases of hepatocellular carcinoma, single-point mutations (i.e., changes to a single base in the DNA chain) show up in the gene p53, which appears in the 17th human chromosome and contains the code for a protein sometimes called “guardian of the genome,” which is a key regulator for DNA repair. In basal cell carcinoma, deletions within the PTCH1 gene, another tumor-suppressor gene, are prominent. In thyroid cancer cases, point mutations appear regularly in the gene BRAF (encodes a protein that helps to regulate cell growth, division, differentiation, and migration) at low radiation doses and in the genes RET and ALK (which also are involved in regulating cellular activity) at higher doses.
Excess deaths not associated with cancers have also been tracked in the LSS study and in longitudinal studies of those exposed in utero. Kamiya, et al.: “The LSS mortality data from 1950 [to] 2003 indicated significant increases in radiation-related mortality due to circulatory, respiratory, and digestive diseases,” particularly in hypertensive and rheumatic heart disease and heart failure.
Among those exposed in utero, “Radiation exposure during 8–15 weeks and 16–25 weeks of gestation was associated with increased risk of intellectual disability, lower intelligence quotient, and poorer academic performance. However, no significant effects were observed in individuals exposed before 8 weeks or after 26 weeks of gestation. Additionally, investigations of in utero exposure indicated increased risk of small head sizes in individuals exposed before 15 weeks of gestation, but no significant increase in those exposed at 16 weeks or later…In utero radiation exposure was also found to significantly impair physical development, particularly with respect to height reduction, with no clear relationship between age of gestation and radiation sensitivity.” Enhanced risks for various causes of mortality were also observed among those exposed in utero, but most of these may not be anomalous in comparison with general public members with small head size, low birth weight, and loss of parents. The one enhanced risk that survives such comparison at a statistically significant level is that of cancer deaths among females exposed in utero.
RERF studies of the children of Hiroshima and Nagasaki survivors have to date provided no clear evidence that the radiation doses received by parents negatively impact the health of offspring conceived after the bombs fell. “So far, analyses of cancer incidence (1958–1997), cancer, and non-cancer disease mortality (birth to 2009), and prevalence of multifactorial diseases including hypertension, hypercholesterolemia, diabetes, angina, myocardial infarction, and stroke have indicated no significant increase in any of these diseases related to parental radiation exposure among the offspring of atomic bomb survivors. However, as the children of atomic bomb survivors are now reaching ages where cancer and lifestyle-related diseases occur more frequently, continued long-term follow-up and careful investigation remain essential.”
III. Nuclear Weapons Tests
Impacts of atmospheric tests:
Eight countries carried out a total of 528 atmospheric tests of thermonuclear weapons between 1952 and 1980, amounting to a cumulative yield of 440 megatons of TNT. One of the major issues with atmospheric bomb tests is that the nuclear fallout can be dispersed widely by currents in the air, oceans, and waterways. For example, the U.S. carried out a total of 67 bomb tests over Enewetak and Bikini atolls in the Marshall Islands (see map in Fig. III.1). The tests began in 1946 with the first atomic bombs tested after Hiroshima and Nagasaki. Before the first test the U.S. evacuated all inhabitants of these two atolls, with the promise that they could return after the tests were completed. But contamination of the atolls proved far too intense to allow inhabitants back and Enewetak and Bikini remained uninhabited for decades after testing in the Marshall Islands was completed.
Radioactive fallout was hardly confined to Enewetak and Bikini atolls. A systematic reconstruction of the radiation found that “Permanent residents of the southern atolls who were of adult age at the beginning of the testing period received external doses ranging from 5 to 12 mGy on average; the external doses to adults at the mid-latitude atolls ranged from 22 to 59 mGy on average, while the residents of the northern atolls received external doses in the hundreds to over 1,000 mGy.” Recall that 1,000 mGy is a dose sufficient to induce radiation sickness and death in most cases.
The largest of the U.S. tests was called Castle Bravo. In March 1954 it tested a new thermonuclear bomb design using dry fuel and produced a yield of about 15 megatons, 2.5 times greater than was predicted. The fallout “blanketed inhabitants of nearby islands with radiation and ash – which they termed ‘Bikini snow’.” The fallout rendered Rongelap, Rongerik, and Utrik Atolls unsafe for inhabitants. Many of the inhabitants of those atolls, who were evacuated only two or three days after the blast, began to show symptoms of radiation sickness. The fallout from Castle Bravo ended up depositing traces of radioactive material in Australia, India, Japan, and parts of the U.S. and Europe. 23 crew members of a Japanese fishing boat were contaminated by the “Bikini snow” 90 minutes after the detonation and all became ill with acute radiation sickness. When one of those fishermen died six months later, leading to a mass media campaign to ban nuclear testing, Edward Teller, the driving force behind the U.S. hydrogen bombs, commented: “It’s unreasonable to make such a big deal over the death of a fisherman.”
Concern over the widespread radioactive fallout from all the weapons tests being carried out in the 1950s led Drs. Louise and Eric Reiss to begin the Baby Tooth Survey in St. Louis in 1959, looking for evidence of strontium-90 from fallout. Over the next 12 years, the survey collected 320,000 baby teeth donated by children who received the button shown in Fig. III.2. A remarkable 314 megatons of thermonuclear weapons tests were carried out worldwide from 1958 to 1962 by the USSR, U.S., U.K., and France, during a period when tensions between the U.S. and Soviet Union over nuclear weapons were at a peak. That made results from the Baby Tooth Survey for babies born in 1963 particularly impactful, showing that strontium-90 levels in U.S. children born in 1963 were 50 times higher than those in children born in 1950, before widespread weapons testing got under way. A new study published in 2011 found “that the donors from the original project who died of cancer by age 50 had more than twice the average strontium 90 levels of donors who were healthy at 50, suggesting that higher levels of the chemical raised cancer risk.” The Baby Tooth Survey demonstrated that radioactive fallout from atmospheric tests was a worldwide problem and helped to convince President John F. Kennedy to sign the 1963 Partial Test Ban Treaty banning atmospheric tests with the USSR and U.K.
The carbon-14 bomb pulse:
While most of the focus during the 1960s was on fission products from nuclear weapons tests, because they did not occur naturally and were easy to detect in the laboratory, an even greater concern arose for the production of carbon-14 in atmospheric tests. 78% by volume of Earth’s atmosphere consists of nitrogen gas molecules and 99.6% of naturally occurring nitrogen resides in the isotope nitrogen-14. The fission and fusion reactions occurring in a thermonuclear weapon release many neutrons. When those neutrons interact with nitrogen-14 nuclei in the air, they produce the radioisotope carbon-14 at a rate 45 times higher than that for the fission product cesium-137. Carbon-14 normally accounts for about one-trillionth of all the carbon in the atmosphere, as a result of cosmic ray neutrons interacting with nitrogen. But the atmospheric nuclear weapons testing led to the so-called “bomb pulse” illustrated in Fig. III.3, nearly doubling the natural level of carbon-14 in the atmosphere by 1963. The 1963 Partial Test Ban Treaty led to a reduction in carbon-14 production, though France and China continued to do atmospheric testing after 1963. The falloff in carbon-14 levels was gradual over the ensuing decades because the carbon dioxide molecules it forms last for many decades in the atmosphere and the carbon-14 nuclei decay slowly, with a half-life of 5,700 years.
Starting in 1958, two Nobel Prize winners, Andrei Sakharov in the Soviet Union and Linus Pauling in the U.S., issued warnings about the serious human health consequences of the atmospheric carbon-14 buildup. Sakharov, who was a key contributor to the Soviet development of the hydrogen bomb, estimated that the carbon-14 from a cumulative 50 megatons of atmospheric tests would, in his assumed future global population of 30 billion humans, cause serious health impacts initially to about 40 individuals per year, but integrating over the full lifetime of the carbon-14 would eventually cause serious problems for a cumulative 330,000 people. Pauling estimated that the result of 30 megatons of cumulative atmospheric tests would lead to 55,000 children born worldwide with “gross physical or mental defects.”
Later estimates of the health effects of atmospheric testing were made after China’s last atmospheric test in 1980 by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). Biological effects are determined by the equivalent doses of radiation, measured not in Gray but in Sieverts, where the Sievert unit (Sv) takes into account the different biological impacts of different energies and types of radiation (i.e., alpha vs. beta vs. gamma). Various characteristic radiation doses are plotted in milliSieverts in Fig. III.4. UNSCEAR’s estimate of the cumulative equivalent dose from bomb-induced carbon-14 over the lifetime of the radioactive isotope is 26 million person-Sievert, with 90% of that dose projected to be delivered to humans living after 2050, assuming a global population of 10 billion people. In comparison, all of the radioactive fission products from atmospheric tests would have delivered an additional cumulative dose of 4 million person-Sievert, while heavy radioisotopes of plutonium and americium would cause a dose of 0.2 million person-Sievert. But while most of the dose from fission products has already occurred, carbon-14 is, in principle, a “gift” that will keep on giving for many generations.
The health effects of such radiation doses have been estimated by the National Academies of Science (NAS) in 2006. Their estimate is that the cancer risk from low-rate whole-body exposure to beta- and gamma-radiation in the U.S. is 5-20% per Sievert. Furthermore, the NAS study estimated that the mortality rate from the mix of cancers that would be caused would be about 50%, although medical advances since 2006 probably reduce cancer mortality rates from that estimate. The study also found that such radiation doses notably increase the likelihood of deaths from “heart disease, stroke, and diseases of the digestive, respiratory and hematopoietic systems.” They also predicted some genetic heritability of radiation effects, though no evidence of that has as yet been reliably observed among Hiroshima and Nagasaki offspring. In all, the likely cumulative impact of the bomb pulse seen in Fig. III.3 would be several million humans suffering serious health effects and possible death over many future generations.
However, humans have unwittingly reduced the health impacts of atmospheric carbon-14 on future generations by their continuing burning of fossil fuels. The carbon dioxide concentration in the atmosphere has increased by about 40% since the 1960s as a result of fossil fuel burning and those emissions contain naturally occurring carbon without the radioactive isotope carbon-14. Hence, the relative carbon-14 concentration in the atmosphere has by now been diluted to about the cosmic-ray background level before the atmospheric tests. Thus, most of the health impacts of the bomb pulse have likely already been experienced.
The health impacts of the nuclear weapons tests may well have been most severe on women and girls and their reproductive success. Survivors of the Marshall Islands tests have reported “alarmingly high rates of stillbirths, miscarriages, birth defects, and reproductive problems in their communities.”
Other legacies of nuclear testing:
In addition to the physical impacts of the nuclear weapons tests, there has been extensive psychological, cultural, and environmental damage. The survivors of weapons tests have experienced major mental health issues that persist for many years, including post-traumatic stress disorder, anxiety, clinical depression, preoccupation with somatic symptoms, and neurasthenia (fatigue, weakness, dizziness, headaches). They have often been forced into permanent relocation by the contamination of their lands.
Many Marshall Islanders have left the archipelago permanently. The current population of all the islands and atolls today is 42,000, while 25,000 expatriate Marshall Islanders live in Arkansas alone (see Fig. III.5). According to Benetick Kabua Maddison, who is the Executive Director of the Marshallese Educational Initiative, “A lot of the children who are born in the US or who come here at a very young age, like myself, lose their language. They lose their ancestral ways. Many do not know the ancient chants. Many do not know how to make traditional medicines or how to navigate or how to build traditional homes. And so these are some of the larger consequences of the testing.”
British nuclear weapons tests were carried out in regions of South Australia and over islands off the coast of Western Australia. Sue Coleman-Haseldine, an elder of the Kokatha nation of Australia, has pointed out that “Many Aboriginal people were still living close to the test areas and received no forewarning of the explosions. The tests permanently disconnected Aboriginal people from their homes and traditional way of life. They cannot return to their ancestral land, and their children and their children’s children will never know the special religious places it contains.”
The International Campaign to Abolish Nuclear Weapons (ICAN) began in Australia in 2007 but by now includes non-governmental organizations in one hundred countries. The organization received the 2017 Nobel Peace Prize for “work to draw attention to the catastrophic humanitarian consequences of any use of nuclear weapons” and “ground-breaking efforts to achieve a treaty-based prohibition of such weapons.” ICAN notes that “Advocacy by survivors, in particular – as living witnesses to the horrors of the atomic age – has awakened decision-makers to the brutal reality of what nuclear weapons do to human bodies and our planet.” Groups of indigenous survivors have stated that authorities in the nuclear power nations have “subjected our peoples to epidemics of cancers, chronic diseases and congenital abnormalities,” and in some cases “denied us access to adequate medical care and even our own medical records.” ICAN notes further that “Few survivors of nuclear testing anywhere in the world have ever been compensated for their suffering. Where efforts have been made to clean up former test sites, they have been woefully inadequate.”
Nuclear weapons treaties:
While efforts such as those of ICAN and indigenous survivors of bomb tests have created somewhat of a taboo against the use of nuclear weapons, the history of nuclear weapons treaties is checkered. The Partial Test Ban Treaty originally signed by the U.S., U.K., and Soviet Union in 1963 remains in force and by now has 126 signees, though ten of the signees have not ratified the treaty. The treaty bans non-underground nuclear weapons tests, but all of the declared nuclear weapons countries have continued underground tests since 1980. The history of some other international treaties restricting nuclear weapons is summarized in Fig. III.6.
The international treaty pushed by ICAN is the Treaty for the Prohibition of Nuclear Weapons (TPNW), which bans “developing, possessing, stationing or threatening to use nuclear weapons.” It went into force in January 2021 but has been signed by only 75 countries, including no current nuclear-armed states and no NATO members. The signees are mostly from the global South and include South Africa, the only independent country that has actually disarmed a previous nuclear arsenal. (Ukraine, Belarus, and Kazakhstan each housed significant numbers of nuclear warheads while they were part of the Soviet Union. When they became independent countries in the 1990s they transferred all of their nuclear arsenal to Russia, where the warheads have either been dismantled or incorporated into Russia’s nuclear management system.)
The Non-Proliferation Treaty (NPT) officially recognized only 5 nuclear weapons states – the U.S., USSR/Russia, U.K., France, and China, all of whom had tested weapons by 1967 – and committed all other countries to renounce acquisition or use of the bomb, in exchange for civil nuclear cooperation overseen by the International Atomic Energy Agency. Although the treaty remains in effect with 191 parties, it has not been terribly successful. India, Pakistan, and Israel never signed the treaty and have each developed nuclear weapons outside it. North Korea joined but then withdrew in 2003 in order to pursue its own nuclear weapons. A number of other countries – Sweden, Switzerland, Taiwan, Argentina, Brazil, Libya, and Iraq — have had ambitions to develop nuclear weapons but stopped development as a result of changes in internal political leadership or geopolitical conditions, or as part of negotiated ends to hostilities. Iran’s possible pursuit of nuclear weapons is one of the motivations for the U.S. and Israel to have launched their ongoing war with Iran.
The Comprehensive Test Ban Treaty (CTBT) would have banned all nuclear weapons tests forever but it has never gone into effect. It required ratification by 44 named states but 9 of those have not ratified it, including the U.S., China, and Russia, which revoked its ratification in 2023. The CTBT includes a monitoring network of 300 worldwide stations. The only verified test since its 1996 signing is by North Korea. But the U.S. has recently accused China of a secret low-yield test in 2020. Furthermore, Donald Trump ordered testing “on an equal basis” on Oct. 30, 2025 and six days later Vladimir Putin ordered Russian proposals for future weapons tests.
In addition to the international treaties the U.S. and Russia (previously, the USSR) have negotiated a series of mutual agreements to limit their nuclear arsenals since the early 1970s. It is worth noting that while atomic bombs were first developed in Los Alamos, New Mexico as part of the Manhattan Project, the leader of that effort, Robert Oppenheimer, and several more recent Directors of Los Alamos National Laboratory, including Harold Agnew and Sieg Hecker – all three scientists who have a first-hand understanding of the impacts of nuclear weapons — have been vigorous proponents for a ban or limits on nuclear weapons. The history of the U.S.-Russia agreements is summarized in Fig. III.7. The most recent of these, the New START agreement signed in 2010, would have limited each country to a total of 1,550 deployed nuclear warheads and 700 deployed launchers, to be checked by on-site inspections. However, Russia suspended its participation in 2023, inspections never resumed, and the agreement officially expired on Feb. 5, 2026. “For the first time since 1972, nothing legally limits the two largest arsenals.”
— Continued in Part II —
