Residual radiation is defined as radiation emitted more than one minute after the detonation. If the fission explosion is an airburst, the residual radiation will come mainly from the weapon debris. If the explosion is on or near the surface, the soil, water, and other materials in the vicinity will be sucked upward by the rising cloud, causing early (local) and delayed (worldwide) fallout. Early fallout settles to the ground during the first 24 hours; it may contaminate large areas and be an immediate and extreme biological hazard. Delayed fallout, which arrives after the first day, consists of microscopic particles that are dispersed by prevailing winds and settle in low concentrations over possibly extensive portions of Earth’s surface.

A nuclear explosion produces a complex mix of more than 300 different isotopes of dozens of elements, with half-lifes from fractions of a second to millions of years. The total radioactivity of the fission products is extremely large at first, but it falls off at a fairly rapid rate as a result of radioactive decay. Seven hours after a nuclear explosion, residual radioactivity will have decreased to about 10 percent of its amount at 1 hour, and after another 48 hours it will have decreased to 1 percent. (The rule of thumb is that for every sevenfold increase in time after the explosion, the radiation dose rate decreases by a factor of 10.)

Electromagnetic pulse

A nuclear electromagnetic pulse (EMP) is the time-varying electromagnetic radiation resulting from a nuclear explosion. The development of the EMP is shaped by the initial nuclear radiation from the explosion—specifically, the gamma radiation. High-energy electrons are produced in the environment of the explosion when gamma rays collide with air molecules (a process called the Compton effect). Positive and negative charges in the atmosphere are separated as the lighter, negatively charged electrons are swept away from the explosion point and the heavier, positively charged ionized air molecules are left behind. This charge separation produces a large electric field. Asymmetries in the electric field are caused by factors such as the variation in air density with altitude and the proximity of the explosion to Earth’s surface. These asymmetries result in time-varying electrical currents that produce the EMP. The characteristics of the EMP depend strongly on the height of the explosion above the surface.

EMP was first noticed in the United States in the 1950s when electronic equipment failed because of induced currents and voltages during some nuclear tests. In 1960 the potential vulnerability of American military equipment and weapons systems to EMP was officially recognized. EMP can damage unprotected electronic equipment, such as radios, radars, televisions, telephones, computers, and other communication equipment and systems. EMP damage can occur at distances of tens, hundreds, or thousands of kilometers from a nuclear explosion, depending on the weapon yield and the altitude of the detonation. For example, in 1962 a failure of electronic components in street lights in Hawaii and activation of numerous automobile burglar alarms in Honolulu were attributed to a high-altitude U.S. nuclear test at Johnston Atoll, some 1,300 km (800 miles) to the southwest. For a high-yield explosion of approximately 10 megatons detonated 320 km (200 miles) above the center of the continental United States, almost the entire country, as well as parts of Mexico and Canada, would be affected by EMP. Procedures to improve the ability of networks, especially military command and control systems, to withstand EMP are known as “hardening.”

Discovery of nuclear fission

Following the discovery of the neutron by the British physicist James Chadwick in 1932 and artificial radioactivity by the French chemists Frédéric and Irène Joliot-Curie in 1934, the Italian physicist Enrico Fermi performed a series of experiments in which he exposed many elements to low-speed neutrons. When he exposed thorium and uranium, chemically different radioactive products resulted, indicating that new elements had been formed rather than merely different isotopes of the original elements. Many scientists concluded that Fermi had produced elements beyond uranium, then the last element in the periodic table, and so these elements became known as transuranium elements. In 1938 Fermi received the Nobel Prize for Physics for his work.

Meanwhile, in Germany, Otto Hahn and Fritz Strassmann discovered that a radioactive barium isotope resulted from bombarding uranium with neutrons. The low-speed neutrons caused the uranium nucleus to fission, or break apart into two smaller pieces; the combined atomic numbers of the two pieces—for example, barium and krypton—equaled that of the uranium nucleus. To be sure of this surprising result, Hahn sent his findings to his colleague Lise Meitner, an Austrian Jew who had fled to Sweden. With her nephew Otto Frisch, Meitner concurred in the results and recognized the enormous energy potential.

In early January 1939, Frisch rushed to Copenhagen to inform the Danish scientist Niels Bohr of the discovery. Bohr was about to leave for a visit to the United States, where he reported the news to colleagues. The revelation set off experiments at many laboratories, and nearly 100 articles were published about the exciting phenomenon by the end of the year. Bohr, working with John Wheeler at Princeton University in Princeton, New Jersey, postulated that the uranium isotope uranium-235 was the one undergoing fission; the other isotope, uranium-238, merely absorbed the neutrons. It was discovered that neutrons were also produced during the fission process; on average, each fissioning atom produced more than two neutrons. If the proper amount of material were assembled, these free neutrons might create a chain reaction. Under special conditions, a very fast chain reaction might produce a very large release of energy—in short, a weapon of fantastic power might be feasible.

Producing a controlled chain reaction

The possibility that an atomic bomb might first be developed by Nazi Germany alarmed many scientists and was drawn to the attention of U.S. Pres. Franklin D. Roosevelt by Albert Einstein, then living in the United States. The president appointed an Advisory Committee on Uranium, which reported on November 1, 1939, that a chain reaction in uranium was possible, though unproved. Chain-reaction experiments with carbon and uranium were started in New York City at Columbia University, and in March 1940 it was confirmed that the isotope uranium-235 was responsible for low-speed neutron fission in uranium. The Advisory Committee on Uranium increased its support of the Columbia experiments and arranged for a study of possible methods for separating the uranium-235 isotope from the much more abundant uranium-238. (Naturally occurring uranium contains approximately 0.7 percent uranium-235, with most of the remainder being uranium-238.) The centrifuge process, in which the heavier isotope is spun to the outside, at first seemed the most useful method of isolating uranium-235. However, a rival process was proposed at Columbia in which gaseous uranium hexafluoride is diffused through barriers, or filters; slightly more molecules containing the lighter isotope, uranium-235, would pass through the filter than those containing the heavier isotope, slightly enriching the mixture on the far side. Using the gaseous diffusion method, more than a thousand stages, occupying many acres, were needed to enrich the mixture to 90 percent uranium-235.

During the summer of 1940, Edwin McMillan and Philip Abelson of the University of California, Berkeley, discovered element 93 (naming it neptunium, after the next planet after Uranus, for which uranium was named); they inferred that this element would decay into element 94. The Bohr and Wheeler fission theory suggested that one of the isotopes of this new element might also fission under low-speed neutron bombardment. Glenn T. Seaborg and his group, also at the University of California, Berkeley, discovered element 94 on February 23, 1941, and during the following year they named it plutonium, made enough for experiments, and established its fission characteristics. Low-speed neutrons did indeed cause it to undergo fission and at a rate much higher than that of uranium-235. The Berkeley group, under physicist Ernest Lawrence, was also considering producing large quantities of uranium-235 by turning one of their cyclotrons into a super mass spectrograph. A mass spectrograph employs a magnetic field to bend a current of uranium ions; the heavier ions (such as uranium-238) bend at a larger radius than the lighter ions (such as uranium-235), allowing the two separated currents to be collected in different receivers.

In May 1941 a review committee reported that a nuclear explosive probably could not be available before 1945. A chain reaction in natural uranium was probably 18 months off, and it would take at least an additional year to produce enough plutonium and three to five years to separate enough uranium-235 for a bomb. Further, it was held that all of these estimates were optimistic. In late June 1941 President Roosevelt established the Office of Scientific Research and Development under the direction of the scientist Vannevar Bush, subsuming the National Defense Research Committee that had directed the nation’s mobilization effort to utilize science for weapon development the previous year.

In the fall of 1941 the Columbia chain-reaction experiment with natural uranium and carbon yielded negative results. A review committee concluded that boron impurities might be poisoning it by absorbing neutrons. It was decided to transfer all such work to the University of Chicago and repeat the experiment there with high-purity carbon. This eventually led to the world’s first controlled nuclear chain reaction, achieved by Fermi and his group on December 2, 1942, in the squash court under the stands of the university’s Stagg Field. At Berkeley, the cyclotron, converted into a mass spectrograph (later called a calutron), was exceeding expectations in separating uranium-235, and it was enlarged to a 10-calutron system capable of producing almost 3 grams (about 0.1 ounce) of uranium-235 per day.