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Radionuclides 157

and destroyed the emergency power back-up systems. Severe cooling problems developed in
the nuclear reactors of Units 1-3, and these led to explosions in the three reactors in the
subsequent days. Temperatures inside the reactor became so high that meltdown occurred
in units 1 and 2. As a consequence, radioactive materials were released to the atmosphere,
groundwater, and sea. Most of the releases were uncontrolled, but some materials were
released deliberately: for example, to reduce gas pressure or to discharge contaminated
137
cooling water. Most of the Cs released into the atmosphere was deposited on the ocean
northeast of Fukushima. Terrestrial deposition of 137 Cs occurred primarily northwest of
Fukushima (Yasunari et al., 2011; Figure 8.4).

8.3.3 Nuclear weapons
Nuclear fission weapons work similar to nuclear fission in nuclear reactors, except that the
process is not moderated, so the chain reaction can proceed unhindered and rapidly expands.
As the atoms that undergo fission with each step of the chain increase exponentially, the
energy release rapidly becomes sufficient to vaporise the warhead, causing the explosion.
Typically, 1–3 percent of the fissile material actually splits; once vaporised, the atoms are too
far apart to sustain the chain reaction. Another process that is used in nuclear weapons is
nuclear fusion. Fusion bombs (also called hydrogen bombs or thermonuclear bombs) explode
with enormous power using uncontrolled self-sustaining chain fusion reactions. Under
2
3
extremely high temperatures, deuterium (D = H) and tritium (T = H) fuse to helium
providing the energy:
D + T 4 He + n (8.7)
2
In principle, a mixture of D, T, and lithium-6 heated to very high temperature and confined
to a high density will start a chain fusion reaction. The explosive process begins with a fission
238
chain reaction of U, which produces a temperature of several million degrees. When the
temperature of the mixture reaches 10 000 000 K, fusion reactions take place. Fusion causes
the temperature to rise, and neutrons released in fusion cause further fission of 238 U that
releases more energy and radioactive fallout .
Nuclear weapons have only been deployed twice in war. In August 1945 the USA
detonated two fission bombs over Hiroshima and Nagasaki in Japan. Although these
weapons killed a large number of people, the principal source of contamination by man-
made radionuclides at a global scale has been the releases during nuclear weapons testing
in the open atmosphere. In total, 541 atmospheric tests have been performed by the
USA, USSR, UK, France, and China. In 1962, USA, USSR, and UK ended atmospheric
testing; no atmospheric test has been performed since 1980. Since 1980 only underground
nuclear tests have been performed. The major test sites for the atmospheric testing of
nuclear weapons include Novaya Zemlya (Russia), Semipalatinsk-21 (Kazakhstan), Nevada
test site (USA), Lopnor (China), and several test sites in the Pacific Ocean (Bikini and
Eniwetok atolls (USA), Mururoa (France), Christmas Island (UK)). Fission bombs were
tested at Novaya Zemlya (USSR) and the Bikini and Eniwetok atolls (USA). Explosions
of thermonuclear bombs caused substantial releases of tritium into the atmosphere and
atmospheric explosions of fission bombs released significant amounts of fissile materials
238
90
( U, 238 U, 239 Pu) and fission products (e.g. 131 I, 137 Cs, Sr, and many others). The
released radioactive materials have been dispersed over large areas and have contributed to
an overall background contamination level of long-lived fission products and transuranic
radionuclides in the environment. It has been estimated that about 12 percent of the fallout
was deposited close to the test site, about 10 percent was tropospheric fallout deposited
in a relatively narrow band around the latitude of the test site, and 78 percent was global

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