Topic 5 Controlling the atom

Nuclear technologies need to be controlled in a variety of ways. As with many non-nuclear technologies, if used carelessly or maliciously, they can be very harmful to humans and other life forms. Doctors practising nuclear medicine need to protect themselves and their patients from excessive exposure to radiation during treatment. Even more care must be taken when designing and operating large nuclear facilities such as nuclear reactors. Major accidents could affect large numbers of people. Safety precautions are carefully designed, with multiple levels of protection against all the possible results of mishaps that may or may not happen. These safety standards are enforced by legal and institutional controls. Similarly, governments around the world work hard to limit the proliferation of nuclear weapons, and hopefully avoid these being used again.

Protecting people

People working with radioactive material take careful precautions to avoid exposure or contamination. Doctors and researchers using dissolved radioactive tracers must take particular care not to allow any material to enter their bodies by swallowing or breathing them in. They wear protective clothing and goggles, never eat or drink while working, wash thoroughly, and monitor themselves with a radiation monitor at all times. They also use Perspex and lead shielding to block most of the radiation. The most powerful sources such as the fuel rods of a nuclear reactor can only be handled using robotic arms that are controlled from an adjacent room. Neutrons produced in a nuclear reactor are stopped by thick layers of concrete or water around the reaction vessel in the nuclear reactor.

Figure 5.1 Working with radioactive substances. ANSTO image.

Figure 5.1 Working with radioactive substances requires careful safety precautions, depending on the type and activity of the source.

Radiation workers limit the time they spend near radioactive materials, although working quickly is less important than working carefully. They wear a badge-like device that records their accumulated radiation dose. This is checked every few months to ensure each worker's exposure remains below the dose limits (an average of 20 mSv per year over 5 years with no year over 50 mSv). This is about thirteen times the amount an average person receives each year from natural background exposure. In practice, very few workers ever reach these levels.

Nuclear reactors

Keeping major nuclear facilities safe and secure requires careful planning, good design and sound operating practices. A nuclear power reactor is particularly challenging, because of the intense radiation, and the very high temperatures and pressures reached in the reactor core. The central task in controlling a nuclear reactor is to maintain a suitable temperature, and to control the rate of the nuclear chain reaction in the core. (This is discussed in Topic 7 Nuclear energy.) If the chain reaction becomes too vigorous – ie supercritical – it can soar out of control and cause a nuclear meltdown. The superhot, radioactive nuclear fuel could melt through the steel reactor vessel and escape. This is what happened at Chernobyl in 1986, accompanied by a fire that burnt for 10 days and spread radioactive material across Europe.

A meltdown can also occur if the reactor vessel runs dry of coolant, as happened at the US's Three Mile Island reactor in 1979. Unlike Chernobyl, almost all the radioactive material from Three Mile Island remained within the containment building. Only some radioactive gases such as iodine-131 escaped into the environment. Ensuring the integrity of the steel and concrete containment building is therefore an important part of the reactor's safety. Reactor sites are carefully chosen to minimise the risk of earthquake, and to retain a buffer zone to residential areas. The containment building must also be secured against deliberate sabotage or infiltration by terrorists.

The Chernobyl accident was a terrible disaster causing many direct casualties. An accident at Tokaimura in Japan in 1999 also caused a small number of deaths. However, despite this, for the most part, the controls on nuclear reactors have functioned properly. Accidents have been fewer than for many other industries. However, there are some groups who claim that the risks from rare accidents at nuclear reactors are unacceptably high.

Figure 5.2 The Australian Nuclear Science and Technology Organisation's High Flux Australian Reactor (HIFAR) has not had accidents of any significance in its 45-year history.

The fuel cycle

Nuclear reactors support a large infrastructure of other nuclear technologies, which also need careful management. Production of the fuel rods used in a reactor requires uranium to be mined, and then enriched at highly specialised facilities. After use, the radioactive waste must be disposed of safely. This chain of activities is referred to as the nuclear fuel cycle, and often involves transportation of radioactive materials by land and sea. In addition, radioactive materials are used every day in hospitals, research laboratories, military facilities and elsewhere, and these materials often need to be transported by air, land and sea. Worldwide there are tens of millions of such transports every year and there has never been an accident affecting human health. Keeping all this material safely contained is quite a challenge. It requires careful protocols and well-designed containment systems, ranging from lead pots to geological repositories. (See the discussion of radioactive waste in this section.)

Figure 5.3 Nuclear fuel cycle (typical power reactor).

Radioactive waste from reactors needs special attention because it is more radioactive than the original fuel, and some types remain so for thousands of years. This waste is also hot because of the continuing reactions going on inside. Initially this waste is kept on site to cool off, and to become less radioactive as the different radioisotopes decay. Eventually, the bulk of it must be permanently stored. There is still some resistance to the long-term storage and disposal of the waste.

Figure 5.4 Graph showing radioactivity diminishing over time.

Figure 5.4 High level radioactive waste remains extremely radioactive for thousands or even millions of years. It requires permanent management. This graph shows the radioactivity from 1 tonne of fuel from the most common type of nuclear reactor. The graph shows radioactivity 10 years after unloading; immediately after unloading, it would be about 100 times more radioactive, and extremely hot!

One innovative solution to the radioactive waste problem, initially described by the Australian National University and developed by ANSTO, is 'synroc' (synthetic rock). This incorporates high level radioactive waste into a ceramic block that has similar properties to the natural minerals that have contained radioactive atoms for millions of years. By locking radioisotopes into the crystal structure of the synroc, the radioactive waste can be kept trapped within the solid block without the danger of leakage. This is a very effective approach to the storage of high level waste. Other countries lock up the radioactive material in a vitrified glass matrix. It is planned that high level waste will be buried in deep repositories built in very stable and dry geological areas. At present, most high level waste and other very hot and radioactive waste is stored at the site of the reactor where it was produced. The dumping of radioactive waste at sea was banned internationally in 1993, as a result of publicity campaigns by environmental organisations such as Greenpeace.

Lower level waste can include radioisotopes, contaminated soil, protective equipment and used scientific equipment, which can be stored in shallow burial repositories.

Weapons proliferation

Despite attempts since 1945 at controlling proliferation, several countries have developed nuclear weapons, and much more powerful nuclear weapons have been invented. Presently ten nations are thought to have acquired nuclear weapons: the USA in 1945, the USSR (the former Soviet Union, which included Russia) in 1949, the UK in 1951, then France (1960), China (1964), India (1974), as well as Pakistan, South Africa and probably Israel and North Korea.

Figure 5.5 Build up of strontium-90 following weapons testing.

Figure 5.5 These graphs show the build up of the radioisotope strontium-90, in the atmosphere and then on the Earth's surface, as a result of atmospheric weapons testing. You can see atmospheric levels drop rapidly following the Test Ban Treaty in 1963. From the 1970s onwards, surface levels gradually begin to drop due to radioactive decay.

Two international agreements have been reasonably successful in controlling arms proliferation. They are the 1963 Test Ban Treaty (TBT), and the 1968 Nuclear Non-Proliferation Treaty (NPT). The TBT banned all atmospheric testing of nuclear weapons. The NPT signatories must agree not to acquire nuclear weapons (if they don't have them already), or to help others acquire them. In return, they can choose to develop nuclear reactors and nuclear technology for civilian applications. These conditions are enforced by inspections by the United Nations and the International Atomic Energy Agency (IAEA). The NPT has 188 signatories.

However, not all countries have signed the NPT, and some who have signed are thought to be secretly developing nuclear weapons. International and national authorities watch very carefully for black market trade in nuclear weapons technology – including weapons-grade plutonium – and equipment for uranium enrichment. This has been more difficult since the 1991 break up of the USSR, which possessed more than 10,000 nuclear warheads.

Laws and institutions

Nuclear technologies around the world are subject to political controls as well as physical and procedural precautions. Nuclear activities are carefully monitored by international agencies such as the IAEA, which monitors the use of nuclear materials globally, and provides standards to ensure that nuclear reactors around the world are run safely and securely. ANSTO's small nuclear research reactor is subject to these international controls, and is also regulated by Australia's own government and the Australian Radiation Protection and Nuclear Safety Agency.

Figure 5.6 Inspecting drums of nuclear material: Kirstie Hansen/IAEA.org Image Bank.