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Dictionary

Waste Elimination

Nuclear waste is radioactive material which is dangerous for human health and the environment, and is subject to special laws and regulations with regards its disposal, transport and storage. Half a million tonnes of high-level nuclear waste has been generated in the world's 500 nuclear power stations. After more than half a century of nuclear power, a corresponding stockpile of political, economic and environmental liabilities have accumulated to mountainous dimensions.

The optimism of the 1950s in nuclear power foresaw two things:

  1. Nuclear power would provide nearly limitless, inexpensive energy, and
  2. Technological advances would solve the problem of nuclear waste and other risks.

Needless to say, neither of these has been achieved - far from it:

  1. Nuclear power never produced more than 5% of world electricity, and is one of the most expensive forms of energy, and
  2. Technology has not found a definitive solution for the safe disposal of half a million tonnes of useless waste, leaving the world with a dangerous, hard to manage legacy for tens of thousands of years.

Two systems are proposed for handling waste from nuclear fission in reactors: geological storage and transmutation.

Storage

The fuel in a nuclear reactor core generates heat from the chain reaction of neutrons from one fission event striking other nuclei, which in turn undergo fission, generating neutrons, which strike other nuclei... and so on, millions of times per second.

When a nuclear reactor is shut down, the fuel rods are removed and placed in a temporary storage tank, under water. The water cools the rods and absorbs the neutrons which are still leaving the uranium or plutonium fuel. Although the chain reaction has ceased, since most of the neutrons are being absorbed by the water, and not reaching the nuclei of other uranium atoms, there is still a fair amount of heat being produced by the beta decay of the fission products in the fuel. This heat generation continues until the unstable isotopes have all been transmuted to stable isotopes, which do not decay.

This decay heat is initially 7% of the chain reaction energy, and within a day is only 4%. The rate of heat generation slowly decreases over time. Although this rate is small compared to the reactor chain reaction heat, the fuel rods need to be kept for a matter of years before they are ready to be placed in more permanent storage.

According to the IAEA, by 2020 there will be worldwide a total of 445 kt (approx. 20 000 m3) of spent fuel, for which the only solution is long-term deep geological depository. Storage is so far only in temporary water tanks, but storage capacity is close to exhaustion.

Transmutation

Transmutation is the conversion of radioactive isotopes to less dangerous isotopes, usually through bombardment with neutrons. An example of transmutation is the conversion of technetium-99c. The target is bombarded with neutrons to create the isotope technetium-100Tc, which has a very short half-life, decaying to the non-radioactive ruthenium-100. So far, transmutation is not a viable solution to uranium and plutonium waste.

Vienna Joint Convention on Radioactive Waste

The Joint Convention on the Safety of Spent Fuel Management and on the Safety of Radioactive Waste Management (Vienna, 1997) is an IAEA global treaty covering the transport and storage of radioactive waste. It currently has 71 state parties.

Download: Vienna Joint Convention - English (pdf 67 kB)

Full name: Joint Convention on the Safety of Spent Fuel Management and on the Safety of Radioactive Waste Management

Radioactive wastes are divided into three classifications: high-level (over 1015 Bq m-3), medium-level (1010 to 1015 Bq m-3), and low-level (less than 1010 Bq m-3). High-level wastes generate high levels of heat through ongoing decay processes, typically in the range 2-20 kW m-3.

There are 7 long-lived radio-isotopes in spent nuclear fuel: selenium-79, zirconium-93, technetium-99, palladium-107, tin-126, iodine-129, and caesium-135.

IsotopeHalf-life /MyDecay modeDecay energy /MeVDecay productYield (U-235) /%Note
7934Se0.327 *1β-0.157935Br0.045bio-accumulating with nitrate
9340Zr1.53β- γ0.0919341Nb5.46low soil mobility, suitable for geological storage
9943Tc0.211β-0.2949944Ru6.14environmentally mobile, significant component of nuclear waste, may be transmuted artificially
10746Pd6.5β-0.03310747Ag1.25not amenable to disposal by nuclear transmutation, less environmentally mobile I and Tc
12650Sn0.230β- γ4.05012651Sb0.108gamma emitted from decay product (antinomy-126)
12953I15.7β- γ0.19412954Xe0.841high long-term risk since environmentally mobile and long-lived, potential for transmutation (neutron bombardment or lasers) under study
13555Cs2.3β-0.26913556Ba6.911disposal by nuclear transmutation difficult, intense medium-term radiation

*1 Uncertainty in the half-life of Selenium-79 gives measurements/estimates in the range 6.5 × 104 to 1.13 × 106 years.

Half-life

The half-life is a measure of the activity of a radioactive substance. Since nucleus decay rates are exponentially decreasing over time, a mass will theoretically never lose all of its radioactivity. Statistically, the decay rate is best described by the period of time it takes for half of the original population to decay - the half-life.

For any mass of a radioactive substance, there is an exponentially decreasing number of decay events through time. After one half-life, half of the isotopes have decayed. After a second half-life has elapsed, half of the remaining isotopes (i.e. one-quarter of the original amount) will have decayed (leaving only 1/4). After a third half-life, one-eighth remain. Four half-lives: 1/16, and so on.

The most abundant isotope of uranium is U-238. This isotope has 92 protons and 146 neutrons, and accounts for 99.274% of all naturally-occurring uranium. The next most abundant isotope is U-235, with only 0.720% abundance. The difference in abundances can be understood by comparing the half-lives: the half-life of U-238 is 4.47 x 109 years, while that of U-235 is 7.04 x 108 years.