Nuclear Waste - Types

Classification of Radioactive Waste

Different schemes can be used to classify nuclear wastes the following are four commonly used methods to classify radioactive waste.

State of Phases of Waste

Nuclear wastes can be gases, liquids and solids, and may be converted from one form to another during the nuclear fuel cycle. Eg, most fission products are solids when created in the fuel element; they are converted to liquid to separate them from Uranium and Plutonium; then are converted back to solids for disposal.

Half-Life

Radioactive wastes may be classified by length of half-lives, short, intermediate and long-lived. For the nuclear fuel cycle, short-lived components, eg fission products would decay to innocuous levels in months or years; intermediate-lived components eg fission products Sr and Cs, in centuries; and long-lived components from Uranium mill tailings and transuranic such as Plutonium in millennia.

Radiation Emission

Radionuclides decay by different modes, e by emitting alpha or beta particles, gamma rays, neutrons, or some combination of these. These radiations have different penetration distances and pose different exposure risks. A recent practice is to distinguish between alpha wastes (which present an inhalation and ingestion exposure risk) and beta-gamma wastes (which present external as well as internal exposure risks).

Radioactivity Level

Nuclear wastes are often classified by radioactivity level:

• High (about 1 Ci/L)
• Intermediate (about 10^(-4) Ci/L)
• Low (about 10^(-6) Ci/L)

Radioactivity levels between low and high levels range over six orders of magnitude.

When classification is based on the radioactivity levels of the waste produced as a result of nuclear power:

• High level waste
  • Consists mostly of spent fuel rods
  • Includes highly radioactive sludge-like residue produced as a by-product in reprocessing.
  • Highly radioactive Transuranic wastes. e.g. Plutonium
• Intermediate level waste
  • Mainly from Uranium processing and enrichment plants, and from nuclear power plants
• Low level waste
  • Includes the aqueous solutions from the second and third cycles of the reprocessing plants
  • This waste can often be buried near the surface of the earth.

Waste during Differing Phases of the Nuclear Power Cycle

Mining Wastes

Significant quantities of radon diffuse from ore bodies. Consequently, in underground mines, high ventilation rates must be used to protect miners from excessive concentrations. The ventilation air is exhausted above ground, therefore increasing the normal atmospheric radon concentration in the vicinity.

Milling Wastes

A Uranium mill recovers a fraction of a percent of Uranium from the ore, with the residual going to tailings ponds and piles. This waste, containing most of the non-Uranium radioactivity of the ore. Atmospheric emissions occur from:

• The ore storage area;
• Tailings ponds and piles.

Liquid wastes may also be released as a result of seepage from tailings ponds.

Enrichment

Uranium Hexafluoride gas from a conversion facility is processed through hundreds of stages of gaseous diffusion to increase the concentration of Uranium-235 from about 0.7% -3.5%. Virtually all the radioactivity coming into an enrichment plant in the Uranium Hexafluoride feed material is from Uranium, although there are minor amounts of Uranium daughter products from ingrowth after purification.
However large quantities of Uranium is still present in the feed material after the process is complete and thus will be present in the tailings

• Uranium oxide concentrate from mining is not very radioactive. It is refined to form yellowcake (U3O8), then converted to Uranium Hexafluoride gas. As a gas, it undergoes enrichment to increase the U-235 content from 0.7% to about 3.5%. It is then turned into a hard ceramic oxide (UO2) for assembly as reactor fuel elements.
• The main by-product of enrichment is depleted Uranium, principally the U-238 isotope, with some U-235.
• Some is used in applications such as the keels of yachts, and anti-tank shells. It is also used (with recycled Plutonium) for making mixed oxide fuel and to dilute highly enriched Uranium from weapons stockpiles which is now being redirected to become reactor fuel.

Nuclear Power Plant Wastes

Radioactive materials produced at nuclear power plants consist of four major types:

• Fission Products, such as Strontium-90and Cesium-137, produced by splitting of Uranium-235 under neutron bombardment;
• Transuranic Elements, such as Plutonium, produced by neutron absorption in Uranium-238 and subsequent beta decay
• Essentially, all fission products and transuranic elements are retained inside the fuel elements.
• Activation Products produced as a result of neutron absorption by corrosion products and impurities in the primary coolant by the coolant itself and by structural materials.

Essentially, all fission products and transuranic elements are retained inside the fuel elements.

Nuclear Wastes from Reprocessing Plants

In the closed fuel cycle, the valuable "unburned" Uranium-235 and Plutonium-239 are recovered at reprocessing plants. Uranium and Plutonium are separated from high-level wastes by solvent extraction processes. Uranium is converted to Uranium Hexafluoride and Plutonium to Plutonium oxide for potential reuse in nuclear fuel. Spent fuel from nuclear power reactors contains valuable quantities of the fissionable element U235 and Pu239. In fact, there is enough of this Uranium and Plutonium recovered from three nuclear reactors to fuel a fourth reactor. The high level waste is concentrated and stored in liquid form to allow decay heat to subside; then it is solidified. Waste consists of remaining any reaming Uranium and Plutonium and any other solutions from the plant decontamination operation such as mercury nitrates.

Interaction with matter: human and environmental effects

Two energy transfer mechanisms are the main source of radiation damage to living organisms. The first is ionisation in which an orbital electron is ejected form its atom to form an ion pair. The second is excitation in which an orbital electron is raised to a higher energy orbit. The energy liberated by these two mechanisms may cause radiation damage depending on the organism. Damage may occur to individual molecules or to macromolecules, such as proteins and DNA

Humans

If a human were to receive doses of 50 rem or more radiation within a few days, it may result in readily observable effects within minutes to weeks. Effects include increased temperature, vomiting and death. Doses resulting in such acute effects are unlikely to occur in routine nuclear fuel cycle operations.
However low-level radiation exposure results in an increase in the frequency of various types of cancer. Cancers caused by low levels of radiation delivered at low dose rate are usually indistinguishable from cancers caused by other factors and can be observed only statistically.

Environment

Radiation effects on animals and plants are much the same as on humans. Usually, the effect for a given radiation level is much less for nonhuman organisms. Thus, if radiation levels in the environment are maintained at a level low enough to protect individual humans, there will be no significant damage to nonhuman population.