High Level Wastes
The term high level waste (HLW) is used, generally, for the
highly radioactive solutions of fission products separated during chemical
processing of irradiated fuel elements. Reprocessing involves solvent extraction
and these wastes occur as raffinate from the first solvent extraction cycle (FSEC).
HLW generate significant amounts of decay heat and require special cooling for a
number of years after separation and concentration. HLW is concentrated and
stored in cooled tanks pending solidification into a form suitable for disposal.
HLW contain more than 99% of non-volatile fission products. With good commercial
practice they contain no more than 0.5% of uranium and plutonium present in the
spent fuel. The other actinides formed by transmutation of uranium and plutonium
in reactors (neptunium, americium, curium etc.) are not separated presently, and
remain in the HLW.
These wastes consist of solid fragments of fuel element
cladding (i.e. zircaloy and stainless steel) and other structural components of
the fuel assemblies. They contain neutron activation products, and are
contaminated by actinides and fission products. Their volume after compaction is
similar to the volume of solidified HLW from the same fuel. At present cladding
hulls are, usually, stored under water.
These solid or solidified wastes (which may be termed actinides or transuranium wastes) are produced essentially at reprocessing plants and mixed oxide (UO2/PuO2) fuel fabrication plants, and their common characteristic is a concentration level of the alpha emitters, which require special management procedures. They also may be contaminated with b - g emitters at various levels, both high level wastes and cladding hulls will have to be considered eventually as alpha wastes.
Other Types of Wastes
An additional category of long-lived radioactive wastes is uranium ore tailings. Considering the low concentration of radionuclides and the nature of the wastes, deep geologic disposal is not considered feasible for the bulk of these wastes. Tailings are produced during the mining and milling of uranium ore. They contain small amount of unrecovered uranium and most of the original daughter products, among which 222Rn , 226Ra and 230Th present the greater radiological hazard. At present tailings are, usually, left on the surface. In order to limit radon emanations and wind-borne dispersion of radioactive particles, the piles may be covered with earth and stabilized
Isolation of the wastes from the biosphere for millions of years is an unattainable and irrational objective, because at a certain time the radiotoxicity of the waste becomes controlled by natural radioactive elements and is no longer greater than the radiotoxicity of the fissile material (with daughters) destroyed by fission. In reality, radiotoxicity is not an indicator of the hazard associated with waste, because no detriment can take place unless the radionuclides reach the biosphere. Therefore, the mobility of the radionuclides in the geosphere, in the situation of geologic disposal, or the biologic availability after disposal into the environment, control the expected health effects produced by the radionuclides. Still, the variation of radiotoxicity with time can be used to give some perspective to the waste isolation problem, by allowing comparisons with deposits of naturally radioactive ores and other radioactive substances, provided the limitation due to the relative availability of the materials is borne in mind.
Available data show that in less than 1000 years the radiotoxicity of HLW becomes lower than that of uranium (with daughters) that had to be mined to produce the fuel that generated the waste. However, the mined uranium is not a fair term for comparison because most of it is in existence yet, and will continue to represent a risk for man and environment. A more appropriate term for comparison is uranium destroyed by fission because the risk due to this material is no longer in existence, at least once the daughters have decayed, and has been replaced by the risk associated with the waste. The radiotoxicity of the waste reaches the level of uranium destroyed in the reactor after several hundred thousand years. After about 50,000 years of decay the radiotoxicity of these wastes is already similar (only 2 to 3 times greater) to that of the consumed uranium. In practice, the radiological consequences of water intrusion into repository and waste leaching after 50,000 to 100,000 years of decay would not be significantly different from the consequences of leaching a uranium deposit containing an amount of uranium equivalent to that which has been consumed to produce the waste. It also must be considered that many uranium deposits mined to produce nuclear fuel are relatively close to the surface, and are less isolated from the biosphere than a repository. Uranium ore tailings, which presently, are left at the surface, are even less isolated from the biosphere, and are likely to cause higher collective doses than either undisturbed uranium deposits or deep waste repositories.