up to trivalent state and transferred into the aqueous fraction. Plutonium is separated from uranium.
4. Re-extraction of uranium from the organic fraction by diluted nitric acid. Uranium transfers into the aqueous fraction.
No more than three cycles of the extraction – re-ex traction process are traditionally used. The number of the cycles can substantially change the content of radioactive fission products in the reprocessed fuel materials, i.e. proliferation-resistance of the extracted plutonium can be changed. If the extracted plutonium is remarkably contaminated with radioactive and heat-generating fission products, then plutonium becomes unsuitable material for manufacturing of a weapon-grade nuclear explosive device. The low number of the extraction – re-extraction cycles well corresponds with the “dirty fuel – clea n waste” concept.
Specific features in reprocessing of spent fuel assemblies discharged from fast reactors. Spent nuclear fuel discharged from fast reactors (SNF-FR) is characterized by the higher values of fuel burn-up in comparison with spent nuclear fuel discharged from thermal reactors (SNFTR). In fast reactors fuel burn-up can reach 100 GWd/t HM via 40-50
GWd/t HM in thermal reactors. As a consequence, SNF-FR contains the larger quantities of plutonium (up to 20% via 0,7% in SNF-TR) and
fission products (up to 10% via 4-5% in SNF-TR). That is why SNF-FR reprocessing encounters the following technological challenges:
1.The more intense radiolysis of the SNF solutions and organic extractants.
2.The larger content of volatile fission products (I, Kr, Xe, T) requires applying the advanced gas-absorption systems at SNF chopping and dissolving.
3.The larger plutonium content can degrade TBP efficiency due to the lower solubility of plutonium dioxide.
4.The more intense radioactivity requires applying the advanced systems for remote control of the extraction – re-extr action process.
5.Volumes of liquid high-level wastes (HLW) are about five times larger than those released in the SNF-TR reprocessing. Nearly 10 m3 HLW per one SNF ton are produced in the SNF-FR reprocessing via 1-
3m3 HLW per one SNF ton in the SNF-TR reprocessing.
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1.6. Treatment and ultimate disposal of radioactive wastes
All nuclear technologies are related with use or generation of radioactive substances. For example, fresh fuel assemblies of nuclear reactors contain radioactive isotopes of uranium; spent fuel assemblies contain radioactive isotopes of uranium, plutonium, transuranium elements and fission products. Some radioactive isotopes can be recovered from spent fuel and profitably used. Fissile isotopes can be repeatedly used (recycled) in fresh fuel compositions. Some fission products and transuranium elements are widely applied as heat sources, sources of ionizing radiation in medicine and various industrial branches. The remaining radioactive substances, whose profitable applications are unfeasible yet, are usually regarded as radioactive wastes (RAW). Thus, RAW are those radioactive substances whose profitable applications are unfeasible now.
Therefore, the following materials and products can be included into RAW composition:
1.Those products of nuclear technologies which are unsuitable now for any profitable applications.
2.All the materials and products which are contaminated with radioactive substances before their decontamination.
Specific peculiarity of RAW is a principal impossibility of their extermination by means of any traditional technology (incineration, conversion into any other chemical form). RAW remain to be radioactive in any chemical forms. Traditional technologies can only transform RAW into the forms suitable for ultimate disposal in deep underground geological repositories. Non-traditional methods of RAW extermination presume construction of the dedicated nuclear facilities where RAW are bombarded by ionizing radiation (neutrons or gamma-rays) with the only aim to transmute (convert) long-lived radioisotopes into short-lived or stable isotopes.
The most dangerous RAW are by-products of spent fuel reprocessing. These RAW are dangerous materials both in respect of their quantity and intensity of radiation emitted mainly by fission products. FP quantity in SNF discharged from thermal and fast reactors are equal to about 40-50 kg/t and up to 100 kg/t, respectively. Appropriate specific
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radioactivities of SNF discharged from thermal and fast reactors are equal to ~6 MCi/t and 20 MCi/t, respectively.
For comparison:
1.Total release of radioactive materials after Chernobyl accident is evaluated as 90 MCi.
2.Total release of radioactive materials after Kyshtym accident (explosion of liquid RAW storage) is evaluated as 20 MCi.
1.6.1. Classification of RAW
Classification of radioactive wastes
RAW are classified depending on their state of aggregation (liquid, gaseous and solid RAW) and on their specific radioactivity (low-level, middle-level and high-level RAW). The norms used by Russian regulatory bodies for classification of RAW are presented in Tables 1.7, 1.8.
Table 1.7
Classification of liquid and gaseous RAW
|
Category |
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Specific activity, Ci/l |
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Liquid |
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Gaseous |
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Low-level |
|
£ 10-5 |
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£ 10-13 |
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Middle-level |
|
10-5 – 1 |
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10 -13 - 10-9 |
|
|||
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High-level |
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|
> 1 |
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> 10-9 |
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||
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|
|
|
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Table 1.8 |
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Classification of solid RAW |
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Category |
Dose |
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Type of radiation |
|
|||
|
rate, R/h |
|
a, Ci/kg |
|
|
b, Ci/kg |
g, Gr/h |
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||
|
|
|
|
|
|
|||||
|
Low-level |
< 0,2 |
|
|
2×10-7-10-5 |
2×10-6-10-4 |
3×10-7- |
|
||
|
|
|
|
|
|
|
|
3×10-4 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Middle-level |
0,2-2 |
|
|
10-5-10-2 |
|
10-4-10-1 |
3×10-4-10-2 |
|
|
|
High-level |
> 2 |
|
|
> 10-2 |
|
> 10-1 |
> 10-2 |
|
|
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Main mission of RAW treatment is to protect humans and the environment against negative effects of radioactive materials. The most significant negative effects include ionizing radiation, heat generation and chemical toxicity.
1.6.2. Treatment of high-level RAW
There are the following two main forms of HLW:
1. HLW from radiochemical SNF reprocessing facilities.
These wastes are mainly liquid RAW because the industrial-scale SNF reprocessing is primarily based on the aqueous solvent-extraction PUREX-like technologies. As is known, the solvent-extraction reprocessing of SNF discharged from nuclear power reactors can produce about 45 m3 of liquid HLW, 150 m3 of liquid middle-level wastes (MLW) and up to 2000 m3 of liquid low-level wastes (LLW) per one ton of spent fuel.
2. Spent fuel assemblies discharged from nuclear power reactors.
In the USA, where the moratorium has been decreed on radiochemical reprocessing of spent fuel from commercial NPP, these assemblies are considered as a form of the transport RAW containers completely ready for interim storage and, further, for ultimate disposal in deep underground geological repositories.
Main stages of the HLW treatment
1.Interim storage:
a.Spent fuel assemblies are placed into the water storage pools at NPP or at SNF reprocessing plants.
b.Liquid HLW are poured into the steel storage tanks. The storage tanks are put under strict control of heat generation rate (if necessary, forced heat removal must be provided) and elemental composition of the gas cushion over the HLW level (if necessary, air blowing-through is carried out to remove explosive hydrogen produced by water radiolysis).
2.Evaporation of liquid HLW.
The HLW evaporation process provides 200-fold reduction of the HLW volume. However, the following negative effects arise:
a. Specific radioactivity of the evaporated HLW increases.
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b.Specific heat generation rate of the evaporated HLW increases too. The larger heat generation rate warms up the evaporated HLW.
c.Corrosion activity of the evaporated HLW intensifies due to the higher corrodent concentrations and to the elevated temperature.
d.Gas release intensifies too due to the radiolysis of water and some liquid HLW components.
The following countermeasures are usually undertaken:
a.Control of explosive hydrogen content in the gas cushion above the HLW level in the storage tanks.
b.Periodical air blowing-through for dilution and removal of explosive hydrogen.
c.Control of the gas cushion temperature (< 600С).
d.Forced heat removal.
e.Application of corrosion-resistant alloys and stainless steels as structural materials of the HLW evaporation facilities and the HLW storage tanks.
f.Introduction of the corrosion inhibitors into the evaporated HLW.
g.Disposition of the HLW storage tanks below the earth level on the concrete saucers.
3.Solidification of the evaporated HLW.
Main mission of the HLW solidification is to implant the HLW into a stable inert material (matrix) that can reliably prevent the HLW release into the environment and, finally, into the food chains. Migration ability of the HLW must be substantially weakened, or a reliable HLW immobilization must be guaranteed.
At present, the HLW implantation into some glass compositions, or the HLW vitrification, is considered as the most suitable form for the HLW immobilization. The following two technologies of the HLW vitrification are the most well-known:
1. One-step technology.
The liquid concentrated HLW are poured into a refractory crucible together with the glass-producing additives. Under gradual warming up, the mixture undergoes the following changes:
a.Ultimate HLW evaporation.
b.Calcination of dried HLW at 300-4000С.
c.Glass-mass melting at 1100-11500С.
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