the cladding surface. Then, these MA-containing films can be dissolved and removed by alkaline or acidic solutions.
3.Re-melting of fuel claddings into metal ingots in electrical furnaces.
4.Placement of metal ingots into the steel containers, interim storage and ultimate disposal.
One else form of solid RAW is constituted by radioactive deposits on internal walls of technological equipment units (mixers, settlers, connecting pipelines, etc.) at the SNF reprocessing plants. The radioactive deposits can be formed by the following processes:
1.Sorption of radioisotopes from the SNF solutions. Radioactivity of the walls gradually increases and can reach the values comparable with radioactivity of the SNF solutions.
2.The walls are gradually saturated with radioisotopes. After each decontamination the process of radioisotope sorption and gradual saturation reiterates.
3.Time-dependent evolution of basic thermal, physical and chemical conditions causes hardening the radioactive deposits. Only RAW components with the highest mechanical strength, chemical, radiation and thermodynamic resistance can remain on internal walls of technological equipment units. The most significant radioactive deposits contain zirconiumand silicon-based compounds (zirconates and silicates of fission products and fuel components) which are able to polymerize with formation of jelly-like materials (salts of silicic acid H2SiO3, oxides of silicon, magnesium, calcium and some alkaline metals).
Technological equipment of the SNF reprocessing plants is mainly decontaminated by the chemical desorption of solid radioactive deposits with liquid desorbing reagents. Radioactivity of the walls must be reduced to the levels acceptable for the repair or dismantling works. The desorbing process can transform solid deposits into liquid solutions for their further treatment as liquid RAW. The desorbing process is usually performed by the multiple washing-outs of technological units. At first, low concentrated solution of nitric acid HNO3 is used to dissolve and remove spent fuel residuals. Afterwards, the multiple alternation of the walls treatment by liquid desorbing solutions is undertaken to weaken the deposits and, then, to dissolve and remove them.
The most widely applied decontamination technology is based on the multiple alternation of the washing-out processes by alkaline solutions
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(chemical dissociation of ill-soluble deposits, hydration of high-density oxides and salts) and by acidic solutions (dissolution of the most friable deposits and removal them from technological units for further treatment as liquid RAW).
Control questions to Chapter 1
1.Call main categories of natural uranium resources.
2.Call main stages of hydro-metallurgical treatment of uranium ore.
3.What is the sorption process of uranium recovery based on?
4.What is the extraction process of uranium recovery based on?
5.What is the chemical precipitation process of uranium recovery based on?
6.What is the in-situ leaching process of uranium ore based on? Call main conditions for the ISL applicability.
7.What is the aqueous extraction affinage process? Call its main stages.
8.Write the material balance relationships for isotope uranium enrichment.
9.What are the basic ideological propositions for determination of the separative work scope and for determination of the separation potential function?
10.Call properties of uranium hexafluoride which are the most important for isotope uranium enrichment.
11.What technologies are used to convert uranium oxides into uranium hexafluoride?
12.What is the gas diffusion technology of isotope uranium enrichment based on?
13.What is the gas centrifuge technology of isotope uranium enrichment based on?
14.What is the separation-nozzle technology of isotope uranium enrichment based on?
15.Call main stages of the atomic vapor laser isotope separation proc-
ess.
16.Call main stages of the molecular laser isotope separation process.
17.How can the chemical methods of isotope separation be used?
18.What is the plasma technology of isotope separation based on?
19.Call main advantages and shortcomings of uranium dioxide fuel.
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20.Call main stages of the process applied for manufacturing of UOXfuel pellets.
21.What specific features can you call in the manufacturing process of MOX-fuel pellets?
22.Call main advantages and shortcomings of metal uranium fuel.
23.Call main missions of refueling in nuclear power reactors.
24.Call main stages of the batch irradiation, partial batch and uniformly partial refueling strategies.
25.Call main stages of the scatter and modified scatter refueling strategies.
26.Call main stages of the out-in refueling strategy.
27.How are the refueling works performed in thermal LWR-type reactors?
28.How are the refueling works performed in fast LMFBR-type reactors?
29.How are the refueling works performed in thermal CANDU-type reactors?
30.How are the refueling works performed in thermal RBMK-type reactors?
31.Describe briefly a typical transport cask for spent fuel assemblies.
32.What requirements must the transport cask design satisfy?
33.What tests must a typical transport cask undergo?
34.Call main missions of spent fuel reprocessing.
35.Call main categories of the spent fuel reprocessing technologies.
36.Call main stages of the aqueous solvent-extraction technology.
37.How does TBP radiolysis occur? What negative consequences does TBP radiolysis lead to?
38.What technology is applied to separate plutonium from uranium?
39.Call main RAW components.
40.Call main RAW categories.
41.Call main stages of high-level RAW treatment.
42.What requirements must geological formations satisfy to be suitable for ultimate disposal of RAW?
43.What geological formations are under estimation now as potential candidates for ultimate disposal of RAW? Call their main advantages and drawbacks.
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44.What are the main difficulties for ultimate disposal of RAW in the Yucca Mountain geological repository?
45.Call main stages of middle-level and low-level RAW treatment.
46.What materials are currently used for RAW immobilization? Range them according to their relative immobilization quality and explain it.
47.Call main stages of solid RAW treatment.
48.What technology is used to decontaminate technological equipment of the SNF reprocessing plants?
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CHAPTER 2. NEGATIVE ECOLOGICAL EFFECTS AT VARIOUS STAGES OF THE CLOSED NUCLEAR FUEL CYCLE
All enterprises of nuclear fuel cycle deal with nuclear materials which are potentially able to impact negatively on the environment and human organisms. Even the least harmful nuclear material – uranium ore – is a radioactive substance, weak emitter of α-particles and spontaneous fission neutrons from decays of uranium isotopes 235U and 238U.
Acceptable specific activity of α-emitters equals 0.2 µCi/kg. As it was noted above, the mined uranium ore contains, in average, about
0.1% U, i.e. 1 kg of uranium ore contains 1 g U which, in its turn, contains 993 mg 238U, 7 mg 235U and 0.05 mg 234U. Radioactivity A of ra-
dionuclide with mass m (grams), atomic weight M (a.m.u.) and half-life Т1/2 (s) can be calculated by using the formula:
А(μCi) = |
m |
× Na |
× |
ln 2 |
× |
10−4 |
; |
|
T |
|
|||||
|
M |
|
3.7 |
|
|||
|
|
|
1/ 2 |
|
|
|
|
where Na – the Avogadro number, 6.023 ·1023 nuclei per a mol.
It is easy to determine the following contributions of uranium iso-
topes into total specific radioactivity of uranium ore: А(238U) ≈ 0.332 µCi/kg; А(235U) ≈ 0.015 µCi/kg; А(234U) ≈ 0.332 µCi/kg.
As is seen, contribution of only uranium isotopes (~0.7 µCi/kg) into specific radioactivity of uranium ore exceeds the acceptable level by a factor of 3.5. However, uranium ore contains radioactive daughter products of 235U and 238U decay families. The chains of radioactive decays start from 235U and 238U, many new radionuclides are produced which, in their turn, undergo decays via their own channels (α and β-decays, isomeric transitions) and so on. Final products of 235U and 238U decay chains are stable lead isotopes 207Pb and 206Pb, respectively. The decay rate (radioactivity) of each intermediate radionuclide equals the decay rate (radioactivity) of its predecessor since for the elapsed many billion years the so-called eternal equilibrium established between all members of decay families. So, radioactivity of any intermediate radionuclide equals radioactivity of start radionuclide. Thus, total uranium radioactivity can be calculated as radioactivity of 238U multiplied by the number of its daughter
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