Апсе Нуцлеар Течнологиес 2014

products. Since the ternary fissions can occur with very low probability, spent fuel contains only about 2×10-5 % 3H.

The following properties of tritium should be taken into account:

1.Tritium is a source of low-energy b-radiation.

2.Tritium can actively enter into the isotope exchange reaction with light water thus forming tritium water HTO or T2O at the SNF reprocessing. Therefore, tritium is present in all liquid RAW produced by aqueous technologies of the SNF reprocessing.

The following methods are used to remove tritium from gaseous RAW composition:

1.Voloxidation of spent fuel before its dissolution: oxidation by air at the elevated temperatures, 450-6500C. The air humidity can bind tritium into tritium water for further treatment as liquid RAW.

2.Chemisorption of tritium water by zeolite.

3.Light-water washing-out of organic TBP-containing fraction after the solvent-extraction process.

Treatment of volatile aerosols and dust. Volatile aerosols and dust

are gas-like materials with specific content of solid and liquid particles within the range from 10-2 g/m3 to 10 g/m3. The following methods are used to treat volatile aerosols and dust:

1.Gravitational deposition in the dust-collecting chambers.

2.Centrifugal removal of solid and liquid particles in cyclones. The gas flow enters into a cylindrical vessel at an angle to its vertical axis. Solid and liquid particles can strike against the wall and drop out of the gas flow.

3.Electrostatic deposition (imparting an electrical charge to solid and liquid particles and their deposition by electrical field).

4.Gas washing-out in scrubbers.

5.Ultra-filtration with application of the dedicated filters made of fiberglasses, polymers, metal-tissue and metal-ceramic materials.

Treatment of solid RAW

The following materials and products are the main components of solid RAW:

1.Details of nuclear equipment, construction materials, rubbish and working clothes before their decontamination.

2.Ion-exchange resins and filters.

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3.Metal claddings of fuel rods.

4.Deposits on internal wall surfaces of technological equipment. The following methods are used to treat solid RAW:

1.Reduction of RAW volume:

a.Incineration with up to 100-fold reduction of RAW volume.

b.Pressing with up to 10-fold reduction of RAW volume.

In total, RAW volume can be decreased by a factor of about 1000.

2. Placement of the remaining RAW into the steel containers, interim storage and ultimate disposal.

A special technology is used to treat radioactive claddings of fuel rods. The following processes caused their radioactivity:

1.Neutron irradiation in nuclear power reactors can transform some stable isotopes of iron, chromium, nickel, molybdenum and other constituents of stainless steels into appropriate radioisotopes.

2.Fission products and minor actinides can migrate from fuel meat to

fuel cladding. That is why fuel claddings can contain α-active radioisotopes.

3. Residuals of undissolved spent fuel.

Treatment of fuel claddings includes the following main stages:

1.Interim storage in concrete shelters under water layer (zirconium particles are pyrophoric in air).

2.Chemical treatment by hydrofluoric acid HF at the elevated temperatures (550-6000С). This treatment can form superficial friable films on 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.

Decontamination of technological equipment at the SNF reprocessing plants

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:

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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 (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

1.Call main RAW components.

2.Call main RAW categories.

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3.Call main stages of high-level RAW treatment.

4.What requirements must geological formations satisfy to be suitable for ultimate disposal of RAW?

5.What geological formations are under estimation now as potential candidates for ultimate disposal of RAW? Call their main advantages and drawbacks.

6.What are the main difficulties for ultimate disposal of RAW in the Yucca Mountain geological repository?

7.Call main stages of middle-level and low-level RAW treatment.

8.What materials are currently used for RAW immobilization? Range them according to their relative immobilization quality and explain it.

9.Call main stages of solid RAW treatment.

10.What technology is used to decontaminate technological equipment of the SNF reprocessing plants?

List of references

1.Sinev N.M., Baturov B.B. Economics of nuclear power industry. Fundamentals of nuclear fuel technology and economics. - Moscow, Energoatomizdat, 1984.

2.Zemlyanukhin V.I., et al. Radiochemical reprocessing of spent nuclear fuel. - Moscow, Energoatomizdat, 1989.

3.Rahn F.J., Adamantiades A.G., Kenton J.E., Braun C. A guide to nuclear power technology. A resource for decision making. – New York, John Wiley & Sons, Inc., 1984.

4.Waltar A.E., Reynolds A.B. Fast breeder reactors. – Pergamon Press, 1981.

5.Waltar A.E., Todd D.R., Tsvetkov P.V. Fast spectrum reactors. – Springer Science and Business Media, 2012.

6.Gardner G.T. Nuclear nonproliferation. A primer. – Boulder, Lynne Rienner Publishers, Inc., 1994.

7.Pshakin G.M., et al. Nuclear non-proliferation. - Moscow, MEPhI, 2006.

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