Крючков Фундаменталс оф Нуцлеар Материалс Пхысицал Протецтион 2011

oxide fuel. The starting inventory of a 1 GWe reactor is about 100 t of UO2. It generates about 200 kg of plutonium per GWe×year.

Channel-type reactors (RBMK). RBMK reactors using graphite moderator and light-water coolant run on uranium oxide fuel of low enrichment (1.8–2 % 235U). The first core of a 1 GWe reactor contains about 150–180 t of UO 2. It produces approximately 250 kg of plutonium per GWe×year.

The proliferation hazard presented by RBMKs lies in the possibility of channel-by-channel refueling without reactor shutdown.

4. Heavy water CANDU reactors

CANDU reactors are fuelled with natural uranium. The first core of a 1 GWe reactor takes approximately 100 t of UO2. Every year, it will generate 350 kg/GWe.

CANDU reactors operate under conditions of continuous refueling carried out without reactor shutdown. This can pose certain difficulties for inspecting the way primary fuel is used and secondary fuel accumulated.

5. Fast breeder reactors

Fast reactors run on uranium oxide fuel of medium enrichment (15–25 % 235U). The inventory of a 1 GWe reactor is 10–15 t, wh ich means that the

core contains 2–3 t of 235U. It generates plutonium at a rate of about 1500 kg/(GWe×year) in an open fuel cycle. If the fuel cycle is closed, about 80 % of plutonium will be recycled, and its net output will approach 250 kg/(GWe)×year.

Natural NM mining and primary processing

High chemical activity of uranium and thorium explains why they occur in nature only in chemical compounds. All in all, about 200 uranium and thorium minerals were discovered.

The known rich uranium deposits are relatively scarce. The total uranium reserves in the Earth crust are estimated at 1014 t, with another ~ 4×109 t found in the water of seas and oceans (3.3 mg/m3 on the average).

Ore varies in uranium content, appearing as:

·very rich ore: more than 1 % of U;

·rich ore: 0.5–1 % of U;

·medium-grade ore: 0.25–0.5 % of U;

·run-of-mine ore: 0.09–0.25 % of U;

·lean ore: less than 0.09 % of U.

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Generally, mined ore contains about 0.1 % of uranium – these are lean and run-of-mine ores.

Uranium reserves are estimated by two cost criteria as:

∙cheap uranium, costing under $ 80 per 1 kg U3O8; and

∙expensive uranium, costing over $ 80 per 1 kg U3O8.

The threshold value of $ 80 per 1 kg U3O8 was adopted at the time when it marked the border of NPP competitiveness against coal-fired plants.

More than 80 % of proven and probable uranium reserves are found in 8 countries of America (USA, Canada, Brazil), Australia and Africa (SAR, Niger, Angola, Namibia). Two countries, namely, Canada and Australia, produce over 50 % of globally mined uranium. The CIS accounts for about 15 % of the world’s resources. The annual uranium output is not large enough to meet the requirements of global nuclear power. The deficit is offset by using reserves, waste processing, and recycling.

There are four methods of uranium production, i.e.:

∙underground mining;

∙quarrying;

∙in-situ leaching;

∙recovery from seawater.

The stage of ore mining is followed by its hydrometallurgical processing. Uranium can be extracted owing to high solubility of uranium oxides in acid and alkaline solutions.

Hydrometallurgical processing results in dry concentrate of uranium oxides (mainly, U3O8). The concentrate will contain all the uranium found before in the ore, as well as impurities (with 95–9 6 % accounted for by uranium oxides and 4–5 % by impurities). The impuri ties, which have to be removed from the concentrate, include powerful neutron absorbers: B, Cd, Hf and rare-earth elements (Eu, Gd, Sm).

The next stage is uranium concentrate purification using a refining process.

The most common method is refining by extraction, with tributylphosphate (TBP) serving as an extracting agent. The TBP density (0.973 g/cm3) is slightly lower than that of water. To reduce its viscosity, TBP is dissolved in neutral organic compounds. An important property of TBP is its capability of selectively extracting uranium compounds. Uranyl nitrate UO2(NO3)2 is extracted by this agent from a mixture 104 times more efficiently than are impurities.

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One of the shortcomings of uranium hexafluoride is its high chemical activity. It reacts with air and steam, giving rise to uranium tetrafluoride UF4 (powder), which deposits in process circuits. That is why circuits and containers should be made leak-tight, should be desiccated and degreased. Ni, Al, Mg, Cu and their alloys as well as teflon show resistance when exposed to UF6.

Uranium oxide conversion to uranium hexafluoride. For isotopic enrichment, the concentrate of U3O8 has to be turned into UF6. This can be done in a two-step process. First, a reaction with fluorine gas yields uranyl fluoride UO2F2:

U3O8 + 3F2 → 3UO2F2 + O2 at 350–370 оС,

whereupon uranium hexafluoride is formed by a reaction of uranyl fluoride with fluorine at a lower temperature of 270 оС:

UO2F2 + 2F2 → UF6 + O2.

A single-stage process (flame method of direct fluorination) can only take place with excess fluorine and at much higher temperatures (900–1000 оС):

U3O8 + 9F2 → 3UF6 + 4O2 .

Production of uranium hexafluoride from uranium dioxide (UO2) is achieved by using another two-step process. First, UO2 reacts with hydrofluoric acid (HF) at 500–600 оС to form uranium tetrafluoride (UF4):

UO2 + 4HF → UF4 + 2H2O, whereupon UF4 reacts with fluorine at 400 оС:

UF4 +F2 = UF6.

1. Uranium enrichment by gas diffusion (GD)

The enrichment GD technology is based on different speeds of thermal motion of heavy and light molecules, and on the ease with which the latter go through thin porous partitions.

In a mixture of two gases with the same temperature, light and heavy molecules have the same average kinetic energy:

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The advantage of this method over GC lies in the absence of rotating components, but the small size of separation slots (a fraction of a millimeter) calls for precision assembly. The uranium enrichment factor for the separating nozzle is e¢ = 0.025.

4. Laser methods

Laser methods of uranium separation are based on the difference in the arrangement of excited energy levels of electrons in atoms of 238U and 235U, which is made possible by use of a monochromatic laser. Excitation of electron shells leads to selective intensification of physical or chemical processes (intensified ionization of excited atoms or intensified dissociation of excited molecules). The conditions for laser-based enrichment include:

1.Presence of an excited electron level unique to one isotope. This level should be sufficiently far removed from other spectrum lines and from lines of other isotopes.

2.Availability of a laser tuned to an appropriate radiation frequency.

3.Existence of processes separating excited atoms and molecules.

Laser-based enrichment of uranium vapors. This enrichment method consists of the following stages:

·evaporation of U atoms in vacuum (an electron beam knocks out U vapors from the U–Re alloy);

·irradiation by a xenon laser with ensuing selective excitation of 235U atoms;

·irradiation by a krypton laser with ensuing selective ionization of excited 235U atoms;

·collection of ionized 235U atoms on a charged plate.

Laser-based enrichment of UF6 molecules. This process has three stages:

·cooling of the UF6 + H2 mixture to 30 К for the UF6 molecules to be mostly in an unexcited state;

·irradiation by an infrared laser with ensuing selective excitation of 235UF6 molecules;

·irradiation by an ultraviolet laser with ensuing selective dissociation of excited molecules:

2 × 235UF6 * ® 2 × 235UF5 + F2.

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White powder of 235UF5 (“laser snow”) precipitates from the gas mixture.

The enrichment factor of laser methods is as high as 3 to 15. They offer a possibility to use the waste of GD plants (containing 0.2–0,3 % of U) for recovery of uranium suitable for light water power reactors.

5. Chemical enrichment methods

Chemical methods depend on different stability of isotopes in compounds. A contact between chemical compounds of one element with different valences leads to isotope exchange. Thus, boron can be enriched in isotope 10B via an isotope exchange reaction:

BF3 + BF3O(CH3)2 ® 11BF3 + 10BF3O(CH3)2 ,

in which 10B builds up in the organic phase.

Heavy water generation results from an isotope exchange reaction between light water and hydrogen sulfide:

H2O + HDS ® HDO + H2S,

with deuterium accumulating in the water phase.

The USA and Japan are developing a uranium enrichment process involving UF6 and NOUF6 as contacting compounds. This is essentially oxidation–reduction chromatography achieved by alte rnation of oxidation reactions (addition of oxygen gas) and reduction reactions (addition of

hydrogen gas). In this case it is possible to separate compounds including ions of UO2++(with 6-valent 235U) and U4+(with 4-valent 238U). The

experimentally obtained separation factors reached 1.08 and power consumption proved to be up to 150 KWh/SWU.

6. Plasma method

This method is based on the effect of ion cyclotron resonance.

Charged particles (ions) moving about in a magnetostatic field experience the action of a force

F = q × [V × B],

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causing them to spiral about magnetic field lines. The orbital radius is found from the following equation:

F = q × V × B = mV 2/R ;

R = m× V /(q × B) = (2m × E)1/2/(q × B),

while the rotation frequency w = V/R = q × B/m is the ion cyclotron frequency (ICF).

If a variable electric field is applied, with its frequency equal to the ICF of a specific isotope, this will be the only isotope to take up the field energy. A higher energy of ions of a certain isotope will increase the radius of its rotation about magnetic field lines. It is then possible to separate ions of different isotopes and to accumulate ions of 235U and 238U selectively on appropriately arranged collectors.

Uranium enrichment technologies viewed in the context of non-proliferation of nuclear weapons

Gas diffusion:

a)technically sophisticated process;

b)high power requirements (2300–2600 kWh/SWU); one GD facility in the USA consumes about 5 GWe;

c)relatively low enrichment factor (e¢ = 0.0043);

d)unlikely secret construction of a GD facility.

Gas centrifugation:

a)technically sophisticated process;

b)low power requirements (100–300 kWh/SWU) and hig h enrichment

factor (e¢= 0.2–0.3) make this technology hazardous from the viewpoint of non-proliferation.

Separating nozzle:

a)less sophisticated than GD and GC processes;

b)low enrichment factor (e¢ = 0.025) and high power requirements (3000 kWh/SWU) make this method a lesser proliferation hazard as compared with GD and GC processes.

Laser methods:

a)the highest enrichment factor (e¢ = 3–15) and the lowest energy requirements (10–50 kWh/SWU);

b)the most sophisticated process; the most promising enrichment technology and the most hazardous process from the non-proliferation point of view.

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