6. Isotropy of crystalline lattice that can simplify the process of hightemperature sintering.
Main shortcomings of uranium dioxide:
1. Low hat conductivity and its sharp reduction at the elevated temperatures (8,4 W/m×К at 450С and 2,4 W/m×К at 13270С). Such low values and temperature dependency of UOX heat conductivity results in very large temperature gradients inside of very thin (R ~ 3 mm) fuel pellets (DТ ~ 15000С at the distance of 3-4 mm).
2.Intense oxidation ability by wet air at ambient temperature (hygroscopicity). This effect requires an inert dry environment 0r vacuum for UOX-fuel pellets manufacturing. Otherwise, superficial layers of UOXfuel pellets can be saturated with water and oxygen. Later on, during the reactor operation, the moisture released from the pellet surface can cause hydration of the cladding materials and destruction of fuel rods.
3.The presence of oxygen in UOX-fuel composition softens neutron spectrum and, thus, decreases the secondary fuel production rate.
1.3.1. Pelletization of uranium dioxide
The following processes are used now to produce UOX-fuel pellets: 1. Conversion of uranium hexafluoride into uranium dioxide. Two conversion technologies have been developed and currently used:
a.“Wet” technology of the AUC-process:
·Barbotage of gaseous uranium hexafluoride through aqueous solution of ammonium carbonate (NH4)2CO3 followed by precipitation of solid insoluble deposit of ammonium-uranyl-carbonate (AUC) -
(NH4)4UO2(CO3)3.
·Heat treatment of AUC at 550-6500С followed by thermal AUC dissociation with the formation of finely dispersed UOX powder.
b. “Dry” technology:
·Hydrolysis of uranium hexafluoride by water vapor at 150-3000С with the formation of uranyl-fluoride UO2F2:
UF6 + 2H2O ® UO2F2 + 4HF.
· Pyrohydrolysis of uranyl-fluoride by hydrogen at ~5500C with the formation of finely dispersed UOX powder and hydrofluoric acid.
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UO2F2 + H2 → UO2 + 2HF.
Thus, the finely dispersed UO2 powder is produced. Unfortunately, such small-sized powder is unsuitable for manufacturing of UOX-fuel pellets by pressing because of too small dimensions of the powder particles (below 0,5 micron). The following procedures should be performed to enlarge the powder particles:
2.Mixing of UO2 powder with an organic plasticizer (polyvinyl, glycidol and so on).
3.Hydro-compaction of the powder-plasticizer mixture: the mixture is placed into a plastic form; the plastic form is placed into a reservoir filled up with water, uniform omni-directional pressing, and production of the powder-plasticizer briquettes.
4.Granulation of the briquettes by milling.
5.Annealing at 600-8000С for removal of organic plasticizers.
6.Cold pressing of pellets (p = 1500-2000 atmospheres).
7.Sintering of UOX-fuel pellets at 1600-17000С.
8.Quality control of UOX-fuel pellets (sizes, content of carbon as a residual of organic plasticizers, stoichiometry).
The manufacturing process of UOX-fuel pellets usually associated with the manufacturing process of mixed oxide (MOX), mainly ura- nium-plutonium, fuel pellets. In principle, the following three MOXfuel compositions are feasible:
1.PuO2 + 238UO2, where plutonium is taken from the weapon-grade nuclear materials (weapon-grade plutonium).
2.PuO2 + 238UO2, where plutonium is extracted from spent fuel of nu-
clear power reactors (reactor-grade plutonium).
3. 235UO2 + 238UO2, where 235U is taken from the weapon-grade nuclear materials (weapon-grade uranium).
Anyway, there is a distinction of principle in the manufacturing process of UOX-fuel pellets from a single feed flow and the manufacturing process of MOX-fuel pellets from two different feed flows. In the former case, natural uranium is a single feed material which, after a series of technological operations, converts into the enriched uranium dioxide and, then, into UOX-fuel pellets. At all these operations, 235U nuclei were uniformly mixed with 238U nuclei. In the latter case, on the
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contrary, the manufacturing process of MOX-fuel pellets is based on technological operations with two different feed flows:
1.The fertile material fraction, i.e. 238UO2 powder made of depleted or natural uranium.
2.The fissile material fraction, i.e. PuO2 powder made of weapon-grade or reactor-grade plutonium, or 235UO2 powder made of weapon-grade uranium.
Here, the homogeneity of the fertile-fissile fractional mixture is not guaranteed. So, a high degree of homogeneity must be ensured in the blending process of the fertile and fissile components. The blending process of two different feed flows is the only stage that distinguishes the MOX-fuel manufacturing technology from the UOX-fuel manufacturing technology.
The homogeneous mixture of the fissile and fertile components can
ensure the safer operation of nuclear power reactors because fertile isotope 238U and fissile isotopes 235U, 239Pu can cause quite different reactivity effects under accidental conditions. If the reactor power increased, then both fuel components warmed up but fissile isotopes, main contributors into the chain fission reaction, warmed up in the first turn. Fertile isotopes can warm up with some time delay, and the better homogeneity of fuel composition results in the shorter time delay of the fertile component warming up. Temperature increasing of the fissile component causes the Doppler effect that leads to the energy extension of the resonances in neutron capture and fission cross-sections. As a rule, the Doppler effect of fissile isotopes can cause a relatively small but positive reactivity change (increment). As a rule too, the Doppler effect of fertile isotopes can cause a large and negative reactivity change (decrement). If the Doppler effects of fissile and fertile isotopes occur simultaneously (the best case) or with only short time delay, then the reactivity stabilization effect of fertile isotopes can be in a due time for neutralization of the positive reactivity change caused by fissile isotopes warming up. If the time delay between actuation of the reactivity increment caused by fissile isotopes and the reactivity decrement caused by fertile isotopes would be remarkably long (the worst case), then the reactivity increment of fissile isotope can have a sufficiently long time interval to increase the reactor power up to an unacceptably high level
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till the stabilizing reactivity decrement of fertile isotopes would begin acting.
At initial stage of nuclear power development in the world some two-purpose thermal reactors were fueled with metal natural uranium (the UK “Magnox” reactors, for instance). These rea ctors were characterized by relatively low values of fuel burn-up and thermal energy generation rate.
Metal uranium has the following advantages:
1.High density (18,7 g/cm3 via 10,96 g/cmм3 of uranium dioxide).
2.The better neutron balance in the reactor core leads to the lower values of uranium enrichment and annual uranium consumption.
3.High heat conductivity (30 W/m×К via 3 W/m×К of uranium dioxide).
4.High heat generation rate and, thus, small sizes of the reactor core.
5.The higher values of breeding ratio.
6.Simplicity and cheapness of metal uranium fuel manufacturing. However, the developers had to decline the further usage of metal
uranium fuel in contemporary designs of nuclear power reactors and decided to use UO2-based fuel preferentially. This decision was validated by the following shortcomings of metal uranium fuel:
1.Incompatibility with light-water coolant. If some defects in the fuel cladding appeared, then metal uranium is intensely dissolved by hot water, and radioactive fission products can release from fuel meat and spread into NPP circuitry.
2.Instability of fuel sizes under high values of fuel burn-up, neutron flux and fluence. The radiation damages of metal uranium fuel pass the following three consecutive phases as the fuel temperature increases:
a. Irradiation-induced anisotropic growth of the metal grain sizes and irradiation creep at temperatures below 5000C.
b. Cavitation swelling within the temperature range from 3700C to 5000С. The cavitation swelling is caused by the formation of irregular pores within the temperature range where mechanical stresses caused by the irradiation-induced growth of the metal grain sizes still take place also. As a consequence, mechanical strength of metal uranium fuel is substantially weakened, especially along the metal grain boundaries.
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c. Gas swelling at temperatures above 5000C. The gas swelling effect of metal uranium fuel is caused by gaseous fission products which are preferentially accumulated on the grain boundaries.
Specific volumetric swelling V/V of metal uranium fuel covers the range of 4-6% per one percent of fuel burn-up. Typical values of maximal fuel burn-up in thermal reactors are equal to 4-5% HM. Therefore, initial porosity of metal uranium fuel must be equal to 25% for neutralization of the swelling effect. In fast reactors maximal fuel burn-up can reach 10% HM. So, initial porosity of metal uranium fuel must be increased up to 50% for the same purpose. It seems unreasonable to deal with so porous fuel. The high-density advantage of metal uranium fuel practically disappears.
Uranium dioxide is superior to metal uranium in specific swelling values. Specific swelling of uranium dioxide is about 1,4-1,5% V/V only per one percent of fuel burn-up. At 10% fuel burn-up in fast reactors initial porosity of uranium dioxide fuel can be below 15%. For comparison, specific swellings of uranium nitride and uranium carbide are equal to 1,5-1,6% V/V and 1,7-1,8% V/V per one percent of fuel burn-up, respectively, i.e. only slightly larger than that of uranium dioxide.
A lot of experimental studies have been carried out to eliminate this shortcoming of metal uranium by its alloying. Some promising results were obtained with the alloying components such as molybdenum, zirconium, silicon, iron, aluminum and fissium (imitator of FP composition). Metal uranium alloying with molybdenum and zirconium (up to 10%) allowed it to upgrade corrosion resistance in water and stability of the grain sizes for temperatures up to 6000С.
Metal uranium is produced in reaction of uranium tetra-fluoride UF4 with high-purity metals (calcium or magnesium):
UF4 + 2 Mg → 2 MgF2 + U.
Magnesium fluoride as a light slag is easily removed from surface of a metal uranium ingot.
The next step is a vacuum melting of the uranium ingot for removal of volatile impurities and introduction of the alloying components,
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