Uranium Radiation Properties
(last updated 13 Dec 2006)
Contents:
> see also:
Uranium is a metal of high density (18.9 g/cm3). The earth's crust contains an average of about 3 ppm (= 3 g/t) uranium, and seawater approximately 3 ppb (= 3 mg/t).
Naturally occuring uranium consists of three isotopes, all of which are radioactive: U-238, U-235, and U-234. U-238 and U-235 are the parent nuclides of two independent decay series, while U-234 is a decay product of the U-238 series.
Properties of the Natural Uranium Isotopes
| U-234 | U-235 | U-238 |
| half-life | 244,500 years | 703.8 · 106 years | 4.468 · 109 years |
| specific activity | 231.3 MBq/g | 80,011 Bq/g | 12,445 Bq/g |
Isotopic Composition of Natural Uranium
| U-234 | U-235 | U-238 | Total |
| atom % | 0.0054% | 0.72% | 99.275% | 100% |
| weight % | 0.0053% | 0.711% | 99.284% | 100% |
| activity % | 48.9% | 2.2% | 48.9% | 100% |
| activity in 1 g Unat | 12,356 Bq | 568 Bq | 12,356 Bq | 25,280 Bq |
see also:
only major decays shown
* in addition, all decays emit gamma radiation
Average decay energies of U-238 series
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only major decays shown
* in addition, all decays emit gamma radiation
Average decay energies of U-235 series
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Ac: Actinium
Bi: Bismuth
Pa: Protactinium
Pb: Lead
Po: Polonium
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Ra: Radium
Rn: Radon
Th: Thorium
Tl: Thallium
U: Uranium
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In natural uranium, these decay chains generally are in secular equilibrium. This means that in 1 g of natural uranium each nuclide of the U-238 series has an activity of 12,356 Bq and each nuclide of the U-235 series an activity of 568 Bq.
In the various processing steps of nuclear fuel production, the equilibrium is destroyed.
In an uranium ore deposit, secular equilibrium obtains between U-238 and its decay products, and between U-235 and its decay products. The equilibrium may be somewhat disturbed by geochemical migration processes in the ore deposit.
An ore grade of 1% U3O8 is equivalent to 0.848% U, and 1 million lbs U3O8 are equivalent to 385 metric tonnes of U.
(see also Unit Converter: Uranium concentration (wt.) · Uranium weight)
Uranium Ore Activity (U-238 series)
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In case of an undisturbed uranium deposit, the activity of all decay products remains constant for hundreds of millions of years. (see also: Uranium Decay Calculator)
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The radiation is virtually trapped underground; exposures are only possible if contaminated groundwater, that is circulating through the deposit, is used for drinking. Radon is of no concern for deep deposits, though it can travel through underground fissures, since it decays before it can reach the surface.
The situation changes completely, when the deposit is mined: Radon gas can escape into the air, ore dust can be blown by the wind, and contaminants can be leached and seep into surface water bodies and groundwater.
The alpha radiation of the 8 alpha emitting nuclides contained in the U-238 series (and to a lesser degree, of the 7 alpha emitters in the U-235 series) presents a radiation hazard on ingestion or inhalation of uranium ore (dust) and radon. The gamma radiation mainly of Pb-214 and Bi-214, together with the beta radiation of Th-234, Pa-234m, Pb-214, Bi-214, and Bi-210, presents an external radiation hazard. For ingestion and inhalation, also the chemical toxicity of uranium has to be taken into account.
See also Impacts of Uranium In-Situ Leaching
The uranium concentration in the solution produced from in-situ leaching wells depends on a number of parameters, an important one being time from startup. The initial concentration from an individual well soon peaks in a few days at values of typically 300 - 600 mg/l and then declines rapidly. The decline slows down as the concentration reaches 30 - 50 mg/l. The well is usually shut in when the concentration reaches 10 - 20 mg/l after 8 - 18 months operation. Average uranium concentrations are typically 40 - 70 mg/l. [IAEA1989 p.17]
Various decay products of uranium are also leached and can reach considerable activities in the leaching solution, depending on the leaching agent used. During processing of the solution, large amounts of the radon contained escape into the atmosphere, while the other decay products are transferred to the waste solutions. Those solutions usually are dumped in deep aquifers through disposal wells, or evaporated in ponds, resulting in a concentrated waste slurry.
See also: Radon Individual Dose Calculator
Most of the radiation hazard results from the inhalation of the short-lived radon progeny (Po-218, Pb-214, Bi-214, and Po-214). Radon itself is of minor concern, since most of the inhaled radon is exhaled. And most of the longer-lived Pb-210 (22 year half-life) and its progeny are eliminated from the body before they decay.
Radon and Progeny Activity
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Long-lived Radon Progeny Activity
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There are a number of units in use to describe radon and radon progeny activity concentration in air: (see also Unit Converter: Activity conc. radon <-> radon progeny)
- Bq/m3 and pCi/l for the activity concentration of radon-222 in air:
1 pCi/l (pico-Curie or 10-12 Curie per liter) is equivalent to 37 Bq/m3 (Becquerel per cubic meter)
- J/m3, WL and Bq/m3 EEC for the activity concentration of radon-222 progeny in air:
1 WL (Working Level) is equivalent to 101.3 pCi/l (3746 Bq/m3) of radon-222 in equilibrium with its short-lived decay products.
1 J/m3 (Joule per cubic meter) is equivalent to 1.8 · 108 Bq/m3 of radon-222 in equilibrium with its short-lived decay products.
EEC (equilibrium equivalent concentration) is defined as the activity concentration of radon in radioactive equilibrium with its short-lived progeny that has the same potential alpha energy concentration as the actual non-equilibrium mixture.
The unit of WLM describes exposure to radon-222 progeny:
1 WLM (Working Level Month) is defined as the exposure to 1 WL during 170 hours.
The Equilibrium Factor describes the fraction of potential alpha decay energy of the short-lived radon decay products, compared to secular equilibrium. The equilibrium factor is defined as:
F = (0.106 cPo-218 + 0.514 cPb-214 + 0.380 cBi-214) / cRn-222
where cx stands for the activity concentration of the nuclide x.
Indoors, the equilibrium factor is depending on the ventilation rate; outdoors it is depending on distance from the source and wind speed.
Typical values are 0.4 for indoors or work, and 0.8 for outdoors.
The uranium ore concentrate ("Yellow Cake") produced in the milling process contains a mixed oxide usually referred to as "U3O8" (UO2 · 2 UO3). Due to a number of impurities contained, it needs further refining before it can be used for nuclear fuel production. 1 t U3O8 is equivalent to 0.848 t U.
Natural Uranium Activity (U-238 series)
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Initially, it only contains the uranium isotopes. Within a few days, Th-231 (U-235 series), and within a few months, Th-234 and Pa-234m (U-238 series) grow in. The activity then remains stable for more than 10,000 years.
After this time, Th-230 and all other decay products of the U-238 series, and Pa-231 and all other decay products of the U-235 series grow in. This could, however, only occur with residual ore concentrate not consumed for nuclear fuel production. (see also: Uranium Decay Calculator)
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The alpha radiation of the uranium isotopes U-238, U-235, and U-234 presents a radiation hazard on ingestion or inhalation of natural uranium. The beta radiation of Th-234 and Pa-234m, together with the weak gamma radiation emitted by all nuclides, presents an external radiation hazard. For ingestion and inhalation, also the chemical toxicity of uranium has to be taken into account.
(see also: Introduction: Uranium Mill Tailings Deposits)
Uranium mill tailings are the residual waste from the process of uranium extraction from the uranium ore. Since only uranium is extracted, all other members of the uranium decay chains remain in the tailings at their original activities. In addition, small residual amounts of uranium are left in the tailings, depending on the efficiency of the extraction process used.
Uranium Mill Tailings Activity (U-238 series)
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Initially, the total activity in the tailings amounts to about 85% of that in the ore. Within a few months, the isotopes of Th-234 and Pa-234m decay to the value given by the residual activity of the U-238. The total activity in the tailings then remains constant for more than 10,000 years at about 75% of that in the ore.
Only after several hundred thousand years, when the Th-230 has decayed to the level of the residual U-234, a major decrease of total activity takes place.
After this time, the activity of all members of the U-238 chain is equal to that of the residual U-238 and U-234, and it remains at this level for some billion years. (see also: Uranium Decay Calculator) |
Compared to uranium ore, the alpha radiation of uranium mill tailings and thus the radiation hazard on ingestion or inhalation of tailings (dust) is approx. 25% lower, while the hazard from radon is unchanged. The external radiation hazard from gamma radiation remains nearly unchanged, while that from beta radiation is reduced. The chemical toxicity of uranium plays a minor role only in tailings.
For use in heavy water reactors (HWR, such as CANDU type, or pressurized heavy water reactors - PHWR), the uranium is needed in the form of UO2. This is obtained from the uranium ore concentrate by refining and conversion. 1 t of UO2 is equivalent to 0.8815 t U.
In addition to the radiological and chemical hazards, UO2 presents a hazard from spontaneous ignition of finely divided particles (pyrophoricity).
For use in Magnox reactors (graphite moderated, cas cooled), the uranium is needed in the form of uranium metal. This is obtained from the uranium ore concentrate by refining and conversion.
In addition to the radiological and chemical hazards, uranium metal presents a hazard from spontaneous ignition of finely divided particles (pyrophoricity).
For the use in light water reactors (LWR, such as boiling water reactors - BWR, or pressurized water reactors - PWR), the fissile isotope of uranium-235 contained in the natural uranium has to be enriched. For the enrichment process, uranium is needed in the form of uranium hexafluoride (UF6). This is obtained from the uranium ore concentrate by refining and conversion. 1 t of UF6 is equivalent to 0.676 t U.
At ambient temperature, UF6 is a crystalline solid, but at a temperature of 56.4°C, it sublimates (becomes a gas). Chemically, UF6 is very reactive: with water (atmospheric humidity!) it forms the extremely corrosive hydrofluoric acid and the highly toxic uranyl fluoride (UO2F2). The hydrofluoric acid causes skin burns, and, after inhalation, damages the lungs. Further health hazards result from the chemical toxicity of the uranium to the kidneys, and from the alpha radiation of the uranium. The external radiation hazard from UF6 is higher than from uranium ore concentrate, since the uranium's alpha radiation produces neutron radiation in an (Alpha,n)-reaction with fluorine, view animation (see also: Alpha-Neutron Reaction Calculator).
Additional hazards exist, if the uranium hexafluoride contains uranium recovered from reprocessing of spent nuclear fuel. In this case, the uranium hexafluoride is contaminated with fission products (mainly ruthenium-106 
and technetium-99 ), with artificial uranium isotopes (U-232 , U-233 , U-236 , and U-237 ), with transuranics (such as neptunium-237 and plutonium-239 ), and with the decay products of all these nuclides.
Uranium-232 Series Activity
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Uranium-232 is of special concern, since some of its decay products are strong gamma emitters (in particular thallium-208 ). While the activity of the fission products slowly decreases with time due to radioactive decay, the activity of the U-232 progeny (and thus its gamma radiation) strongly increases during the first 10 years, until secular equilibrium with U-232 is obtained. (see also: Uranium Decay Calculator)
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Composition of uranium isotopes in spent fuel [weight-percent]
(initially enriched to 3.5%, after burnup of 39 GWd/tHM)
| U-232 | U-233 | U-234 | U-235 | U-236 | U-237 | U-238 |
| after reactor unload | 6.59 · 10-8% | 1.58 · 10-7% | 0.0175% | 0.846% | 0.472% | 0.0013% | 98.664% |
| after 5 year delay | 1.88 · 10-7% | 2.59 · 10-7% | 0.0184% | 0.846% | 0.472% | 4.83 · 10-9% | 98.664% |
Source: [Neghabian1991] p.83
The UF6 is shipped in steel cylinders containing up to 12.7 tonnes. If cylinders are involved in long-lasting fires during accidents, large amounts of UF6 can be released within a short time.
> see also Uranium Hexafluoride Hazards.
For use in pressurized water reactors (PWR), uranium is enriched to between 3.6% and 4.1%, and for use in boiling water reactors (BWR), between 3.0% and 3.2% weight-percent uranium-235; that is around 4 to 6 times the natural concentration. As a side effect, the concentration of uranium-234 is enriched at an even higher ratio, according to its lower atomic weight.
Composition of uranium isotopes in enriched uranium from enrichment of natural uranium
(enrichment to 3.5%)
| U-234 | U-235 | U-238 | Total |
| weight % | 0.02884% | 3.5% | 96.471% | 100% |
| activity % | 81.8% | 3.4% | 14.7% | 100% |
| activity in 1 g Uenr | 66,703 Bq | 2,800 Bq | 12,005 Bq | 81,508 Bq |
If the UF6 feed contained uranium recycled from spent fuel, then the lighter uranium nuclides U-232 and U-233 mainly and U-236 partly end up in the enriched UF6 product. Any fission products present, such as technetium-99, completely end up in the enriched UF6 product.
Composition of uranium isotopes in enriched uranium from enrichment of uranium recycled from spent fuel
(initially enriched to 3.5%, after burnup of 39 GWd/tHM and delay of 5 years after unload)
| U-232 | U-233 | U-234 | U-235 | U-236 | U-237 | U-238 | Total |
| weight % | 1.055 · 10-6% | 1.45 · 10-6% | 0.09281% | 3.82% | 1.602% | - | 94.485% | 100% |
| activity % | 3% | 0.0018% | 77.7% | 1.1% | 13.9% | - | 4.3% | 100% |
| Activity in 1 g Uenr | 8,360 Bq | 5 Bq | 214,670 Bq | 3,056 Bq | 38,384 Bq | - | 11,763 Bq | 276,238 Bq |
after [Neghabian1991] p.90
In addition to the hazards already described, handling of the enriched uranium presents a criticality hazard: if too large amounts of enriched uranium are accumulated in one place, uncontrolled chain reactions can occur, causing heavy releases of neutron and gamma radiation.
The waste product from the enrichment process is depleted in uranium-235, it is therefore referred to as "depleted uranium". Typical concentrations of U-235 in depleted uranium (the "tails assay") are 0.2 to 0.3 weight-percent; that is around 30 - 40% of its concentration in natural uranium. The concentration of uranium-234 is depleted to an even lower ratio, according to its lower atomic weight.
The tails assay is a parameter that can be adjusted to economical needs, depending on the cost of fresh natural uranium and on the enrichment cost (expressed in $ per separative work unit - SWU).
> See graphs: Cost balance of uranium enrichment · Optimal tails assay
(Note: feed cost includes uranium price plus conversion cost)
> See also: Uranium Enrichment Cost Optimizer
For current prices, see Ux Industry Spot Prices
Composition of uranium isotopes in depleted uranium from enrichment of natural uranium
(from enrichment to 3.5%, tails assay of 0.2%)
| U-234 | U-235 | U-238 | Total |
| weight % | 0.0008976% | 0.2% | 99.799% | 100% |
| activity % | 14.2% | 1.1% | 84.7% | 100% |
| activity in 1 g Udep | 2,076 Bq | 160 Bq | 12,420 Bq | 14,656 Bq |
Depleted Uranium Activity (U-238 series)
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Within a few months, the isotopes of Th-234 and Pa-234m grow in to the value given by the activity of the U-238. The total activity in the depleted uranium then remains constant for around 10,000 years.
Then, Th-230 with all its decay products starts growing in. After around 100,000 years, U-234 grows in to the activity level given by the U-238, further promoting the ingrowth of Th-230 and decay products.
After around 2 million years, all nuclides are in secular equilibrium, and the total activity reaches a maximum and remains at this level for a billion years.
From residual U-235, Th-231 grows in within a few days. After around 10,000 years, Pa-231 and all other decay products of the U-235 series start growing in. (see also: Uranium Decay Calculator)
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Depleted uranium thus has the unusual property that it becomes more hazardous with time - an effect that has to be taken into account for its long-term management as a waste.
If the UF6 feed contained uranium recycled from spent fuel, then the heavier uranium nuclides U-236 and U-237 partly end up in the depleted UF6 tails. Any transuranics present, such as neptunium-237 and plutonium-239, mainly end up in the tails.
Composition of uranium isotopes in depleted uranium from enrichment of uranium recycled from spent fuel
(initially enriched to 3.5%, after burnup of 39 GWd/tHM and delay of 5 years after unload, tails assay 0.2%)
| U-232 | U-233 | U-234 | U-235 | U-236 | U-237 | U-238 | Total |
| weight % | - | - | 0.001939% | 0.2% | 0.2266% | - | 99.571% | 100% |
| activity % | - | - | 20% | 0.71% | 24.1% | - | 55.2% | 100% |
| activity in 1 g Udep | - | - | 4,485 Bq | 160 Bq | 5,429 Bq | - | 12,396 Bq | 22,470 Bq |
after [Neghabian1991] p.90
Most of the depleted UF6 produced so far is being stored in steel cylinders in so-called cylinder yards near the enrichment plants. In the storage yards, the cylinders are subject to corrosion. The integrity of the cylinders must therefore be
monitored and the painting must be refreshed from time to time. This maintenance work requires moving of the cylinders, causing further hazards from breaching of corroded cylinders, and from handling errors. (see Cylinder Storage of Depleted UF6)
As a worst-case scenario, the crash of an airplane into a cylinder yard must be assumed.
Unloading of UF6 cylinders usually is accomplished by heating the cylinder in an autoclave. The UF6 then sublimates (becomes a gas) and is fed into the receiving plant. However, there are also gamma-emitting decay products of the U-238 and U-235 present in the cylinder, namely Th-234, Pa-234m, and Th-231. They have grown in within a few months after the chemical separation of the uranium, and they do not form gaseous compounds with fluorine. They rather tend to concentrate in a residue called "heels" which is not removed from the cylinder.
These decay products (in particular Pa-234m) happen to be the major source of gamma radiation in the cylinder; the uranium itself emits only smaller amounts of gamma radiation. In a full cylinder, only a small fraction of the gamma radiation generated reaches the cylinder surface, since most of the gamma radiation is shielded by the uranium contained. In an "empty" cylinder, however, the major source of gamma radiation is still present in the heels and now reaches the cylinder surface nearly unhindered.
> View animation
Only approx. half a year after unloading of the UF6, these decay products have mostly decayed away.
For cylinders carrying recycled UF6, the heels comprise the even stronger gamma emitter thallium-208 (Tl-208) from the U-232 series, which takes somewhat more than 10 years to decay away.
In a re-conversion process, the enriched uranium hexafluoride is converted to the oxide form of UO2. In this form, the uranium is used for the production of nuclear fuel for light water reactors. 1 t of UO2 is equivalent to 0.8815 t U.
If the UF6 contained uranium recycled from spent fuel, then the nuclear fuel may be contamined with the artificial uranium isotopes of U-232, U-233, and U-236, and with fission products such as technetium-99.
Production and handling of this material, as of all enriched uranium, presents a criticality hazard: if too large amounts of enriched uranium are accumulated in one place, uncontrolled chain reactions can occur, causing heavy releases of neutron and gamma radiation.
In addition to the radiological and chemical hazards, (enriched) UO2 presents a hazard from spontaneous ignition of finely divided particles (pyrophoricity).
In a re-conversion process, the depleted uranium hexafluoride can be converted to the form of uranium metal.
If the UF6 contained uranium recycled from spent fuel, then the depleted uranium may be contaminated with the artificial uranium isotopes U-236 and U-237, and with transuranics such as neptunium-237 and plutonium-239.
In addition to the radiological and chemical hazards, (depleted) uranium metal presents a hazard from spontaneous ignition of finely divided particles (pyrophoricity).
In a re-conversion process, the depleted uranium hexafluoride can be converted to the oxide form of UO2 or U3O8. In these forms, the depleted uranium is chemically more stable and suitable for long-term storage or disposal (see Waste Management - Depleted Uranium).
1 t of UO2 is equivalent to 0.8815 t U, and 1 t U3O8 is equivalent to 0.848 t U.
If the UF6 contained uranium recycled from spent fuel, then the depleted uranium may be contaminated with the artificial uranium nuclides U-236 and U-237, and with transuranics such as neptunium-237 and plutonium-239.
In addition to the radiological and chemical hazards, (depleted) UO2 (other than U3O8) presents a hazard from spontaneous ignition of finely divided particles (pyrophoricity).
[Neghabian1991] Verwendung von wiederaufgearbeitetem Uran und von abgereichertem Uran, von A.R. Neghabian, H.J. Becker, A. Baran, H.-W. Binzel, Der Bundesminister für Umwelt, Naturschutz und Reaktorsicherheit (Hg.), Schriftenreihe Reaktorsicherheit und Strahlenschutz, BMU-1992-332, November 1991, 186 S.
[IAEA1989] In Situ Leaching of Uranium: Technical, Environmental and Economic Aspects, IAEA-TECDOC-492, IAEA Vienna 1989, 172 p.