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(last updated 13 Oct 2014)
Contents: Introduction · Decay Series · Uranium Compounds (see schematic below) · References
> see also:
|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|
|activity in 1 g Unat||12,356 Bq||568 Bq||12,356 Bq||25,280 Bq|
* in addition, all decays emit gamma radiation
Average decay energies of U-238 series
* in addition, all decays emit gamma radiation
Average decay energies of U-235 series
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.
|Uranium Ore Activity (U-238 series)
||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)|
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.
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.
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
||Long-lived Radon Progeny Activity
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)
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 0.6 for outdoors.
|Natural Uranium Activity (U-238 series)
||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)
The alpha radiation of the uranium isotopes U-238, U-235, and U-234 presents a radiation hazard on ingestion or inhalation, while the beta radiation of the short-lived decay products 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.
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)
||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.
In addition to the radiological and chemical hazards, UO2 presents a hazard from spontaneous ignition of finely divided particles (pyrophoricity).
In addition to the radiological and chemical hazards, uranium metal presents a hazard from spontaneous ignition of finely divided particles (pyrophoricity).
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.
The alpha radiation of the uranium isotopes U-238, U-235, and U-234 presents a radiation hazard on ingestion or inhalation, while the beta radiation of the short-lived decay products Th-234 and Pa-234m, together with the weak gamma radiation emitted by all nuclides, presents an external radiation hazard. In addition, the beta radiation produces secondary X-rays (Bremsstrahlung) in the UF6 and in the cylinder wall.
The external radiation hazard from UF6 is even 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).
For ingestion and inhalation, also the chemical toxicity of uranium has to be taken into account.
Additional hazards exist, if the uranium hexafluoride contains reprocessed uranium recovered from 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.
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).
|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.
|weight %||1.055 · 10-6%||1.45 · 10-6%||0.09281%||3.82%||1.602%||-||94.485%||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|
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.
Enriched uranium (hexafluoride) presents a proliferation hazard, as the separative work required to enrich a certain amount of reactor-grade uranium further to bomb-grade (> 90 wt-% U-235) is lower than that required to produce the reactor-grade uranium from natural uranium in the first place. (see also Uranium Enrichment Calculator)
|activity in 1 g Udep||2,076 Bq||160 Bq||12,420 Bq||14,656 Bq|
|Depleted Uranium Activity (U-238 series)
||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)
|Depleted Uranium Gamma Decay Energy Rate
(for 1 g DU)
|The rise of the gamma decay energy rate is even sharper, as the strongest gamma emitters of the series are among the decay products (in particular Bi-214, see Decay Series). (generated with Uranium Decay Calculator)|
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.
|activity in 1 g Udep||-||-||4,485 Bq||160 Bq||5,429 Bq||-||12,396 Bq||22,470 Bq|
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.
|UF6 Heels Activity (U-238 series)
(in 10 kg of residual UF6 in 48Y cylinder initially filled with natural UF6)
|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.
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).
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).
In addition to the radiological and chemical hazards, (enriched) UO2 presents a hazard from spontaneous ignition of finely divided particles (pyrophoricity).
Spent fuel activities:|
(1 t Heavy Metal, from UO2 fuel, burnup: 33 GWd/t U in Light Water Reactor, short-lived nuclides omitted)
It takes 10 million years, before the artificial reaction products have decayed away and the activity approaches the level of the residual uranium and its decay products.
(see also: Universal Decay Calculator (Vers. B))
Comparison of activities in spent fuel and in corresponding amounts of wastes arising in fuel production:
(Spent fuel: 1 t Heavy Metal, from UO2 fuel / MOX fuel, burnup: 45 GWd/t HM in Pressurized Water Reactor)
In the beginning, the activities in spent fuel exceed the activities in the wastes from fuel production by several orders of magnitude, but after approx. 1 million years, the activities in depleted uranium become highest.
At certain times, the activities in spent fuel generated from MOX fuel are up to 5 times higher than those in spent fuel from UO2 fuel, and it takes up to 10 times longer until the spent fuel from MOX fuel reaches the activity levels in spent fuel from UO2 fuel.
The uranium recovered in the PUREX process (as used in the Sellafield (UK) and La Hague (France) reprocessing plants) has the form of uranyl nitrate (UO2(NO3)2). This has to be converted to the form of U3O8 for further use.
Uranium recovered from reprocessing of spent nuclear fuel 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
||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)|
|at reactor unload||U-232||U-233||U-234||U-235||U-236||U-237||U-238|
|weight %||6.59 · 10-8%||1.58 · 10-7%||0.0175%||0.846%||0.472%||0.0013%||98.664%|
|activity %||1.329 · 10-6%||1.436 · 10-9%||1.031 · 10-4%||1.724 · 10-6%||2.879 · 10-5%||99.999834%||3.128 · 10-5%|
|activity in 1 g Urep||522 Bq||0.564 Bq||40,495 Bq||677 Bq||11,304 Bq||3.927 · 1010 Bq||12,284 Bq|
|after 5 year delay||U-232||U-233||U-234||U-235||U-236||U-237||U-238|
|weight %||1.88 · 10-7%||2.59 · 10-7%||0.0184%||0.846%||0.472%||4.83 · 10-9%||98.664%|
|activity in 1 g Urep||1,490 Bq||0.925 Bq||42,578 Bq||677 Bq||11,304 Bq||145,914 Bq||12,284 Bq|
Like enriched uranium, plutonium presents a criticality hazard: if too large amounts are accumulated in one place, uncontrolled chain reactions can occur, causing heavy releases of neutron and gamma radiation.
Since plutonium mostly consists of fissile material (only comparable to highly enriched uranium), it presents a most serious proliferation hazard.
Due to the higher activity of plutonium, MOX fuel presents a higher radiation hazard than uranium oxide fuel.
Like uranium oxide fuel, MOX fuel presents a criticality hazard: if too large amounts are accumulated in one place, uncontrolled chain reactions can occur, causing heavy releases of neutron and gamma radiation.
Due to the possibility of recovering the plutonium by chemical processing, MOX fuel presents a proliferation hazard.
[IAEA1989] In Situ Leaching of Uranium: Technical, Environmental and Economic Aspects , IAEA-TECDOC-492, IAEA Vienna 1989, 172 p.
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