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Uranium Radiation Properties

(last updated 11 Jan 2026)

Introduction · Uranium Compounds · References

 

> see also:

• Uranium Toxicity
• Uranium Radiation Exposure
• Health Effects of Radiation: Dose and Risk
• Glossary

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Introduction

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Uranium

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: U-238 (more than 99%), U-235, and U-234, all of which are radioactive and have very long half-lives, i.e. decay very slowly.
• 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.
• Only U-235 (contained in natural uranium at 0.711 weight %, or 0.72 atom%) is fissionable, and therefore able to release energy in a nuclear fission reactor.

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: Nuclide Mix Calculator (mass/activity conversions)   ·   Nuclear Data Viewer

 

Uranium Decay Series

Uranium-238 Decay Series (image ) Nuclide Half-Life Radiation * U-238 4.468 · 109 years alpha Th-234 24.1 days beta Pa-234m 1.17 minutes beta U-234 244,500 years alpha Th-230 77,000 years alpha Ra-226 1,600 years alpha Rn-222 3.8235 days alpha Po-218 3.05 minutes alpha Pb-214 26.8 minutes beta Bi-214 19.9 minutes beta Po-214 63.7 microseconds alpha Pb-210 22.26 years beta Bi-210 5.013 days beta Po-210 138.378 days alpha Pb-206 stable - only major decays shown * in addition, all decays emit gamma radiation

Uranium-235 Decay Series Nuclide Half-Life Radiation * U-235 703.8 · 106 years alpha Th-231 25.52 hours beta Pa-231 32,760 years alpha Ac-227 21.773 years beta Th-227 18.718 days alpha Ra-223 11.434 days alpha Rn-219 3.96 seconds alpha Po-215 778 microseconds alpha Pb-211 36.1 minutes beta Bi-211 2.13 minutes alpha Tl-207 4.77 minutes beta Pb-207 stable - only major decays shown * in addition, all decays emit gamma radiation Ac: Actinium Bi: Bismuth Pa: Protactinium Pb: Lead Po: Polonium Ra: Radium Rn: Radon Th: Thorium Tl: Thallium U: Uranium

> See also: Uranium Decay Calculator   ·   Universal Decay Calculator   ·   Nuclear Data Viewer
> See also: Unit Converter :   Specific activity <-> half-life   ·   Decay <-> half-lives   ·   Decay <-> time

In natural uranium contained in an undisturbed uranium ore deposit, these decay chains generally are in secular equilibrium. This means that in 1 g of natural uranium (almost) 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:

• Chemical separation of uranium (e.g. in a uranium mill) removes the decay products, while the isotopic composition of the uranium remains unchanged. However, within a few months, the short-lived decay products Th-234, Pa-234m, and Th-231 grow in, again. So, in most practical situations, their presence has to be taken into account, as well. All further decay products require millenia to grow in, and therefore can be neglected in most practical situations.

• Uranium enrichment (e.g. in a centrifuge plant) changes the isotopic composition of the uranium, increasing the concentration of U-235 in the product stream, while decreasing its concentration in the waste stream (depleted uranium, or tails).

The uranium isotopes (U-238, U-235, and U-234) and many of the decay products mainly emit alpha radiation and only little gamma radiation, while several decay products mainly emit beta radiation. Two of the beta emitters (Pb-214 and Bi-214) moreover are the main source of gamma radiation.

Alpha radiation

• Alpha radiation is a short-range particle radiation. It has a heavy impact on organic matter, when incorporated.

Beta radiation

• Beta radiation is a limited-range particle radiation. It mostly presents an external radiation exposure hazard. In addition, it may cause X-ray radiation by interaction with matter (Bremsstrahlung).

Gamma radiation

• Gamma radiation is a penetrating long-range electromagnetic radiation. It presents an external radiation exposure hazard.

 

The alpha radiation emitted from the decay of uranium can initiate an alpha-neutron reaction in some of the lighter elements contained in a uranium compound (in particular fluorine in UF₆, or, to a lesser degree, oxygen in UO₂ or U₃O₈), transforming certain nuclides to others, while emitting a neutron. Such uranium compounds thus produce some neutron radiation, even if not irradiated.
> See: Alpha-Neutron Reaction Calculator

In addition, the isotope U-238 undergoes spontaneous fission at a rather low frequency. Each fission produces approx. 2 neutrons, contributing some neutron radiation independent of the type of uranium compound.
> See: Alpha-Neutron Reaction Calculator (also covers spontaneous fission)

Neutron Radiation

• Neutron Radiation is a long-range particle radiation. It mostly presents an external radiation hazard.

 

In addition to the radiation hazard, the heavy metal uranium also presents a chemical toxicity hazard.

 

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Uranium Compounds

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(select from process schematic)

In-situ leach Mining    or  Open pit     /   Underground Mining   → Waste Rock ↓ ↓ Uranium In-situ Leach Solution Uranium Ore ⏐ ⏐ or   ⏐ or ↓ ⏐ ⏐   ⏐ Ore Sorting   ▩ → Ore Sorting Reject ⏐ ↓   ⏐ ⏐ ⏐ Heap Leaching   ⎯   ⦘⏐⦗ ⎯ ⎯   ⎯  ⦘⏐⦗  ⎯   ⎯ → Spent Ore (Ripios) ⏐ ↓   ⏐ ↓ ⏐ Heap leach solution   ⏐ Ore Sorting Accept ↓ ↓   ↓ ↓ Processing   ⚗ Milling     ⚗ → Uranium Mill Tailings ↓ ↓ Uranium Ore Concentrate (UOC) ↓ Refining   ⚗ → Refining Waste ↓ Refined Uranium Ore Concentrate (UO3) ⏐ or ↓ ⏐ Conversion   ⚗ → Conversion Waste ⏐ ↓ ⏐ Natural Uranium Hexafluoride (UF6) Heels ⏐ ↓ ⏐ Enrichment   ⟳ → Depleted Uranium Hexafluoride Heels ⏐ ↓ ⏐ ⏐ Enriched Uranium Hexafluoride (eUF6) Heels ⏐ ↓ ↓ ↓ HWR/Magnox Fuel fabrication   ⚗   LWR Fuel fabrication   ⚗   De-conversion   ⚗ ↓ ⏐ ↓ HWR Fuel (natural UO2) Magnox Fuel (natural U metal) ↓ Depleted Uranium Oxide (UO2 or U3O8) Depleted Uranium Metal LWR Fuel (enriched UO2) ⏐ ↓ ↓ HWR/Magnox Reactor   Light Water Reactor (LWR)   ↓ ↓ Spent HWR/Magnox Fuel Spent LWR Fuel ↓ Reprocessing Plant   ⚗ → High Level Waste ↓ and ↓ Plutonium Reprocessed Uranium ↓ ↓ Uranium (various forms) → MOX Fuel fabrication   ⚗   RepU conversion   ⚗ ↓ ↓ MOX Fuel RepU UF6 Heels ↓ RepU enrichment   ⟳ → Depleted RepU Hexafluoride Heels ↓ Enriched RepU UF6 Heels ↓ RepU fuel fabrication   ⚗   ↓ RepU fuel

 

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Uranium Ore

Uranium ore contains uranium of natural isotopic composition in concentrations ranging from less than 0.03% to over 20%.
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)

In an uranium ore deposit, secular equilibrium obtains between U-238 and its decay products, and between U-235 and its decay products, respectively. The equilibrium may be somewhat disturbed by geochemical migration processes in the ore deposit.

 

Uranium Ore Activity (1 t ore at grade of 0.1% U)
(logarithmic time scale)

In case of an undisturbed uranium deposit, the activity of all decay products remains rather constant for hundreds of millions of years. (see also: Universal Decay Calculator )

 

In the ore deposit, 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, and the neutron radiation, mostly resulting from spontaneous fission of U-238, present an external radiation hazard.
For ingestion and inhalation, also the chemical toxicity of uranium as a heavy metal has to be taken into account.

> See also: Radiation Exposure for Workers at Uranium Mine
> See also: Radiation Exposure for Residents at Uranium Mine

 

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Waste Rock

Waste material left from open pit or underground mining:

• removed overburden, often containing more than the 2 - 3 ppm U₃O₈ background concentration typical for material from layers not influenced by a uranium deposit, and
• mined material with a grade too low for economic uranium extraction; the cutoff concentration depends on the economics of the mine and is typically somewhere in the range of 50 - 300 ppm U₃O₈.

Like in uranium ore, secular equilibrium obtains between U-238 and its decay products, and between U-235 and its decay products, respectively, unless the equilibrium has been disturbed by geochemical migration processes in the ore deposit.

 

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Ore Sorting Reject

Low-grade fraction residue from sorting of mined uranium ore, with a grade too low for economic uranium extraction. The amount of rejected material often is many times over the material accepted for further processing.

> See also: Uranium ore beneficiation
> See also: Uranium Ore Sorting Calculator

 

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Spent Ore (Ripios)

Leached ore remaining from heap leaching of uranium ore.
Ripios has many properties in common with uranium mill tailings: like in tailings, the uranium isotopes U-238 and U-235 are no longer in secular equilibrium with their respective decay products. Due to the poorer uranium recovery of the heap leaching process, the fraction of residual uranium remaining in ripios is higher than in tailings, though. In addition, the ripios are coarser than the finely ground tailings.
Upon termination of the designated leaching period, the material initially still is saturated with leaching liquid, leading to continued leaching of uranium and other constituents (many of which are radioactive and/or toxic).

> See also: Uranium Heap Leaching

 

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Uranium In-situ Leach Solution

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: Impacts of Uranium In-Situ Leaching

 

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Radon

Radon is a gaseous radioactive element generated from the decay of radium contained in uranium ore. The most important isotope is Rn-222 generated from the decay of Ra-226 (a member of the U-238 decay series).
Radon presents an inhalation hazard, but a much higher 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 (logarithmic time scale)

Long-lived Radon Progeny Activity (logarithmic time scale)

There are a number of units in use to describe radon and radon progeny activity concentration in air:

• 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 c_Po-218 + 0.514 c_Pb-214 + 0.380 c_Bi-214) / c_Rn-222
where c_x 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.

> See also: Unit Converter : Activity conc. radon <-> radon progeny
> See also: Radon Individual Dose Calculator

 

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Uranium Ore Concentrate (UOC)

The uranium ore concentrate (UOC) produced in the milling process contains a mixed oxide usually referred to as "U3O8" (UO2 · 2 UO3). It is obtained fromYellow Cake by calcining. Due to a number of impurities contained (UOC contains at least 90% U₃O₈), 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 (1 g U)
(logarithmic time scale)

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 rather 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: Universal 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. An additional external radiation hazard is present from neutron radiation, mostly resulting from spontaneous fission of U-238.
For ingestion and inhalation, also the chemical toxicity of uranium as a heavy metal has to be taken into account.

Uranium ore concentrate is usually packaged in 205 to 250 litre steel drums containing up to approx. 430 kg UOC. These drums are then shipped in twenty foot (20') sea containers.

> See also: Radiation Exposure for Workers at Uranium Mine · Uranium Mill
> See also: Radiation Exposure for Residents at Uranium Mine · Uranium Mill

 

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Uranium Mill Tailings

(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 (1 t tailings from ore grade 0.1% U at 90% extraction)
(logarithmic time scale)

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 rather 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: Universal 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.

> See also: Radiation Exposure for Workers at Uranium Mill
> See also: Radiation Exposure for Residents at Uranium Mill ·Uranium Mill Tailings

 

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Refining Waste, Conversion Waste

These wastes arise during the refining and subsequent conversion of Uranium Ore Concentrate (U₃O₈) to uranium hexafluoride (UF₆).
Like uranium mill tailings, these wastes also are slurry-like and are held in large impoundments. The waste arisings vary widely with the kind of conversion process used. They contain a variety of nuclides from the uranium decay series and other toxic substances and must be safely managed in the long term.

 

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Heavy Water Reactor Fuel (natural UO2)

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).

> See also: Radiation Exposure for Workers at Heavy Water Reactor Fuel Fabrication Facilities

 

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Magnox Reactor Fuel (natural U metal)

For use in Magnox reactors (graphite moderated, gas 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).

> See also: Radiation Exposure for Workers at Magnox Reactor Fuel Fabrication Facilities

 

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Enriched Uranium

> See: Enriched UF₆ · Enriched RepU UF₆

 

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Depleted Uranium (DU)

> See: Depleted UF₆ · Depleted RepU UF₆ · Depleted Uranium Metal · Depleted Uranium Oxide (UO₂ or U₃O₈)

 

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Uranium Hexafluoride (UF6)

General · Natural UF₆ · Enriched UF₆ · Depleted UF₆ · UF₆ heels

UF6 - General

For the use in light water reactors (LWR, such as boiling water reactors - BWR, or pressurized water reactors - PWR), the fissile isotope of U-235 contained in the natural uranium has to be enriched. For the enrichment process, uranium is required in the form of uranium hexafluoride (UF₆). 1 t of UF₆ is equivalent to 0.676 t U.
Uranium hexafluoride is obtained from uranium ore concentrate by refining and conversion.

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,n)-reaction with fluorine produces more neutron radiation (view animation ), in addition to that resulting from spontaneous fission of U-238. (see also: Alpha-Neutron Reaction Calculator )

For ingestion and inhalation, also the chemical toxicity of uranium as a heavy metal has to be taken into account.

For uranium hexafluoride made from reprocessed uranium (RepU), see here

> See also: Radiation Exposure for Conversion and Enrichment Plant Workers

 

Natural UF6

> For general properties of uranium hexafluoride (UF₆), see here

Natural uranium hexafluoride (enrichment feed) is obtained from uranium ore concentrate by refining and conversion.

The natural UF₆ is shipped in Type 48Y steel cylinders containing 12,500 kg UF₆ (8,452 kg U). 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).

 

Enriched UF6

> For general properties of uranium hexafluoride (UF₆), see here

Enriched uranium hexafluoride (enrichment product) is obtained by enrichment of natural UF₆ or re-enrichment of depleted UF₆.

For use in light water reactors (LWR), uranium is enriched to between 3 and 5 weight-percent U-235; that is around 4 to 7 times the natural concentration. As a side effect, the concentration of U-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

(see also JOL's Friendly Enrichment Calculator )

The enriched UF₆ is shipped in Type 30B steel cylinders containing 2,277 kg UF₆ (1,540 kg U). 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).

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 )

 

Depleted UF6

> For general properties of uranium hexafluoride (UF₆), see here

Depleted uranium hexafluoride is a by-product from uranium enrichment, as tails from enrichment of natural UF₆, or as secondary tails by re-enrichment of depleted UF₆ .

The waste product from the enrichment process is depleted in U-235, it is therefore referred to as "depleted uranium". Typical concentrations of U-235 in depleted uranium (the tails assay) are in the range of 0.15 to 0.3 weight-percent; that is around 21 - 42% of its concentration in natural uranium. The concentration of U-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
> View Current Uranium 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

(see also JOL's Friendly Enrichment Calculator )

 

Depleted Uranium Activity (for 1 g DU with tails assay 0.2%)

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 rather 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: Universal Decay Calculator )

 

Depleted Uranium Gamma Decay Energy Rate (for 1 g DU with tails assay 0.2%)

The rise of the gamma decay energy rate is even sharper, as the strongest gamma emitters of the series are among the decay products (Pb-214, and 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.

Most of the depleted UF₆ produced so far is being stored in Type 48Y (or similar) steel cylinders containing 12,500 kg UF₆ (8,452 kg U) 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

> For general properties of uranium hexafluoride (UF₆), see here

Uranium hexafluoride heels are obtained as a residue from unloading of containers holding UF₆ of any isotopic composition.

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 .
In addition, the beta radiation of the decay products (in particular Pa-234m) produces secondary X-rays in the cylinder wall (Bremsstrahlung).

 

UF₆ Heels Activity (in 48Y cylinder initially filled with 12,500 kg natural UF₆)
(logarithmic time scale)

Only approx. half a year after unloading of the UF₆, these decay products have mostly decayed away.

> see also: Universal Decay Calculator

 

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Depleted Uranium Metal

In a de-conversion process, the depleted uranium hexafluoride can be converted to the form of uranium metal.

If the UF₆ contained reprocessed uranium (RepU), 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).

 

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Depleted Uranium Oxide (UO2 or U3O8)

In a de-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.

An external radiation hazard is present from gamma radiation and from neutron radiation (mostly resulting from spontaneous fission of U-238).
> View: Radiation Exposure at Depleted Uranium Storage Buildings: Residents · Workers

If the UF₆ contained reprocessed uranium (RepU), 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).

Depleted U₃O₈ is typically stored in type DV70 steel containers containing 7.8 to 11.8 t U₃O₈ (6.6 to 10 t U).

 

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Light Water Reactor Fuel (enriched UO2)

In a de-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 reprocessed uranium (RepU), 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).

> See also: Radiation Exposure for Workers at Light Water Reactor Fuel Fabrication Facilities

 

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Spent LWR Fuel

While spent fuel still consists mostly of uranium, the irradiation in the reactor has increased the activity inventory by several orders of magnitude, due to the formation of artificial uranium isotopes, transuranics, activation products, fission products, and all their decay products. During the first time, the activities are so high, that the decay heat keeps the temperature at high levels, necessitating adequate cooling.
Spent fuel emits a wide variety of ionizing radiations, including neutron radiation.

 

Spent fuel activities:
(1 t Heavy Metal, from UO₂ fuel, burnup: 33 GWd/t U in Light Water Reactor, at time of unload, 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 )

 

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.
(see also: Nuclear Fuel Chain Waste Activity Calculator )

 

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High Level Waste (HLW)

Reprocessing of spent fuel recovers only reprocessed uranium (RepU) and plutonium, while all other constituents are discarded as High Level Waste (HLW). HLW still generates heat, and it is a strong emitter of radiation of all sorts. It therefore requires safe storage for hundreds of thousands of years.

 

High Level Waste activities:
(from reprocessing of 1 t Heavy Metal, from UO₂ fuel, burnup: 33 GWd/t U in Light Water Reactor, reprocessed 5 years after unload)

(see also: Universal Decay Calculator )

 

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Reprocessed Uranium (RepU)

Residual uranium contained in spent nuclear fuel can be recovered in a reprocessing plant.

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 UO₃ for further use.

For long term storage, reprocessed uranium is converted to U₃O₈ and stored in 213 liter barrels containing 295 kg U₃O₈ (250 kg U).

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.

The radiation hazard to employees thus is much higher in the processing of RepU rather than natural uranium.

 

RepU Activity (for 1 g RepU, initial enr. 3.5%, 39 GWd/tHM, 5 years after unload)
(logarithmic time scale)

U-232 is of special concern, since some of its decay products are strong gamma emitters -- in particular thallium-208 (Tl-208 ), which is hidden away in a minor branch towards the end of the decay chain. The activity of the U-232 progeny (and thus its gamma radiation) reaches a peak after 10 years, increasing the gamma energy release from the RepU more than 5-fold. (see also: Uranium Decay Calculator )

 

RepU Gamma Decay Energy Rate (for 1 g RepU, initial enr. 3.5%, 39 GWd/tHM, 5 years after unload)
(logarithmic time scale)

 

Composition of uranium isotopes in uranium contained in spent fuel
(initially enriched to 3.5%, after burnup of 39 GWd/tHM)

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 %	0.6955%	0.0004317%	19.87%	0.3160%	5.276%	68.11%	5.734%
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

(after [Neghabian1991] p.83)

 

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"Perfumed" Uranium

"Perfumed" or "Blended" uranium is an industry term used for uranium that contains traces of reprocessed uranium (RepU). The traces may have been acquired by processing of unirradiated material in a system that has handled irradiated material before. As a result of the exposure to the system, the uranium that is produced is contaminated with artificial uranium isotopes, fission products and transuranics.

 

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Hexafluoride (UF6) from reprocessed uranium (RepU)

General · Raw RepU UF₆ · Enriched RepU UF₆ · Depleted RepU UF₆ · RepU Heels

RepU Hexafluoride (UF6) - General

> For general properties of uranium hexafluoride (UF₆), see here

Additional hazards exist, if the uranium hexafluoride contains reprocessed uranium (RepU) recovered from spent nuclear fuel. In this case, the uranium hexafluoride may be 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.

During the conversion process, most of the fission products and transuranics are removed, but residual contamination may remain.

> See also: Radiation Exposure for Conversion and Enrichment Plant Workers

 

Raw RepU Hexafluoride (UF6)

> For general properties of reprocessed uranium hexafluoride (UF₆), see here

If the uranium hexafluoride is made from reprocessed uranium (RepU), then it still contains all of the artificial uranium isotopes recovered during reprocessing.

Composition of uranium isotopes in uranium contained in 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
weight %	1.88 · 10-7%	2.59 · 10-7%	0.0184%	0.846%	0.472%	4.83 · 10-9%	98.664%
activity %	0.6955%	0.0004317%	19.87%	0.3160%	5.276%	68.11%	5.734%
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

(after [Neghabian1991] p.83)

 

Enriched RepU Hexafluoride (UF6)

> For general properties of reprocessed uranium hexafluoride (UF₆), see here

If the UF₆ feed contained reprocessed uranium (RepU), 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.

To obtain the same reactivity, RepU must be enriched higher than natural uranium to compensate for the presence of U-236, a neutron absorber.

Composition of uranium isotopes in enriched uranium from enrichment of reprocessed uranium (RepU)
(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; see also JOL's Friendly Enrichment Calculator )

 

Depleted RepU Hexafluoride (UF6)

> For general properties of reprocessed uranium hexafluoride (UF₆), see here

If the UF₆ feed contained reprocessed uranium (RepU), 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 reprocessed uranium (RepU)
(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; see also JOL's Friendly Enrichment Calculator )

 

RepU Heels

> For general properties of reprocessed uranium hexafluoride (UF₆), see here

For cylinders carrying UF₆ made from reprocessed uranium (RepU), 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.

 

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RepU fuel

Reprocessed uranium (RepU) can be used to replace natural uranium in nuclear fuel manufacturing. For this purpose, the uranium recovered in the reprocessing plants by the PUREX process first has to be converted from the form of uranyl nitrate (UO₂(NO₃)₂) to U₃O₈. Like natural uranium, it then is converted to UF₆ and enriched, and finally manufactured into nuclear fuel.

There are, however, some caveats, involving the fission products, artificial uranium isotopes, and transuranics contained in RepU:

• The radiation hazard to employees is much higher in the processing of RepU rather than natural uranium.
• The processing facilities for RepU must be strictly kept separated from those processing natural uranium, for not risking to contaminate natural uranium with any contaminants contained in RepU. Otherwise, the natural uranium would become "Perfumed" Uranium, possibly making it unfit for the intended use.
• To obtain the same reactivity, RepU must be enriched higher than natural uranium to compensate for the presence of U-236 , a neutron absorber.

> See also: Radiation Exposure for Workers at Light Water Reactor Fuel Fabrication Facilities

 

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Plutonium

Plutonium contained in spent nuclear fuel can be recovered in a reprocessing plant. The recovered plutonium consists of several nuclides, only some of which (Pu-239 and Pu-241 ) are fissile. Due to the shorter half-life of the plutonium isotopes (Pu-239: 24,065 years), the specific activity of plutonium is much higher than that of uranium, leading to a higher radiation hazard. In addition, radiation hazards can result from short-lived plutonium isotopes, in particular Pu-241, which decays with a half-life of 14.4 years to the gamma-emitter Am-241 .

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.

 

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Mixed Oxide (MOX) fuel

Plutonium recovered from spent fuel can be used to replace the fissile uranium isotope U-235 in reactor fuel. The plutonium is mixed with natural or depleted uranium to produce the Mixed Oxide fuel.
As only part of the plutonium recovered is fissile (e.g. 66% by weight), and the reactivity of the fissile plutonium (Pu-239 and Pu-241) is lower than that of U-235 (e.g. 66% by weight), approx. twice the amount of plutonium is required to replace the U-235 in the fuel.

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.

 

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REMIX fuel

REMIX (regenerated mixture) fuel is made from a mixture of reprocessed uranium (RepU) and plutonium, both extracted from spent nuclear fuel, with addition of small volumes of enriched uranium.

 

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References

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[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.

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