Radiological And Chemical Properties Of Uranium
Radiological and ChemicalProperties of Uranium1
Module Objectives Recognize the basic chemical, physical, radiologicalproperties of uranium and other radioactive materialspresent at uranium recovery facilities. Describe the properties of decay products of uranium. Describe the processes and health physics concerns atconventional mills and in-situ recovery facilities.2
General3
Natural Uranium There are three naturally occurring isotopes of uranium:U-234U-235U-238 All threeth are longllivedli d alpha-emitters.l hitt U-238 is the head of the uranium decay series of which U-234 is amember. U-235 is the head of the actinium decay series. These decay series include alpha, beta and gamma emitters.4
Natural Uranium Uranium is relatively abundant in nature. Typical concentrationsof uranium in soil: U-238 1 pCi/g of soilU-234 1 pCi/g of soilU-235 0.05 pCi/g of soil Uranium bearing ore is ranked by the amount of U presentpresent. In the U.S., most mined ore contains from 0.1% to 1% of U. Since 1 pCi/g U-238 3 ppm of uranium:1 pCi/g U-238 0.0003% of uranium100 pCi/g U-238 0.03% of uranium1000 pCi/g U-238 0.3% of uranium5
Isotopes of Natural UraniumIsotope% by weightHalf-lifeU-2340.00552.47 x 105 yearsU-235U2350.727.10 x 108 yearsU-23899.284.51 x 109 yearsEnriched uranium would have more U-234 and U-235 and less U-238Depleted uranium would have less U-234 and U-235 and more U-2386
Radiological Properties(Daughter Products & Equilibrium) All of the uranium isotopes decay to shorter-lived decayproducts often referred to as “daughters.” U-238 and U-235 together with their decay productsform a “decaydecay chain”chain or “series”series the final decay productof which is a stable isotope of lead. Natural uranium has two decay chains:Actinium series (U-235)Uranium series (U-238)7
U-238U-2344.47 x 109 y2.45 x 105 yPa-234mTh-23424.1 d1.17 minTh-2307.7 x 104 yRa-226Uranium Decay Series1600 yRn-2223.8 dPo-218Po-214Po-2103.11 min163.7 us138.4 dBi-2105.013 dPb-21426.8 minBi-21419.9 minPb-21022.26 yPb-206(stable)8
U-2357.04 x 108 aPa-2313.28 x 104 aTh-231Th-2271.063 dAc-22718.68 d21.77 aRa-223Actinium Decay Series11.435 dRn-2193.96 sPo-2151.78 msPb-21136.1 minBi-2112.14 minTl-207Pb-207(stable)4.77 min9
Equilibrium Equilibrium occurs when a fixed ratio exists between the activityof a parent radionuclide and that of its radioactive decayproduct(s). When equilibrium exists, the activity of the decay product(s) isdecreasing according to the half-life of the parent. There are two types of equilibrium: secular equilibriumtransient equilibrium In the case of the uranium and actinium decay series, secularequilibrium is the thing of interest. Nevertheless, we will alsoconsider transient equilibrium and the situation where noequilibrium is possible.10
Secular Equilibrium Secular equilibrium is possible if the parent’s half-life is muchgreater than that of the decay product. When secular equilibrium is achieved, the activity of the decayproduct equals that of the parent. Starting with nothing but the parent, the time to reach secularequilibrium is roughly five to ten halfhalf-liveslives of the daughterdaughter. Example: if one curie of pure radium 226 is placed in a sealedcontainer, the activity of radon 222 will increase until it alsoreached one curie. This would take 20 to 30 days given the 3.8 dayhalf life of Rn-222.11
Secular Equilibrium100AActivityParentActivityNo equilibriumSecularequilibrium10Decay ProductActivity1Time (t)12
Transient Equilibrium If the parent nuclide’s half life is greater, but not “much”greater, than that of the daughter, a condition known astransient equilibrium can be established. The time to reach transient equilibrium is roughly five tot half-livestenh lf lioff theth daughter.d ht When transient equilibrium is achieved:The daughter activity exceeds that of the parent.The daughter activity decreases according to thehalf-life of the parent.13
Transient Equilibrium100AActivityNo EquilibriumTransient Equilibrium10Decay Product ActivityParent Activity1Time (t)14
No Equilibrium If the parent’s half life is shorter than that of the daughter,equilibrium cannot be achieved. The daughter activity increases from zero, reaches a maximum,and decreases at a rate primarily determined by its own half life. There is no increase in the total activity with time,time the activity willonly decrease.15
No EquilibriumActivity100Parent ActivityNo Equilibrium(curves never parallel)10Decay Product Activity1Time (t)16
Radiological Properties ofUranium17
U-238U-2344.47 x 109 a2.45 x 105 aPa-234mUranium Decay Series1.17 minTh-23424.1 dTh-2307.7 x 104 aThe most radiologically significantradionuclides associated with naturaluranium ore are Ra-226 and its short-liveddecay products.Ra-2261600 aRn-2223.8 dPo-2183.11 minPo-214Po-210163.7 us138.4 dBi-214Bi-21019.9 min5.013 dPb-214Pb-21026.8 min22.26 aPb-206(stable)18
Uranium Ore - Daughter ProductsRa-226 and its short-lived decay products are the mostradiologically significant radionuclides associated withuranium ore because: The external gamma hazard is primarily due to Pbgamma raysy are of214 and Bi-214. Some of their ghigh energy. Ra-226 is readily assimilated by the body (f1 0.2) Rn-222 is a noble gas. Its decay products becomeattached to airborne particulates which can beinhaled.19
Uranium Ore - Daughter ProductsIn some situations the long lived decay products of Rn222 ( Pb-210, Bi-210 and Po-210) might be an internalradiological concern because: Over time, they can accumulate on surfaces in areaswith high radon levels Pb-210 is readily assimilated by the body (f1 0.2)20
Purified Uranium - Daughter Products When natural uranium has been chemically purified, orotherwise separated from the ore, it consists of threenuclides: U-238, U-234 and U-235. By mass, it is almost all U-238. ByB activity,ti it it isi roughlyhl 50% U-238U 238 andd 50% UU-234.234 Over time, the activity of their decay products (initiallyzero) will increase.21
Purified Uranium - Daughter ProductsU-234 Th-230 (75,400 a)Although U-234 represents almost half the activity of pureuranium, the ingrowth of the U-234 daughters is relativelyunimportant because: The decay product of U-234, the pure alpha emitter Th230, has such a long half-life that any measureableincrease in its activity will take tens of thousands ofyears.22
Purified Uranium - Daughter ProductsU-235 Th-231 (1 d) Pa-231 (32,800 a)Th-231, the immediate decay product of U-235 has a oneday half life. As such, it will be in secular equilibrium withU-235 within a week. Nevertheless, the ingrowth of the U235 daughters is relatively unimportant because: There is much less U-235 than U-238. Th-231 only emits low energy betas and gamma rays. The Th-231 gamma rays are of very low intensity. Any measureable increase in the Pa-231 activity will taketens of thousands of years.23
Purified Uranium - Daughter ProductsU-238 Th-234 (24.1 d) Pa-234m (1.17m)When considering ingrowth of the daughters of purifiednatural uranium, Th-234 and Pa-234m are the mostsignificant. Theyy achieve secular equilibrium with U-238 inapproximately six months. They are beta-gamma emitters Initially purified uranium emits only alpha particles, butafter six months there are two betas for every alpha.24
Purified Uranium - Daughter ProductsU-238 Th-234 (24.1 d) Pa-234m (1.17m)Th-234Beta:GGamma:Pa-234mBeta:Gamma:106 keV107 keV199 keV63 kkeVV92 keV6%14%78%4%4%2270 keV766 keV1001 keV98%0.3%0.8%25
26
Purified Uranium - Daughter ProductsRegulatoryGuide 8.3027
Specific Activity Specific Activity (SA) is the activity per unit mass of apure radionuclide. The units of specific activity might beCi/g, Bq/g, etc. It provides an indication of the concentration ofradioactivity, or the relationship between the mass ofradioactive material and the activity.28
Calculation of SAEquation for specific activity (Bq/g):SA N λN number of atoms/gram 6.022 x 1023 (atoms/mole) / atomic mass (g/mole)λ ln 2/ T1/2 (seconds)For convenience, the equation can be rewritten to use theT1/2 in years by converting from years to secondsλ ln 2/ (T1/2 y x 3.156 x 107 s/y) 2.1965 x 10-8 / T1/2 y29
Calculation of SA (continued)SA N λ (in Bq/g) (6.022 x 1023/atomic mass) x (2.1965 x 10-8 / T1/2 years) 1.3227 x 1016/ (T1/2 years x atomic mass)To calculate the SA in curies per gram:SA (Ci/g) 3.575 x 105/ (T1/2 years x atomic mass)30
Calculating the Specific Activity of U-234, U235 and U-238SA of U-234 3.575 x 105/ (T1/2 years x atomic mass) 3.575 x 105 / (2.455 x 105 x 234.04) 6.22 x 10-3 Ci/gSA of U-235 3.575 x 105 / (7.038 x 108 x 235.04) 2.16 x 10-6 Ci/gSA of U-238 3.575 x 105 / ( 4.468 x 109 x 238.05) 3.36 x 10-7 Ci/g31
Total Specific Activity of Natural UraniumIsotope% 22.16 x 10-61.56 x 10-8U-23899.280.99283.36 x 10-73.34 x 10-7Total10016.91 x 10-76.91 x 10-70.000055 6.22 x 10-3Activity pergram ofuranium (Ci)3.42 x 10-732
Weight vs. Activity of Natural UraniumIsotope% byweightSA(Ci/g)U-234U-235U-2380.00550.7299.286.22 x 10-32.16 x 10-63.36 x 10-7Activity pergram ofuranium (Ci)3.42 x 10-71.56 x 10-83.34 x 10-7% Activity49.52.348.333
Chemical Properties ofUranium34
General The chemical properties of uranium are importantbecause they can affect the hazard of the material. The main concern is an internal intake of uranium ratherthan an external exposure. The dose and physiological consequences of such anintake are dependent on the several factors of which thechemical form is one.35
UO2 Uranium Dioxide Known as brown oxide or uraninite. The uranium in the ore body is primarily in a tetravalentstate (U4 ) and can be considered to consist of UO2 One of the ooxidesides that make upp yellowcakeello cake Widely used to form nuclear fuel pellets. Low solubility in water. Clearance class Y (S)36
UO3 Uranium Trioxide Known as orange oxide. When heated in air will decompose to U3O8, whenheated to high temperature (greater than 700 degree C)in the presence of H2 will convert to UO2. During the leaching process, uranium dioxide in the orebody is oxidized to uranium trioxide. One of the oxides that make up yellowcake. Moderate solubility in water. Clearance class W (M)37
UO2(CO3) Uranium Carbonate Generic form of several uranium carbonates . Carbonates are usually formed as charged complexes that reactwith other ions to form minerals. The charged complexes are very soluble and mobile with lowaffinity for soil.soil After the uranium in the ore body has been oxidized it isconverted to a uranium carbonate complex. It is in this form thatthe uranium is extracted from the ground at an ISL facility. There is no “official” ICRP or NRC clearance class. Little to nodata available.38
UO4-nH2O Uranium Peroxide Pale yellowish compound. Commonly found at ISL mills as part of the process ofprecipitating uranium from the pregnant eluant. ModeratelyModeratel solublesol ble in water.ater No “official” ICRP or NRC clearance class. Somestudies indicate that Class W might be appropriate(Biokinetics and Analysis of Uranium in Man. UnitedStates Uranium Registry, USUR-05 HEHF-47, 1984)39
((NH4)2U2O7) Ammonium Diuranate One of the intermediate chemical forms of uraniumproduced during yellowcake production. The name 'yellowcake' was originally given to this brightyellow substance, now applies to mixtures of uraniumoxides which are actually hardly ever yellow. It is precipitated during production by adding aqueousammonium hydroxide. No “official” ICRP or NRC clearance class. Some studiesindicate that Class D and W might be appropriate(Biokinetics and Analysis of Uranium in Man. UnitedStates Uranium Registry, USUR-05 HEHF-47, 1984)40
Effect of Calcining Temperature on Ammonium DiuranateDecompositionRadiological Issues at ISR Facilities (Burrows, 2009)41
U308 Triuranium Octoxide Pure U308 is greenish black in color. This is the most stable form of uranium and used forstorage and disposal. PrimaryPrimar component (ca.(ca 70 – 90%) of yellowcake.ello cake Low solubility in water. Clearance class Y (S)42
Yellowcake Uranium concentrate. The end product of uraniumrecovery operations. Originally, the term referred to ammonium diurinate, butnow it refers to the final product of purification. A mixture of uranium oxides: UO2, UO3, and U3O8 The main component is U3O8. But it is the other oxidesthat are responsible for the yellow to orange color.43
YellowcakeNRC Online Glossary: “The solid form of mixed uranium oxide, which isproduced from uranium ore in the uranium recovery(milling) process. ThThe materialt i l iis a mixturei toff uraniumi oxides,id whichhi h canvary in proportion and color from yellow to orange to darkgreen (blackish) depending on the temperature at whichthe material is dried (which affects the level of hydrationand impurities), with higher drying temperaturesproducing a darker and less soluble material.”44
45
Effect of Chemical Form on Hazard The chemical form of uranium does not affect the external dosehazard. It does affect the internal hazards when uranium is inhaled oringested. Different chemical forms of uranium are classified by theirclearance class (aka solubility class). This describes how quickly the chemical form of uranium isremoved from the deepest portion of the respiratory system (thepulmonary region).46
Class D Material Clearance Class D (or F) material is usually referred toas soluble uranium this includes UF6, UO2F2, and UNH Class D material clear rapidly from the body. DDuee to rapid removalremo al from the bodybod uraniumrani m in this formmay be more hazardous from a chemical toxicitystandpoint than a radiological one.47
Class W Material Clearance class W (or M) material is moderately solubleand includes UO3, UF4, and uranium peroxide. Class W material will clear more slowly from the body. Class W material is considered insolubleinsol ble for internaldosimetry and toxicity purposes.48
Class Y Material Class Y (or S) material is also known as insolubleuranium and includes U3O8, and UO2. Class Y material is slowly cleared from the body. Class Y material is usuallys all a radiological hazardha ard withithlower chemical toxicity hazards. Under normal environmental conditions, class Dmaterial might be chemically converted to class W, butclass W material would not be converted to class Y.49
Effect of Chemical Form on Hazard The Annual Limit on Intake (ALI) and the Derived AirConcentration (DAC) depend on the clearance class. If there is a mixture of chemical forms, it might benecessary to determine the percent makeup of themixture in order to calculate the dose.50
Effect of Chemical Form on Hazard10 CFR 20 Appendix B51
Guide of Good Practices for Occupational Radiological Protection inUranium Facilities (DOE-STD-1136-2009)52
Effect of Chemical Form on HazardRegulatory Guide 8.30 “Yellowcake dried at low temperature, which is predominantlycomposed of ammonium diuranate, or in the new processes uranylperoxide, both are more soluble in body fluids than yellowcakedried at higher temperature; and a relatively large fraction is rapidlytransferred to kidney tissues . . . For purposes of compliance with10 CFR Part 20, yellowcake undried or dried at low temperatureshould be classified as soluble.”53
Effect of Chemical Form on HazardRegulatory Guide 8.30 “Yellowcake dried at high temperature is a mixture of compoundsthat contains a major portion of more insoluble uranium oxides.Radiation dose to the lung and other organs is the limitingconsideration rather than chemical toxicity; this is primarily due tothe large insoluble component. For compliance purposes,yellowcake dried at 400 C (752F) and above should be classified asinsoluble54
Effect of Chemical Form on HazardRadiological Issues at ISR Facilities (Burrows, 2009)“NRC Staff DAC Recommendations: 400 C remains a valid transition temperature between inhalationclasses (based on UO3 decomposition). LLackingki siteit specificifi ddata,t hhydrogendperoxideid precipitatedi it t dyellowcake, dried at 400 C, should be considered a Class Wcompound for radiation protection purposes. Lacking site specific data, hydrogen peroxide precipitatedyellowcake, dried at 400 C and higher, should be considered aClass Y compound for radiation protection purposes.”55
Physical Form The external hazard of uranium is not affected by itsphysical form. Nondispersible solids (e.g., metals or ceramics ofuranium) have minimal hazard from inhalation. Dispersible forms can be easily made airborne andrepresent a potential internal hazard (e.g., powders, andliquids).56
Human Response Indicators to UraniumExposure Uranium has been recognized as a chemical hazard since the1800s. The main chemical hazard as with most heavy metals is toxicityto the kidney. This was first observed in the 1800s when UNH was used to treatdiabetes. Excessive intake of UNH resulted in kidney damage. This is the primary concern from a chemical toxicity standpoint.57
Uranium Toxicity Limit Due to the potential for kidney toxicity the NRC haspromulgated a limit to uranium based on U toxicity. 10CFR20 1201(e) states that “ In addition to the annualdose limits, the licensee shall limit the soluble uraniumintake by an individual to 10 milligrams in a week inconsideration of chemical toxicity.” 10CFR20 Appendix B footnote 3 also addresses thisissue more in depth.58
Natural Uranium There are three naturally occurring isotopes of uranium: U-234 U-235 U-238 All th l li d l hAll three are long lived alpha-emitters. U-238 is the head of the uranium decay series of which U-234 is a member. U-235 is the head of the actinium decay series. The
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