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1
Brazil, monazite sands
Along the Atlantic coast of South and Southeastern Brazil there are several beaches where heavy minerals form placer deposits (black sand) with high concentrations of natural radioactivity. In Brazil the exposure to natural radiation is considered to be therapeutic, since a physician introduced a treatment for arthritis and rheumatism based on the exposure to natural radiation. As a result thousands of tourists visit the beaches throughout the year, preferably around the city of Guarapari, where deposits of radioactive black sand frequently occur.
The main source of gamma radiation is the frequently found mineral monazite ([Ce,La,Th]PO4) which may contain considerable amounts of U and Th. The measured absorbed dose rates on areas with black sand are up to a few hundred times higher than the background values. A comparison of activity concentrations with data of tailings from a Czech uranium processing plant shows that the radium content of the monazite sand is higher than in these tailings, which are generally considered to be radiologically and environmentally harmful.
C-01
2
Brazil, monazite processing
Bioassays for Th of background population (Rio de Janeiro), workers and local population. Th concentrations in faeces of workers is 3.6 times higher than the average for inhabitants and 10 times higher than for background areas.
J-01
3
Brazil. monazite sands
Area of high natural radiation background in the North of Rio de Janeiro (large deposits of monazite sands), inhabitants of Buena are exposed to thorium through ingestion. The foodstuffs consumed by the population are basically composed of local products. A study that included the analysis of complete prepared meals has shown average concentrations of Th-232, Th-228, Ra-226 and Ra-228 of 12 mBq/kg fresh, 126 mBq/kg fresh, 167 Bq/kg fresh and 481 mBq/kg fresh, respectively. The results of the analyses of the urine and feces samples from volunteers have shown, in average, urine to feces excretion ratios for thorium higher than the predicted by ICRP models.
The absorption of an element incorporated into food is higher than its ingestion in inorganic form. The use of standard parameters may lead to an underestimation of the internal radiation dose of the population.
M-04
4
European Union, thorium minerals and compounds
Minerals: monazite - (Ce,La,Nd,Th)PO4; bastnaesite (Ce,La,…)(CO3)F.
Monazite concentrate is obtained by gravimetric & electromagnetic separation from other heavy mineral sands. Strong acids or alkaline solutions are then used and lanthanides are precipitated.
Bastnaesite ore concentrate is obtained by a wet process in which it is washed and separated in water. This concentrate is washed with hydrochloric acid and calcined in order to produce a crude oxide containing 90% of lanthanide oxides.
Monazite concentrate, Th-232 chain: 8-3000 Bq/g, U-238 chain: 6-40 Bq/g. During the extraction process isotopes of radium are co-precipitated with barium sulphate to form a radium-bearing by-product (Ra-228 ~3000 Bq/g, Ra-226 ~450 Bq/g)
Doses:
Normal conditions, min act conc 0.002 mSv/y, Normal conditions, max act conc 3500.0 mSv/yr
Unlikely conditions, min act conc 0.01 mSv/y, Unlikely conditions, max act conc 9800.0 mSv/yr
Thorium compounds
Decomposing concentrate with acids to produce thorium salts, these are raw materials for the metallic thorium. Use - thoriated tungsten welding electrodes, magnesium-thorium alloys for aerospace industry. Thorium nitrate - manufacture of gas mantles.
Activity concentration of thoriated tungsten welding electrodes ~100 Bq/g of Th-232 &Th-228. Gas mantled contain ~ 1000 Bq of Th-232 and Th-228 each. Special alloys for jet engines may have an activity ~ 70 Bq/g
Doses
Normal conditions, min act conc 0.09 mSv/y, Normal conditions, max act conc 14.0 mSv/yr
Unlikely conditions, min act conc 1.2 mSv/y, Unlikely conditions, max act conc 28.0 mSv/yr
D-02
5
UK
Rare earths in catalysts
Catalyst for the oil industry - incorporating a rare earth mixture into a zeolyte, deposit of isotopes of radium in tanks and pipes.
Thoriated tungsten welding electrodes
The addition of thoria improves the striking performance of tungsten welding electrodes. Pure thoria used in their manufacture, use: electrodes need to be occasionally ground to restore a conical tip - possibility of dust generation.
Thorium magnesium alloys for aero engine components
Hardened light alloy. Typical concentrations (Bq/g):
Thoria: Th-232 3600, Ra-228 1800; Thorium magnesium alloy: Th-232 160.0, Ra-228 80.0; Thoriated tungsten: Th-232 160.0, Ra-228 80.0. Estimated typical dose for working with Th/Mg alloys: 9.1 mSv/year.
H-01
6
Germany, thoriated electrodes
Specific activity of Th-232 ranges from less than MDL (~0) for WT-10 to 149 Bq/g for WT-40 electrodes. Possible exposure would be in range 5 - 20 mSv/y.
L-03
7
Australia, monazite separation
External gamma radiation
Principal source Tl-208 from Th chain. Significant only when monazite is in relatively concentrated form. Separation plant 0.5-5.0 microGy/hr, monazite separation section up to 20-30 microGy/hr, centre of full bag store ~250 microGy/hr
Internal exposure to dust (alpha)
Typical numbers 0.1 Bq/m3, some above 1.0 Bq/m3
Surface contamination
Settled dust or loose mineral. Potential contribution to external gamma exposure plus dust re-suspension.
Radon/thoron + daughters
Th in monazite is chemically bound within the grain. There is little opportunity for emanation. Thoron is particularly unlikely to escape due to its half life of 55 sec. Measurements made under worst case scenarios have indicated a thoron emanation rate of <1%. Routine monitoring results are consistently low, <0.005WL thoron daughters, <0.0005WL radon daughters - around natural background.
Water contamination
Levels are less than ones applicable for members of the public
W-03
8

Australia, monazite separation
Radiation exposure associated with monazite, external radiation 1-10 microGy/hr in plants, >150 microGy/hr in storage areas. Dust 0.05 - 5.0 Bq/m3
in 1989 ~15% of workers were receiving doses above 15 mSv/yr, small percent either approaching or exceeding 50 mSv/year.
H-05
9
Australia, monazite separation
Some workers may have been exposed over their employment to a mean committed effective dose from inhalation in excess of 50 mSv/yr (since mid-1970's)
H-08
10
Australia, monazite separation
Study of historical radiation exposure measuring thoron in breath of 62 workers. 8 workers (13%) >20 mSv/yr for many years; 2 workers (3%) >50 mSv/yr for many years.
T-02
11

Australia, monazite separation
Ra-226 concentration strongly depends on grain diameter and decreases when the size of the grain is increased. Surface type of distribution. Radon emanation coefficient for a surface type of distribution and for the size range of monazite grains (70-130 microns) is about 0.5. A comparatively high emanation rate of radon from monazite was detected.
M-06
12

Australia, monazite separation
In the past, occupational exposure to level of 50 mSv/yr were not uncommon.
U-01
13

Australia, monazite separation
Monazite, being softer and finer than the zirconium and titanium minerals, may fragment during the numerous sizing and physical separation processes, becoming preferentially concentrated in airborne dust.
Bioassay measurements of thorium in the urine and blood serum of mineral sands workers and unexposed persons were carried out. Levels of Th are much lower than predicted by ICRP-30. Discrepancy between measured & expected bioassay values up to an order of magnitude or higher. No correlation was found between the bioassay results and cumulative thorium concentration.
H-09
14

Australia, monazite separation
An attempt to correlate thorium excreted in the faeces of two male workers in the monazite section of mineral sands dry separation plant over a 10-day period with personal air sampling measurements (inspirable dust filter, integrated personal dosimeter, personal cascade impactor).
Th faecal analyses are able to detect acute and chronic exposures to the inhalation of Th-bearing dusts and to confirm the amount of inhaled thorium predicted from air sampling and metabolic models.

T-03
15

Australia, monazite separation
In 1987 - positional samples - particle size characterisation, Activity Median aerodynamic Diameter (AMAD) average = 4 microns
In 1988 personal cascade impactor was used to determine the actual AMAD size = 14 microns
Another study returned the following AMAD data: personal 15.7 microns, positional indoors 4.6, positional outdoors 2.7
Further information on personal cascade impactors is available from [R-02]

M-08
K-03
16

Radiobiological studies of workers involved in the extraction and milling of monazite (Brazil. 300 workers, ~14 mSv/year external dose) showed a significant difference in the proportion of chromosome aberrations between the control group and employees working in the physical/chemical treatment section of the plant.
Epidemiological survey of >2,000 dust-inhaling Chinese miners was unable to establish a relationship between the relatively low thorium lung burden and lung cancer. However, an association was found between pneumoconiosis (stage 1) and an estimated Th lung burden of about 5.5 Bq.
Mortality studies among US workers engaged in the production of thorium and rare-earth chemicals from monazite sand showed a statistically significantly elevated standardised mortality rate for pancreatic cancer.
S-08
17

Australia, monazite separation
Airborne concentrations of Rn-222 and Rn-220 and their short-lived decay products are of no radiological concern.
Typical gamma (microGy/hr): primary separation tails 0.2; heavy mineral concentrate stockpiles 1-2; secondary separation tails 2-7; stockpiled monazite >200
1987-1993 - external exposure decreased by 25-35%, an overall 4-fold decrease in the internal exposure, mostly due to the effect of the implemented engineering control measures of dust control.
K-02
18

Australia, monazite processing
Monazite treatment waste can contain up to 1,100 Bq/g radium
Monazite processing. Monazite contains up to 10% Th and 0.3% U. Concentration factor for waste can be up to 2, thus waste could contain 20%Th and 0.6%U
H-11
19

Monazite ore - exposure pathways: (1) Gamma - external, (2) Dust re-suspension - internal, (3) External beta risk to skin because of the energetic beta emitters in Th ore (beta particles with energy >1 MeV emitted by Ac-228, Bi-212, Tl-208). It can be shown that a contamination of skin of 1 Bq/cm2 (gross beta) of Th ore can give rise to a beta dose rate of 5 microGy/hr to the basal cells of the skin.
Monazite processing
Finely ground monazite (50% <10 microns) is generally treated by alkaline digestion. The caustic process involves cracking of the phosphate matrix with a hot 70% NaOH solution to dissolve the phospahte and convert the rare earths, Th and U to insoluble hydroxides. Crystallisation is used to recover the original hydroxide solution as a trisodium phosphate by-product.
The filter cake from cracking (intermediate product) is subjected to an acid leach to selectively dissolve the rare earths leaving a crude residue containing hydrous U/Th oxides. Common leachants include HCl, in which case a RE chloride results, and HNO3, which results in RE nitrate product.
After the acid leach, soluble radioactive isotopes such as Ra can be directed to the insoluble Th waste stream by the addition of BaCl2/H2SO4. Alternatively, the Ra stream can be allowed to enter the solvent extraction circuit where it will be rejected into the raffinate. The main solid waste is the sludge from the RE dissolution. It contains the same quantities f Th and U as the monazite but in a smaller mass.
The rare earths are finally separated and purified by solvent extraction or ion exchange.
Radiological problems:
Potential of liberation of Rn-220 and Rn-222 is significantly enhanced in grinding and cracking stages.
Practically all Th and most of U remain as insoluble hydroxides. The specific activity of Th/U solid residues - 2-3 times higher than for monazite. Gamma- levels from fresh gangue will depend upon the Ra amount reporting to the gangue. If less than 5% of Ra is present in gangue then initially the waste will contain 20-30% of the total radioactivity in monazite. With time due to ingrowth of Ra228, gamma will increase to ~300 microGy/hr.
Without Ra removal, a very concentrated Ra waste stream (100-150 kBq/l Ra-226 and 1500-2000 kBq/l Ra-228) could be produced during the latter stages of processing (ie. Reporting to raffinate from solvent extraction).
Highly active Ra residues in radium removal circuits. 30-2000 microGy/hr (2000 - in close proximity)
Doses - external:
India. Two workers > 2mSv/week, some 0.5-1.5 mSv/week. Annual >100 mSv.
France. 5-10 mSv/yr, no workers exceed 30 mSv/yr
USA. Doses for workers 10-20 mSv/yr
Malaysia. 10-12.5 mSv/yr, job rotation.
Brazil. 8-35 mSv/yr
Doses - internal:
India ~7.2 mSv/yr
Brazil. 78-300 mSv/yr
Estimated doses 10-1200 mSv/yr
H-12
20

Australia, monazite processing
1987-1992 dose assessment in minerals sands processing plant - all doses below 50 mSv/yr limit, and all below 15 mSv/yr.
Tailings - 2 microGy/hr, small areas - 20 microGy/hr
A-06
21

Australia, monazite processing
Surveys in New South Wales.
1965, gamma up to 150 microGy/hr, alpha dust up to 5 Bq/m3
1971 gamma up to 150 microGy/hr (monazite bags), alpha dust 0.02 - 4 Bq/m3
1973, alpha dust <1 Bq/m3, but close to magnetic separator 7 Bq/m3, dose ~32 mSv/yr
1984, gamma 2-60 microGy/hr (monazite stockpiles), alpha dust 0.8-20 Bq/m3
Available data suggest that annual doses >100 mSv/yr have occurred.

C-06
22

Russia, rare earth metals production
Specific activities (Bq/g)
Raw materials
Loparite: Th-232 1.3, Ra-228 1.4, U-nat 1.9, Ra-226 0.14, Ac-227 0.11
REM Chloride alloy: Th-232 2.3, Ra-228 3.2, U-nat 1.4, Ra-226 0.42, Ac-227 0.26
Products
REM carbonates: Th-232 0.6, Ra-228 0.08, U-nat 1.5, Ra-226 0.05, Ac-227 0.18
REM carbonates from monazite: Th-232 0.32, Ra-228 0.47, U-nat 6.4, Ra-226 0.09, Ac-227 0.25
"Ftoropol": Th-232 0.21, Ra-228 0.03, U-nat 1.7, Ra-226 0.04, Ac-227 0.25
REM oxides: Th-232 0.07, Ra-228 0.01, U-nat 1.6, Ra-226 0.01, Ac-227 0.06
Wastes
Fabric of the filtration (1 filter): Th-232 45,000, Ra-228 60,000, U-nat 100, Ra-226 4,900, Ac-227 1,200
Fabric of the filtration (filter 2): Th-232 0.7, Ra-228 1.6, U-nat 1,6, Ra-226 0.16, Ac-227 0.22
Sediments: Th-232 1.8, Ra-228 3.0, U-nat 1.6, Ra-226 0.07, Ac-227 0.17
S-13
23

Germany, thorium glass mantles manufacture
Former GDR 1950's -1985, building heavily contaminated.
In walls from 0.04 Bq/g for both Ra-228 and Th-228 in mortar to 263.3 Bq/g for both Ra-228 and Th-228 in cardboard.
In floors (Bq/g) Th-232 2.8, Ra-228 63.38, Th-228 82.74, Th-230 3.17
Gamma dose rates 0.16-3.43 microSv/hr, with background of 0.14; outside the building 0.2-6.0 microSv/hr
Neibourhood ground up to 0.1 Bq/g per radionuclide, max 0.6 Bq/g
C-06
24
 Germany, thoriated electrodes
Thoriated electrodes are used in Tungsten Inert Gas (TIG) shielded welding. Thorium is added to electrodes to facilitate arc-starting and to increase arc stability. Aerosols are emitted during welding, plus electrodes must be grinded to sharpen them, as only a sharp electrode will provide the desired quality. Specific activity of thorium is always less than 500 Bq/g.
Personal and positional air monitoring and time-motion studies were carried out. Annual intake during grinding is in range between 0.02 and 30.2 Bq/year, annual intake during welding - between 0.07 and 144.0 Bq/yr, Total annual intake is in range from 0.09 to 150 Bq/yr. Assessment index was determined by dividing the intake by ALI (Annual Limit of Intake) and the results were in the range between 0.01 and 10.2.
L-11
25
 Malaysia
Rare earth industry operated in Malaysia in the period 1982-1992 and there is a substantial amount of waste left: xenotime sludge (190 Bq/g of Ra-226, 250 Bq/g of Ra-228), thoria (0.45%U, 15%Th), lead cake (26 Bq/g of Ra-226, 350 Bq/g of Ra-228), tri-calcium phosphates (0.1 Bq/g of Ra-226, 0.6 Bq/g of Ra-228). Regulation - if the dose to the public < 1 mSv/y - disposal exempted from control. In practice, however, a dose constraint of 0.3 mSv/y is used.
O-04
26
 Germany, production of thoriated electrodes and lamps
Depending on the type of industrial production and specific operational process such as sintering, pressing, grinding of wires or rods or mixing and filling powders, concentrations of Th-nat obtained were in the range of less than 1 mBq/m3 - 2500 mBq/m3, the latter ones arising from filling thorium/tungsten powder in the tubes used for pressing welding rods. The concentration limit for occupationally exposed persons with 50 mSv annual dose limit is 42 mBq/m3 for a 2000 working hours year.
C-12

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