External Dosimetry

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Module Objectives

• State the most common source of external dose as a result of handling uranium.

• Compare the relative hazard of beta versus gamma radiation at uranium facilities.

• State the appropriate use of personal protective equipment (PPE) to be used when handling uranium.

• Identify the appropriate survey instrumentation to assess external dose rates.

• Identify methods used to reduce external dose at uranium facilities.

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External Dose Hazards

• External dose control is a necessary component of the radiation protection program at a uranium facility.

• Internal dose due to the inhalation and ingestion of uranium is typically viewed as the greater hazard.

• However, at facilities like uranium recovery facilities handling natural uranium, most of the total dose is primarily from external radiation.

• Dose is usually highest to:

• Skin

• Extremities

• Lens of the eye

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Dose Limits

• The applicable annual dose limits are specified in 10 CFR 20.1201.

• Total Effective Dose Equivalent (TEDE) 5 rem

• The TEDE is composed of the deep dose equivalent (DDE) and the committed effective dose equivalent (CEDE) from intakes

• Shallow Dose Equivalent (SDE) of 50 rem to the skin or extremities.

• This is the shallow dose averaged over the contiguous 10 square centimeters of the skin receiving the highest dose.

• “Extremities” means the hands, elbows and forearms, foot, knees, lower legs, and feet.

• Eye lens dose equivalent (LDE) of 15 rem

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External Dose Hazards

• The alpha radiation from uranium will not penetrate the dead layer of skin (“stratum corneum”). The sensitive cells are in the basal layer at an average depth of 0.007 cm, or 70 µm, and beyond alpha range in tissue (~40 µm).

• The majority of the dose is from the beta radiation arising from th d d t f i i il P 234 Th b tthe decay products of uranium, primarily Pa-234m. These betas can reach the basal layer, but cannot penetrate to internal organs. The dose evaluated at 70 µm depth in tissue (absorber thickness of 7 mg per sq. cm) is called the “shallow dose”

• The gamma dose is delivered to the whole body; the majority of the gamma dose is usually from radium accumulation and not the uranium itself.

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Radionuclide Beta (Max) Gamma

U-238

Th-234

Pa-234m

None

0.103 (21%)0.193 (79%)

2.29 (98%)

None

0.063 (3.5%)0.093 (4%)

0.765 (0.3%)

Major Uranium Beta and Gamma Emissions (MeV)

( )1.00 (0.6%)

U-235

Th-231

None

0.140 (45%)0.220 (15%)0.305 (40%)

0.144 (11%)0.186 (54%)0.205 (5%)

0.026 (2%)0.084 (10%)

U-234 None 0.053 (0.2%)

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Radionuclide Maximum Beta Energy (MeV)

Decay Fraction MeV per Transformation

%MeV per Transformation

U-238 None 0.00 0.00 0.0

Th-234 0.103 0.19 0.020 0.8

Th-234 0.193 0.73 0.141 5.9

Pa-234m 2.29 0.98 2.244 93.3

Total MeV/Transformation = 2.405

U-235 None 0.00 0.00 0.0

Th-231 0 14 0 45 0 063 28 9Th 231 0.14 0.45 0.063 28.9

Th-231 0.22 0.15 0.033 15.1

Th-231 0.305 0.40 0.122 56.0

Total MeV/Transformation = 0.218

U-234 None - no significant beta contributions

Assumption:The mix of radionuclides seen above is what might be expected from the "pure" uranium isotope in which the short-lived daughters have had time to grow.

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Sources and Detection of Beta Radiation

• In the uranium series, U-238 decays to Th-234 and Pa-234m.

• At low enrichments, most of the beta dose rate arises from:

• Pa-234m (where “m” indicates a metastable state)

• Pa-234m emits a high energy (2 29 MeV max ) beta particlePa 234m emits a high energy (2.29 MeV max.) beta particle

• Th-234 contributes a much smaller beta dose component

• At higher enrichments, the beta dose rate is lower due to less U-238 per g uranium and therefore less Pa-234m.

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Beta Dose Rates from Various Compounds

Source Surface Dose Rate*(mrad/hr)

Natural U metal slab 233

UO2 207

UF4 179

UO (NO ) 6H O 111UO2(NO3)26H2O 111

UO3 204

U3O8 203

UO2F2 176

Na2U2O7 167

*Beta surface dose rate in air through a polystyrene filter 7 mg/cm2 thick.

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Sources and Detection of Beta Radiation

• Pa-234m beta is easy to detect due to its high energy

• G-M detector is a good choice

• Ion chamber with beta window can also be used

• Beta correction factors need to be applied to calculate absorbedBeta correction factors need to be applied to calculate absorbed dose rate

• Correction factors will vary based on:

• Instrument type

• Geometry

• Uranium compound

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Beta Instrumentation(G-M Pancake)

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Beta Instrumentation(G-M Sidewall)

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Beta Dose Correction

• Beta correction factors must be applied to most instruments.

• These factors account for:

• the window thickness of the detector being greater (or less) than the ideal to match the human depth (“ideal” window thickness ~5 mg per sq. cm),

• the attenuation of the beta radiation in a large detector, i.e, we want to know the energy absorption in the next 2 mg per sq. cm of the detector, not the average in its entire volume,

• and the fact that many detectors show a high angular dependence to beta radiation due to thicker walls around the beta window.

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Beta Correction Factors

• Several beta correction factors have been determined for instruments measuring beta radiation fields around uranium.

• Ideally each facility would determine such correction factors based on the types of instruments that have and the types of uranium compounds that they are measuringuranium compounds that they are measuring.

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Instrument Window (mg/cm2)Beta* Correction

Factor Exposure Geometry

Victoreen 471 1.1 1.4 30 cm from U foils

Eberline RO-2A 7 4.0 Contact with depleted uranium (DU) slab

Al W ll d GM 30 1 7 30 f U f ilAl Walled GM 30 1.7 30 cm from U foils

"Teletector" 30 (low range)

50 Contact with DU slab

Eberline PIC-6A 30 40 Contact with DU slab

* True reading/measured value.

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External Dose Hazards

• In addition any process that separates the uranium from its decay products can increase the external dose rate where the decay products are concentrated.

• This is due to both the beta and gamma radiation emitted by the decay products.

• The most common beta is the 2.29 MeV beta (average energy of the beta is 0.8 MeV) from Pa-234m.

• This is a short-lived decay product of Th-234 (24 d) that is in secular equilibrium with its parent.

• During melting or casting operations, these isotopes frequently concentrate on the surface on the material.

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Sources and Detection of Gamma Radiation

• Betas are emitted by several uranium decay products but…

• Energies and effective yields are low

• Low radiation fields result

• Gamma emitters include U-235 Th-231 U-234 and decayGamma emitters include U 235, Th 231, U 234 and decay products

• Bremsstrahlung radiation can also be a contributor when the radioactive material is near high atomic number (Z) materials (lead, steel, etc.)

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Gamma Radiation

• The higher energy gamma-rays arise primarily from the progeny of Ra-226 and Rn-222, namely Pb-214 and Bi-214.

• Radium plate out can cause widespread gamma fields that can cause areas to be posted as radiation areas.

• Storage of large quantities of purified uranium can lead to widespread low-level gamma radiation fields because of the low-abundance, and (usually) low energy gammas from U-238, the grow in of Th-234, and U-235 and Th-231.

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External Dose Hazards

• There are areas of concern for external radiation at ISR facilities.

• Radium filter press at in situ leach facilities

• Problem: accumulation of radium and radon progeny in the filter

• Drummed yellowcake in confined areas

• Process piping in recovery areas

• Problem: external hazard may increase during shutdown due to liquid removal (less shielding)

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Sources and Detection of Gamma Radiation

• Uranium gamma rays are typically emitted with energies of less than 250 keV

• Pancake G-M detector can be used for detection or measurement purposes

• When G-M detectors are calibrated to provide a measurement p(e.g., mR/hr):

• G-M detectors will over-respond if not calibrated to same source/energies encountered in the field

• Alternative is to use:

• Ionization chambers

• Energy-compensated G-M20

Gamma Instrumentation(Ionization Chamber)

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Average Ion Chamber Response to Gamma Photon Radiation

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Gamma Instrumentation(Energy-compensated G-M)

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Average GM Response to Gamma Photon Radiation

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Personnel Dosimetry

• Typically, there is nothing particularly unique about dosimetry selection at uranium facilities relative to other facilities

• Examples include:

• Thermoluminescent dosimeters (TLD)

• Optically stimulated luminescent dosimeters (OSLD)

• Pocket ionization chambers

• Extremity dosimeters (TLD)

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Personnel Dosimetry(TLD)

• TLD are made of either lithium fluoride (LiF) or calcium fluoride (CaF).

• Radiation interacts with the crystals and causes electrons to move to a higher energy state where they are trapped by impuritiesimpurities.

• When the TLD is heated the electrons are released and return to ground state, emitting light (typically IR) which is measured.

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Personnel Dosimetry(OSLD)

• OSL dosimeters use aluminum oxide powder and work like TLDs.

• The difference is that OSLDs are “read” by shining a green light on a part of the dosimeter and a blue light is given off that is proportional to the radiation dose. Unlike a TLD, an OSLD can be re readre-read.

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Personnel Dosimetry(Extremity Badge)

• Extremity badges or finger rings usually use a TLD chip and is worn on the finger to measure dose to hands.

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Dose Reducing Principles

• Dose reduction at uranium facilities requires application of the ALARA philosophy

• ALARA dose reducing measures include:

• Decreasing exposure time

• Increasing distance

• Employing shielding (e.g., safety glasses for beta radiation)

• Reducing quantities of uranium materials where possible

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Material Approximate Material Thickness Required to Stop Pa-234m Betas (cm)

Air 850.

Aluminum 0.41

Lead 0.10

Lucite 0.92

Uranium Beta Shielding

Pyrex Glass 0.49

Polyethylene 1.2

Stainless Steel (347) 0.14

Water 1.1

Wood 1.7 (approximate)

Uranium 0.06

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Item Penetration Factor

Vinyl surgeon’s gloves 0.95

Latex surgeon’s gloves 0.87

Lead loaded (10 mil Pb equivalent) 0.77

Lead loaded (30 mil Pb equivalent) 0.13

Uranium Beta Dose Penetration Factors

Pylox gloves 0.62

Leather (medium weight) 0.62

White cotton gloves 0.89

"Tyvek" coveralls 0.98

"Durafab" paper lab coat 0.96

65% Dacron/35% cotton lab coat 0.91

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Absorber Dose Rate (mrad/hr) Percent Transmitted (%)

None 199 100

2 pairs coveralls 160 80

Reduction of Beta Radiation Dose

2 pairs coveralls 160 80

2 pair gloves + liner 120 60

Face shield 81 41

* Source: Natural Uranium Metal Slab at 1 cm

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Surveys for External Radiation

• Most workers at uranium recovery facilities receive external gamma doses of less than 1 rem per year.

• Gamma exposure rates are generally below 1 mR/hour for ore and 1.2 mR/hour for fresh yellowcake.

• During the buildup of the uranium daughters the radiation levels increase for several months.

• According to RegGuide 8.30 “Health Physics Surveys in Uranium Recovery Facilities” gamma surveys should be performed semi-annually throughout UR facilities to determine if radiation areas exist and determine external dosimetry requirements.

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Surveys for External Radiation

• Surveys should be conducted at new UR facilities shortly after startup of operations.

• If the semi-annual surveys reveal radiation areas ( areas where gamma radiation rates are high enough that a major portion of the body of an individual could receive a dose in excess of 5the body of an individual could receive a dose in excess of 5 mrem in an hour at 30 cm) then gamma surveys should be conducted quarterly.

• The most likely place for this condition to exist is near the filter press where Ra-226 accumulates.

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Surveys for External Radiation

• Gamma surveys should be representative of where workers might stand so that their whole-body radiation exposures can be estimated.

• Measurements should be made at 30 cm (12 inches) from the surfacessurfaces.

• Surface “contact” exposure rate measurements are not required for establishing radiation area boundaries or estimating whole-body exposures.

• A list of radiation levels in each area should be prepared after each survey.

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Surveys for External Radiation

• Beta surveys of specific operations that involve direct handling of large quantities of aged yellowcake should be conducted to evaluate the extremity and skin doses to workers.

• Beta dose rates, as opposed to gamma surveys should be conducted on contact with the surface or near the surfaceconducted on contact with the surface or near the surface.

• Beta surveys need to be conducted only once for an operation but should be repeated for an operation any time the equipment or operating procedure is modified in a way that may have changed the beta dose that would be received by a worker.

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Yellowcake Dose Rates

• The beta dose rate on the surface of yellowcake just after separation for the ore is negligible but the dose rate rises steadily afterwards.

• The beta dose rate from yellowcake that has aged for a few months after chemical separation when the short lived decaymonths after chemical separation when the short lived decay products are in equilibrium is about 150 mrem/hr.

• It is acceptable to use the following tables to evaluate beta doses in place of beta surveys using radiation survey instruments.

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Beta Dose Rate on the Surface of Yellowcake

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Beta Dose Rate From Aged Yellowcake Versus Distance

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Summary

• A health physics program at a uranium facility must consider external dose control

• Beta radiation often predominates as the principle source of external exposure from processed uranium

• The major beta emitting radionuclide of interest to us is Pa 234m• The major beta-emitting radionuclide of interest to us is Pa-234m from the U-238 decay chain

• Pa-234m emits a high energy beta, representing a hazard to the skin, extremities, and lens of the eye

• Operations that separate uranium from its decay products can create high external dose rates – especially in “empty” casks or cylinders.

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Summary, con’t

• Facilities handling large quantities of uranium can create widespread, low-level gamma radiation fields

• External dose reducing principles consist of application of the ALARA philosophy in concert with time, distance, shielding and consideration of reduced uranium quantities where possible (especially in concentrated locations)

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