Chemical Engineering 412

Introductory Nuclear Engineering

Lecture 20Health Physics

Radiation Hazard Assessment

Spiritual Thought2

“Nothing is going to startle us more when we pass through the veil to the other side than to realize how well we know our Father and how familiar His face is to us.”

Ezra Taft Benson

Exposure Definition

• Units (Conventional and SI)– Exposure

• Pertains to gamma rays (x-rays) outside the body, which ionize air and biological systems (such as humans)

– Conventional unit = Roentgen (R) = 2.58x10-4

coul/kg = 1 esu charge/cm3 air– SI unit = exposure unit = 1 coulomb/kg of air =

3876 R

massionstotalofsum

mqx )&( −+=

∆∆

=

Roentgen is a measure of exposure with dimensions of charge/kg

Imparted Energy (Dose)

• Biological effects generally scale with deposited energy

• Absorbed dose rate

• conventional unit = rad (radiation absorbed dose) = 0.01 J/kg = 100 erg/g; mrad is for millirad

• SI unit = gray = 1 Gy = 1 J/kg; mGy is milligray• Absorbed dose rate = D = Gy/s, mrad/hr, etc.

𝐷𝐷 ≡ limΔ𝑚𝑚→0

Δ 𝜖𝜖Δ𝑚𝑚

𝜖𝜖 = Σ𝐸𝐸𝑖𝑖𝑖𝑖 − Σ𝐸𝐸𝑜𝑜𝑜𝑜𝑜𝑜 + 𝑄𝑄

Gray or rad is a measure of dose with dimensions energy/mass

Kerma

• Acronym for kinetic energy of radiation absorbed per unit mass

• A concept most associated with non-charged radiation (neutrons, gamma rays).

• Sum of initial kinetic energies of all charged, ionizing particles released by indirectly ionizing radiation (neutrons, gamma rays) per unit mass.

• Same units as dose• Easier to measure and compute• Commonly nearly identical to dose, which is generally

the most important quantity.

𝐾𝐾 ≡ limΔ𝑚𝑚→0

Δ �𝐸𝐸𝑜𝑜𝑡𝑡Δ𝑚𝑚

Insufficient

• These metrics are insufficient for bio impact:– Dose– Kerma– Exposure

• Why?• Biological damage varies with:

– radiation type– deposition profile

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Linear Energy Transfer

• The linear energy transfer (LET) is a second important characteristic. It is large/high for charged or massive particles (alpha particles, neutrons) and small/low for light particles (beta and gamma).

• Neutrons and alpha particles are more damaging at the same dose than are beta and gamma particles

• These differences are referred to as different qualities.

collisionsdxdELET

=

Biological Characterization

• Relative biological effectiveness (RBE) = effect normalized to 200 keV gamma rays.

• RBE depends on tissue, biological effect, dose, and sometimes dose rate.

• More useful in scientific studies than in general radiation protection due to complexity.

Quality Factor

• A poor man’s RBE• Assigned based on average RBE behavior.• Weight factor is appx.

equiv.

Equivalent Dose

• Quality-factor or weighting factor modified dose• NCRP distinguished between equivalent dose (average

dose in organ weighted by weighting factor) and dose-equivalent (absorbed dose at a point in tissue weighted by quality factor determined from LET of radiation at that point).

• Any given tissue should have about the same response to an equivalent dose, regardless of radiation source.

• Different tissues differ in responses, even if equivalent dose is similar.

• Measured in rem (roentgen equivalent man - by NRC) or sievert (SI), where 1 Sv = 100 rem.

DQFH ⋅=

Equivalent Rate

• Symbol is and is given as product of dose rate and quality (dose equivalent rate)/weighting factor (equivalent dose rate).

• 1 rem effective dose has about a 0.055% chance of causing cancer, which is high. The mrem (millirem) is a more common unit and equal to 10 µSv.

• Population dose or collection dose, Hpop is the dose equivalent to a group of people, measured in man/person-rems, man-Sieverts

H

( )∫∞

=0

dHHHNHpop

Example

0.7 mCi sample of 137Cs emits 0.662 MeV gamma rays from 137mBa with a frequency of 0.845 per decay. If the source propagates through 0.5 m of water, find (a) the exposure rate, (b) the kerma rate in air, and (c) the dose equivalent rate.Linearly interpolate water data in Ap. C to find 𝝁𝝁 = 𝟎𝟎.𝟎𝟎𝟎𝟎𝟎𝟎𝟎𝟎𝟎𝟎/𝒄𝒄𝒄𝒄 – then find uncollided flux density (Eq. 7.23)

𝜙𝜙0 =𝑆𝑆𝑝𝑝 exp−𝜇𝜇𝜇𝜇4𝜋𝜋𝜇𝜇2

=

0.0007 𝐶𝐶𝐶𝐶 3.7 × 1010 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝐶𝐶𝐶𝐶 0.845 𝛾𝛾𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 exp−0.08604 × 50

4𝜋𝜋 50𝑑𝑑𝑚𝑚 2

et= 9.433 1/(cm2 s)

Exposure Rate

Linearly interpolate the interaction data for air in Ap. C for E=0.662 MeV to get 𝝁𝝁𝒆𝒆𝒆𝒆

𝝆𝝆 𝒂𝒂𝒂𝒂𝒂𝒂= 𝟎𝟎.𝟎𝟎𝟎𝟎𝟎𝟎𝟎𝟎𝟎𝟎

Eq. 9.9

��𝑋 = 1.835 × 10−8𝐸𝐸𝜇𝜇𝑒𝑒𝑖𝑖𝜌𝜌 𝑎𝑎𝑖𝑖𝑡𝑡

𝜙𝜙0

= 1.835 × 10−8 0.662 0.02931 9.433 = 3.36 ×10−9𝑅𝑅𝑠𝑠

= 12.1𝜇𝜇𝜇𝜇/ℎ

Kerma rate in airLinearly interpolate the interaction data for air in Ap. C for E=0.662 MeV to get 𝝁𝝁𝒕𝒕𝒂𝒂𝝆𝝆 𝒂𝒂𝒂𝒂𝒂𝒂

= 𝟎𝟎.𝟎𝟎𝟎𝟎𝟎𝟎𝟎𝟎𝟎𝟎

Eq. 9.5��𝐾 = 1.602 × 10−10𝐸𝐸

𝜇𝜇𝑜𝑜𝑡𝑡𝜌𝜌 𝑎𝑎𝑖𝑖𝑡𝑡

𝜙𝜙0

= 1.602 × 10−10 0.662 0.02937 9.433 = 2.94 ×10−11𝐺𝐺𝐺𝐺

𝑠𝑠= 0.106 𝜇𝜇𝜇𝜇𝑑𝑑/ℎ

Equivalent Dose Rate

Assume charged particle equilibrium so kerma rate is the same as absorbed dose rate, ��𝑲 = ��𝑫. Equivalent dose is quality factor for photons (1) times the absorbed dose in tissue. Approximate tissue with water. Linearly interpolate the interaction data for air in Ap. C for E=0.662 MeV to get 𝝁𝝁𝒕𝒕𝒂𝒂𝝆𝝆 𝒘𝒘𝒂𝒂𝒕𝒕𝒆𝒆𝒂𝒂

= 𝟎𝟎.𝟎𝟎𝟎𝟎𝟎𝟎𝟎𝟎𝟎𝟎

Eq. 9.5 & 9.10

��𝐻 = 𝑄𝑄𝑄𝑄 × ��𝐷 = 1.602 × 10−10𝐸𝐸𝜇𝜇𝑜𝑜𝑡𝑡𝜌𝜌 𝐻𝐻2𝑂𝑂

𝜙𝜙0

= 1 1.602 × 10−10 0.662 0.03260 9.433

= 3.26 × 10−11𝑆𝑆𝑆𝑆𝑑𝑑

=0.117𝜇𝜇𝑆𝑆𝑆𝑆

ℎ

Radiation ExposureAverage annual radiation exposure (millisievert)

Radiation United Nations Princeton U of Washington MEXT

Type SourceWorld Typical

rangeUSA USA Japan Remarkaverage

Natural

Air 1.26 0.2-10.0a 2.29 2 0.4 mainly from radon, depends on indoor accumulation of radon gas

Internal 0.29 0.2-1.0b 0.16 0.4 0.4 mainly from food (K-40, C-14, etc.)(b)Depend on diets

Terrestrial 0.48 0.3-1.0c 0.19 0.29 0.4 depend on soil and building material

Cosmic 0.39 0.3-1.0d 0.31 0.26 0.3 from sea level to high elevation

sub total 2.4 1.0-13.0 2.95 2.95 1.5

Man made

Medical 0.6 0.03-2.0 3 0.53 2.3

Fallout 0.007 0 - 1+ - - 0.01peak at 1963 and spike at 1986. still high near test and accident sites. US; Fallout is included in others

others 0.0052 0-20 0.25 0.13 0.001average occupational exposure 0.7mSv, mining workers are high, population near nuclear plant 0.02mSv

sub total 0.6 0 to tens 3.25 0.66 2.311Total 3 0 to tens 6.2 3.61 3.81

Radon Exposure

Exposure Limits

Limits for Exposures Exposure

Occupational Dose limit (US - NRC) 5,000 mrem/year

Occupational Exposure Limits for Minors 500 mrem/year

Occupational Exposure Limits for Fetus 500 mrem

Public dose limits due to licensed activities (NRC) 100 mrem/year

Occupational Limits (eye) 15,000 mrem/year

Occupational Limits (skin) 50,000 mrem/year

Occupational Limits (extremities) 50,000 mrem/year

Allowed exposure above background (300 mrem)

• 25,000 mrem/yr– Astronauts, per Space Shuttle mission– Annual occupational limit for adults through 1950.

• 15,000 mrem/yr– 1950 to 1957 occupational limit per year for adults,– changed in 1957 to 5,000 millirems.

• 5,000 mrem/yr– Occupational limit per year for adult radiation workers– ALARA – "as low as reasonably achievable”– lifetime cumulative exposure not to exceed the age multiplied by 1,000 millirems.

• 500 mrem/yr– Occupational limit per year for a minor under 18 exposed– Cumulative total for Embryo or fetus of a pregnant worker (Jan. 1, 1994)– Fetus should be limited to 50 millirems above background levels per month.

Radiation Health Risks

Health Risk Est. life expectancy lostSmoking 20 cigs a day 6 yearsOverweight (15%) 2 yearsAlcohol (US Ave) 1 yearAll Accidents 207 daysAll Natural Hazards 7 daysOccupational dose (300 mrem/yr) 15 daysOccupational dose (1 rem/yr) 51 days

Occupational Health Risks

Industry type Est. life expectancy lostAll Industries 60 daysAgriculture 320 daysConstruction 227 daysMining and quarrying 167 daysManufacturing 40 daysOccupational dose (300 mrem/yr) 15 daysOccupational dose (1 rem/yr) 51 days

Comparison of Risks

Procedure Effective Dose (Sv)

Effective Dose (mrem)

Risk of Fatal Cancer

Equivalent to Number of Cigarettes Smoked

Equivalent to Number of Highway Miles Driven

Chest Radiograph 3.2 x 10-5 3.2 1.3 x 10-6 9 23

Skull Exam 1.5 x 10-4 15 6 x 10-6 44 104Barium Enema 5.4 x 10-4 54 2 x 10-5 148 357

Bone Scan 4.4 x 10-3 440 1.8 x 10-4 1300 3200

Radiation Sources (I)

Sleeping next to someone

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Living near a nuclear plantEating a banana

Living near a coal plantDental x-ray

Natural daily background

Living in Brick, Stone, Concrete Building for 1 year

Flight from LA to New York;Living in Tokyo after Fukushima (weeks)

Radiation Sources (II)24

Natural Background Radaition (1 year)

Previous sources

Chest CT Scan

Safe Dose Limit (1 year) Radiation Workers

Biological Effect Classification

• Stochastic– Probability of occurrence (not severity)

depends on equivalent dose– Cancer and genetic mutations are examples– Has minimum threshold (contested)

• Non-stochastic– Deterministic effects with severity that scales

with dose– Skin damage (erythema), cataracts, blood

composition are examples.– May have threshold

Human Physiology

• Wide ranges in active cell division rates in mature adults.– Intestinal lining, bone marrow, skin, and reproductive

systems – most active– Other organs and tissues – low rates in adults.– Developing embryo, children, and youth

• many regions of cell division• activity level varying greatly depending on system and age• But all much higher than in adults.

Effects of absorbed Doses (I)27

Erythema28

Effects of absorbed Doses (II)29

Radiation Sickness

• General dose radiation damage is most severe in activity reproducing cells– Broad exposure generally affects reproductive cycles (can be

temporary at modest exposure), intestinal lining (can recover), and bone marrow (can recovery – sometimes with surgery)

– Common symptoms of non-lethal exposures are changes in reproduction fertility and virility, nausea an diarrhea, and leukemia.

– Cancers are long-term, stochastic issues.• Specific dose radiation damage has fewer general trends

– Thyroid cancers common because of radio-iodine– Basal cell cancers common but can have long latencies– Very little reliable data on humans

Cancer rates

0.1 Gy dose (Low LET) Excess Cancers

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Example Problem

What are the probabilities a 25-yr old male who receives a sudden accidental gamma-ray exposure of 2 rad (0.02 Gy) will eventually die from (1) radiogenic leukemia and from (2) any cancer caused by his exposure?

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LET impacts on cells

Dose responses

Blood effects

Blood response