Practical methods for reducing
exposure
1. Time
– minimizing the amount of time spent in a radiation on a field will limit the
total radiation received. Practical applications of the time factor include
working efficiently when handling radioactive materials and practicing new
procedures with non–radioactive materials until the desired speed and accuracy
are attained.
2. Distance
– increasing the distance from a radiation source will decrease the amount of
radiation received. The radiation dose rate (the dose per unit time) from a
small volume radiation source varies inversely with the square of the distance
from the source. By doubling the distance, the radiation dose rate is reduced
to ¼ of the original. If the distance is halved, the dose rate is increased to
4 times the original.
I1 = (d2)2
I2 (d1)2
Where:
I1 – dose rate
(intensity) at distance d1, from the source
I2
– dose rate (intensity) at distance d2, from the source
a. If
10 mR/hr is measured at 3m, what is the dose rate at 0.5m?
10 mR/hr = (0.5 m)2
I2 (3.0 m)2
I2 x 0.25 m2 = 10
mR/hr x 9m2
I2 = 10
mR/hr x 9m2
0.25 m2
= 360 mR/hr
b. If
150 mR/hr is measured at 20cm, at which distance will 95 mR/hr be measured?
150 mR/hr = (d2)2
95 mR/hr
(20 cm)2
(d2)2 = 150 mR/hr x 400 cm2
95 mR/hr
d2 = 632 cm2
= 25 cm
3. Shielding
– involves the use of a material to absorb radiation transmitted from a source.
a. Alpha
particles can be completely stopped by a sheet of paper.
b. Beta
particles can be absorbed completely by a few millimeter of plastic (e.g.
syringe)
Pure beta
emitter should not be used with lead because bremsstrahlung will be produced
Bremsstrahlung
is
radiation that results from the deceleration of the beta particles as they
approach the nuclei of the lead atoms in the shield. As the beta particles slow
down, they lose energy that is released in the form of X –rays.
c. Gamma
& X–ray particles, lead shield are used to reduce rather than completely
absorb the radiation emitted from a source.
Half–Value
layer (HVL)
– is defined as thickness of a material required to reduce the radiation
intensity to half its original value. This is the ability of a material to
absorb or attenuate x–rays and gamma rays.
HVL of lead for
selected radionuclides
I–125 0.04
mm
Tc–99m 0.27
mm
Cr–51 2.0
mm
I–131 3.0
mm
F–18 4.1
mm
Cs–137 6.5
mm
Personnel Monitoring Device
1.
Film badges
The film
consists of a plastic holder in which a strip of photographic film is held
between a set of filters. Following development, the amount of darkening
(density) on the film is proportional to the amount radiation absorbed. The
three or four filters are each made of a different material (lead, copper,
aluminum, plastic) so that the energy range and penetration of the radiation
striking the badge can be assessed.
The film badge
holder also has an opening where the film can be exposed to beta particles and
low energy photons. Film badges are effective in measuring radiation exposures
of 10 mrem to several hundred rems.
Film badges
should be worn between the waist and the shoulder at the site of the highest
exposure rate to the body.
2.
Themoluminescent
Dosimeter (TLD)
– uses lithium fluoride crystals that emit a quantity of light proportional to
the amount of radium absorbed by the crystals. The light is released and
measured when the crystal are heated to an extremely high temperatures. This is
less affected by humidity and temperature.
3.
Pocket
Ionization Chamber – used as a supplementary monitoring device.
Immediate radiation exposure measurements are provided through a separate
reading device. The amount of ionization is directly related to the amount of
radiation exposure.
NRC Regulations
A. Occupational
Dose Limits
– are based on the concept of “acceptable risk” which is the dose levels cited
in the regulations which are below to develop radiation–induced diseases such
as cancer or genetic changes. This is formerly called maximum permissible dose.
1. Dose
equivalent – product of absorbed dose and effective quality factor
Dose
equivalent = absorbed dose x
effective quality factor
= rem or Sv (SI unit)
Effective
quality factor – differences in biological effectiveness for different
types
of radiation
– unit
(x–ray, gamma and beta)
Effective dose
equivalent (EDE) – sum of the weighted dose equivalents for irradiated tissues
or organs. It takes into account the different risk associated with irradiation
of different organs and tissues.
2. Total
Effective Dose Equivalent (TEDE) – sum of the deep dose equivalent (DDE) and
the committed effective dose equivalent (CEDE)
DDE +
CEDE = TEDE
Deep Dose
Equivalent – refers to external exposure, considered when EDE of 1 cm in tissue
or organ
a. Lens
Dose Equivalent – DE to the lens of eye of 0.3 cm.
b. Shallow
Dose Equivalent – EDE of 0.007 cm (and average over an area of 1 m2)
Committed
Effective Dose Equivalent (CEDE) – refers to internal exposure
a. Committed
Dose Equivalent – total DE over 50 years period resulting from the intake of a
radionuclide into the body.
b. Committed
Effective Dose Equivalent (CEDE) – sum of the weighted CDE’s for each
irradiated organ or tissue.
c. Annual
Limit on Intake (ALI) – amount of radionuclide taken into the body during a
year.
LDE = 5 rem
TODE = 50
rem
d. Derived
Air Concentration (DAC) – concentration of radionuclide in air breathed for 1
working year (2000 hrs) would results in an intake by inhalation of one annual
limit or intake.
Terms used to
describe INTERNAL and EXTERNAL exposure
1. Total
Effective Dose Equivalent (TEDE) – radiation exposure to an organ or tissue
resulting from both external and internal radiation
TEDE =
DDE + CEDE
2. Total
Organ Dose Equivalent (TODE) – organ receiving the highest dose.
TODE =
DDE + CEDE
Occupational
Dose Limits (Annual)
1. Adults
TEDE = 5 rems
TODE = 50 rems
LDE = 15 rems
SDE = 50 rems
2. Minors
10% of annual
dose
3. Embryo
0.5 rems
4. Public
exposure 0.1 rems per year
B.
Posting
requirements for radiation area
1. Unrestricted
Area – is one in which access is not limited by or under the control of the
radioactive materials licensee. Radiation levels in an unrestricted area must
be such that anyone who is in the area will receive less than 2 mrem (0.02 mSv)
in any 1 hour.
2. Restricted
Area – if radiation level is more than mrem in one hour
a.
“Caution:
Radioactive Materials”
These words
are used to indicate any area in which certain quantities of radioactive
materials are used or stored.
Quantities of
Selected Radionuclide Requiring Posting with “Caution: Radioactive Materials”
Radionuclide
Quantities
exceeding
(mCi)
Co–57 1
Ga–67 10
St–89 0.1
Mo–99 0.1
Tc99m 10
I–123
1
Xe–133
10
Cs–137
0.1
b.
“Caution:
Radiation Area”
These words
are used to denote areas in which an individual could receive more than 5 mrem
(0.05 mSv) in 1 hour at 30 cm.
c.
“Caution: High
Radiation Area”
These words
are used to designate areas in which an individual could receive more than 100
mrem (1 mSv) in 1 hour at 30 cm.
d.
“Grave Danger:
Very High Radiation Area”
These words
are used to post an area in which an individual could receive an absorbed dose
of more than 500 rads in 1 hour at 1mm.
C.
Other NRC
Regulations
1. Surveys
must be performed to monitor for external radiation exposure and surface
contamination in locations where radioactivity is routinely stored or used.
RECEIPT OF
RADIOACTIVE MATERIALS
NRC requires that all shipments of
radioactive materials display a radioactive label be wipe tested
Types of Radioactive Labels
1. Yellow
I (white) – <0.5 mRem/hr from package surface
2. Yellow
II (yellow) – 1 mR/hr at 1 m from package surface
3. Yellow
III (yellow) – 200 mR/hr from package surface
4. Type
A – > 20 Ci of Mo99
– 100 Ci of Tc99m
– 10 Ci of I–131, Cs–137, Ir–192
– 70 Ci of I–125
5. Type
B – used for very large quantities of radioactive material
TI – is
referred to Transport Index and should appear as label of the radioactive
package.
Procedure for Wipe Testing
1. Visually
inspect the package for signs of damage or moisture.
2. Measure
exposure rates at the package surface and at a distance of 1m from the surface.
If the exposure rates exceed 200 mRem/hr at the surface or 10 mRem/hr at 1 m
from the surface, notify the NRC and the carrier delivering the shipment
3. After
opening the package, verify that the contents match the packaging ship and
check the integrity of the radionuclide container.
4. Wipe
the external surface of the container, assay the wipe and decontaminate the
container and exterior surface as required. If removable contamination exceeds
22 dpm/cm2 x 300 cm2 (6600 dpm or 0.003 µCi) of container surface,
notify the NRC and the carrier through the RSO.
5. Monitor
the packaging material for contamination and remove the radiation labels before
discarding the packaging material. If the materials are contaminated, dispose
them as radioactive waste.
RADIOACTIVE
WASTE DISPOSAL
Methods of Waste Disposal
1.
Decay in
Storage
In this
method, radioactive waste is separated according to half–life. After the waste
has been allowed to decay to background radiation levels, the radiation symbol
are defaced and the waste is disposed of along with other non–radioactive
trash.
If the waste
contains potentially biohazardous materials, then it should be incinerated
after the radioactivity has been decayed to background levels.
According to
NRC, only radionuclides with a half–life of less than 65 days maybe decayed in
storage. Materials must be held for a minimum of 10 half-lives and be monitored
to ascertain that there is no measurable radiation emitted from waste before
final disposal.
2.
Sewage
Disposal
The amount of
soluble waste that maybe discharged into sewer system is based on the rate of
waste–water discharge from a facility. NRC regulations specify that maximum
concentrations and total amounts that may be disposed of by this method.
It is
important to note that radioactive urine or feces are exempt from any
limitations and may be discarded through sewer system.
3.
Incineration
Special
permission is required by this method.
4.
Transfer to
authorized recipient
Long lived
radioactive waste can be disposed of by transferring the material to an
authorized commercial waste handler. Such companies either bury the material at
an approved site or incinerate the waste. Any radioactive materials transferred
for disposal must be packaged and labeled according to the US DOT requirements.
5.
Release to
atmosphere
Radioactive
gases may be discharged directly into the atmosphere in limited quantities.
Maximum airborne concentrations are specified in the NRC Regulations.
DECONTAMINATION
PROCEDURES
Decontamination is a procedure of
removing radioactive contaminant from a surface.
1.
Minor
contaminations
a. Clear
the area where the spill has occurred and notify other personnel in the
immediate vicinity
b. Wear
disposable, protective clothing such as shoe covers, gloves and lab coats.
c. Contain
the spill in a limited area. Place absorbent material over liquids as soon as
possible
d. Place
all materials used to mop up the spill into plastic bags.
e. Remove
any contaminated clothing or protective gear and discard with other
contaminated materials.
f.
Label all radioactive materials
and dispose of them properly with other contaminated materials.
2.
Major contaminations
a. To
prevent the spread of airborne contamination, shut off fume hoods, ventilation,
heating and air conditioning equipment
b. Remove
contaminated articles before leaving the area.
c. Evacuate
and close all doors to the area.
d. Lock
the doors and post warning signs
e. Begin
contamination of personnel involved.
f.
Notify the RSO immediately.
Clean up and decontamination of major spills should be conducted under the
supervision of the RSO.
3.
Personal
a. Contaminated
clothing should be removed immediately before leaving the area of the spill.
b. Mild
soap and warm water should be used to remove contaminations on the skin.
c. The
use of abrasive cleansers and brushes to clean the skin should be avoided.
d. The
treatment of serious injuries always takes precedence over decontamination.
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