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I- Radioactive sources found - photos and characteristics
II- Identification of radionuclides and quantities
Due to unfortunate past events 1,2 and 3,
recoverers and recyclers of scrap metal including steel mills and
foundries want to prevent the entry of any radioactive source which
could lead to possible melting of such sources, decontamination and
disposal work and costs that has sometimes required the temporary
shut down of the plant. They also want to avoid transfer of such
radioactive products to their clients. Waste management sites also
want to keep away radioactive material from coming in.
Many radioactive sources found by recoverers and recyclers originally
come from nuclear installations (fission and activation products) as
well as from industrial and research irradiator activities (Co 60, Cs
137), teletherapy (Co 60, Cs 137), industrial radiography (Co 60, Ir
192, Tm 170, Yb 169), medical brachytherapy (Ra 226, Co 60, Cs 137,
Ir 192), humidity gauges (Am 241/Be, Ra/Be) and density gauges (Cs
137), industrial gamma gauges (Co 60, Cs 137) and various beta
gauging (Sr 90), and well logging (Am 241/Be, Cs 137, Cf 252). One
can also find pipes contaminated with uranium and thorium and from
the potash industries (K 40).
There can be other radioactive sources from these fields and
elsewhere. Radioactive sources found in the recovering and recycling
metals business usually have very long half-lives and energies.
The first 6 photographs of the figure shows
below are one old teletherapy unit, a gauge, one portable and two
mobile industrial radiography cameras, and transport
containers, all of which might contain
radioactive sources, when recovered and recycled. These photographs
are a courtesy of the Atomic Energy Control
Board of Canada.
The last two photographs represent a typical stationary radiation
detection system and a gamma spectrometer to identify radioactive
sources and do other functions needed as discussed later. They are a
courtesy of Exploranium.
Detection system and
installation
High performance fixed radiation detector systems are available
commercially. They are designed to detect penetrating gamma radiation
from radioactive sources. The detectors are usually installed at the
weighting scale of the vehicles (trucks or railroad wagons and
sometimes at the shredder or at the conveyor site. Before purchasing
a radiation detection system, it is best to obtain and compare
pertinent technical and commercial data from suppliers and also to
consult users.
A stationary radiation system is usually made of two separate
detector boxes, each having one or two scintillators with similar
electronic components as well as a system console and outside optical
sensors for vehicle movements. All these components are related by
means of electric cables with underground conduits.
Each detector box is fixed to a strong stationary stand at mid-height
of the usual vehicle loads expected. The distance between these
vertical detector boxes is somewhat larger than the maximum vehicle
width expected. Each of these detector box assemblies has its outside
surface parallel to the lateral sides of the vehicles. The system
console can be mounted in the scale house or at another indoor or
outdoor monitoring location. In each detector assembly box, there is
at least one scintillator which is usually a large and thick 18 L PVT
plastic plate. For PVT scintillators, there are often two per box. In
order to optimize sensitivity of the system, we must use very high
grade photo multiplier tubes, special electronic design and a very
good geometry with regards to vehicle-detector distance. In order to
reduce natural outside background and thus increase the detection
capability, a thick steel or another low-background shield should be
added behind the detector box and on its four sides.
Gamma radiation emitted in all directions from a radioactive source
in a slowly moving loaded vehicle first crosses its own shielding, if
any. This radiation then crosses some metal scrap, each lining of the
vehicle and of the detection box and the scintillator lining. The
scintillator absorbs much of the incoming gamma radiation. This
absorption produces light flashes called scintillations which are
then reflected on the light proof reflective surface of each of the
18 L PVT scintillators and reach one or preferably two windows in
contact with corresponding high quality PMTs. These PMTs have the
ability to convert the light pulses into electric pulses which are
analysed and transformed by the electronic system into useful
information for the system console.
The purpose of the scintillators, the PMTs and the incorporated
electronics in the combined detector box assemblies is to detect even
shielded gamma and possibly neutron, radioactive sources bury in a
load of scrap metal. Triggering by the loaded vehicle leads to the
vehicle's refusal at the plant entrance unless it is a false alarm or
noise alarm or a radioactive driver who had a previous nuclear
medicine procedure.
In radiation detection procedures with vehicles a serious difficulty
encountered is the very small additional radiation level added to the
existing background. Because of this, small sensitive hand-held
detectors are inadequate for general vehicle inspection because they
can only measure radiation well above background levels. They are
however useful as complements after a source has been detected by the
stationary installation, and only to verify its location within the
vehicle.
The console has a front panel providing the user interface to the
system and usually access doors, such as to the power switch. It has
a printer which usually records hard copies of alarms and warning
messages. The suggested components of a modern radiation detector
system console are:
- Bright console display for system information and alarm display
- Button keypad to set the system parameters and retrieve alarm data
and for a password entry
- Red alarm press button lighting during radiation alarm and is
silenced by resetting after the alarm data are recorded.
- Yellow status light in normal operation with power on. On certain
units, a button may flash when the system detects a faulty component
or if vehicle speed is too high (e. g. exceeding 5 km/h)
- Audio buzzer to warn of radiation alarms and system errors on some
units
The system console collects and monitors the gamma radiation
information from the detectors and displays the data on the front
panel LCD of the console display in a chart recorder. The purpose of
using sometimes in a detection system with two PMT per scintillator
is to provide a fail-safe backup electronics for radiation detection
security and to use such coincident circuit to reduce electronic
noise, thus bringing the useful level down to some 3 standard
deviations above the vehicle - in background at the detectors' sites.
If the system determines that a gamma emitting radioactive source is
present, an audio alarm is sounded and the alarm information is
displayed on the console display.
In order to detect the very low gamma energies from shielded sources
found into a shipment of scrap metals, detection can only occur below
average background levels. Indeed, such system continuously takes
background measurements prior to the entry of the vehicle into the
portal between the two detector panels. As the first optical sensor
is tripped, vehicle presence is acknowledged by the system and
measurements are recorded as the vehicle and its contents block or
shield the detectors from the background radiation. Depending on the
density of the material in the vehicle, background radiation can be
depressed as much as 40 %. This is called Vehicle-IN Background.
After the vehicles passes the portal, and all measurements are taken,
the microprocessor goes to work analyzing the data and triggering an
alarm, only if the below background alarm threshold is exceeded.
Some systems have been designed to monitor all internal components
and to automatically alert the user of any system malfunction. With
such continuous system diagnosis, if a component failure is detected,
signals can be rerouted to take advantage of back-up systems. These
detected system faults are displayed on the console display for
servicing. With the above feature, the system should be able to
operate, thus giving the user the maximum detection capability even
during this period. Internal modem facilitates servicing if
adjustments are necessary. The console usually has an alarm display
and in some recent installations, they could have the following
functional elements:
- Alarm diagnostics which is displayed on LCD and printed at the
console and/or at the remote printer; it could be stored in memory
for retrieval and printed for immediate record keeping
- Stored alarm information and history life that can be downloaded
via software to a PC
- Vehicle logging software to record and store data for each segment
analysis including vehicle speed and acceleration in and out of the
detection area
- System that can be automated to release radiation compliance
tickets which cannot be bypassed
- Provision of X-Y coordinate analysis to assist in source location
of the source within the truck or wagon
There are certain ways to identify and localize a
radioactive source3. Many containers having a radioactive source can be
identified visually because of their known shape, size, labels and
writings. Certain detected radioactive sources can also be identified
visually such as an one meter rod of radium previously used as an
anti-electrostatic bar in lithography. Acquired experience and use of
photographs can help in identifying radioactive sources with or
without its shielding component. If the source and/or the detected
radioactive gauge is carefully removed by vehicle unloading over a
non porous surface, and the load searched for, under controlled
conditions, it becomes easier to identify the source. The ideal way
is certainly by means of a portable gamma spectrometer, especially if
it has been programmed to analyze the emitted radiation and identify
the corresponding radionuclides. The results of spectrometry, dose
rate and accumulated dose over a given period of time can be
displayed on the LCD screen and put in the memory for future
retrieval on the display or sent to a PC.
The last photograph of the figure shows the Exploranium GR-130 which
can do the above functions and others at different locations and be
stored and retrieved later on the meter or on a PC.
Concerning the quantity of radioactive material in MBq of an
identified source, it can be obtained simply by combining the
measured dose rate at one meter away from source, and the photonic
specific emission rate as published for the identified radionuclide.
If the source is inside a nuclear device, the results calculated will
significantly underestimate the actual quantity in MBq because of
shielding.
On a metal scrap site, we should have in advance a
certain number of useful procedures, internally and externally.
Internal ones include procedures for general use, for activated
alarms activated, alarm responses, controlled area dose rate,
internal investigation, personal protection, etc. There should be
also procedures on training and educational aspects for the radiation
safety officers, employees, alarm monitor first responders, lab
technician, etc. There should equally be procedures on record
keeping, emergency management and for management and administration
and public relations.
Concerning the external procedures, there should be especially a
joint ones with regards to the company of the vehicle driver, and the
radioactive waste management company in case of need and most
importantly a joint procedure with the national nuclear agency with
regards to national and sometimes international aspects.
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1. |
OCDE/OECD |
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2. |
Lubenau J. O. and Yusko J. G. |
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3. |
IAEA |
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