Detection of radioactive sources in scrap metal

 

Jean-Marc Légaré, Ph. D.

Radioprotection J.-M. Légaré

 

Main Page Jean-Marc Légaré

Commercial Radiation Protection Services

I- Radioactive sources found - photos and characteristics

II- Identification of radionuclides and quantities

III- Procedures

IV- Bibliography

 

Radioactive sources found - photos and characteristics

 

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

Identification of radionuclides and quantities

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.

Procedures

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.

Bibliography

1.

OCDE/OECD
Nuclear decommissioning - Recycling and reuse of scrap metals, 60 p., 1996.

2.

Lubenau J. O. and Yusko J. G.
Radioactive materials in recycled metals. Review paper. Health Physics 68, 440 - 451, 1995.

3.

IAEA
Methods to identify and locate spent radiation sources, TECDOC-804, 81 p., 1995, Vienna.

 

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