APPLICATION OF RADIOISOTOPES IN

INDUSTRY

BY:UPVITA PANDEY

INTRODUCTIONThe use of radioisotopes in industry ensures good

quality products and brings down the cost of manufacture by ways of sensitive non-destructive testing and efficient in-process control.

Industrial applications of radioisotopes and radiation can be broadly classified into three categories :

RADIATION PROCESSING

NON-DESTRUCTIVE

TESTING

RADIOTRACER APPLICATION

WHAT IS RADIATION PROCESSING ? Radiation processing is controlled application

of energy of short wave length radiations of the electromagnetic spectrum known as ionizing radiations and includes gamma rays, accelerated electrons and X-rays to have desired effect on the product.

USES OF RADIATION PROCESSING RADIATION STERILISATION OF MEDICAL

PRODUCTSIonisation radiation has been effectively employed as

bactericide for sterilisation of medical products and devices on commercial scale.

This technique is mainly a cold process. It is used for disposable medical devices made from heat

sensitive plastics. A dose of 25 kGy is given to the sealed packages

containing the products to achieve sterilisation. The gamma rays penetrate through these sealed packages and destroy micro-organisms, if present in the product.

Major advantages of this process over conventional methods employing steam and chemicals are :

- high inactivation factor - the ability to sterilise the product in its final package - enhancement of shelf-life of the product - no carry over of toxic residuesSome of the products regularly sterilised are syringes,

surgical dressing, blades, sutures, absorbent cotton wool, infusion sets, surgical kits and certain antibiotics and ophthalmic ointments.

In India, currently three such plants are in operation in Mumbai, Bangalore and Delhi and offer sterilisation services to industry. The ISOMED plant at BARC completed 25 years of operation in 1999.

SLUDGE HYGIENISATION

Sewage sludge is rich in organic matter, nutrients and trace elements and merits recycling for economic considerations.

The raw sludge contains a very high concentration of pathogens, which are responsible for causing infectious diseases like typhoid, cholera and hepatitis. Hence the sludge needs to be treated before being released for use as manure.

The pathogen concentration in sewage sludge is reduced by treatment with about 3-5 kGy of radiation dose.

Advantages over conventional methods of aerobic and anaerobic digestion:

-reduce the pathogen concentration to safe levels.

Sludge Hygienisation Research Irradiator (SHRI) facility, containing about 500 kCi(18.5 x 10^15 Bq) of Co-60 for treatment of sewage has been in operation in Baroda.

TREATMENT OF FLUE GASES

WHAT IS FLUE GAS ? The flue gas refers to the combustion exhaust gas

produced at power plants. Its composition are nitrogen , carbon dioxide, water vapour. It further contains a small percentage of a number of pollutants  such as particulate matter (like-soot), carbon monoxide, nitrogen oxides, and sulfur oxides.

A technique is available using gamma ray irradiation to convert the gaseous pollutants into useful fertiliser constituents like ammonium sulphate and ammonium nitrate. The flue gas from the boiler is passed through a mechanical filter and saturated with water vapour before admission into an irradiation chamber. In the presence of radiation field and ammonia, the oxides are converted to their salts.

A radiation dose of around 10-20 kGy is required for this purpose.

RADIATION IN MANUFACTURING

Radiation processing technique is used for manufacturing improved quality products like polyethylene foam, cables, treated leather and semi-conductors. The radiation doses required vary w.idely depending upon a particular treatment.

The general advantages are: -ease of flexibility in process control, -better wear resistance, -cost effectiveness and -environmental safety.

Some important applications are given below:

Radiation is now widely used for vulcanisation of natural rubber latex instead of the conventional method using sulphur, which results in the formation of carcinogen in rubber.

Radiation cross-linking of polymers imparts dimensional stability to polymers thereby extending the working temperature range.

When cross-linking is induced in some polymers like polyethylene, it tends to shrink on heating.

By irradiation with gamma-rays or with electron beam (EB), it is possible to impart chain scission in polytetrafluoroethylene (PTFE) which is otherwise extremely resistant to pulverisation. These degraded PTFE products are useful for specialised applications as lubricants.

WHAT IS NON-DESTRUCTIVE TESTING ? Quality control method that does not damage or destroy

the material or product being tested. It uses infrared radiation, radiography, ultrasound, x-rays, and

other techniques to detect fatigue effects, structural flaws, and other such defects. 

The difference in the intensity of the radiation after passing through a test material is useful to get information such as thickness, density and defects of the material under study.

Based on this principle, the materials are tested non-destructively. Sealed sources of radioisotopes are used for non-destructive testing.

USES OF NON-DESTRUCTIVE TESTING GAMMA RADIOGRAPHY

When gamma radiations pass through matter, its intensity is reduced and the extent of reduction depends on the density and atomic number of the interposed material.

In the case of a metal cast, if there is a crack, then the reduction pattern will be different than in the normal case. Thus gamma radiography is useful for the non-destructive examination of welds and castings and is a well established technique.

Highly penetrating gamma rays from radioisotope sources such as 60Co (1173 and 1332 keV) and 192Ir (296, 308, 317 and 468 keV) are used in radiography techniques for examination of a variety of industrial products of greater thickness as in boilers, pressure vessels, ship and aeroplane components.

Gamma radiography cameras containing upto 100 Ci

of 192Ir or 20 Ci of 60Co are being used in India in thermal and nuclear power stations, fertiliser and petrochemical plants for checking welds/casts.

COMPUTED TOMOGRAPHY

This system is useful to obtain cross sectional images of the internal structure of test objects.

The computed tomography imaging system consists of a gamma ray source, a collimated detector assembly, a precisely controlled mechanical manipulator and a data acquisition system along with a PC.

The method involves collection of transmission data of the penetrating radiation through an object at different planes. Subsequent reconstruction using the two dimensional planar profiles of the effective linear attenuation coefficients at designated points leads to a 3D image.

ADVANTAGES1. CT completely eliminates the superimposition of

images of structures outside the area of interest.2.  Because of the inherent high-contrast resolution of CT,

differences between tissues that differ in physical density by less than 1% can be distinguished.

3. Data from a single CT imaging procedure consisting of either multiple contiguous or one helical scan can be viewed as images in the axial, coronal, or sagittal planes, depending on the diagnostic task. This is referred to as multiplanar reformatted imaging.

DISADVANTAGE1. The radiation used in CT scans can damage body cells,

including DNA molecules, which can lead to cancer

GAMMA SCANNING Gamma scanning is used for on-line trouble-shooting

of industrial columns without disturbing the on-going production activity.

A collimated source and a detector are positioned in the horizontal plane either across the diameter (in tray type columns) or across the different equal-length chords (in packed bed columns). Both the source and detector are then moved synchronously along the length of the column and radiation intensity is recorded at desired elevations.

The analysis of the data with reference to the internal loading and hardware configuration of the column gives information about the column.

‘Signature scans’ obtained under normal functioning or during pre-commissioning trials are used for comparison and to derive useful information of the internal configuration of the column.

Gamma scanning techniques have wide applications in trouble shooting, de-bottlenecking, preventive maintenance and for optimisation of the design of industrial columns.

NUCLEONIC GAUGING Attenuation of the radiation passing through any material

is dependent on the mass interposed. This property can be used to measure and monitor thickness of sheet materials such as films and sheets of metals and plastics during the manufacturing process.

A sealed source of suitable radioisotope is placed on one side and the detector on the opposite side.

The degree of attenuation of radiation is used for controlling the desired parameters.

The levels of liquids in closed or large tanks can be conveniently measured and controlled using radioisotope gauges kept external to the tanks. Similar concepts are applied in controlling the filling of gases in cylinders and detergent powder in packages.

SMOKE DETECTOR A tiny source of radioisotope (e.g., 241Am) is used in

a suitable ionisation chamber in the detector. Interaction of radiation with matter produces -

ionisation/excitation. The extent of ionisation depends on density and

atomic number of the medium. If the density of the medium is increased, ionisation current also increases. e.g., smoke and air. This increase is utilised to trigger an alarm above a set threshold value.

Ionisation chamber of smoke detectors is divided into two compartments: one is closed and the other has openings for continuous flow of air.

•When air containing smoke enters this compartment, the resultant ionisation current is compared with the ionisation emit from the closed compartment.

•Alpha particles emitted by the decays of 241Am ionize the air (split the air molecules into electrons and positive ions) and generate a small current of electricity that is measured by a current-sensitive circuit. When smoke enters the detector, ions become attached to the smoke particles, which causes a decrease in the detector current. When this happens, an alarm sounds. 

INDUSTRIAL RADIOTRACER TECHNIQUES

WHAT IS A RADIOTRACER ? A radioactive tracer is a chemical compound in which

one or more atoms have been replaced by a radioisotope so by virtue of its radioactive decay. It can be used to explore the mechanism of chemical reactions by tracing the path that the radioisotope follows from reactants to products.

Radioisotopes of hydrogen, carbon, phosphorus, sulphur, and iodine have been used extensively to trace the path of biochemical reactions.

USES OF RADIOTRACER TECHNIQUES LEAK DETECTION The leakage in long and buried pipelines or those in

industrial plants can be monitored with a suitable short-lived isotope like 82Br (t1/2 = 35.3 h).

This involves isolating segments of the pipelines, introducing the tracer and holding the liquid column under pressure, flushing the pipeline and scanning the surface / soil for leaked out radioactivity. Even minute leaks have been detected leading to enormous savings of cost, energy and man-hours. Similarly, leaks in dams and reservoirs can also be detected.

CHEMICAL REACTORChemical reactors are vessels designed to

contain chemical reactions. The mixing time and mean residence time of materials

in process containers are of vital importance for production efficiency and product quality in chemical industry.

A small quantity of the radiotracer is introduced at the input point and monitored as a function of time to determine the mixing rate and residence time.

Such measurements can be made on streamas well as by sampling. These studies are being routinely used in chemical and cement industries.

24Na, 60Co and 198Au are used.

MATERIAL INVENTORY

The material inventory of mercury in caustic soda plant has been determined using the principle of isotope dilution analysis.

The short-lived 197Hg (t1/2 = 64.14 h) is mixed with the mercury and the measured radioactive concentration after thorough mixing is used for estimating the total quantity of mercury in the cell.

SILT MOVEMENT IN HARBOURS Dredging operations in harbours are carried out

periodically for keeping the shipping channels clear. In order to select suitable sites for dumping the dredged silt and for checking the suitability of the alignment of any shipping channel in a new harbour, it is required to study the movement pattern of the silt under water.

A radiotracer like 46Sc (t1/2 = 83.79 d) is mixed with a sample of silt and deposited in the sea bed. The movement of the silt is monitored by measuring radioactivity of 46Sc using underwater radiation detectors. All the major port authorities in India have used such techniques.

OFF-SHORE CONTAMINATION MOVEMENT Similarly radiotracers are used to study the pattern of

dispersal of waste in water bodies. The timing of and the distance at which discharge is preferred can be reliably established. Such investigations would help ensure safe and acceptable means of waste disposal and improve environmental protection measures. In collaboration with NEERRI (National Environmental Engineering Research Institute, BARC has carried out studies off Mumbai coast.

For this, 82Br (t1/2 = 35.3 h) as NH4Br was used as tracer.