RADIATION SAFETY PROCEDURES IN INDUSTRIAL RADIOGRAPHY

Contents Introduction Basic principle and applications of radiographic techniques Industrial radiographic equipment Control of external radiation exposures Safe working procedures Summary

Introduction Radiography technique - ensuring the integrity of vessels, pipes, welded joints and metal castings. Radiography produces high energy/penetrative radiations - person accidentally exposed to these radiations would result in radiation injury. To avoid unnecessary radiation exposure certain procedures should be introduced, through: proper radiation safety programme Training maintenance of sources and devices.

Basic Principle and Applications of Radiographic Techniques Three basic items needed: radiation source, object, and film. Basic principle: x-rays or gamma rays interact with a test object, some of the radiation is absorbed and another portion passes through un-deviated to form an image. Applications: to detect defects in welding, casting and building structure. Radiation Source Specimen Film

Industrial Radiographic Equipment X-Ray radiographic equipment. X-ray tube; Transformers or high voltage source to produce the required voltage; and A set of control panel. Gamma radiography sources and containers.

Industrial Radiographic Equipment X-ray radiographic equipment: X-ray tubes The x-ray tube is a vacuum tube in which electrons are accelerated to a high velocity by means of electrostatic field and then suddenly stopped by collision with a target. Result of this collision, x-rays are emitted. To prevent x-rays from becoming a hazard and create scattered radiation, the x-ray tube is shielded with lead (the window remains unshielded).

Industrial Radiographic Equipment X-ray radiographic equipment: X-ray tubes (cont.): The basic components of a x-ray tube are: a sealed glass tube envelope, a cathode, and an anode Target (Anode) Filament (Cathode) Glass tube envelope X-ray tube window

Industrial Radiographic Equipment X-ray radiographic equipment: X-ray tubes (cont.): A sealed glass tube envelope: Made of glass or metal-ceramic having high melting point to withstand the intense heat generated at the anode. High vacuum environment: To prevent oxidation of the electrode materials; To permit ready passage of the electron beam without ionisation of gas within the tube; and To provide electrical insulation between the electrodes.

Industrial Radiographic Equipment X-ray radiographic equipment: X-ray tubes (cont.): A cathode: Cathode incorporates focusing cup and filament. Focusing cup acts as a lens to direct the electrons in a beam towards the anode. Filament is heated by AC current from a separately controlled transformer. A change in the voltage (kV) applied to the filament varies the filament current (in A) and the number of electrons emitted. Current passing between the cathode and anode by means of the high-speed electrons, called tube current (in mA).

Industrial Radiographic Equipment X-ray radiographic equipment: X-ray tubes (cont.): An anode: Anode - metallic electrode of high electrical and thermal conductivity. Target materials - tungsten, gold or platinum. Tungsten - most common anode material because it has a high atomic number, high melting point and high thermal conductivity. Generation of heat if not controlled would quickly cause the surface of the target to erode. To avoid overheating the target, the tungsten is embedded in a mass of material with high thermal conductivity, such as copper.

Industrial Radiographic Equipment X-ray radiographic equipment: the X-ray control panel (cont.): The three controls that govern a radiographic exposure using x-rays equipment are the timer, the current (mA) knob and the voltage (kV) knob.

Industrial Radiographic Equipment X-ray radiographic equipment: the X-ray control panel (cont.): Timer: The timer is usually calibrated in minutes. The exposure time for an exposure is preset; when the equipment is activated, the timer counts down from the preset value.

Industrial Radiographic Equipment X-ray radiographic equipment: the X-ray control panel (cont.): Current knob: The current knob - controls the intensity or quantity of x-rays. When current flow through the filament is increased - the filament get hotter resulting in an increase in the intensity of electrons released. The greater the intensity of electrons striking the target, the greater the intensity of the x-rays produced.

Industrial Radiographic Equipment X-ray radiographic equipment: the X-ray control panel (cont.): Voltage knob: The voltage knob governs the energy or quality (penetrating power) and quantity (intensity) of x-rays produced. The voltage meters on the control panels for conventional x-ray equipment are peak voltage values measured across the tube

Industrial Radiographic Equipment X-ray radiographic equipment: generation of x-rays (cont.): Anode Cathode Electrons High positive charge Electron bombardment at target material Focusing cup Hot body Electrons cloud Electrons cloud around a heated filament

Industrial Radiographic Equipment X-ray radiographic equipment: generation of x-rays (cont.): Apply kV, the electrons are quickly accelerated towards the anode. Electrons will strike a target material and is brought rapidly to halt. More than 97% electrons’ kinetic energy converted to heat; and less than 3% to x-ray photons.

Industrial Radiographic Equipment X-ray radiographic equipment: generation of X-rays (cont.): Continuous X-ray High speed electron Continuous x ray

Industrial Radiographic Equipment X-ray radiographic equipment: generation of X-rays (cont.): Characteristic X-ray Low speed electron Ejected electron High speed electron Characteristic x- ray

Industrial Radiographic Equipment X-ray radiographic equipment: generation of X-rays (cont.): Effect of kV: Intensity or quantity of x-ray increased with increasing kV. Energy of x-ray increased with increasing kV. INTENSITY High kV Low kV Min.A Min.B WAVELENGTH Intensity of radiation (with the same ) increases with increasing kV  added by increasing kV

Industrial Radiographic Equipment X-ray radiographic equipment: generation of X-rays (cont.): Effect of mA: Intensity or quantity of x-ray increased with increasing mA. Energy of x-ray remains constant with increasing mA. Wavelength () INTENSITY High mA Low mA  min

Industrial Radiographic Equipment X-ray radiographic equipment: types of X-ray equipment: Directional X-ray units Panoramic X-ray units Linear accelerators Microfocus X-ray system Flash X-ray equipment Betatron

Industrial Radiographic Equipment X-ray radiographic equipment: types of X-ray equipment (cont.): Directional and panoramic conventional X-ray units. Directional x-ray tube assemblies are fitted with suitable window/collimators Without the collimator the tube is called panoramic x-ray tube.

Industrial Radiographic Equipment Gamma radiography sources and containers: ISO Standard 2919, ISO Standard 3999 – designed and tested standards for gamma ray sources containers used in industrial radiography. Activity of the source after time t can be calculated by using these formulas: a). At = Ao.e-0.693t/T1/2 Where At = activity after time t; Ao = original activity; T1/2 = half life or b). 2n = Ao/An Where Ao = original activity; An = activity after n half life; n = the numbers of half-life

Industrial Radiographic Equipment Gamma radiography sources and containers (cont.): Radionuclides (sealed source) commonly used in industrial radiography: Isotope Half-life Typical Activity Gamma constant (Gyh-1GBq-1 at 1 m Ir-192 74 days 3.7 TBq 130 Co-60 5.3 years 1.8 - 3.7 TBq 351 Yb-169 32 days 185 GBq 34 Se-75 120 days 1.48 TBq 56

Industrial Radiographic Equipment Gamma radiography sources and containers (cont.): Exposure container: Gamma containers classification according to mobility: Class P (portable, less 50kg, carried by hand) Class M (Mobile, example by trolley) Class F (Fixed, installed in an enclosure) ISO 3999 specifies dose rate limits for Class P, M and F containers.

Industrial Radiographic Equipment Gamma radiography sources and containers (cont.): Exposure container: Dose rate limits for the various classes of exposure containers (According to ISO 3999)   Maximum dose equivalent rate Sv/h Class On external surface of container At 50mm from external surface of container At 1m from external surface of container P 2000 500 20 M 1000 50 F 100

Industrial Radiographic Equipment Gamma radiography sources and containers (cont.): Exposure container: Class M – mobile Typical Co-60 Class P – Portable Typical Ir-192

Industrial Radiographic Equipment Gamma radiography sources and containers (cont.): Types of gamma ray projectors or cameras: Removable Plug Type Unit - Available with capacities up to 74GBq of Co-60 (or 3.7TBq of Ir-192). D-type Unit - Available with capacities up to 277.5GBq of Ir-192 or 37GBq of Cs-137. Remote Control Unit - Operated from a remote distance. Units that can hold very large Ir-192 sources and up to 18.5TBq Co-60 are also available.

Industrial Radiographic Equipment Gamma radiography sources and containers (cont.): Source assemblies source pigtail connector source assembly

Industrial Radiographic Equipment Gamma radiography sources and containers (cont.): Remote controls Crank Type Pistol Type

Industrial Radiographic Equipment Gamma radiography sources and containers (cont.): Guide and extension tubes

Industrial Radiographic Equipment Gamma radiography sources and containers (cont.): Collimators - made from: Tungsten Depleted uranium Lead

Industrial Radiographic Equipment Pipe crawler equipment: Application - used to radiograph welds of pipelines. The machines/mobile carriage carry either an x-ray tube assembly or a gamma source. The crawler is powered by batteries on the carriage, an internal combustion engine, or trailing cables from a generator. The crawler is activated and controlled by the radiographer from outside the pipe using a control source, Cs-137 sealed source.

Industrial Radiographic Equipment Safety features of radiographic equipment and shielded enclosure: The equipment used (example: projector, source assembly, sealed source, and all ancillary equipment) should meet the current specifications stipulated in ISO 3999, ISO 2919 or other equivalent national standards. Gamma projectors should not be used in conditions other than what they were designed for. X-ray equipment, electrical safety needs to conform to national and international electrical safety standards.

Industrial Radiographic Equipment Safety features of radiographic equipment and shielded enclosure: For shielded enclosures, the safety system provided should have the following items: Interlocking devices should prevent exposure from taking place unless the door is properly closed, or immediately terminate the exposure when the door is opened. Cut-off switches or other means inside the enclosure, which allows for control of an exposure by any person inadvertently trapped inside.

Industrial Radiographic Equipment Radiation safety equipment: Personal monitoring instruments: They dose monitoring instruments, are small devices, designed to be worn by an individual radiographer to measure the exposure received by them, Examples of these instruments are: pocket dosimeter, film badge, and thermoluminescent dosimeter (TLD)

Industrial Radiographic Equipment Radiation safety equipment (cont.): Monitoring instruments / dose rate meters: Dose rate monitoring/measuring instruments or survey meter are those that measure the time rate, at which exposure is received. A meter should be available for use with each source of ionizing radiation. All meters should have a battery test position and the battery state should always be checked prior to operation. Dose rate meters are expensive and delicate instruments, and should be treated with care and respect at all time. Only calibrated instrument shall be used.

Industrial Radiographic Equipment Radiation safety equipment (cont.): Monitoring instruments / dose rate meters: The survey meter should be used to achieve the following objectives; To check initial level at the safety barriers. To monitor on a routine basis the dose rate at the safety barriers, particularly when the radiographic location varies. To check that a source is fully shielded after use or that a source is fully retracted. To help locate a lost source.

Industrial Radiographic Equipment Radiation safety equipment (cont.): Source changer: A source changer - a device used to transport new or old sources from the manufacturer to the operating organisation and vise-versa. Source changing: Performed in a controlled area by trained and authorised workers. It will be done according to instruction manual given by the manufacturer.

Industrial Radiographic Equipment Radiation safety equipment (cont.): Source changer:

Control of External Radiation Exposure The three principle methods used to control radiation exposure are: Time Distance Shielding

Control of External Radiation Exposure Time: The exposure received by an individual working in an area is directly proportional to the amount of time that the individual spends in the area. The individual’s exposure will be equal to the product of the radiation intensity and the amount of time spent in that radiation intensity. This can be mathematically expressed as: Exposure (E) = Intensity (Dose Rate, I) x Time (t)

Control of External Radiation Exposure Distance: Dose rate is inversely proportional to the square of the distance from the source. This is known as the inverse square law. Mathematically it can be expressed as: (I1/I2) = (d2/d1)2 The above formula can also be used if the specific gamma ray constant given. The gamma ray constant represents the radiation dose rate from one GBq source at a unit distance, usually one meter.

Control of External Radiation Exposure Gamma ray constant for radioisotopes used in industrial radiography Isotope Half-life Typical Activity Gamma constant (Gyh-1GBq-1 at 1 m Ir-192 74 days 3.7TBq 130 C0-60 5.3 years 1.8 – 3.7TBq 351 Yb-169 32 days 185GBq 34 Se-75 120 dys 1.48TBq 56

Control of External Radiation Exposure Shielding: The effectiveness of shielding materials depends on the atomic number, the density of the material and the thickness of the material. The shielding efficiency is also dependent on the energy of the gamma rays / x-rays. Higher energies are less likely to interact with electrons. Thus, they are more penetrating.

Control of External Radiation Exposure Ix x,  Shielding (cont.): Formulas for shielding calculation: Ix = Io exp (-x) Where Ix = dose rate after passing thickness x; Io = dose rate before shielding;  = linear attenuation coefficient; x = thickness Intensity (Ix) = Intensity (Io) x Transmission factor () Half-value layer (HVL), 2n = (Io/ Ix) Where n = the number of HVL Tenth-value layer (TVL), 10n = (Io/ Ix) Where n = the number of TVL

Control of External Radiation Exposure Shielding (cont.): HVL and TVL values for radiographic radioisotopes and common shielding materials Shielding Material and Thickness (cm) Source Ir-192 Co-60 Concrete TVL 14.74 22.86 HVL 4.82 6.85 Steel 2.90 7.36 0.87 2.20 Lead 1.62 4.11 0.48 1.24 Uranium 0.93 2.29 0.28 0.69

Control of External Radiation Exposure Shielding design for an exposure room: It is an enclosed space engineered to provide adequate shielding from ionizing radiation. It is important to plan the design of the exposure room for immediate and foreseeable future needs before commencing the construction. The shielding design should also consider both the primary and scattered radiations. The amount of shielding should be calculated with reference to the dose rate, use factor and occupancy factor. Once the design of the exposure room has been established, no subsequent changes that affect radiation safety are made unless approved by the Regulatory Authority.

Safe Working Procedures Preparation prior to commencement of work: Personnel monitoring: All personnel must wear appropriate personnel monitoring at all times during radiography. The film badges should be stored in a radiation free area when not in use. Any accidental exposure or damage to the film badge due to mishandling shall be reported immediately to the safety officer in-charge.

Safe Working Procedures Preparation prior to commencement of work (cont.): Radiation survey meter: The response of the instrument should be appropriate to the type of radiation. Only calibrated instruments shall be used. (Refer to the instrument's certificate of calibration). Ensure that the instrument's battery is in good working condition.

Safe Working Procedures Preparation prior to commencement of work (cont.): Warning signals: It used to warn people around the area for the presence of radiation. It should be in the form of light or audible signals or both. The light or audible signal shall be distinguishable for the following situations: When a sealed source is about to be exposed or when an X-ray machine is about to be energised. When a sealed source is exposed or an X-ray machine is energized.

Safe Working Procedures Preparation prior to commencement of work (cont.): Warning and radiation signs : Warning and radiation (Tri-foil) signs of adequate size must be made available. These signs are used to identify and define restricted areas. It is suggested that the name, address and telephone number of the person responsible for the site to be included on each warning sign.

Safe Working Procedures Operating procedures for exposure rooms: Only authorised workers who have received the appropriate training are allowed access to exposure rooms. If the exposure room is designated as a controlled area, it is appropriate for the authorised workers to have had medical examinations. Written operating procedures should be made available as appropriate or required.

Safe Working Procedures Radiographic work procedure in open sites: (prior to work) Movement of radiation sources outside the premise shall comply with the Radiation Protection (Transport) Regulations 1989. On arrival at site, the radiographer must obtain permit-to-work. Inform the area supervisor on the following items: Type and strength of radiographic source. Model and serial number of radiographic equipment. Name of radiographer. Type of radiation safety equipment to be used. Period of works. A sketch of the site plan indicating the location of the works and intended safety boundary.

Safe Working Procedures Radiographic work procedure in open sites (cont.): (prior to work) After the location of the radiographic work has been identified, the following requirements shall be satisfied: Boundaries for controlled area shall be clearly defined with barriers of rope or other means. Warning lights shall be displayed to indicate that radiographic exposure is underway. Warning and radiation signs shall be prominently displayed. The area shall be kept under surveillance at all times during an exposure.

Safe Working Procedures Radiographic work procedure in open sites (cont.): (during work) The radiographer must at frequent intervals measures the dose rate to ensures that the rope is correctly placed. If the dose rate exceeds the permissible limit, position of the barrier must be adjusted. The radiographer must take every effort to minimise the radiation risk. This can be achieved by observing the following parameters: Time (as short as possible) Distance (as far as possible) Shielding (use collimator or shielding materials whenever possible)

Safe Working Procedures Effective communication between radiographers in a crew is essential

Safe Working Procedures Radiographic work procedure in open sites (cont.): (after work completed) Source returned to container Use survey meter to confirm safe position of source Lock the container Container returned to store/vehicle Barriers and signs dismantled Inform area supervisor

Safe Working Procedures Establishment of radiographic boundary: Initial calculations – safe working distance using inverse square law. Make preliminary boundary and place radiation warning signs at the boundary. Energize the x-ray machine or expose the radioactive source. Check the dose rate at the boundary using a survey-meter and adjust the boundary if necessary.

Safe Working Procedures Radiation Sign Gamma Projector Specimen Survey Meter Winding cable Rope

Safe Working Procedures Storage of radiographic equipment on sites: Storage for gamma sources: Specially constructed storage pit is needed. Safety and security of this pit shall be the responsibility of authorize person appointed by the NDT company. Sufficient radiation warning notices shall be displayed clearly. Names, telephone numbers of the responsible persons should be included together with the company’s name, address and telephone number. Storage of gamma source can be classified into two types, namely; long term and temporary storages.

Safe Working Procedures Storage of radiographic equipment on sites (cont.): Storage for gamma sources - long term: Source may be stored in its own containers that are used during local transport or in storage pit Store should be provided with sufficient shielding to ensure adequate protection for all persons. Dose rate outside the store < 0.02mSv/hr at 1 meter from the source or 0.2mSv/hr at 5cm from the outer surface of the protective container or any additional shielding used around it.

Safe Working Procedures The radioisotope storage pit (on site) Exposure device Storage pit Fence

Safe Working Procedures Storage of radiographic equipment on sites (cont.): Storage for gamma sources - short term/temporary: Containers used for temporary storage of gamma sources should be: Permanently marked for the intended use (nuclides, maximum permitted activity). Permanently marked with a warning if they do not fulfil the requirement for long-term storage.

Safe Working Procedures Storage of radiographic equipment on sites (cont.): Storage for X-ray equipment: A storage facility like a small lockable storeroom is sufficient. Protection is only required against theft or vandalism.

Safe Working Procedures Gamma source changing: Only person authorised by regulatory authority can perform this source changing. Gamma source changing is done using a source changer. This is to transfer: New source from source changer into exposure device; and Decayed source back into source changer. Radiation surveys shall be carried out during source changing.

Safe Working Procedures Transportation of gamma sources: Transport within company premises: Gamma sources should be carried in its container (exposure device). The appropriate label and warning signs should be placed on the container. Information such as the type, nature, activity and radiation level of the sources being transported should also be indicated on the container. Transport of gamma sources should be carried out only by the designated radiation workers.

Safe Working Procedures Transportation of gamma sources (cont.): Transport outside company premises: It should comply to with Radiation Protection (Transport) Regulations 1989. Ensure that the vehicle to be used for transporting the source is in good working condition. The radiation levels in the driver’s compartment shall not exceed 0.02 mSv/hr. The driver may need to be given a personal dosimeter such as film badge or pocket dosimeter. The vehicle may also be used as a storage facility en-route, radiation levels external to the vehicle should also satisfy the requirements for an unrestricted area.

Safe Working Procedures Transportation of gamma sources (cont.): Transport outside company premises: All emergency equipment - radiation signs, rope, a fire extinguisher and a radiation survey meter should be made available in the vehicle. In case of an accident, make an immediate radiation survey. If any abnormal radiation levels exist, establish and secure the restricted area, and notify the responsible officer.

Safe Working Procedures Transportation of gamma sources (cont.): Transport outside company premises: If the vehicle is carrying a package bearing a radioactive Yellow II and III labels, it must be labelled with a radioactive placard at the left, right and the rear sides. No other persons (beside the driver and/or his assistant) is/are allowed travelling in the same vehicle transporting/carrying the radiation source. Any loss of gamma sources during transport should be reported immediately to the RPO.

Summary

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