Helena Janžekovič
Slovenian Nuclear Safety Administration Litostrojska 54
1000 Ljubljana, Slovenia helena.janzekovic@gov.si ABSTRACT
Industrial radiography is still one of radiation practices with the highest frequency of accidents, e.g. according to UNSCEAR around 30-40% of all reported accidents related to clinical consequences are a result of an event in industrial radiography. As consequences related to unsafe handling of industrial radiography sources might span from exposures bellow deterministic thresholds for radiation effects up to deaths as happened in 1984 all involved in this specific practice are taking lessons learned identifying mistakes leading to accidents. The analysis identified five major mistakes, namely lack of using survey meters and other radiation measuring equipment, inappropriate management of safety and warning systems of shielded enclosures, inappropriate maintenance or replacement of industrial radiography equipment, lack of procedures in recovery operations and poor communication with clients. This analysis might be used for training purposes of radiographers. It might facilitate the communication between industrial radiography companies and their clients, e.g. NPP operators, as well as for improvements licensing procedures of this practice.
1 INTRODUCTION
One of the widely used method of inspecting materials for identifying hidden flaws is industrial radiography where the ability of X-rays, gamma rays and neutrons to penetrate various materials is used. As the method does not require destruction of the material inspected it belongs to a group of non-destructive methods (NDT). The method is needed in many industrial areas, e.g. in manufacturing processes and quality assurance of welding. It is an important part of such industries and a list of international or national standards related to it is long [1, 2]. As the risks associated with this practice is significant, numerous safety measures should be in place. The practice requires constant vigilance not only of industrial radiographers but also others involved in the practice, e.g. producers of sources, exposure devices sometime called gamma cameras and producers of survey meters. A study of lessons learned from accidents, incidents as well as near misses enables identification of major mistakes leading to accidents. It can contribute to better understanding the complexity of this practice.
2 INDUSTRIAL RADIOGRAPY
The method is using ionizing radiation which hits an object to be inspected. The radiation which is not attenuated or is attenuated only partly in the object is detected producing image on radiographic film or digital detector system. This image is processed and analysed. When selecting a source of ionizing radiation few possibilities exist: radioactive material, e.g. Se-75 and Ir-192, X-ray generators using X-ray tubes, accelerators, e.g. NDT electron linac and
nuclear reactors used for neutron radiography. The last two types of ionizing radiation sources, i.e. accelerators and nuclear reactors, are less common. The practice is performed in specially designed enclosures with safety systems or at the site where the object to be analysed is installed. The on-site radiography is applicable when the object cannot be transported to the enclosure, e.g. large oil pipelines and pipes at construction sites. Details of industrial radiography are given elsewhere, e.g. in [3].
3 REGULATORY REGIME
As this NDT method might cause significant risk to the workers as well to the general public which cannot be neglected from the radiation point of view, e.g. photon dose rates applicable might be around Sv/min and gamma sources of Category 2 [4] are used, a use of industrial radiography sources should be under a strict control of regulatory authorities. A risk related to external exposure is significant, but is should not be neglected that in some very particular cases sealed sources were damaged causing a risk of internal contamination of persons and the environment.
Considering requirements of the IAEA GSR Part 3 [5] and EU basic safety standards (BSS) [6] the practice should be authorized. In addition, according to the EU BSS any practice involving a high-activity sealed source should be licensed. Safety of industrial radiography requires a comprehensive list of technical safety measures as well as measures related to organisation of activities including strict requirements for education and training of staff involved. As a rule, in countries with well-developed nuclear and radiation safety legislation a specific part of legal acts talks about mentioned safety measures. Many regulatory authorities publish specific guides such as one at the US NRC web site [7]. Despite somehow narrow description of the industrial radiography given above the practice is linked to other facilities and activities involving a risk caused by ionizing radiation, such as production of sources, e.g.
operation of a reactor where a material for gamma sources is produced, distribution of sources and equipment, import and export of sources, maintenance of equipment, management of permanent and temporary storages of sources and disused sources, radioactive waste management and transport of radioactive material. Only a complete set of safety requirements considering all aspects of activities assures that a use of such sources does not pose unjustified risk. When nuclear materials are used for shields, e.g. in exposure devices and collimators, specific requirements for such materials apply.
In the last ten years few changes in industrial radiography are noted.
Digital radiography has been developed.
A use of more than one source of ionising radiation, e.g. two X-ray tubes, and a use of CT scans have been introduced.
Radioactive sources widely used in the past are replaced by sources using electrical generators to prevent a use of radiation sources for terrorist acts.
Gamma sources in industrial radiography have been a subject of common EU legislation for years. From 1993 a shipment of sealed sources is a subject of a specific control to maintain full control over any movement of them within the EU Member States (MSs) [8]. The regime is similar to regimes described in IAEA documents [9, 10]. As gamma sources used in industrial radiography are high-activity sources additional control has been required in the EU MSs from 2003 when so-called HASS Directive [11] have been published. Sources used in this practice shall also be a subject of appropriate security plans and associated regulatory regimes.
4 RISK ASSOCIATED WITH INDUSTRIAL RADIOGRAPHY
Despite numerous safety requirements this practice is still one of radiation practices with the highest frequency of accidents, e.g. according to UNSCEAR around 30-40% of all reported accidents related to clinical consequences are a result of an event in industrial radiography [12].
Even a quick analysis of about 20 reported events in IAEA NEWS in 2018 reveals that five of them are related to this practice. Consequences of accidents span from exposures bellow deterministic thresholds for radiation effects to death of members of the public. According to the data available the most serious accident related to industrial radiography happened in 1984, when 8 members of the public handling a lost gamma source of 1000 GBq Ir-192 died.
One of the first analysis of accidents in this practice has been published decades ago 1998 e.g. IAEA SRS No. 7 [13]. The document analyses a broad spectrum of accidents describing eight so-called primary causes of reported accidents starting with inadequate regulatory control, five primary causes related to human behaviour and only two primary causes related to the equipment. As each accident is a chain of events the authors analysed so-called initiating events, e.g. bad connection between the source assembly and the drive cable, and contributing factors, e.g. lack of adequate survey meter for detection of a position a source.
A year later a very first review of cases reported in the Ionising Radiations Incident Database, UK, [14] has been published. The database containing 100 events includes accidents, incidents as well as near misses in UK. The authors noted that about 40% of all cases are related to industrial radiography. Only 30 percent of the accidents are related to X-ray radiography, i.e.
it seems that somehow accidents with gamma radiography sources are two times more common than accidents with X-ray equipment. In addition, they reported that 15 cases were related to gamma radiography in enclosures and 14 cases to on-site gamma radiography. From these data it might be concluded that a site of the practice when considering gamma radiography does not play substantial role. On the other hand, regarding X-ray equipment 8 events are related to enclosures and only 2 to on-site radiography. This database contains some events which happened more than 30 years ago, e.g. such as a loss of sources due to bad designs of the equipment. The development of mentioned standards is trying to prevent such accidents.
Today the OTHEA – RELIR database [15] gives a short description of radiological accidents in some EU MSs including accidents in industrial radiography. The IAEA is publishing detailed description of selected accidents, e.g. given in [16, 17]. It also established a working group studying optimisation in industrial radiography within the ISEMIR project [18]. Already mentioned the IAEA NEWS promptly informs regulatory authorities about accidents and about their progress to facilitate recovery operations, e.g. when a gamma source is stolen.
To facilitate understanding of major components of safety in this practice mentioned databases were analysed. Where details of a chain of events leading to an accident were given a use of a flowchart facilitated understanding when and which safety rules were violated. From a comprehensive list of mistakes which span from equipment flaws and involvement of untrained personnel to poor communication with a client responsible for a construction site, five major mistakes were identified. This analysis might be useful for all stakeholders involved in this practice. Such sharing of lessons learned is in line with the conclusions of the IAEA Technical Meeting Radiation Safety in Industrial Radiography [19] where experts identified a long list of actions necessary to improve safety in industrial radiography in 2014.
5 FIVE MAJOR MISTAKES IN INDUSTRIAL RADIOGRAPHY
Generally, the main reason that accident happened lies in the fact that safety rules were not followed, and safety culture was not present when managing sources. As a rule, not only one person or one organisation was involved in an event. Five critical mistakes leading to accidents in the last years should be understood by all involved in industrial radiography, i.e.
not only by industrial radiographers but also by senior management and others who are involved in maintaining high level of safety culture in a NDT company. Some of the mistakes tackle also others involved in this practice or a control of it, e.g. qualified experts and producers of equipment. The mistakes identified are related to safety, i.e. poor security culture enabling malevolent acts is not included in the list by purpose.
5.1 Lack of Using a Survey Meter
A use of equipment for measuring radiation is widely known to industrial radiographers as they are aware of a need to control their occupational exposure. Among workers whose occupational exposure is monitored the industrial radiographers are using the highest number of measuring equipment, i.e. usually they are using four different types of measuring equipment at the same time [3]. However, the importance to use a survey meter regularly, its regular calibration and testing before its use is often underestimated. In particular, when performing gamma industrial radiography on-site under time pressure a lack of a use of survey meter confirming that a source is safely back in an exposure device often contributed to accidents resulting in a loss of a source, handling an exposure device and a guide tube when a source was still in a guide tube or resulting in handling a source which was not fully shielded in an exposure device. Such management of a source resulted in unjustified exposures, injuries or deaths. As a rule, the understanding of a role of all four types of measuring equipment is of a vital importance to perform industrial radiography safely.
A use of a survey meter is threefold. Namely, it is used to confirm the location of a source each time before and after the exposure, e.g. confirming that the gamma radiography source is not lost or stacked in a guide tube or that the X-ray machine is not working. A survey meter is also used to confirm that a boundaries of a control areas are adequate and that a supervised area is properly controlled. In addition, in case of emergency or recovery operation related to gamma radiography source a survey meter is used to confirm that a shield used in a recovery operation is adequate. The NDT licensee should have adequate number of survey meters, i.e. with each source there should be at least one.
In order to be warned about the existence of a higher radiation field each radiographer should wear personal alarm dosimeter. The alarm dosimeter should be able to warn the person even in the environment with very high level of noise such as at construction sites or production halls. Nowadays higher radiation fields should trigger a combination of sound, vibration and visual alarms.
Optimisation of radiation safety can be achieved only if doses received by an industrial radiographer are related to specific daily activities of this radiographer. To have detailed overview how a person is performing the tasks an electronic dosimeter giving daily dose is used. Radiation protection officer can then identify good and bad practices of a NDT company as a whole as well as good and bad activities of each radiographer.
Passive personal dosimeters, e.g. TLDs, are used to measure cumulative dose of an industrial radiographer in a period of time. As a rule, in this practice the period should
not be more than one month. RPO should follow the trends in personal doses and the NDT licensee should have a clear system how the workers are informed about their doses.
Although all four measurement techniques are important, a regular use of survey meter and alarm dosimeters could prevent majority of accidents which resulted in deaths and injuries.
5.2 Inappropriate Management of Safety and Warning System of an Enclosure Enclosures are specially designed rooms where industrial radiography is taking place enabling shielding of workers and the general public during the irradiation form primary and scattered radiation. The industrial radiographer should be outside a shielded enclosure when the irradiation of an object is taking place. The design of a shielded enclosure should be carefully analysed before its building as later it might be difficult to change its use or to install additional safety features. Safety systems and warning systems of enclosures are numerous and include interlocks, such as door interlocks and emergency stop buttons or pull-cords. They should be designed using three principles, i.e. redundancy, diversity and independence. Details are given in [3].
However, analysis of accidents reveals that often NDT companies, i.e. licensees:
do not provide adequate maintenance of safety systems and warning systems, e.g.
warning lamp is not working, dose rate meter in a shielded enclosure is not calibrated and sources used are not in line with limits and conditions given in the authorization,
introduce modifications jeopardising safety, e.g. make a hole in the shield without providing additional shielding,
do not provide training and re-training of industrial radiographers to assure that industrial radiographers understand specific roles of each safety systems and importance to follow procedures.
NDT companies using old shielded enclosures should pay attention to adequacy of safety systems and their ageing and to updating its safety assessment as assumptions used for a safety assessment might be very different nowadays then years ago. In addition, NDT companies should identify bad practices conducted by experienced industrial radiographers as well as assure that all newcomers are adequately trained and that they understand specifics of the safety systems and warning systems of a shielded enclosure.
5.3 Inappropriate Management of Equipment used for Industrial Radiography The sources of radiation in industrial radiography, e.g. X-ray tubes and gamma sources, are only one part of the equipment to be used. A list of other equipment is given in [3].
Equipment maintenance program should be in place. Equipment should be regularly tested, e.g.
by using Go-No-Go gauge. Routine checks of suitability of equipment should be conducted to control ageing and to identify malfunctioning. Equipment is designed for harsh environment such as construction sites. As a rule, equipment might be used for a very long time, e.g. it is not uncommon to use X-ray machines produced 20 years ago. But bad maintenance and lack of regular checks contributed to numerous accidents.
On the other hand, during the procurement of the equipment either for the first time or later when replacement of a part of equipment is needed a NDT company should fully analyse compatibility of equipment. Cases related to accidents and incidents due to a lack of compatibility include for example a use of inappropriate exposure device causing exposure of
passengers at the plane carrying an exposure device with a source in it which was not fully shielded.
The NDT company should have a clear procedure regarding appropriate maintenance and checking of its equipment including checking of compatibility of the equipment.
5.4 Lack of Procedures for Recovery Operations
A very specific group of mistakes is related to recovery operations. When so-called initiating event happens, e.g. a source assembly is detached from the drive cable or heavy object falls on the radiographic equipment when in use, recovery operation must take place. Untrained industrial radiographers might manage the situation in a way that the situation deteriorates, e.g.
in a very specific case a gamma source was cut by the NDT staff when they wanted to recover stuck source causing contamination of personnel and environment.
The NDT company should have a written recovery procedure followed by trained industrial radiographers explaining which actions should be done by them, radiation protection officer of the licensee and others involved. The initiating event should be handled either by licensee staff, e.g. using emergency kit including remote source handling tools and a spare shielded container for gamma radiography source or by qualified services called in such cases.
In addition, a combination of both approaches can be suitable, however it must be well defined which steps should be taken by licensee staff and which by called experts. In all three cases the emergency situation should be handled safely. All recovery operations must be reported, and documented enabling analysis of lessons learned.
Licensee should include initiating events and foreseen recovery operations in its safety assessment. Radiation protection program should not only contain procedures to be followed but regular exercises should be conducted. It is of a vital importance to prevent any ad-hoc solutions when managing recovery operations.
5.5 Poor Management of Communication with On-site Clients
On-site radiography requires specific attention as industrial radiographers might work in specific environment, e.g. hazardous areas such as explosive areas. In addition, industrial radiographers should cooperate with other workers and their supervisors at a site, e.g. in a NPP, which also have their tasks at a site. Their tasks might interfere with the tasks of industrial radiographers. In specific cases even more than one team of industrial radiography company might be at the same time at the same location. Moreover, on-site radiography requires knowledge of surroundings of a site, e.g. location of offices, schools and houses. Accidents related to poor understanding of conditions at the site span from exposure of workers who worked inside the large pipeline unknowing that industrial radiography is taking place at the same time, misunderstanding of warning signals when two teams of industrial radiography were working close to each other or unnoticed audible signals because of a high level of noise at the site. All such accidents might be prevented by carefully analysis of the client environment and a role to all involved at the site.
The NDT licensee should have a specific procedure regarding cooperation with the client including the very first communication between senior staff of the NDT companies and a client emphasising safety. Initial visit of a site might be beneficial. In addition, the industrial radiographers should have a check list assessing the environmental conditions jeopardising safety such as language barriers, unacceptable time constraints, poor visibility or high level of noise. Written protocol defining the roles of different types of supervisors at the site might be
beneficial. This is important if any type of recovery operation, emergency situation or even drills take place.
6 CONCLUSIONS
The analysis of description of accidents revealed five major mistakes which led to accidents in industrial radiography, i.e. lack of using survey meters and other radiation measuring equipment, inappropriate management of safety and warning systems of shielded enclosures, inappropriate maintenance or replacement of industrial radiography equipment, lack of procedures in recovery operations and poor communication with on-site clients. As industrial radiography is still one of radiation practices with the highest frequency of accidents this analysis might be used for training purposes of newcomers or regular retaining of radiographers. It might be also used for facilitation of communication between industrial radiography companies and their clients, e.g. NPP operators, as well as for improvement of licensing procedures.
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