Some differences may exist between french and english regulations. Lessons learned are only a translation of those of french incident and are not adapt to english regulation.
Description of incident
Equipment (radiation source)
The equipment in question is an electrostatic electron accelerator (2.5 MV, 35mA), used for the ionization treatment of fluorinated materials. Dose rates in normal operation are 80,000 Gy/s (electrons).
The equipment was bought second-hand in the USA, by a company X who installed it in Europe, and then sold it again to company Y, a sterilization-by-irradiation company, which was subsequently resold to an individual Z. After legal forclosure, yet another party, the CEO of a local company, buys the buildings and the device and creates a new company, T whose objective is the irradiation treatment of fluorinated materials. Despite this complex path, there is no evidence that this history plays a specific role in the occurrence of the accident.
However, it should be noted that there were a number of anomalies:
- There is no proof that the pressurized (8 bars) metal tank of the accelerator underwent the regulatory pressure testing. Several defects and electrical non-conformities were also detected when the units was commissioned by the various investors.
- The radiation protection authority was advised, and notified the regulatory authorities of these offences and requested sanctions. Company T escapes these by reducing the operating voltage (2.3 MV --> 1.7 MV) and the pressure (8.3 bars --> 4 bars).
- The building was not designed to accommodate this equipment, which caused constraints on the conveyor trajectory.
- The conveyor is a standard design, not intended for work with high radiation levels: the lubricating oils and plastic components degrade under the effect of radiation; and certain materials oxidize in contact with the ozone produced by oxygen ionization.
Safety systems and working methods
When starting the equipment, the operator must follow a specific procedure and route, activating a series of safety switches attesting to his passage, closing interlocked (mechanical and optical) doors. prior to being able to place the console command key in the "on" position.
The course must be completed in less than three minutes or it must be restarted. The accelerator is then placed in the "high voltage" position. Then the beam is delivered by turning the key to "Beam Current".
Teminating the beam automatically leads to a programmed installation stop of 15 minutes (the time necessary to clear the ozone produced by the ionization of the air). To avoid this delay, before entering to fix a ventilation duct directly below the electron beam (see 2nd accident below), the decision was taken to leave the command key in the "high voltage" position. In this position, the power supply to the tungsten filament (electron source) is cut, but the high voltage accelerator maintained. The residual current is very low, but dose rates of around 0.1 Gy/s remain.
In addition, to avoid passing through the metal entry door (which would stop the installation), the persons involved are accustomed to passing through the conveyor exit which does not have an intrusion detector.
Two accidents occurred:
- An aluminum container, containing fluorinated products, sticks in the scanner. The products ignite, and the scanner’s titanium window is damaged and will have to be replaced.
- The team leader C equipped with a mask to protect himself from the hydrofluoric acid (created by interaction of the water steam and fluorine released by the products), goes into the irradiation room to extinguish the fire.
- The origin of the blockage comes from the general design of the conveyor system, the relatively small size of the containers in relation to the width of the conveyor (the containers tend to cross), and the warping of the containers from the heat.
- The aluminum vent duct in question was weakened by radiation. It is brittle and friable. It has already been repaired previously with adhesive tape.
- Mr. A enters through the conveyor exit to repair the vent duct. After 15 minutes of unsuccessful attempts, the team leader C sends Mr. B and, eventually, intervenes himself.
- On several occasions, the three people involved pass their hands, their forearms, their heads and backs under the scanner. Mr. A who was crouched also exposed his legs.
The personal dosemeter films were not developed until after the appearance of the first symptoms of acute irradiation in the three accident victims (see below).
The doses measured are at the limits of total film darkness, and the results (below) are only approximations. These values do not represent the maximum doses received, since the dosimeters (worn on the chest) were not exposed to the primary electron beam.
|Mr A||40 Sv||1 Sv|
|Mr B||9 Sv||250 mSv|
|Team leader||5 Sv||?|
Two days after the accidents, the team leader C notices an abnormal discoloration of his hands and forehead. Shortly after, he progressively loses the hair on the top of his head. Two weeks after, he noted the presence of a blister on his right hand (lifting of the skin under which there was translucent fluid). The affliction then spread to both hands and forearms and then the thorax and the forehead. Radiological burns covered 14 per cent of the skin surface, but they did not require skin grafts.
Mr. B felt itching of the scalp as soon as the intervention ended (after 15 minutes). In a few days, he was abnormally tanned and lost his hair. 2 weeks later, the blisters spread down onto his back and along his trunk. Radiological burns covered 25 per cent of the surface of the body. He was subjected to skin grafts.
Mr. A was the most severely affected – 60% of the body surface area was burned. These burns affected the head, the trunk, the shoulders, all of the left leg, the right thigh and both arms. For six months, including 6 weeks in a coma, he was placed in a sterile chamber. He received a 18 skin transplants.
Analysis of the causes of the incident
Two causal chains can be identified: a chain related to the equipment design, and a chain linked to management and operator behaviour.
Equipment and facility:
The conveyor and the ventilation were not designed to operate in intense radiation fields. Thus, regular failures occurred, requiring interventions. The physical security measures were flawed, since it was possible to enter via the conveyor exit. Such an entry is not without (conventional) risks, since it is necessary to climb 3 rising conveyors to reach the scanner.
NB: One of the doors integrated into the security chain did not have a lock, and was secured by a tool case or a broom.
Behaviour of the operators and company management:
According to testimonies gathered during the trial which followed the accident, the three operators had been accustomed to entering the facility by the equipment exit, following the example of the Executive Director. The reasons include the loss of time caused by the official procedure (productivity pressures), a misunderstanding of risks, and simply habit.
The staff had received no risk training, except perhaps chemical (eg HF).
Lessons to be learned from the incident
It is obvious that the regulations in force were not respected, and there was almost a total absence of any radiation protection culture. Specific failures included:
- The non-notification of this radiation generator , and the absence of any RP information such as risk assessments and safe working procedures. The facility was known by the radiation protection authorities, which had requested sanctions by the official prosecutor. However, this changed little in terms of potential risk (although the doses received would have been even higher if the device had functioned to its full design potential).
- The device was not subject to any radiation safety inspections before being put into service.
- There was no radiation protection training, nor any appointments of persons responsible for radiation safety.
- The appropriate signs intended to inform workers of the risks were in English, rather than the local language.
- The personnel (A and B) were temporary workers, were not classified workers, and not subject to competency requirements, or prior medical examination.