After the occurrence of an explosion and fire accident at Yokkaichi Plant of Mitsubishi Materials Corporation (MMC), the Accident Investigation Committee (Chairman: Dr. Masamitsu Tamura, Emeritus Professor, The University of Tokyo) was formed on January 17, 2014 to investigate the cause of the accident and recommend preventive measures. Through a series of four meetings held from January 22 to March 28, 2014, we have clarified direct causes leading to the accident and recommended preventive measures. Our findings and recommendations are shown in this Interim Report.
- 1. Overview of accident & damages
An explosion and fire accident occurred at our Yokkaichi Plant at about 14:05 on January 9, 2014 when the top channel cover was removed from the water-cooled heat exchanger for cleaning, which was previously removed from the hydrogen recycling process in the manufacturing process for high-purity polycrystalline silicon. As a result of this accident, 5 persons were killed and 13 were injured, and certain facilities were also damaged.
- 2. Water-cooled heat exchanger in the manufacturing process for high-purity polycrystalline silicon
The manufacturing process for high-purity polycrystalline silicon consists of the following processes: chlorination, distillation, reduction, processing/finishing and hydrogen recycling. In the hydrogen recycling process, water-cooled heat exchangers for cooling gases are used to separate and recover hydrogen and chlorosilanes from process gases which were discharged from the reduction process. During gas cooling, chlorosilane polymers adhere to the inside of the heat exchanger and the flow is restricted, thus leading to decreased cooling performance. Therefore, decrease in flow is monitored and cleaning is conducted when cooling performance declines.
(Note) Chlorosilane polymers stated in this report refer to the mixtures of polymerization products of chlorosilanes which contains 2 or more silicon atoms.
- 3. Opening/cleaning of the water-cooled heat exchanger and occurrence of the accident
The heat exchanger involved in the accident was removed and transported from the production line on November 27, 2013. For preparation of opening and cleaning, dry nitrogen and humidified nitrogen were supplied sequentially but separately to suppress the generation of hydrogen chloride and hydrogen.
Next, the bottom channel cover was opened in the morning of the day that accident occurred. In the afternoon, the top channel cover was opened; several seconds later after opening the top channel cover, an explosion and fire accident occurred.
- 4. Analysis for direct causes of the explosion and fire accident
- (1) Analysis of residues in the heat exchanger and their ignition and explosion hazards
- The existence of chlorosilane polymers was confirmed in the residues from the inside of the heat exchanger involved in the accident.
- The hydrolyzed products of the chlorosilane polymers are not single composition compounds; rather, they are estimated to be mixtures of polymers composed of silicon, hydrogen, chlorine and oxygen.
- Chlorosilane polymers have the potential for explosion. However, their explosion energy is relatively small at about 3.4% of TNT.
- It has been confirmed that the hydrolyzed products of chlorosilane polymers have a range of explosion energy equivalent to 0% to 30% of TNT depending on the temperature at the time of hydrolysis. The explosion energy increases when the temperature is low.
- Hydrolyzed products of chlorosilane polymers are prone to have ignition and explosion hazards when exposed to impact or heated under dry conditions. Conversely, under humid conditions, the sensitivity decreases and ignition and explosion cease to occur.
- (2) Investigation of explosion energy and estimation of materials involved in explosion
Explosion energy was estimated from the damages sustained at surrounding facilities and the speed and distance that the top channel cover was blown. The explosion energy was almost equivalent to the explosion energy calculated for the hydrolyzed products of chlorosilane polymers. Therefore, it is presumed that the hydrolyzed products of chlorosilane polymers would be the main materials responsible for the explosion.
Moreover, on the basis of the estimated amount of hydrogen generated during the opening process, it is presumed that hydrogen did not significantly contribute to the explosion.
- (3) Estimation of causes of the accident
- Materials with large explosion energy were generated by the hydrolysis of chlorosilane polymers at low temperature.
- Dry conditions caused an increase in the ignition and explosion sensitivity of hydrolyzed products of chlorosilane polymers. The explosion may result from an unidentified ignition source when the top channel cover of the water-cooled heat exchanger was opened. Possible ignition sources are estimated as impact at the time of opening or autoignition of hydrolyzed products.
- Lack of sufficient and accurate public scientific information regarding the risks of ignition/explosion for the hydrolyzed products of chlorosilane polymers, their generation process, and appropriate humidified processing conditions for chlorosilane polymers, led to insufficient consideration of the appropriate safety measures.
- 5. Recommendations for preventive measures
- (1) Consideration of preventive measures during maintenance of heat exchangers removed from the production line
On the basis of presumed causes of the accident, verification was conducted for all processes in which chlorosilane polymers exist. We found that the production line does not contain any processes with the similar conditions and status. Therefore, as for recommending measures to prevent reoccurrence of direct causes leading to the accident, we focused on methods for opening the heat exchanger. The following points were reviewed.
- In the future, conditions shall be established for a fundamental method in which hydrolysis is conducted while maintaining a humid (full of water) condition, reaction heat is controlled, and generated gases are safely treated.
- The use of specialized facilities with protective barriers shall be considered, and opening of the channel cover shall be conducted through remote operation.
- By assessing the accumulation of chlorosilane polymers inside the heat exchanger, standardization for appropriate timing of opening and cleaning shall be considered.
- (2) Consideration of measures which lowers risks to improve plant safety (including currently implemented measures)
- Strengthening of activities for risk assessment.
- Revision of safety/health standard operating procedures and conduct of comprehensive inspection for all the standard operating procedures.
- Retraining of MMC employees and employees of partner companies in order to maintain safety in the operation.
- Comprehensive assessment of hazardous factors and formulation of measures by using analytical methods to examine causes of the accident.
- Maintenance of periodical conduct of the actions listed above.
- (3) Examination of background factors and fostering of safety culture
In the future, we expect MMC to independently analyze background factors which could have led to the accident and to foster safety culture at Yokkaichi Plant and Mitsubishi Materials Corporation.
MMC's previous report :
- No.3 Feb.5
- Report Number 3; Explosion and Fire at Yokkaichi Plant
- No.2 Jan.24
- Report Number 2; Explosion and Fire at Yokkaichi Plant
- No.1 Jan.9
- Explosion and Fire at Yokkaichi Plant