643 Explosives Mix Explosion in Mustang Nevada USA 1998 6431 Introduction

On 7 January 1998, at 0754, two explosions in rapid succession destroyed the Sierra Chemical Company's Kean Canyon plant east of Reno near Mustang, Nevada, U.S.A. killing four workers and injuring six others. Because of the loss of life and extensive damage, the United States Chemical Safety and Hazard Investigation Board (CSB) sent a team to investigate the explosion in an attempt to understand the causes of this incident. The present account is a brief summary of the comprehensive CBS (1998) report.

6.4.3.2 The Kean Canyon Plant and its Production

The Sierra Kean Canyon plant manufactured explosive boosters, mixed custom flux for gold smelting operations, and repackaged bulk soda ash for sale to the mining industry. When initiated by a blasting cap or detonation cord, boosters provide the added energy necessary to detonate less sensitive blasting agents or other high explosives. The boosters manufactured at the Kean Canyon plant consisted of a base mix and a second explosive mix, called Pentolite, both of which were poured into cardboard cylinders (see Section 6.2.2.2). The operators working in the plant were responsible for the preparation of the explosive mixtures, the operation of the mixing pots, and the pouring of the mixtures into the cardboard cylinders. Figure 6-8 gives an approximate outline of the part of the plant that was demolished by the accidental explosion.

Muter tank

KooMer Room 2

Muter tank

KooMer Room 2

PRTN drying building

NOT TO SCALE!

Figure 6-8 Approximate plan of the Sierra Chemical Company Kean Canyon plant east of Reno near Mustang, Nevada, USA. From CSB (1998).

PRTN drying building

NOT TO SCALE!

Figure 6-8 Approximate plan of the Sierra Chemical Company Kean Canyon plant east of Reno near Mustang, Nevada, USA. From CSB (1998).

The boosters were filled in the two Booster Rooms indicated in the figure. The primary explosives used in the base mix were TNT (2,4,6 trinitrotoluene), PETN (pentaerythritol tetranitrate), and a mixture of TNT and RDX (hexahydro 1,3,5 trinitro 1,3,5 triazine). Pentolite was a mix of TNT and PETN.

6.4.3.3 The Explosions

CSB investigation team identified four possible explosion scenarios that could account for the explosions that occurred, but based on the seismic data, interviews of workers, and the physical evidence observed during the investigation, they considered one of these as being more likely than the others. They concluded that most probably the first explosion occurred in the plant's Booster Room 2, indicated in Figure 6-8, whereas the second, larger explosion followed 3.5 s later in the PETN building, which is also indicated in Figure 6-8. The two explosions were recorded by the Seismology Laboratory at the University of Nevada, Reno, and they were also able to conclude that the site of the first explosion was somewhat further to the north than that of the second, larger explosion, which in accordance with Booster Room 2 being located further to the north than the PETN building. The interval of 3.5 s between the explosions was estimated by the laboratory to be accurate to ± 0.2 s.

Most probably the first explosion was initiated in a mixing pot in Booster Room 2 (see Figure 6-8), when an operator who had left about 25-50 kg of base mix in his mixing pot the day before, turned on the mixing stirrer motor of the pot the next morning. During the evening and night the mix had stratified and solidified, and the explosion initiation occurred when the bottom of the mixer blade, which was embedded in the solidified explosives mix in the pot, forced the explosive material by impact, shearing, or friction against the pot wall. Alternatively, explosive material was pinched between the mixer blade and the pot wall, causing the initial detonation in the pot. The shock wave from this initial detonation then instantaneously detonated the other several thousand kg of explosives in the Booster Room 2, leading to the first main explosion that completely demolished Booster Room 2, and also the rest of the building. Only three walls were left of Booster Room 1, as indicated in Figure 6-9, whereas Booster Room 2 was completely levelled with the earth, as can be seen in Figure 6-10.

When the first explosion occurred, a worker in Booster Room 1 saw a huge fireball engulf a truck that was parked immediately outside the building. This worker was thrown against one of the walls in Booster Room 1, as the ceiling and another wall of the room collapsed on top of him and the four other workers.

Seconds later the same worker heard a second, more powerful (louder) explosion in the PETN building. CSB concluded that this explosion was

Figure 6-9 Remains of Booster Room 1 of the Sierra Chemical Company Kean Canyon plant east of Reno near Mustang, Nevada, USA, after explosives mix explosion in 1998.

Figure 6-9 Remains of Booster Room 1 of the Sierra Chemical Company Kean Canyon plant east of Reno near Mustang, Nevada, USA, after explosives mix explosion in 1998.

most likely initiated by a heavy piece of equipment or burning debris from the first explosion falling through the reinforced concrete roof or the skylight of the building. The site of the PETN building and an adjacent magazine was now transformed to a 13 m x 20 m wide and 2 m deep crater. The explosions were felt as far as 35 km away.

A total of 11 employees were at the site at the time of the explosions. After the explosions, five workers in Booster Room 1 were trapped temporarily under the collapsed building, but were able to crawl out within a few minutes. Three of the five workers were seriously injured, whereas two only received minor injuries. Concerned about possible additional explosions, the workers from Booster Room 1, after calling for other possible survivors, went to the entrance to the facility. There they met two other workers who had been in the gravel pit below the site, approximately 100 m southwest of the PETN building. The other four workers who were believed to have been in or near Booster Room 2 had been killed by the explosions.

The blast effects of the explosions leveled the site and threw structural materials, manufacturing equipment, raw materials from the booster and flux operations, and other fragments up to 1 km away. The legs and cross

Figure 6-10 Remains of Booster Room 2 of the Sierra Chemical Company Kean Canyon plant east of Reno near Mustang, Nevada, USA, after explosives mix explosion in 1998.

Figure 6-10 Remains of Booster Room 2 of the Sierra Chemical Company Kean Canyon plant east of Reno near Mustang, Nevada, USA, after explosives mix explosion in 1998.

bracing from an empty tank, which previously stood at the corner of the changing room, were thrown 300 m away from the production building. The doors of one of the large magazines and a portable magazine located west of the production facility were sprung open by the negative pressure pulse; however, large quantities of explosive materials that were stored inside did not detonate. Many un-detonated boosters had been scattered throughout the site by the explosion. Other hazards remaining after the two major explosions as the after-fire progressed included fires, toxic chemicals, and potential detonation of the explosives in Booster Room 1.

6.4.3.4 Events Leading to the Explosion

The CSB investigation concluded that the following sequence of events led to the explosion catastrophe. The day before the accidental explosion, one melt/pour operator working in Booster Room 2 (see Figure 6-8)

needed to leave work early, at 3 p.m. When he left, there were between 25 and 50 kg of base mix left in one of his mixing pots (no. 5). He mentioned that he had left some explosive mix in the pot to another operator in the room, who later confirmed this by visual inspection. This other worker, from having looked into the mixer, indicated that the depth of explosive mix left in pot no. 5 was about 100 mm, which matched his estimate of there being 25-50 of explosives in the mixer. The mixer blades extended about 50 mm down into the mix. Metallurgical analysis of mixer parts retrieved after the explosion provided further evidence supporting the conclusion that explosive material was left in pot no. 5, because the analysis showed that damage to the hub of the mixing blade was consistent with it having been in contact with detonating explosives.

An over-current protection device on the electrical mix motors in Booster Room 2 would stop the motor if the blade was unable to break up the explosives, but not before the maximum start-up torque had been applied to the explosives. Without continuous agitation, the different explosives and binders of the mix tended to stratify due to their different densities. This stratification would increase the initiation sensitivity of portions of the explosive material left in the pot. Turning on an agitator immersed 50 mm into a solidified mass of stratified explosives would present a high risk of explosion from the impact of the mixer blades.

Leaving material in the mixing pots overnight was not according to common practice in the plant, but this practice was not for safety reasons. Several months before the explosion, when material had been left overnight in the Comp B mixing pot in Booster Room 1, management made it clear that this was an unacceptable practice because it delayed the operation of the day shift workers.

The operator who left the material in his pot had been working in Booster Room 2 for eight weeks prior to the incident. His normal practice was to leave both of his mixing pots empty in agreement with common practice. Because he was the only person working his production line, he would normally know whether his pot was empty when he started work the next day. Since the pots in Booster Room 2 heated material much faster than the pots in Booster Room 1 could do, it is possible that he on the day before the explosion thought that leaving material in the pot would not delay production the next day. It was acceptable practice at the Kean Canyon facility to alter normal processes without discussion or management approval.

At the end of each day, operators were instructed to leave a steam line valve to each pot partially open to keep the boiler cycling, to prevent freezing of condensate in the lines. This amount of steam would be insufficient, however, to maintain any quantity of explosive mix in the pots above its melting point if outside temperatures were below the freezing point. During the night before the explosion, the outside temperature dropped to between -4 and -7°C. The operator who had left explosives in his pot offered the remaining material to the operator on the other production line in Booster Room 2. Because the operator who was leaving did not reach a firm agreement on whether the second operator would use the residual explosives, it is possible that no steam valves were left open that afternoon because leaving the valves open would make the remaining base mix too runny to pour. The operator who left early may have mistakenly thought that his remaining base mix would be used that afternoon and, thus, he failed to look in the pot the next morning before turning on the steam and mixer motor. The CSB investigation team concluded that this was the most likely scenario.

6.4.3.5 Key Findings and Root/Contributing Causes

• U.S. standards require that companies using highly hazardous materials have in place an integrated safety management system. CSB's investigation of the Kean Canyon explosion catastrophe revealed that many essential elements of adequate process safety management were missing or deficient. The process hazard analysis (PHA) of the facility and operations was inadequate, and supervisors and workers from the plant had not been involved. The PHA for Booster Room 1 was conducted by company personnel from other locations and did neither consider safe location of buildings nor human factors issues. These deficiencies in the PHA program allowed unsafe conditions and practices to remain unrecognized and uncorrected. No PHA was conducted for Booster Room 2.

• There was no safety inspection or safety auditing program. Safety "walkthrough" inspections were unfocused and did not examine process safety management (PSM) program effectiveness. The result was that management was generally unaware of unsafe practices and conditions.

• The training programs for personnel working in the plant were inadequate. Worker training was conducted primarily in an ineffective, informal manner that over-relied on the use of on-the-job training. Poor management and worker training led to a lack of knowledge of the hazards involved in manufacturing explosives.

• Written operating procedures were either non-existing or not available to the working personnel. Managers believed that, without using a blasting cap, it was almost impossible to detonate the explosive materials they used or produced. Personnel primarily relied on experience to perform their jobs. Procedures and other safety information were not available in the language spoken by most workers. Operators routinely made changes in the steps they took in manufacturing explosives. This resulted in the use of inconsistent and hazardous work practices. There were no written procedures for Booster Room 2.

• The whole facility was built with insufficient separation distances between different operations, and the construction of buildings was inadequate. Because unrelated chemical operations were located in the same building as Booster Room 2, one additional fatality and additional property damage resulted. Close proximity of structures allowed the explosion to spread to a second building.

• Safety inspections by regulatory organizations were conducted infrequently and inspectors generally did not have expertise in explosives manufacturing safety. This allowed unsafe conditions in the plant to remain uncorrected.

• Employees had not been involved in developing or conducting process safety activities. This resulted in a lack of understanding of process hazards and controls by workers. It also resulted in management not benefiting from the experience and insights of workers. Employees had not been involved in developing or conducting process safety activities. This resulted in a lack of understanding of process hazards and controls by workers. It also resulted in management not benefiting from the experience and insights of workers.

• Reclaimed, demilitarized explosive materials purchased by Sierra sometimes contained foreign objects. The risk of using contaminated explosive materials had not been adequately examined.

See Section 6.5.2 for CSB's recommendations for improvements.

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