Explosion in a plant producing sodium

The event occurred in a plant specialised in the production of metallic sodium and lithium. The plant is clssified Seveso high tier.
An explosion, with generation of smokes, took place in the unit treating the residues of metallic sodium by hydrolysis, which are very reactive and need to be passivated before disposal.

This can happen in a two-stage process, burning followed by hydrolysis, or by direct hydrolysis. Due to a backlog of activity, the day of the event the residues were treated by direct hydrolysis, an operation which produces hydrogen. A shortcoming in the correct venting of the hydrogen was the cause of the explosion.

It did not cause any injury, and the emergency was handled internally. The case appears a type of event occurring regularly on the site. Because of this, and the potential negative effect on public health and environment of toxic substances, the authorities decided to start an in-depth formal investigation.

The INTIATING CAUSE was an excessive production of hydrogen during the passivation of metallic sodium residues.

The operations consisted predominantly in manual actions and obsolete equipment. This, in combination with highly reactive substances, regularl ycasued runaway reactions. On top of that, in the period of the incident, ther was a pressure to work out a backlog of materials. This made opting for the quicker but also more hazardous direct treatment of the residues by hydrolysis. Despite the absence of negative records on personnel health, the inspection found considerable shortcoming in safety management.

reactor

DESCRIPTION OF THE PROCESS
The role of the unit where the hydrogen is to remove the highly reactive residues that are removed either by burning followed by hydrolysis, or by direct hydrolysis. The burning stage produces sodium oxide by combustion in air:
4 Na + O2 -> 2 Na2O
This makes the following hydrolysis less hazardous, because it avoid large production of hydrogen.
Na2O + H2O -> 2 NaOH

The direct hydrolysis process, if not properly controlled, presents a high risk of explosion due to the exothermic reaction between sodium and water, which generates hydrogen:
2 Na + 2 H2O -> 2 NaOH + H2

the roof of the electrolytic cell was damaged and had to be replaced.

The post-incident investigation could not perform a forensic analysis aiming at assessing quantities and sequence of event, because the affected unit was immediately brought back in a functioning state. Nevertheless, it identified several factors contributing to event. The waste treatment management exhibited structural weaknesses:
(1) lack of traceability of treated batches,
(2) manual sorting based on operator judgment,
(3) potentially incomplete manual incineration,
(4) outdated facilities,
(5) significant pressure to reduce an existing stockpile of waste (to address this, the treatment capacity has already been doubled).

The conclusion of the investigation produced a series of recommendations:

On traceability - It is important to identify waste at every stage of the treatment process, the best treatment to apply and precisely recording the progress and the characteristics of the residues treated, including near misses and difficulties encountered and their corrective actions.

Recommendations on reliability of practices -To issue clear and well-managed procedures on burning and crushing and on hydrolysis, based on monitoring and controlling of the process parameters. This includes a preventive maintenance plan and its enforcement.

Recommendations on safety improvements – To launch a research and development program, involving external partners (university, companies specializing in hazardous waste disposal, etc.), to re-examine all safety practices.

Recommendations on safety of current operations– To ensure that sufficient human and material resources are available so that all stages of the process (sorting, burning, crushing, verification, and hydrolysis) can be carried out under safe conditions, also in periods of backlog reduction. This implies the availability of personal protective equipment and training on safety aspects. Further, to organise external industrial safety audits, to identify and prioritise key areas for automation which could facilitate hydrogen management and prevention of fire and explosion risks.