Pollution
Humain
Environnement
Economique

On a petrochemical platform, a temperature rise vacuum test of a new reactor was underway when an explosion measuring several bar occurred at 9:17 am on the “salt cover” placed underneath the device. The equipment involved was a 125-m³, heat-insulated tank used to melt the salt (a mix of sodium nitrate-nitrite and potassium nitrate) by introducing a pin filled with water vapour at 180°C and electrical resistances before injection into the reactor’s double containment as a heat transfer fluid. This set-up was also being used to drain the salt circuit. Supply to the cover’s electrical resistances was cut off. The internal emergency plan activated. Plant personnel, assembled in an on-site confinement shelter for 90 min, were then evacuated while in-house first responders cooled the tank, whose temperature had risen above 350°C. External emergency services were not required. The facility was shut down and a 100-m safety perimeter established. The tank expansion space was swept with nitrogen and mobile gas test meters were installed throughout the zone. The reactor sustained no damage since the blast from this explosion was released via the unbolted manhole and a rupture disc. Salt and debris (the manhole, 2 scaffold bars, pieces of heat-insulating wool) were ejected tens of metres around. No victims were reported and the damage was minor (steam heating coils, insulation, manhole reinforcement hoop, reactor cladding), yet start-up of the new unit was still delayed 2 months. The internal emergency plan was lifted at 11:15 am. Since the explosion was heard outside the facility, the operator issued a press release and informed the inspection authorities for classified facilities and neighbouring nuclear power plant. Damages were appraised at over €1 million.

The investigation conducted by the site operator and an expert body confirmed ignition during the gaseous phase of an organic fuel of the heptane type, stemming from the pyrolysis of a foreign body placed into contact with a highly oxidising saline mix. This fuel was accompanied by the presence of an oxidant (nitrogen oxide) resulting from the thermal degradation of the heating salts (the tank’s large expansion space volume was due to its limited contents: 10 m³ of melted salt). This foreign body might have been either an anti-caking agent (organic product) contained in the salts, or a piece of a 600-g polypropylene big-bag, or a pair of safety goggles (these 2 latter objects had fallen into the tank prior to the accident), or perhaps a gas stemming from a degradation reaction involving both these substances. The organic combustible had accumulated in the tank’s confined atmosphere until reaching its lower explosive limit and then self-ignited, or else burned upon contact with an ignition source present in the tank (product temperature > 370°C, hot spot created from the salts/foreign body contact, or current induced by a poorly grounded welding station). This rapid temperature rise could not be controlled by the assigned safety barriers, since the alarms triggered for exceeding the safety thresholds set at 320° and 370°C were too close to afford pipeline technicians the opportunity to respond. The temperature probe for the salt mix in the tank, which had been positioned too high relative to the low operating level, was not immersed in the liquid at the time of the accident, leading technicians in charge of the test to underestimate the actual mix temperature. Moreover, nitrogen sweeping of the tank’s expansion space had not been activated due to its non-inclusion in the preliminary launch phase protocol, thus explaining the presence of oxygen as an additional combustible. According to the reactor’s supplier, this was the first time such an accident had occurred, as this salt melting operation had only been previously performed upon installation start-up.

Subsequent to this accident, the operator adopted an array of measures, i.e.: adding 2 vents on the upper part of the reactor and upgrading tank vents to improve breathability of the tarp-reactor assembly; digging an offset concrete ditch to drain heating salts circulating inside the reactor; sweeping the expansion space with nitrogen systematically and permanently; using salts without anti-caking agents as the risks of these agents when mixed with melted salts had been underestimated by the project team; installing devices to reduce the risk of salt contamination by foreign bodies (e.g. gratings to prevent falling into the manhole); repositioning the temperature probe to ensure its immersion at lower levels (i.e. at 10 cm from the bottom instead of 44); and revising both the temperature safety thresholds triggering heating shutoff and the tarp settings after reaching the low level.