In addition to resealing the glove box window, a positive pressure of argon gas was maintained inside the glove box while the course of action was planned. Subsequently, the glove box was cleaned up by specialized hazardous materials personnel using natural bristle brushes and plastic utensils. Also, Teflon-coated magnetic stirring bars were used to separate the milling balls from the powder while avoiding metal-to-metal contact.

While no direct evidence has been obtained, it is possible that a small leak in the antechamber seals or back diffusion from the vacuum pump occurred to expose the NaAlH₄ material to oxygen and/or water vapor. Similar sudden reactions within a glove box have been noted by other researchers working with NaAlH₄ where contamination by oxygen / water vapor was suspected. A possible material mechanism is detailed in “Ashby's warning” published in Chemical and Engineering News, V47 (1), 1969. In general, researchers working with NaAlH₄ or other reactive hydrogen storage materials should take extra precautions with regards to sealing and vacuum pump type/performance when holding such materials under vacuum for extended periods of time.

Additional discussion about working with reactive metal-hydride materials in the laboratory can be found in the Lessons Learned Corner on this website and in the Hydrogen Safety Best Practices Manual.

• A SOP should be developed which prohibits drivers from backing into the fill position.
• As a precautionary measure to mitigate similar events in the future, piping barriers should be installed which protect critical H₂ piping components/systems from delivery truck collisions.

• When truck drivers are carrying new trailers, with new load distribution characteristics, they need to exercise caution, especially on hazardous roadways such as the one described in this occurrence. If the trailer stack would have been smashed and plugged or if the trailer shell would have been punctured and ignited, a much more severe accident would have occurred.
• H₂ truck drivers need to be educated on the explosive characteristics of H₂ they are carrying, so they will have a built in incentive to drive with caution in similar situations.
• Company policy should be clearly established to encourage safe driving practices under all conditions. Unsafe driving, in contrast, will not be tolerated.

• All tank trailers should have a safely accessible auxiliary shut off valve in case of spills.
• Emergency personnel need to have access to all of the appropriate protective clothing, including shoes. Liquid hydrogen is stored at 20.28 Kelvin or -423.166 °F, and is cold enough to freeze surrounding air at these temperatures.

Standard procedure must be followed in all cases. Assumptions are made at great risk. Risk also increases with complacency.

Standard procedures must be followed at all times. The importance of doing so should be frequently reinforced through safety communications to all staff.

The use of inerting gas or other means of separation should be employed when conducting mechanical work where hydrogen gas could be present. More importantly, per CGA S1.3, the vessel should be equipped with a dual relief system that can isolate one side from the other and allow a rupture disc to be changed without exposing the operator to hydrogen.

In the second incident, the cracking of the outer mild steel vacuum jacket was more than likely related to the coefficient of thermal expansion of steel, which defines how much the material will contract when its temperature is decreased. The temperature of cryogenic liquid nitrogen is at -195.8 °C (-320.44 °F), and the linear coefficient of thermal expansion of 1020 steel at room temperature is 12 x10^-6 1/0 °C. Thus, the significant contraction in the steel due to the instantaneous temperature reduction created localized stresses, which cracked under the vacuum pressure of the system. Some other method of controlling the fire should have been employed. In addition, the metal would have been made much more brittle due to the low temperature.

All relevant personnel should receive at least basic training on the proper selection of fire extinguishing techniques for the given scenarios they are likely to encounter.

Liquid nitrogen should not be used to put out a hydrogen fire. It is very difficult to put out a gaseous hydrogen fire, plus had the liquid nitrogen not cracked the nearby vessel's shell, it certainly could have cracked the original vessel. It could also have plugged up the stack by freezing at liquid hydrogen temperatures.

Extra caution should be taken working around elevated pressure or low-temperature fluids and storage. Values should be checked and then verified by a second party, if possible.

All valves and connectors should be clearly labeled to minimize chances for mis-connection. All technicians must be trained on proper procedures for both taking systems offline and bringing them back online.

Valves for compressed hydrogen gas service are discussed in the Hydrogen Safety Best Practices Manual.

First, it appears that the system may not have been vented properly. CGA G-5.5 should be used for determining safe locations based on the variables of the specific setup. Also, if the compressor was tied to a storage system, a backflow prevention device may have limited the amount of gas that was released. Finally, it appears that equipment was left in place from previous activities. Such equipment should be evaluated to make sure that it is appropriate and safe for use in new processes.

Because of the near invisibility of a hydrogen flame in daylight and hydrogen's extremely low ignition coefficient, if a known leak is present (e.g., an audible hissing), ignition should always be presumed. The primary cause of this incident derives from the technician improperly performing hot work in the vicinity of a charged flammable gas line. Given the location of the flammable gas line, an alternative to performing hot work or relocating the hot work should have been considered. If such work was necessary at this location, it should have been performed only after the gas supply was verified closed (along with a lock and tag). Also, if this latter option was chosen, then the system should be checked for leaks prior to turning the gas back on.

The tube was 403 stainless steel, which is subject to hydrogen embrittlement. It is requested that all gauges that have bourdon tubes be replaced with 303 stainless steel.

A web-based resource developed by Sandia National Laboratories to provide data on hydrogen embrittlement of various materials is available at Technical Reference for Hydrogen Compatibility of Materials.

The charging characteristics of a battery vary with the condition of the battery and are proportional to the ampere-hour capacity of the battery. During charging, when a battery is close to being fully charged, hydrogen and oxygen both offgas in a flammable mixture. Emergency battery containers require adequate ventilation for all operations, or the battery box should be designed to:

• Eliminate all sparking devices
• Ventilate during charging operations,
• Provide inert gas purge and pressurization methods, and
• Provide adequate pressure relief.

Adequate ventilation of battery charging facilities is addressed in the Lessons Learned Corner on this website.

It is characteristic of silver/zinc batteries to outgas both hydrogen and oxygen for several hours after discharge. Also, in the future, the battery should be properly secured inside the container.

Refueling operations need to be conducted in a careful and attentive manner, and operators need to be aware of the potential consequences of their actions.

The site needs to implement a good Operational and Readiness Inspection procedure. System inspection deficiencies need to be identified, and (if possible) reviewed by a second party before future tests are conducted. This inspection could include the following:

• Check all relief valve set points
• Check potential design flaws
• Check all other relevant operational characteristics

A proper vent system design is as important as the relief valve itself. A vent system should be able to operate with no consequence at all. Proper facility design and venting to a safe location would have made this a near-miss instead of an incident.

Proper bolt identification can prevent similar occurrences in the future. This can be achieved by simply painting the critical bolt heads a certain color (or by purchasing bolts with painted bolt heads). An explanation form should then be clearly posted, which indicates which bolts are critical, and which aren't. This information should then be disseminated to all of the relevant technicians.

Extra caution should be taken when working around elevated pressure storage tanks. Pressure relief valve settings should be checked and then verified by a second party if possible. Proper procedures need to be followed at all times.

• Facilities housing battery charging systems need to ensure the ventilation systems are operational, and delivering enough outdoor air to properly ventilate the enclosure.
• Operators need to be aware of safe practices and proper battery charging instructions for the nickel cadmium batteries they work on. This information needs to be disseminated to all of the technicians working on the batteries and battery charging systems.

Adequate ventilation of battery charging facilities is addressed in the Lessons Learned Corner on this website.

Normally hydrogen fires are not extinguished until the supply of hydrogen has been shut off due to the danger of re-ignition and explosion. During a gas leak/fire, it is important to shut off the source of the hydrogen if it is safe to do so. If the leak cannot be stopped, the danger of a fire (or an explosion if the unburned hydrogen gas is leaking into a poorly ventilated confined space) would be very high. If a leak cannot be stopped, fire extinguishing is impossible and only prevention of fire spreading is possible. It is also difficult to detect a gas leak from thermally insulated piping at an early stage.

Countermeasures which were employed to prevent future occurrences include:

• Improving management of construction at turnaround to prevent improper gasket installations.
• To detect gas leaks in piping systems covered with thermal insulation, inspection windows were installed in the insulation and gas leak checks were strengthened.