An ice plug is a blockage that forms in the neck of a liquid nitrogen dewar when atmospheric moisture freezes at the neck opening, sealing the vessel. It is one of the less frequently discussed but genuinely dangerous hazards of liquid nitrogen storage, and understanding it is important for anyone maintaining cryogenic equipment in a laboratory or IVF clinic.
The neck of a non-pressurised liquid nitrogen dewar is designed to be open, it vents continuously to allow nitrogen gas, produced by the inevitable evaporation of liquid nitrogen, to escape safely. If the neck becomes blocked by an ice plug, this venting pathway is eliminated. Nitrogen gas continues to be produced inside the vessel but has nowhere to go. Pressure builds inside the dewar.
As the NHS Lothian liquid nitrogen procedure notes, ice plugs may form in the neck of open dewar flasks and cause a build-up of pressure. As the pressure rises within the dewar, the ice plug may be expelled at high velocity, or in extreme cases the dewar itself can fail. Either outcome is dangerous. An ice plug expelled under pressure from a dewar neck is effectively a projectile. A dewar that fails catastrophically under internal pressure releases both a pressure wave and a sudden large volume of nitrogen gas.
Ice plugs are most likely to form when a dewar is left in a humid environment with the neck open and not in use, when a dewar has been partially filled and left for an extended period, or when condensation from handling repeatedly freezes at the neck. The correct response when an ice plug is detected is not to attempt to force it open. The dewar should be moved carefully to a well-ventilated outdoor area and allowed to warm slowly until the plug clears naturally.
Preventing ice plugs requires regular inspection of all liquid nitrogen storage tanks, appropriate storage conditions, and staff training that specifically covers this hazard. The defog function integrated into the CryoBank Series helps maintain clear access to the vessel neck during routine operations.
