ANALYSIS OF A SIMULATED LIQUID NITROGEN STORAGE TANK FAILURE.

Technology to cryopreserve and store human gametes and embryos has been in use for over 40 years. Monitoring systems for liquid nitrogen (LN2) storage tanks are essential and widely used, but there is little knowledge of the time frame involved when a tank fails. This study evaluates the time it takes for a standard 47L LN2 storage tank to warm following vacuum jacket failure. Prospective experimental trial. A fully functional MVE XC 47/11 LN2 storage tank (mfg: Jan/1990) was weighed empty (33.6lbs). To simulate normal storage conditions, the six 4" storage canisters were filled with canes & plastic cryo sleeves (11.2lbs) for total tank weight of 44.8lbs. A temperature probe was set low (8cm from the bottom) where vitrified embryos would be located. A second temperature probe was set high (33cm from the bottom) where a level sensor would typically be located. The tank was filled with LN2 to the neck and reweighed at 122.8lbs. To simulate tank failure, a 1/16" hole was drilled through the vacuum port to breech the tank's insulating vacuum jacket. The temperature of both probes was continuously monitored and tank weight measured every 30 minutes until the temperature reached 0oC. The tank was then refilled with LN2 and monitored for a second replicate trial. Upon vacuum jacket breach, hissing was immediately heard as the pressure inside the vacuum equilibrated. Within 5min, LN2 vapor billowed from the tank lid and by 30 minutes, frost was apparent. Full pressure equilibration of the vacuum took longer than expected (∼5h). By 6h, pressure inside the jacket reversed as evidenced by air blowing from the hole as the tank continued to warm. It took 18h for 43.8L of LN2 to evaporate, with the highest loss rate between 3-6h. An overview of results and observations is displayed in Table 1.Table 1Tank Observation DataTime (h)Upper Temp (oC)Lower Temp (oC)Tank Weight (lbs)LN2 Volume (L)Event / Observation0-196.8-197.2122.843.8Full intact tank prior to breach3-197.7 to -191.7-197.0107.635.2Upper probe at LN2 level; Frost on tank lid6-189.1-197.087.223.9Upper probe above LN29-184.7-197.075.217.1Lower probe submerged in LN212-182.1-196.964.411.0Lower probe submerged in LN215-178.6-194.654.25.3Lower probe at LN2 level18-167.3-182.045.60.5Last of LN2 evaporating21-72.5-79.844.80.0Tank above critical temperature24-26.5-30.144.80.0Tank continues to warm27-4.8-5.844.80.0Tank just below 0oC304.94.044.80.0Tank above 0oC Open table in a new tab Cryo tanks can be monitored by temperature or conductivity level sensors &/or tank weight. In this trial, 19h elapsed between vacuum breach and estimated time of sample exposure to critical glass transition phase (approx. -125oC). If this had been a true vacuum failure with an alarm threshold of -185oC, notification would have activated 9h post-breach when the tank was completely frosted over, leaving 10h remaining before sample exposure. Temperature monitoring requires precise threshold limits. Storage tank failure is rare, but consequences are devastating. Probe placement must be optimized for each tank and visual observation is useful for early detection. Study of additional tank failures would provide further valuable data.