The lesson learned (LL) article referenced in the question cites an incident that occurred in December 1969. While there may have been other accidents, the HSP does not have any other LL articles on alkaline water electrolysis explosions. In the LL article that was updated in 2017, the technology described employs a potassium hydroxide (KOH) electrolyte solution. The KOH electrolyte is held by surface tension within a fibrous mat used as a cell separator. A 30 weight % KOH solution is typically circulated through each of many electrolysis cells internally manifolded together within a bipolar electrolysis cell stack. Product hydrogen with KOH solution rises into an overhead degassing tank (drum in the LL article); product oxygen with KOH solution rises into a separate overhead degassing tank. 

The article describes a breakdown of the separator allowing H2 and O2 gases to mix. Slight differences in pressure between the oxygen and hydrogen sides of the cell are enough to permit crossover of gases. A recent Gangwon electrolysis incident involved a semipermeable alkaline membrane technology instead of a fibrous cell separator. Conditions at near idle current may have allowed sufficient cross cell diffusion to occur, permitting an explosive mixture to collect in the storage tanks with the result reported. This event has not yet been published in our Hydrogen Tools LL database.