I did make and use a Free-to-Play account that I streamed on for a while. I never used Det Flags in Randoms so I would always use the Slot 1 Upgrade that reduces Detonation Chance. It really does work quite well.
There is more to the Detonation mechanic than a simple die-roll. The Magazines are modules that exist in the ship. Damage has to be dealt to the magazines in order to cause a detonation chance, so shell placement and type can and do affect the situation.
Someone else mentioned that you had 20 Detonations though show nearly 5k battles. I did look up my Free-to-Play account and found I have a single Detonation out of 283 Random Battles. While this can certainly be a tide-turning event, it is not as simple as a 1-in-10 chance to explode for each shell hit.
Hits must be accurate in that they must interact with the magazine, and further the magazine needs to be damage sufficiently to allow for a chance of Detonation at all.
You can read more about Detonations here: https://wiki.wargaming.net/en/Ship:Detonation
It's recorded as an Achievement in your Profile~ You can view your profile online: https://profile.worldofwarships.com/achievements/1031836257
It looks like you've been Detonated only 3 times <3
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