Well based off the draft resolution passed by the senate a few days ago, a nuclear accident that sends nuclear clouds into a NATO countries airspace would trigger article 5.
So you’re right, it would definitely open that door.
Fallout on NATO territory would likely be considered article 5 worthy. At least that's what I keep reading. Pentagon officials, members of Commons Defense Committee in the UK, ...
Although, I suspect we'll escalate with air power. No fly zone and/or air strikes.
Of course, if that happens, all bets are off and it's likely it all cascades out of control.