Without the bug, quad uranium cells last 4 times as long, causing the design to go for 4 times as long = explode
Again, the reactor planner was already coded from the beginning to have quad uranium cells last the correct amount of time.
A single uranium cell should run the same duration as a quad uranium cell.
Thus a Mark I design that vents an equal or greater amount of heat than is produced will NOT explode, assuming of course the design is made in such a way that the parts don't melt. Which in a Mark I design, they don't.
If you built that 420 design and it blew up, then you changed something other than the running duration of quad/dual uranium cells.