This allows them to utilize helium3 reactor assemblies which are only supposed to be usable by 50MW+ reactors; also, the Nova-25 reactor only accepts L:1-3 fuel.

Going by the preexisting pattern, the Nova-10 should only accept L:1 fuel and the Nova-15 should accept L:1-2 fuel.

This was originally discovered by Xephyr:

george moromisato on 8/15/2015 7:03 PM:

I believe the stock reactors in previous versions allowed L:1-4. I'll check and make sure it is consistent (i.e., we either have to fix the Nova-10 and -15 or the Nova-25).

megas on 8/15/2015 7:30 PM:

Ships without a reactor have default fuelCriteria of L:1-3.

nms on 8/22/2015 9:37 PM:

The other reactors can use fuels equal to or up to three levels lower than their level. Following this pattern would leave lower level reactors with a smaller range, though.

Also note that there are no level 2 fuels and no good level 3 fuels. So for runs where you refuse to upgrade your reactor, there's a big difference between being stuck with level 1 fuel rods and being able to use level 4 reactor assemblies, which have 4 times the fuel per mass.

george moromisato on 8/22/2015 11:32 PM:

Fixed in 1.6.2. Thanks!

I've changed the Nova-10 and -15 to L:1-3. This is consistent with the original reactors.
I've also left the Nova-25 alone, since it is sometimes a starting reactor also.