Insufficient testing on buReducingAssemblyRotation

[drewj-tp]

ARMI provides functionality for a fuel handler to do "burnup reducing" assembly rotation

https://github.com/terrapower/armi/blob/f1f3dbea05c2564404a03d8e053f72c2af15daa9/armi/physics/fuelCycle/assemblyRotationAlgorithms.py#L38-L40

https://github.com/terrapower/armi/blob/f1f3dbea05c2564404a03d8e053f72c2af15daa9/armi/physics/fuelCycle/hexAssemblyFuelMgmtUtils.py#L32-L41

The testing for this functionality is lacking, in my opinion; it checks that that the orientation is different, not "optimal"

https://github.com/terrapower/armi/blob/f1f3dbea05c2564404a03d8e053f72c2af15daa9/armi/physics/fuelCycle/tests/test_assemblyRotationAlgorithms.py#L30-L50

So that could use some improvement.

I propose we add more testing, specifically on the internal getOptimalAssemblyOrientation function, to check that

1. We get the correct, not just different, orientation rotating an assembly "in place" (a is aPrev in the function)
2. We get the correct orientation rotating an assembly in the location of a different assembly (a is not aPrev)

Addendum

This last sentence

https://github.com/terrapower/armi/blob/f1f3dbea05c2564404a03d8e053f72c2af15daa9/armi/physics/fuelCycle/hexAssemblyFuelMgmtUtils.py#L40-L41

is wrong and should be removed. If we don't have any pin information, the getOptimalAssemblyOrientation raises an error

https://github.com/terrapower/armi/blob/f1f3dbea05c2564404a03d8e053f72c2af15daa9/armi/physics/fuelCycle/hexAssemblyFuelMgmtUtils.py#L119-L122

[john-science]

If we had unit tests for this that were of very small reactors (~three rings), would that make it easier to hand-calculate the optimum rotation?

(And yes, ditch that last sentence.)

[drewj-tp]

If we had unit tests for this that were of very small reactors (~three rings), would that make it easier to hand-calculate the optimum rotation?

It's probably good to have. I'm working on a test that picks up a test reactor like

https://github.com/terrapower/armi/blob/b436537edfed991e6ff560a1d1b2d4b018507db1/armi/physics/fuelCycle/tests/test_fuelHandlers.py#L64-L73

So we already have something that's halfway decent. The trick is setting the block data such that you have a known input state (pin powers, max burnup pin index, etc.) such that you can compare the rotation you get vs. the rotation you expect