Aftershock: fix up minireactor and plutonium cells

[dynst]

Summary

Mods "buff minireactor and make plutonium cells more compact"

Purpose of change

A minireactor weighs 27kg and takes up 100 L. To fill a minireactor, making it hold the same potential energy as 10 large_storage_battery, you currently have to load in 500 L of plutonium cells weighing 800 kg, which should be physically impossible.

Describe the solution

Make plutonium cells denser, more compact, and require fewer of them to completely fill up a minireactor, so now you only load 1 L of plutonium cells weighing 20 kg. Also, make a full minireactor equal to 100 large storage batteries instead of 10.

Describe alternatives you've considered

Testing

Additional context

Noticed as part of #75336

[Drew4484]

Stats for the KRUSTY (via < https://en.m.wikipedia.org/wiki/Kilopower>) "The prototype KRUSTY 1 kWe Kilopower reactor weighs 134 kg and contains 28 kg of 235 U. The space-rated 10 kWe Kilopower for Mars is expected to have a mass of 1500 kg in total (with a 226 kg core) and contain 43.7 kg of 235 U."

Assuming incredible advances in materials technology, you could probably trim that weight down, say 100-125kg per kWe. The current minireactor is massively underweight (and also requires no heat management, which is a separate issue). As long as your fuel is respecting the energy density (plus additional weight for a pellet instead of raw plutonium), I don't see an issue with getting more power out of it. But fuel pellets should be heavy and the reactor should be much heavier.