After looking into your arsenal, I have considered adding support to work with this mod. Namely, adding the portable MOX-reactor equipment.
88 mm and 128 mm variants of cannon shells for my heavy & super-heavy tanks are also possible. Just your existing cannon shells are too vastly overpowered in my taste. :D I am not a nuclear physicists (though I have a major in physics), but it seems if there is a thing called "depleted plutonium" (not used in real world because of several concerns, like being quite unstable, but I totally ACCEPT this as a sci-fi game mod) and use it in armour-piercing rounds like in DU-style, they are not that vastly better if considering the physical properties. The penetration power of DU rounds mainly comes from the very high density (19.1 g/cm^3). In comparison, Plutonium has 19.8 g/cm^3, which is just a bit denser. It would not be sufficient for the extra +50% damage over the DU cannon shells. From my unprofessional knowledge in physics, it would be about +11% penetration power. But exaggerating it up to +20% is still fine (gameplay-wise).

But before that, there is one thing bothering me, the license of your mod: AGPL v3.
Before talking about those license terms, first let me list what I have to do with your code & resources:
1. My mod does nothing if your mod is not enabled. Thus, the "new" items are added only when both mods are enabled.
2. Using graphics (icons & spritesheet) from your mod, with some modification. Like adding a lower-left tank mini-icon to indicate the MOX-reactor is a tank-equipment variant. (Though I don't have to "pack" your graphics in my mod actually. Just linking your icons and second overlay for the caliber and tank equipment indicators.)
3. Using part of your code with modification. Since that would be based on your reactor and ammo items.
4. I do not want to change my mod's license to AGPL v3. The terms are somehow ambiguous. I do not want to prevent other people from developing any compatible mods with mine, just because of this. In comparison, currently I can give permissions to people in using my code, not enforcing them to change to the same license. (see below)

I am not a lawyer, my following conclusions are made by reading some (if not a lot of) articles about the license. It seems there are two main opinions which are quite opposite:
1. Since I would be using your code (and resources), this counts as "modifying" and "derivative" work. My work needs to be bounded by the same license (AGPL v3), which is what I want to avoid exactly. (Because it then make any mods doing anything with mine enforced under the same license, like a chain...)
2. It is seen as a "compatible" work. Those parts will not show up without your mod, rather than as independent items. An example (maybe not a good one though) is like OpenOffice having the compatibility to open MS Office documents. OpenOffice obviously has very different license from the copyrighted MS products.

BUT, in the first place, do you want such mods compatibility? ONLY if you agree, then we can start to find out what can be done with the license...

Of course, I agree.
I added this license because I made compatibility with Realistic Nukes. Author of this mod asked me to apply the same license on the Plutonium Energy. I always use MIT because I don't understand anything about licenses. I just applied AGPL to my mod and forgot about this.
I thought about changing licenses (to choose one which will be good for me and others, then apply it to all my mods) but I didn't choose yet.
And to change license I first need to change how compatibility with realistic nukes work...

About balancing: I balanced nearly everything in this mod quite randomly. I read about nuclear processes in Wikipedia. And balancing weapons properly was hard for me without playing actual game with my mod. So I will decrease 50% bonus to 20% bonus. Also I will think about some MOX-reactor changes.

That's good. So let me give you a brief idea about different license. Here is a table that you can compare various licenses quickly in a glance: https://choosealicense.com/licenses/
Let me explain some key points in short. MIT is the most free from what you can choose for Factorio mods. It is meant to permit almost everything, including using part (or full) of your code and resources, without quoting or giving credits to your work. So it is good for spreading your "influence" but not good for protecting your creativity. (I am not a lawyer, so don't quote me on that.)
On the other hand, various GNU licenses are on the other end, to protect the rights of the author. But in my taste they are too restrictive, because any follow up work that are using "modified" code must be bounded to the same license. (And it's like a chain that any follow-up mod supporting yours must also bounded...) Also, the statement is quite unclear about differing "compatible" work to "modified" work (as my previous reply stated). That's why I avoided that, not put any of my mods under GNU licenses.
Creating my own license is too much for me, so I checked how other modders deal with that. Bob is an experienced and famous modder, created his large set of overhaul mods. If there are any licensing problem, he will the one of the first to encounter that. So I believe he handles that much better than most modders. Also, his license is clean and easy to understand, in the modder sense, and fits my taste. Therefore, I just followed his license. It may not be a good choice, but still better than both MIT and GNU ones IMO.

No rush about license choices or degree of balancing though. It would take some time to change things on my end too. So take your time to find out and decide what options you have too.

Ok, so I changed license to Bob's mods copyright.
Also I made a Google spreadsheet with current warfare stats for Plutonium Energy.

Nice, so now license is no longer a problem.

Have you playtested a bit? Are those plutonium shells overkill for behemoth biters or not?
BTW, I have made a new mod Schall Endgame Evolution adding very late game aliens. So they could have enough health to withstand more than 1 shot.

I will add the 88 mm and 128 mm cannon shell from my side. Their damage may go off-scale, if the damage bonus is too high together with damage upgrades from research.

PS: Plutonium-238 is the "fissile/high energy" isotope, right? I feel strange that it uses a darker colour than "low energy" Plutonium-239.

I haven's playtested because I was busy (school, housework, etc.), but now I asked one man to help and playtest it.

Idk how I decided that Pu-239 is a "high-energy" isotope... Maybe I'll just swap them in recipes and swap their icons.

In the last PS, I just determined that relation from your recipes... That was wrong after checking with the handbooks...

Pu-239 is the fissile (high energy) isotope used in plutonium bombs. So the icon is correct that they are "brighter".
But some of your recipes are wrong that having Pu-239 & Pu-238 reversed.

I just checked the recipes and Pu-238 is used more than Pu-239, like in base game (U-238 used more than U-235).

I have no question on recipes of MOX fuel.

But for example, it needs 30× Pu-238 but no fissile Pu-239 to craft a plutonium bomb.
And it needs 1× fissile Pu-239 for the "normal" magazines and cannon shells.

Oh, you're talking about ammo recipes...
I will change the required isotope for ammo.