Got plutonimum? Build a Radioisotope Thermal Generator!


[citation needed]
“You could probably” isn’t compelling, my impression from reading about it is the opposite, “you definitely can’t”. In particular lack of water cooling systems doesn’t mean the reactor is a bare vessel with fuel and heat exchanger systems (like the kilopower system), the fuel “pebbles” are recirculated in and out of the reactor by some kind of automated mechanical system, that’s large and requires space, weight and maintenance.

There are indications that it’s smaller, but if it’s “massive” instead of “titanic”, it doesn’t really change anything.


No, the pebbles do not move or recirculate a significant amount. They are typically static, they HAVE to be for the reactor to work. The fuel pebbles just sit in a pile arranged with particular geometry to allow the radioactive material to reach criticality. The only thing that recirculates to any large degree is gas around and through the pile of pebbles, which serves as the heat transfer and cooling medium. You just have a hopper and a hatch for reloading more fuel pebbles and removing depleted batches, and a screw mechanism that occasionally shifts some pebbles from the bottom to the top. It’s a lot simpler than you think. I work with pumps and mechanical seals IRL, the footprint for the gas recirculation system would not be that big. Biggest part in both terms of mass and volume would likely be the radiation shielding. If you want hard numbers for volume of the gas transfer portion, give me a gas flow rate and I’ll find a suitable system IRL that would handle it.

Edit: For the whole thing, I could give you a rough volume estimate if I knew how much power you want it to generate, and if I can find data to estimate how many pebbles you would need to initiate criticality, (to find the dimensions and reqired shielding thickness) I’m a mechanical engineer, not a nuclear engineer or technician, so it wouldn’t be 100% certain, but probably in the ballpark.


One other thing that reduces its potential volume and weight is the simplifications that would be made for a pebble-bed reactor system marketed to small companies or other consumers. Specifically the pebble recycling system. In a power plant it makes sense to recycle the pebbles and constantly replace them, but it does not for a system designed to power a business, large vehicle, or house. Nobody wants to fool around with disposing of a single spent fuel pebble every week. There are two potential alternatives:

  1. For small systems, there may be sufficiently few pebbles that shifting them will not extend the life of the fuel to be worth it, or it is sufficiently economical to accept replacing them before the pebbles are totally exhausted.

  2. Instead of individually shifting the pebbles, the entire containment geometry slowly rotates, or rotates once every few days. The fuel is replaced all at once at a regular interval, every few years.


I don’t want your estimate, I want sources. I have sources that say RTGs can be this small (but they have poor power density), I have sources that say the kilopower system is this small, I have no sources indicating any other reactor technology is this small. If you want to change my mind, no amount of estimates or arguments are going to do it, you need to provide sources.

This is pointless anyway, as pebble bed reactors (along with most other reactor types) require constant maintenance the player isn’t qualified to provide. There are a few other systems designed for no-to-minimal maintenance, but every time I’m able to find a source for these, they are in the tens of tons range of weight, which is not a reasonable thing for the player to maniulate. That’s the outstanding thing about the kilopower system, it’s designed to operate completely unattended for its entire lifespan AND it’s small enough to deal with.

Alternatives must:

  1. Not require maintenence.
    1a. Not require refueling within the expected player lifespan.
  2. Be transportable by the player.


Sorry to waste your time then.

Could you please clarify for future reference regarding power supplies or other systems/equipment:

How long are you expecting someone to survive?

What is the maximum weight/volume a player could be expected to move given mechanical assistance?

How complex can something be for operation and maintenance to be feasible?
(Ex: We can fix fusion rifles and plasma weapons, but from our various conversations here and elsewhere it seems like the threshold you are shooting for should preclude that).