A K2+ civilization wants to build a fission star, a huge mass of fissile material which doesn't collapse due to the radiation emitted by its fission decay. I know there are reactors like the aqueous homogeneous reactor that are self regulating and even naturally occurring reactors exist, so why not a star?

So basically can I make a self sustaining, spherical, homogeneous fission reactor? I don't care how big it is, for how long does it shines, if it's solid, liquid, gas or plasma, it's also ok if it needs to contains other elements (moderator?) to exist. It's a K2+ so resources are not a problem!

  • $\begingroup$ Related (unintentional self-promotion): worldbuilding.stackexchange.com/questions/22769/…. A couple answers talk about fission. $\endgroup$
    – HDE 226868
    Commented Nov 10, 2016 at 22:00
  • $\begingroup$ @HDE226868 nice, but don't exactly answer my question I think $\endgroup$ Commented Nov 10, 2016 at 22:42
  • $\begingroup$ I understand. I didn't intend to imply it's a duplicate; it's just some material that might prove helpful. $\endgroup$
    – HDE 226868
    Commented Nov 10, 2016 at 23:45
  • $\begingroup$ K2 is not a magic, they are pretty limited by resources and energy of one star system. $\endgroup$
    – MolbOrg
    Commented Nov 11, 2016 at 19:00
  • $\begingroup$ The energy density of fission is simply not enough to run a star. You would get maybe 1% as much oomph per mass as just chucking a pile of hydrogen onto itself. $\endgroup$
    – PcMan
    Commented Mar 4, 2021 at 15:56

3 Answers 3


An artificial fission star could be stellar-sized gaseous nuclear reactor.

A gas nuclear reactor (or gas fueled reactor) is a proposed kind of nuclear reactor in which the nuclear fuel would be in a gaseous state rather than liquid or solid. In this type of reactor, the only temperature-limiting materials would be the reactor walls. Conventional reactors have stricter limitations because the core would melt if the fuel temperature were to rise too high.

However, maintaining a gaseous core nuclear reactor may be compromised by the need for containment of the gaseous fission core.

It may also be possible to confine gaseous fission fuel magnetically, electrostatically or electrodynamically so that it would not touch (and melt) the reactor walls.

Considering how a gaseous core reactor might make and its possible applications does a possible way of implementing an artificial fission star.

The vapor core reactor (VCR), also called a gas core reactor (GCR), has been studied for some time. It would have a gas or vapor core composed of UF4 with some 4He added to increase the electrical conductivity, the vapor core may also have tiny UF4 droplets in it. It has both terrestrial and space based applications. Since the space concept doesn't necessarily have to be economical in the traditional sense, it allows the enrichment to exceed what would be acceptable for a terrestrial system. It also allows for a higher ratio of UF4 to helium, which in the terrestrial version would be kept just high enough to ensure criticality in order to increase the efficiency of direct conversion. The terrestrial version is designed for a vapor core inlet temperature of about 1,500 K and exit temperature of 2,500 K and a UF4 to helium ratio of around 20% to 60%. It is thought that the outlet temperature could be raised to that of the 8,000 K to 15,000 K range where the exhaust would be a fission-generated non-equilibrium electron gas, which would be of much more importance for a rocket design.

If the bulk of the star was composed of helium (He-4) and the equivalent of a gigantic gaseous core reactor was assembled at its centre, this could consist of UF-6 and He-4 mixture, once the fission reaction process was initiated the nuclear reaction core, provided the surrounding He-4 bulk mass could act as a default containment vessel this might be a fission powered star.

Since this model is a conceptual extrapolation of a gaseous core nuclear reactor system to stellar dimensions, there are many factors that are imponderable without extensive analysis. Basically the thermodynamics and hydrodynamics of a stellar-sized nuclear reactor. The engineering issues in assembling such a construct are considerably non-trivial too.

  • $\begingroup$ thanks a lot for the great answer. Don't worry about factors that are difficult to model, it's not marked as hard-science after all. Based on your impressions what would be the size of such a structure? $\endgroup$ Commented Nov 13, 2016 at 16:15
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    $\begingroup$ My first gut feeling was like a brown dwarf. @Feyre's answer gives good ball park figures, if you wanted a fission star with the Sun's output, it will be supermassive. A fission core at 100 solar masses, the outer He layers will be even more massive. Massive enough that He-He fusion will self-ignite. A dim red dwarf might be possible. It depends on the technical capacity of your constructor civilization. What surprised me was that a gaseous fission core star is more possible than I first thought. Grateful to you for the intellectual exercise in playing with this concept. $\endgroup$
    – a4android
    Commented Nov 14, 2016 at 1:09
  • 2
    $\begingroup$ @SilverCookies Or alternatively, if you want a star as massive as the sun, a planet would need to be at a distance of $\frac{AU}{10}\approx 1.5\times10^6km$, where the radius of the earth is $~6.4\times10^3km$. Either way, the ratio of orbital distance to star radius would be at least ten times as small as what we have with our sun. $\endgroup$
    – Feyre
    Commented Nov 14, 2016 at 10:47

Uranium-235 can sustain a natural fission reaction, with a half life on the scale of $10^{15}$ years (1), whereas Uranium-238 has a fission half life on the scale of $10^{17}$ years. This reaction is different from a normal fission reaction which releases all energy in a short time span.(2) This kind of natural slow fission has existed right here on earth, and has been known about since 1972.(3)

Note that these real life examples were based on masses of a combination with U-235 and U-238. They have been inferred to exist through analysis of the reaction products, and the left over ratio between the two forms of Uranium.

Here I assume that such a reaction can be sustained with just U-235, and calculate whether the reaction would be fast enough to sustain the star.

In fission, U-235 releases $3.24\times10^{-11}$ joules. The total energy output of the sun is $3.85\times10^{26}$ watts. To reach this amount of energy output, a U-235 star would have to have $1.18\times10^{34}$ atoms engaging in fission per second.

Taking an approximate equation to prevent underflow, calculate:

Solve[N0 Exp[- λ] == N0-10^34]

{{N0 -> -(20000000000000000000000000000000000/(-2 + 2^( 31535999999999999999999/31536000000000000000000)))}}

Or numerically the amount of initial atoms is $5\times10^{56}$.

As there are approximately $10^{24}$ atoms of Uranium-235 in one kg, that means $5\times10^{32}$ kg, or two orders of magnitude heavier than the mass of the sun.

In other words, a fission star would have to be at least about two orders of magnitude heavier than a regular star for the same energy output. I don't actually think this is very far out.

However, it is not inconceivable that this reaction, as being naturally inhibited can be sped up to increase the energy ouput by mass, and it is possible that the reaction would speed up with a 100% U-235 mass, or under the severe gravity and heat that such a large mass would have.

It is up to OP to decide if this is feasible enough in his situation.

  1. Pure Appl. Chem., Vol. 72, No. 8, pp. 1525–1562, 2000.
  2. Nature's Nuclear Reactors: The 2-Billion-Year-Old Natural Fission Reactors in Gabon, Western Africa ; Evelyn Mervine (sci-am)
  3. http://brendans-island.com/blogsource/20101015ff/a-natural-fission-reactor.pdf ;George Cowan (sci-am)
  • 1
    $\begingroup$ Fission and radioactive decay are different processes, and you've done the computation for a star powered by decay. The question is asking about fission. $\endgroup$
    – Mark
    Commented Nov 10, 2016 at 23:02
  • $\begingroup$ I'm not really asking specifically about energy output. For the record the most massive star is well over 2 orders of magnitute more massive than the sun $\endgroup$ Commented Nov 10, 2016 at 23:05
  • $\begingroup$ @Feyre, that is describing decay by spontaneous fission. Fission reactors work by neutron-induced fission, which happens on a much shorter timescale. $\endgroup$
    – Mark
    Commented Nov 10, 2016 at 23:10
  • $\begingroup$ @SilverCookies I've edited my question to explain why I think this is relevant to your question. $\endgroup$
    – Feyre
    Commented Nov 11, 2016 at 14:19
  • $\begingroup$ The critical mass for bare U-235 (this means no neutron reflector present) is 52 kg. A star made of fissile U-235 with a mass of 5×10^32 kg, is 10^31 times bigger than its critical mass. Not the biggest bang since the Big One, but big enough. $\endgroup$
    – a4android
    Commented Nov 13, 2016 at 9:12

By definition, stars create energy through fusion. So a fission star is a misnomer.

Secondly, anything the size of a star would most definitely gravitationally collapse on itself. In case there is potential fusion fuel in the mixture (anything from hydrogen to manganese), the star would initiate fusion and die like a normal (main sequence) star. In case there is no fusion fuel in the mixture, the gravitational collapse would create a supernova (if the collapse is quick enough) or slowly crush itself into a black hole or neutron star, depending upon the mass.

  • $\begingroup$ gravitational collapse comes from gravity, which in this case should be produced by mass, not by size. $\endgroup$
    – MolbOrg
    Commented Nov 11, 2016 at 18:59
  • $\begingroup$ @MolbOrg: Where did you find in my answer that I was implying a gravitational collapse based on the visual size/volume of the said star instead of its mass? $\endgroup$ Commented Nov 11, 2016 at 19:40
  • $\begingroup$ Secondly, _anything_ the _size_ of a star would most definitely gravitationally collapse on itself. Hydrogen cloud size of sun, at 1 bar, 300K will have mass about 2 earth mass and will most likely not collapse, I have calculate to be sure, but it is my estimation atm. $\endgroup$
    – MolbOrg
    Commented Nov 12, 2016 at 3:23
  • $\begingroup$ In case you don't know, the size of celestial bodies is measured in terms of their mass, not their volume. When it is said that a star is bigger or smaller than another star, it means by mass, not by appearance. $\endgroup$ Commented Nov 12, 2016 at 18:41
  • $\begingroup$ yhea? maybe, I guess lot of people who may read you answer may be not familiar too, so you can improve it by pointing out which size you meant. I have googled keyword "size of stars" - first page is about real size, not mass-size. Size have relations with mass and brightness and is that main sequence or not, so I may see why it may be used interchangeably in some situations, or maybe by some people. I'm just not in that astronomy subject deep enough to say who when and etc. But I'm pretty sure that size of any artificial object will be measured in inches,meters etc. $\endgroup$
    – MolbOrg
    Commented Nov 12, 2016 at 19:23

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