Expert: Philip Stahl - 3/25/2010

Question
QUESTION: Is it possible that someplace in the universe there exists a large celestial body made of fissionable elements such as uranium and thorium?  What would it be like?  How would its temperature compare?  What would its life cycle be like?  How would the radiation it emits compare to a [fusion] star?  It seems like this would be a very exotic object.

ANSWER: Hello,

Nuclear fission is an implausible means for stellar energy simply because of how stars originally form, and their physical conditions. This combination means that nuclear fusion is assured of being the primary stellar energy source, not fission.

Thus, the original "building blocks" for most stars (such as reach the Main Sequence and become stable) are hydrogen and some helium. These gases then contract under the influence of gravity to form a sphere which over time acquires a pressure as well as temperature gradient. That means, its structured like a giant onion with more dense material compacted toward the center, and less dense toward the surface. Pressures in the interior will attain such levels that the nuclei (say protons) are fused together. As this occurs, nuclear fusion is the result - not fission. Fission, in addition, is unlikely to sustain a star in hydrostatic (pressure-radiation) balance long enough - say compared to fusion processes.

You can't start with the basic elements of most stars (H, He) and get fission. Fission typically pertains to highly unstable heavier elements - for example cobalt. (Google or check out "the liquid drop model" of the nucleus, when you get the chance)

What about fissioning of much heavier elements that have already been produced by fusion - say cobalt in the innards of massive (10-20 M_s) stars that are nearing the end of their life cycles? Again, not at all plausible or likely! As these heavy elements are formed it isn't fission which comes into play, but neutron capture (in the so-called r, s processes) which becomes available for a time for addition to elements of the iron (Fe) group.

Now, it is feasible that at some limit of high Z, neutron addition by the r, s process is cut off(since the binding energy of each successive neturon becomes progressivley less as more are added).

Thus, some theorize that there will be some ultimate threshold where fission will kick in. Since most transuranium elements are believed produced via the r-process, it seems the fusion limit and fission start must commence very late in rspect to the periodic table, for stars and other stellar objects - including exploding stars, supernovae.

We beleive this commences with Californium 254, for nuclei with atomic weights 254, 258 etc. so it is at this level that fission occurs predominantly.

Thus, a star evincing this behavior may rightly be called a "fission star" - at least for a time.
But we certainly don't expect a star to *commence* this way!



---------- FOLLOW-UP ----------

QUESTION: Ok, so we have established that it is highly unlikely or impossible that this type of object would ever naturally form.  Can you speculate as to what such an object would be like?  The Earth has an abundance of heavy, fissionable elements in its core.  Suppose that nearly the entire planet were made of these elements and suppose the planet were bigger (but not massive enough to cause a black hole).  What would it be like?

Answer
Even in the scenario you suggest, nothing of note is going to happen. There will be no fireballs, no sudden fireworks or anything to rival the most menial flare on a minor star. Here’s the deal: while planets will be the most plausible places to find heavier elements (since they would have been formed from the exploded stellar material transmuted in massive stars), they can’t act as comparable in magnitude fission reactors, say to stellar fusion “reactors”.

They DO and can act as natural fission reactors, but only in terms of the localized radioactive isotopes found in assorted rocks, media or core. But this comes with limitations.

First, if they are *too massive*, say in excess of 3x Jupiter there is more probability of igniting in fusion as a small star.

Second, what natural nuclear fission does occur in a planet is confined to rock formations (like chondrites), it isn’t pure. We don’t get planets formed with pure uranium 235, 238 or whatever so I wouldn’t even speculate on it except to say – if anything like a pure uranium 235 planet ever did form- given a half life of ~ 700m yrs. (with no other additives, or elements) it would essentially burn itself off via fission in 4.6 billion odd years. It would probably glow like a low grade “furnace” at its origin, but as time (and its radioactive decay) progressed – its mass would shrink in tandem, the “glow” would end, and it would terminate as a much smaller mass with barely recordable radioactivity.

Back to reality. In the case of Earth, 4.6 billion years ago U-235 exhibited a heat production of about 4.6 x 10^-13 W/kg, today that is down to 5 x 10^-14 W/kg.  This means that if there was originally a mass of 10^20 kg of U-235, its heat production would have been around 4 x 10^7 joules/ sec back soon after the Earth formed. This is fairly significant, but nowhere compared to a typical star’s energy production (e.g. for the Sun, about 3.9 x 10^26 joules/sec.)

Even if the original mass were much greater, say 10^24 kg or nearly one fifth the total mass of Earth, the energy production rate would be only around 4 x 10^11 J/s. It is hard to conceive of any planets that would form with such a vast proportion of radioactive nuclide, so I am guessing that would be the edge of realism before one veers into a realm of fantasy.

Given this, one would find much lower concentrations of all such radionuclides  (e.g. K-40, Th 232, U-235, U-238 etc) for any planet that has lasted as long as Earth.

Last, while a planet’s interior heats up as its radio-nuclides release energy in decay, this is a self-limiting process. By the very process of decay, the total amount of a radio-nuclide is constantly decreasing. In the case of Earth, the abundance of U-235 today is about 0.0012 ppm (0.71% abundance), while back 4.6 billion odd years ago (around the time Earth formed) it was at 17% abundance.  Since the lowest concentration of U-235 required for the operation if a bona fide fission reactor is ~1%, a nuclear chain reaction involving U-235 could have occurred as recently as 400m yrs. ago when the putative concentration of the radio-nuclide was at that threshold level.

But this doesn't imply anything remarkable would have been observed, except for some excess heat and radioactivity - say by observers with the appropriate instruments.

Hope this sheds some more light!