Is it impossible for reactor grade uranium to be detonated?

Question

So, this is a question that has been puzzling me for numerous reasons. Be it for advocating nuclear power, or getting a bit of nerd rage when watching a sci-fi work, or having an argument with someone.

Pretty much everyone knows that you need as much U235 you can get in a given concentration of uranium to make a bomb that's feasible and practical since the presence of U238 slows down the reaction. So working that backwards, is there a concentration of U238 to U235 where it's impossible for it to become supercritical because there are too many U238 atoms absorbing the neutrons and thus cannot trigger a nuclear detonation?

I have looked at the wiki for it: https://en.wikipedia.org/wiki/Enriched_uranium#Highly_enriched_uranium_.28HEU.29

If I am reading the second to last sentence on the first paragraph correctly, that would mean any Uranium that is below 5.4 enrichment would be physically incapable of undergoing nuclear detonation regardless of mass of uranium involved. But I'm not sure I am interpreting what they have to say correctly, so I am asking here for further clarification.

Since reactor grade uranium is only 3-4% enriched, that should mean it is impossible for that grade of uranium to ever achieve nuclear detonation, even if you tried. There is also a blurb about this notion (fact?) in this Chernobyl footage, but given it's age, and lack of explanation as to why, I'm not sure this is a valid source. https://youtu.be/Cc-vvhWXL9Q?t=14m50s

So, if I got all my duck in a row and understanding this correctly, assuming 3-4% enriched uranium is being used, would it be accurate to say that nuclear reactors (Of the above parameters), cannot, and won't ever explode in the sense of an atom bomb?

Answer 1

Yes, nuclear grade uranium can never explode in the sense of an atom bomb for various reasons
1) Only Uranium 235 is capable of sustaining nuclear chain reactions and as you said reactor grade uranium has only 3-4% of that whereas the bomb dropped on Hiroshima had well over 80%
2) Critical Mass : The term simply means that there's enough fissile material present to sustain a chain reaction, and a supercritical mass is where enough material is present for the fission rate to increase.
A nuclear weapon is designed to release all its energy in one incredibly destructive blast, which means the material wants to be as densely packed with fissile material as possible in a homogeneous sphere(nearly)
Whereas reactor cores are meant to produce a steady, controlled release of energy, and even the sort of energy buildup needed to produce a meltdown can't ever attain the speed and intensity needed for an explosive nuclear energy release. The geometric arrangement of uranium-235 in a nuclear reactor is just fundamentally not conducive to the spherical arrangement needed for an explosive chain reaction, and the amount of non-fissile uranium-238 in reactor-grade uranium also stops any runaway reactions.

Answer 2

To add to Prabhdeep Singh's correct answer, there's another fundamental reason why a reactor would never explode like a bomb even if using highly enriched uranium. And that is simply that an exploding critical mass is, well, exploding, so it's dispersing and thus quenching the reaction. Moreover, temperature rises also tend to quench the reaction. Critical masses, unless very carefully designed, don't tend to make big nuclear explosions: they tend to blow themselves apart.

If you must make a bomb, then you must design things so that the fissile material stays together in a supercritical mass long enough for a sizeable fraction of it to undergo fusion. In an implosion-type fission weapon, one must set up a huge acoustic spherical wave that crushes the fissile material perfectly symmetrically so that the ingoing momentum of the material keeps it together long enough before explosive reaction quenching puts an end to the whole process.

There have been some famous accidents where critical masses of U235 have been accidentally assembled with gruesome outcomes, but none of these outcomes was an explosion. See my answer here for more details.

Of course, a nuclear reactor releases a huge amount of heat, so, whilst it cannot end in a nuclear explosion, if the operators lose control of the reaction the sheer heat output can lead to catastrophic explosions (usually from flash vaporization of water, as happenned at Chernobyl) or destruction of the plant and escape of radioactive material.

Answer 3

A thermal reactor uses fuel of about 4% enrichment and requires a moderator to be critical; the prompt neutron generation lifetime is too long for such low enriched fuel to be used as a nuclear weapon.

A fast reactor uses higher enriched fuel (about 20%) and does not require a moderator to be critical; it has a sufficiently short prompt neutron lifetime such that it is possible the fast reactor fuel could be taken and configured into a nuclear explosive (See Willrich and Taylor, Nuclear Theft Risks and Safeguards.) However, the enrichment is not nearly as high as the uranium (highly enriched) used in a nuclear weapon. The US currently uses plutonium, not highly enriched uranium (about 90%), as the fission primary for nuclear weapons in the stockpile.

The need for high/highly enriched uranium for an effective nuclear fission weapon is why all the attention is given to the level at which Iran is enriching uranium using centrifuges.

The above discussion addresses taking the reactor fuel and trying to configure it as a weapon, not using the reactor as a weapon. A nuclear reactor cannot explode like a nuclear weapon. In the reactor (fast or thermal) there is no mechanism for creating and maintaining a super prompt critical assembly sufficiently long for significant release of energy from fission. You have to really work hard to assemble the correct material to create a nuclear weapon; you need to create a system that is super prompt critical using fast neutrons and remains so sufficiently long for the chain reaction to produce enough energy before pressure causes dis-assembly into a non-critical configuration. By super prompt critical is meant super critical on the prompt neutrons alone without having to wait for the delayed neutrons to contribute. In operation, reactors are critical but not prompt critical; delayed neutrons are required for criticality.