How is uranium created in a supernova explosion?

Well... it's a little complicated, but.... Try this: http://large.stanford.edu/courses/2013/ph241/rober...

Here's an excerpt:

In order for heavy elements like uranium to be formed, a rapid supply of energetic neutrons must be made available. This is what a supernova explosion is capable of providing. The explosion is initiated by an excessive pressure build up about the iron core, and rapid compression ensues. The gravitational force overcomes the electron degeneracy pressure and begins to drive endothermic (energy consuming) reactions. A process occurs where protons in the nuclei capture electrons and become a neutron. This happens until a sufficient core of neutrons has been built up, reaching a state of neutron degeneracy. The compression wave then bounces off of the neutron core and reflects back as an outward propagating shockwave, travelling at a significant fraction of the speed of light.

As the shockwave moves outward, it passes through various layers: first a dense neutron gas, then the equilibrated iron region, followed by a number of still fusing layers of silicon burning, carbon and oxygen burning, helium burning, and hydrogen burning. The outermost layer is still primarily non-fusing hydrogen. Through each layer that the shock wave interacts, a host of nuclear reactions occur, most notably the rapid neutron capture process.

The r-process is called rapid with respect to the decay mechanisms available to heavy, unstable nuclei. Specifically, it is referenced to the time over which beta minus decay occurs, in which a neutron within the nucleus converts into a proton and emits an electron (and a neutrino). This allows very large nuclei to accumulate in number. The r-process is contrasted with the s-process, a slow neutron capture mechanism, which does not require a supernova explosion to initiate. This process is capable of producing heavy elements up to lead and bismuth, roughly four times the mass of iron. However, the very heavy elements like uranium require the r-process to be formed.

The result of a supernova explosion is that heavy elements are ejected out into space, and are available material for the formation of other celestial objects. Through the natural decay rates of heavy elements, time scales can be estimated for when supernova events may have happened. A few particularly useful elements have half-life decay times on the order of 108 to 1010 years, and they are used in a method called cosmochronology. The useful isotopes for dating nucleosynthesis events that created Earth's material are U-235 and U-238, which have half-lives of 7.13 × 108 and 4.51 × 109 years, respectively. The present day ratio of these two isotopes is roughly U-235/U-238 = 0.007. Given their decay rates, the abundance of these elements at the time the solar system formed (roughly a billion years ago) should have been about 0.3. Within a supernova explosion, models predict that the production ratio should be approximately 1.5. Thus, depending on how many supernova explosions contributed to the abundance of uranium isotopes we see today, the time of their occurrence is estimated to be from 2 billion years ago (if only a single supernova) to 10 billion years ago. Given the timescales discussed above, it is interesting to ponder with respect to the present understanding for the age of the visible universe, which is ~13.77 billion years.

Uranium is probably not created in supernovae, it is created in neutron star collisions.

The same way as other elements are created in a supernova. The high temperature and pressure enables atoms to be fused together to form bigger atoms, and so you get all the elements up to uranium.

Current thinking is that most uranium, along with other heavy metals, was formed in neutron star mergers rather than supernovae. The link is to a paper on how rapid neutron capture creates these elements during the mergers.

"...Supernova and Heavy Elements

In order for heavy elements like uranium to be formed, a rapid supply of energetic neutrons must be made available. This is what a supernova explosion is capable of providing. The explosion is initiated by an excessive pressure build up about the iron core, and rapid compression ensues. The gravitational force overcomes the electron degeneracy pressure and begins to drive endothermic (energy consuming) reactions. A process occurs where protons in the nuclei capture electrons and become a neutron. This happens until a sufficient core of neutrons has been built up, reaching a state of neutron degeneracy. [4] The compression wave then bounces off of the neutron core and reflects back as an outward propagating shockwave, travelling at a significant fraction of the speed of light. [3]

As the shockwave moves outward, it passes through various layers: first a dense neutron gas, then the equilibrated iron region, followed by a number of still fusing layers of silicon burning, carbon and oxygen burning, helium burning, and hydrogen burning. The outermost layer is still primarily non-fusing hydrogen. [4] Through each layer that the shock wave interacts, a host of nuclear reactions occur, most notably the rapid neutron capture process.

The r-process is called rapid with respect to the decay mechanisms available to heavy, unstable nuclei. Specifically, it is referenced to the time over which beta minus decay occurs, in which a neutron within the nucleus converts into a proton and emits an electron (and a neutrino). This allows very large nuclei to accumulate in number. The r-process is contrasted with the s-process, a slow neutron capture mechanism, which does not require a supernova explosion to initiate. This process is capable of producing heavy elements up to lead and bismuth, roughly four times the mass of iron. However, the very heavy elements like uranium require the r-process to be formed. [4]

The result of a supernova explosion is that heavy elements are ejected out into space, and are available material for the formation of other celestial objects. Through the natural decay rates of heavy elements, time scales can be estimated for when supernova events may have happened. A few particularly useful elements have half-life decay times on the order of 108 to 1010 years, and they are used in a method called cosmochronology. The useful isotopes for dating nucleosynthesis events that created Earth's material are U-235 and U-238, which have half-lives of 7.13 × 108 and 4.51 × 109 years, respectively. The present day ratio of these two isotopes is roughly U-235/U-238 = 0.007. Given their decay rates, the abundance of these elements at the time the solar system formed (roughly a billion years ago) should have been about 0.3. Within a supernova explosion, models predict that the production ratio should be approximately 1.5. Thus, depending on how many supernova explosions contributed to the abundance of uranium isotopes we see today, the time of their occurrence is estimated to be from 2 billion years ago (if only a single supernova) to 10 billion years ago. [4] Given the timescales discussed above, it is interesting to ponder with respect to the present understanding for the age of the visible universe, which is ~13.77 billion years. [5]..."

http://large.stanford.edu/courses/2013/ph241/rober...

"...The r-process ('r' for 'rapid') is the creation of elements heavier than zinc through the intense bombardment of other elements by neutrons. It occurs on a timescale so short compared to the time needed for nuclear decay via the emission of a beta particle that it enables chains of reactions involving highly unstable intermediate nuclei to take place. It is believed to occur in supernovae, which unleash a fierce flux of neutrons, and to be responsible for the manufacture of most of the elements in the periodic table more massive than iron...."

http://www.daviddarling.info/encyclopedia/R/r-proc...

"...The s-process is a type of nuclear reaction, believed to take place inside massive stars, in which elements heavier than copper are formed by the slow ("s") absorption of neutrons by atomic nuclei. The capture of neutrons occurs on time scales that are long enough to enable unstable nuclei to decay via the emission of a beta particle before absorbing another neutron. This contrasts with the r-process that is thought to operate in supernovae. Prominent s-process elements include barium, zirconium, yttrium, and lanthanum.

Since the s-process starts with existing iron-group nuclei, it is only expected to take place in second-generation stars that have collapsed out of the residue of previous supernova explosions. A flux of neutrons is required, and it is most likely that these neutrons come from various reactions in the helium-burning region of a red giant. The seed isotope Z, A, from the iron region absorbs a neutron, changing from A to A + 1. If the new isotope is stable, it can absorb another neutron, going to A + 2. If it is unstable, it is assumed that the neutron capture rate is low enough that the nuclide has plenty of time to decay to Z + 1 by beta emission before the next capture. The same neutron-absorption process is then repeated for Z + 1. Thus, the nuclides produced lie in the "valley of beta stability" of the chart of the nuclides...."

http://www.daviddarling.info/encyclopedia/S/s-proc...

The production of uranium requires TWO different process. not one the s-process takes time ( the "s' process is for "slow", But r-process ("r" is for rapid) what happens quickly when stars go supernova..

https://en.wikipedia.org/wiki/S-process

https://en.wikipedia.org/wiki/R-process