Uranium-233, Uranium-235, and Plutonium-239

Up to now, we have looked at spontaneous radioactive decay that leads to either alpha radiation or gamma radiation. Nuclear fission, on the other hand, is about triggering externally a breakup of the nucleus.

The basic idea is to hit the nucleus with a high-energy neutron that will split the atom and eject alpha particles and neutrons. Large amounts of energy can be released in this process. The source of that energy ultimately stems from the nuclear potential energy but we usually just say mass energy. In fission the atom you start with weighs more than the broken pieces. This is a large source of energy that we can tap in nuclear reactors.

There are three main fuels that we can easily do fission with in nature.

  1. U-235: This is a radioactive isotope of Uranium. It occurs naturally in nature. You could find it in your backyard . . .

  1. U-233: This is another radioactive isotope of Uranium. It does not exist in nature but it can be created by bombarding Thorium-232 with neutrons.

  1. Plutonium-239: It does not exist in nature but it can be created by bombarding Uranium-238 with neutrons.

Uranium Cycle

The current system used in nuclear fission relies on U-235 and it is called the Uranium Cycle. Here are the basics:

We first mine for Uranium-238, which can be found in pretty large quantities (40 x more uranium on Earth than silver). It is almost stable:  t½ is about 4.468 billions years old.

A typical sample of U-238 will contain 0.7% of U-235 with a half-life t½ of about 703,800,000 years. In order to effectively do fission we need more U-235 than that and so we have a procedure called "Uranium enrichment" in which we increase the % of U-235 in th U-238 sample. We then create a rod with this enriched uranium and we send high-speed neutrons toward it.

When U-235 absorbs a neutron, it becomes U-236 which is really unstable and the whole atom breaks into Krypton and Barium (see table of element for Kr and Ba); see Fig 11.

The process also releases three neutrons. All the particles in the end process have a lot of kinetic energy. This is heat and the nuclear reaction can be used to work a steam engine and create electricity.

Q. Let's estimate how much energy is released during fission of U-235. Look at the binding energy curve again and the table of elements. U-235 has an average binding energy per nucleon of 7.5 MeV while Krypton and Barium are around 8.5 MeV. Given this energy, how much energy do you think will be released?

  1. 8.5 MeV.

    No. Too low.

  2. 7.5 MeV.

    No. Too low.

  3. 16 MeV.

    No. Too low.

  4. 235 MeV.

    Correct. We gain about one MeV per nucleon of binding energy. We started with 235 nucleons so the final energy will be reduced by 235 MeV roughly.

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