Nuclear Fission PhET

Directions

Part 1 - Nuclear Fission

1. Click Here to run the sim.

2. Click the tab "One Nucleus" and you will see a single U-235 Nucleus staring down the barrel of a neutron gun. Click the red button on the gun, and give the nucleus an extra neutron. Notice that this raises the energy level of the nucleus in the energy diagram, and the nucleus eventually is so unstable that it breaks apart into two chunks.

3. Notice also that when it breaks apart into two pieces, it also release a few neutrons. This is because the stable nuclei of lighter elements have fewer neutrons relative to protons, and so all the neutrons from the U-235 don't have a place. It's a bit like musical chairs, only there suddenly are 2-5 seats missing.

Part 2 - Chain Reactions

1. Now click the tab at the top that says "Chain Reaction" (The fun part!) and on the right notice that you can change how many U-235 atoms (Don't add U-238 yet). Slide the slider to put 100 nuclei on the page, and fire the neutron gun. If it misses all the nuclei, you can aim the gun by dragging the muzzle of it.

2. Hooeeeee!! We just did that for fun, really, didn't we? Notice that the released neutrons from the fission reactions all set off subsequent reactions. This is what we mean by a chain reaction. Do this a few times. Notice that every now and then there is a nucleus that doesn't get nailed by a neutron.

3. Now "Reset Nuclei" and drag the slider to 50 and set it off again. Notice that is is much more common to have a reaction that is not 100%

4. Now do a 25 atom chain reaction. You can aim the gun by dragging the muzzle.

5. Now try only 10 atoms. Now it's hard to get a chain reaction going, isn't it?

6. Mess around with the number of U-235 atoms and see how many you need to start with to end up with fewer than half left. There seems to be a transition just North of 25. Notice too that it is highly variable, and dependent on which atom you hit first. Sometimes you can hit an atom at the edge, and it can fail to set off any further reactions.

7. We are exploring the idea of a "critical mass" here - a critical mass is idea that there is a minimum number of atoms you need to sustain a nuclear fission reaction. It depends on the shape of the piece of Uranium as well. A hollow sphere the same mass as a solid sphere might not be critical when it is hollow, but it can be critical when it is imploded and made into a solid sphere. The bomb that killed hundreds of thousands of Japanese in Hiroshima used this trick - they imploded a hollow sphere of enriched Uranium to make it critical.

Part 3 - Not Pure U-235

1. Now put 50 U-235 and 50 U-238 nuclei on the screen. U-238 is stable. If it gets hit by a neutron, it just absorbs the neutron and becomes U-239. If it gets hit by a second neutron (in this sim) it ignores it. (not sure if that is true in real life)

2. Pop a neutron into one of the U-235s on your screen and set it off. Notice that you typically have far more U-235s left. Do it a few times, and compare the result to just a screen of 50 U-235s and 0 U-238s.

3. This difference is because the U-238 just absorbs the neutrons without releasing any. They are a damper. Right now, Iran is trying to purify Uranium which is a mixture of U-235 and U-238 (among other isotopes) into more pure U-235.

4. Keep the number of U-238 at 50 nuclei, and mess with the number of U-235 nuclei. Notice that there seems to be a transition of efficiency somewhere between 50 - 60 U-235s. The results can be highly variable, so repeat and repeat.

Part 4 - a Nuclear Reactor

1. Now click the tab at the top for "Nuclear Reactor". Notice that there are now nuclei in fuel rods. (very small fuel rods) and the fuel rods are interlaced with control rods that absorb neutrons.

2. Drag the control rods all the way down (You were going to do it anyway...) and click the red "Fire Neutrons" button and watch it go Chernobyl on you. (and be slightly disappointed) A real reactor could actually get so hot that the fuel would melt together if you did that. Do an image search for "elephant's foot Chernobyl" and you will get a picture of some poor (dead) person kneeling next to the melted fuel from the reactor.

3. What is missing from this sim is the notion that the released neutrons all interact with U-235 nuclei. In a real reactor, we would need to slow the neutrons down to get them to "stick" to a nucleus. Fast neutrons just go through the nucleus without "sticking" and fail to cause a reaction. Most reactors use water as a moderator, but the reactor at Chernobyl used graphite which can burn if you get it hot enough...

4. Try a few different positions of the control rods to see that we control reactors that way.

5. Dive as deep as you want into the Chernobyl disaster. "Girl of Speed" is believed to be a bit apocryphal, but there are good sites - start with Wikipedia.