A Nuclear Primer When energy leaves an atom, it’s usually called radiation; that scares civilians. Caused by electrons: light, x-rays, low energy electrons Caused by nuclear processes:  helium nucleus  electron or positron  high energy photon p,n proton or neutron Other energetic particles possible

Radiation Units Geiger counter video - watch dial curie (Ci) and becquerel (Bq) are certain numbers of decays per second rad and gray (Gy) measure how much energy is deposited in a kg of tissue. rem and sievert (Sv) measure how much damage is done. 1 Sv = 100 rem rem = rad * RBE It’s rems that harm you

Radiation Sickness Consider whole body doses < 100 rem no short-term illness 100 - 200 nausea, hair loss 300 LD50 > 1000 survival unlikely Alexander Litvinenko

Cancer and the linear hypothesis 2500 rem whole body gives 100% chance of excess cancer So 25 rem whole body gives 1% chance of excess cancer Give 100 people 25 rem each, expect one extra cancer (20 from other causes) Most people assume linear hypothesis at all dose levels, so 25 mrem gives 0.001% chance of excess cancer.

Fission and fusion energy Fission occurs when a nucleus splits Fragments have less mass than the original nucleus; E = mc2 Fusion occurs when two lighter nuclei fuse into a single heavier nucleus. Again, mass goes missing and comes out as energy (lots of it) How can this happen in both cases?

Fusion generates lots of relatively clean power, but requires high temperature and probably magnetic confinement Fusion is the power source of stars Fusion bombs are the most powerful ever made

Chain reactions The power of repeated doubling Neutrons initiate fission, and fission creates more neutrons Runaway chain reaction is a bomb Critical mass is required (15 kg for U-235, 5 kg for Pu-239) to ensure enough neutrons are absorbed Make critical mass with “gun” or with implosion

How to make a bomb Enrich uranium to increase U-235 Use plutonium Gas centrifuges Modern, efficient, but need strong material for their construction Calutrons Slow but sure; Saddam tried this method Use plutonium Implosion must be used; hard to do Use fission bomb to start a fusion reaction (H-bomb)

The only winning move is not to play the game Bomb damage 20 kiloton about a mile away Light/x-ray pulse Thermal pulse Blast wave Radioactive fallout New York City example The only winning move is not to play the game

A reactor hodge-podge Fast vs. slow neutrons Moderators Bombs need fast neutrons to get enough doublings in a short time Reactors generally use slow neutrons, since they are absorbed more readily by the smaller fraction of U-235 Moderators Used to slow the neutrons down - “thermal” Water, heavy water, graphite

A reactor hodge-podge, p.2 Control rods Absorb neutrons to control the reaction Neutron absorbed by U-238 gives Pu-239 -- “breeder” reactor Sometimes uses fast neutrons. Less safe but more efficient. Depleted uranium “bullets”