Introduction to Nuclear Weapons

1. Introduction to Nuclear Weapons Physical Science

2. I. Nuclear Physics • Key Concepts • The atom: Nucleus surrounded by electrons (a.k.a. beta particles)

3. 2. The Nucleus: Protons and Neutrons • Electro-magnetism holds electrons in orbit (electrons are negatively charges, protons are positive) • “Strong nuclear force” holds protons and neutrons together (137 times as strong as electro-magnetism)

4. 3. Elements • Definition: Elements are atoms with the same # of protons in nuclei (their atomic number) • Change # protons = change element • Atomic weight = protons + neutrons + electrons (trivial weight) • Change # neutrons but not protons = same element but different atomic weight  isotope (Carbon-12, Carbon-13, Carbon 14, etc.)

5. 4. The novelty of nuclear weapons • Chemistry – Elements are combined into compounds (atoms become molecules), which can release electro-magnetic energy as heat, light, etc. ALL weapons before 1945 use chemistry – explosives, napalm, toxins, etc. • Nuclear weapons use the strong nuclear force for destruction  inherently more powerful than any possible chemical reaction (by weight)

6. B. Fission: Splitting a Nucleus • Heavy nuclei are unstable – Put too many protons together and they repel each other. Too many (or too few) neutrons can increase this repulsion. • Spontaneous fission: Unstable heavy nuclei can randomly fission – break into two smaller nuclei (different elements).

7. 3. Induced fission • Throw a neutron at an unstable nucleus and: • It might escape (pass by without being captured by nucleus) • Be absorbed into the nucleus • Trigger fission of the nucleus into two nuclei (shown)

8. 4. The Fission Chain Reaction • More energy is required to hold one heavy nucleus together than two moderate-sized nuclei. • Therefore, splitting a heavy nucleus releases a great deal of energy (strong nuclear force). • If neutrons cause fission, and fission creates more neutrons, a chain reaction may ensue. Small initial energy (a few neutrons) cascades to trillions of split nuclei. • Uncontrolled chain reaction = fission explosion. Requires Critical Mass (enough nuclei close together for neutrons to be more likely to hit nuclei than fly out of the mass without hitting anything) • Critical mass varies by element, isotope, shape (spheres work best), and density (so compressing sub-critical mass can make it “go critical” and explode)

9. Example: Chain Reaction in U-235

10. C. Fusion: Combining Nuclei • It takes more energy to hold two light nuclei together than a single moderate-sized nucleus. • Therefore, forcing two light nuclei together into one nucleus generates energy. • In general, fusion produces more energy than fission (which means bigger bombs)

11. Curve of Binding Energy: Note energy increase in fusion (light elements) compared to fission (heavy elements)

12. 4. The problem of fusion • Fission is easy – just throw some neutrons at inherently-unstable nuclei and they split • Fusion is hard – Hydrogen doesn’t just randomly slam into itself with the energy level of the sun’s core. About 100 million degrees required to overcome strong nuclear force. • All efforts to create controlled fusion use more energy to force the nuclei together than they extract from fusion • BUT we do have one tool to generate huge amounts of uncontrolled energy – a fission chain reaction! (Even this just barely provides enough energy – limiting fusion weapons to very light elements like hydrogen)

13. II. Weapon Design • The most basic fission weapon (aka atomic bomb) – The U-235 weapon • U-235 is fissile – Only low-energy neutrons are needed to split the nucleus. Other types of uranium (U-238, the most common type) require very high-energy neutrons for fission (= nearly impossible to create a chain reaction) • Critical mass of U-235 = 50 kg (about 110 pounds) in a sphere.

14. Advantage of U-235 over U-238

15. 3. The gun-type nuclear weapon • Principle = Quickly mash two sub-critical pieces of U-235 together into one piece above critical mass. Detonation ensues. • Simplified design:

16. 4. Barriers to building a gun-type weapon • Getting the U-235 • 99.3% of Uranium is U-238. Must enrich uranium to increase % of U-235 • Combine uranium with fluorine to make uranium hexafluoride gas (“hex”). Then put hex in a container surrounded by a membrane. Slightly more U-235 will diffuse out than U-238. Also useful…

17. Gas Centrifuges • Since U-235 is lighter than U-238, spinning hex rapidly pulls the U-238 to the edge and leaves more U-235 in the middle • US cascade of centrifuges 

18. b. The danger of “fizzle” • Difficult to eliminate the last U-238 from the U-235 (Hiroshima bomb was 80% U-235 / 20% U-238) • U-238 spontaneously fissions, generating neutrons • Danger = chance that U-238 will start a partial chain reaction just before critical mass is reached. Blows U-235 apart before most of it has a chance to fission. Result = small explosion. • Solution = assemble critical mass so quickly that U-238 is unlikely to spontaneously fission at the wrong moment (we now know Hiroshima bomb had just under a 10% chance of fizzle – the U-238 in the weapon spontaneously fissioned about 70 times/second) • Similar problem makes U-233 gun-type bombs difficult to build (contaminated with U-232, which fissions too rapidly) and Pu-239 ones impossible (contaminated with Pu-240) • More complex designs reduce – but do not eliminate – chance of fizzle. DPRK test probably fizzled (very small blast)

19. c. Safety problems • Accident-prone: Two subcritical masses kept in close proximity to explosives • Accidental moderation: Seawater moderates (slows) neutrons, and slower neutrons are more likely to cause fission before escaping the core. Result = drop bomb in seawater = potential detonation! • Terrorist’s dream: Easy to use U-235 to improvise a nuclear device

20. B. The Basic Implosion-Type Fission Weapon • Why bother? • Desire to use Pu-239 (can be made using nuclear reactors, so no separation necessary) • Compressing material takes 1/10 the time of slamming it together (helps prevent fizzle) • Less fissile material is required if it can be compressed • Much safer – accidental detonation can be made impossible • Allows flexibility: some or all charges can be detonated, compressing material to different degrees

21. Advantage of Pu-239 

22. 2. The basic components • Subcritical mass of Plutonium (any isotope), U-233 (rarely), U-235, Np-237 (similar to U-235 but easier to obtain), or Am-241 (theoretically) surrounded by explosives  nearly all designs use Pu-239 or U-235 • Explosives are shaped, layered, and timed to generate a spherical shock wave • Neutron initiator supplies neutrons to begin fission at right moment – too soon causes fizzle, but so does too late (material rebounds after compression) • Tamper between explosives and Pu-239 helps to reflect neutrons and hold compression for a moment or two to maximize yield

23. Simplified Implosion Design

24. 3. Maximizing Efficiency (Proportion of material that fissions before the whole thing blows itself apart into sub-critical pieces) • Neutron reflector: Surrounds fissile material below tamper to bounce stray neutrons back into the core • Levitating core: Empty space between tamper and core to allow tamper to build up momentum (standard in today’s weapons) • External neutron trigger (particle accelerator outside the sphere) – also useful if you want to put something else in the center of the core….

25. C. Boosted Fission Weapons: Using Fusion to Increase Power • Problem: Most fissile material wasted (only 1%-20% fission before it blows itself apart – Hiroshima bomb was 1.4% efficient). More neutrons needed! • Solution = fill core with isotopes of H that fuse easily: Deuterium (D or H-2 -- 1 proton, 1 neutron) and Tritium (T or H-3 -- 1 proton, 2 neutrons) can fuse into He-4 (2 protons, 2 neutrons), creating energy and 1 extra neutron. Fusion energy generated is trivial in these weapons, but… • The “boost”: Extra neutrons hit the fissile material and cause more of it to fission before blowing itself apart. Result = much larger explosion (about double the explosive power). • Advantages: Higher yield for equal mass – which also means weapons can be miniaturized (up to a point), “dial-a-yield” through control of D/T injected into center.

26. Schematic of Primary Part of Boosted Fission Weapon Hollow core, where D (H-2) and T (H-3) are injected for boosting. Fissile material (U-235 or Pu-239) Beryllium reflector (2 cm) Tamper (tungsten or uranium) (3 cm) High explosive (10 cm) Aluminum case (1 cm)

27. C. Boosted Fission Weapons: Using Fusion to Increase Power • Problem: Most fissile material wasted (only 1%-20% fission before it blows itself apart – Hiroshima bomb was 1.4% efficient). More neutrons needed! • Solution = fill core with isotopes of H that fuse easily: Deuterium (D or H-2 -- 1 proton, 1 neutron) and Tritium (T or H-3 -- 1 proton, 2 neutrons) can fuse into He-4 (2 protons, 2 neutrons), creating energy and 1 extra neutron. Fusion energy generated is trivial in these weapons, but… • The “boost”: Extra neutrons hit the fissile material and cause more of it to fission before blowing itself apart. Result = much larger explosion (about double the explosive power). • Advantages: Higher yield for equal mass – which also means weapons can be miniaturized (up to a point), “dial-a-yield” through control of D/T injected into center.

28. D. Staged Fusion Weapons: The Thermonuclear or Hydrogen Bomb • Parts: • The “primary stage” – A fission device • The “secondary stage” – designed to fuse when bombarded with radiation • The casing: Usually made of U-238

29. 2. Inside the Secondary • Radiation channels filled with polystyrene foam surround the capsule • The capsule walls are made of U-238 • Spark plug of plutonium boosts fusion

30. 3. Radiation Implosion • Primary ignites  high-energy X-Rays • X-Rays fill the radiation channels, turn polystyrene to plasma • Tamper is heated  outside ablates (vaporizes – think of an inside-out rocket). Ablation compresses the nuclear fuel. • Plasma helps keep the tamper from blocking the radiation channels, increasing duration of compression

31. 4. The fusion explosion • Compressed fuel must still be heated • Plutonium “spark plug” in center of fusion fuel is compressed, becomes super-critical and fissions (raises temperature inside case) • Result = huge pressures and temperatures produce fusion, which releases far more energy than fission PLUS “fast fission” of spark plug from fusion-produced neutrons

32. 5. The fuel • Early designs (first US test) used deuterium and tritium – but this required cryogenic machinery (D and T are gases at room temperature) • Modern designs use solid Lithium Deuteride instead. Enriched fuel (lots of Li-6) much more effective. • The fusion process: Neutrons from fission turn some D into T, which then fuse together, generating more neutrons. Some D and T also fuses with Lithium (but this generates less energy).

33. E. Enhanced Fusion Weapons • Fission-Fusion-Fission designs: Make the bomb case out of U-238 or even U-235 and it will detonate when neutrons from the fusion capsule hit it, greatly enhancing yield (doubling power is easy) • Multi-stage weapons: Use the secondary stage to compress a tertiary stage, and so forth. Each stage can be 10-100 times larger than previous stage (= unlimited explosive potential)

34. III. Detonation Parameters • Yield – A measure of explosive power • Expressed as kt or Mt of TNT • Measures power not weight – 20 kt weapon is equivalent to detonating 20,000 TONS of TNT all at once. 1 Mt means the equivalent of a million tons of TNT detonating at once.

35. Examples: “Tiny” to Huge • Oklahoma City non-nuclear bomb (.002 Kt) • Davy Crockett nuclear rifle (.01 kt) • British tactical nuclear weapon (1.5 kt) • The nuclear cannon (15 kt) • Hiroshima (15 kt) and Nagasaki (20 kt) • Max pure fission: Orange Herald (720 kt) • Chinese (3 Mt) and British (1.8 Mt) H-Bombs • Largest deployed weapon (25 Mt) • Tsar Bomba, the largest bomb tested (58 Mt)

36. Comparative fireballs by yield

37. B. Height: Air-Burst vs. Ground-Burst Zones of destruction (1 Mt weapon) Groundburst (energy concentrated at ground zero): Airburst (energy distributed over wider area):

38. IV. Effects of Nuclear Weapons • Prompt effects • Thermal and visible radiation (heat and light) • Initial pulse = 1/10 second (too quick for eyes to react). Few killed, but many blinded • Second pulse = most heat damage, lasts up to 20 seconds for large weapons

39. c. Biological effects i. “Flash burns” – Most prominent on exposed areas (i.e. dark areas of kimono worn by this victim)

40. Burns 1.5 miles from hypocenter in Nagasaki

41. Add 20% for 1st degree burn range, subtract 20% for 3rd degree burn range

42. ii. Blindness: Most far-reaching prompt effect • Flash blindness (temporary) and retinal burns (permanent) from light focused on retina

43. iii. Fire Storms • Heat ignites flammable materials • If large enough area burns, it creates its own wind system, sucking in oxygen to feed the flames • Natural example in Peshtigo, WI (1871): “A wall of flame, a mile high, five miles (8 km) wide, traveling 90 to 100 miles (200 km) an hour, hotter than a crematorium, turning sand into glass.” • Firestorms in Hiroshima (but not Nagasaki), Dresden, Tokyo in World War II. • Result: Large numbers of people not burned by nuclear detonation will be burned by subsequent firestorms sweeping through city

44. 2. Blast damage • Heat of fireball causes air to expand rapidly, generating a shock wave • Shock wave hits and damages buildings, and is followed by… • Low-pressure area follows and sucks everything backwards (blast wind)

47. Note the Mach Front:

48. 1 Mt

49. d. Biological Effects • Few likely to die from blast wave itself, but flying debris may kill many • Lung damage occurs at about 70 KPa (double the pressure needed to shatter concrete walls) • Ear damage begins at 22 KPa (as brick walls shatter) • In general, heat will kill anyone close enough to experience primary blast damage. Crushed buildings will kill many outside this zone.

50. 3. Ionizing Radiation • For most weapons, immediate radiation (gamma rays and neutrons) will only kill those very close to the explosion • More on biological effects later…