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The good , the bad , and the ugly : Use and abuse of nuclear physics

The good , the bad , and the ugly : Use and abuse of nuclear physics. Nuclear power Nuclear bombs Nuclear waste. Ch. 15. What does it take to generate 1 GW of power ?. Hydroelectric power: 60,000 tons of water per second. Burning Coal: 10,000 tons of coal per day. Nuclear reactor:

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The good , the bad , and the ugly : Use and abuse of nuclear physics

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  1. The good, the bad, and the ugly:Use andabuse of nuclearphysics • Nuclear power • Nuclear bombs • Nuclear waste Ch. 15

  2. What does it take to generate 1GW of power? • Hydroelectric power: • 60,000 tons of water per second. • Burning Coal: • 10,000 tons of coal per day. • Nuclear reactor: • 100 tons of uranium per year.

  3. Nuclear energy from fission • Adding a neutron to a heavy nucleus can causeittosplitintotwo mid-sized nuclei. • Energyisreleased,because each nucleon has a higher energy in a heavier nucleus. • Since the heavy nucleus is neutron-rich, excess neutrons are emitted.These split other heavy nuclei.

  4. Nuclear fuel • Fission(=splitting)ofaheavy nucleus works with uranium (235U)and plutonium (239Pu). • Fission is initiated by neutrons,and it also produces more neutrons. • If this process remains under control, it can be used to generate nuclear power. • If not,arunaway‘chain reaction’occursand splits more and more nuclei,eventually cau-sing the meltdown of a nuclear reactor.

  5. Nuclear reactor • The reactor in a nuclearpowerplant produces heat,likea coal-fired plant. The heat is used to create steam, which drives a turbine that generates electricity(Lect.12,Slides 7,8). • Basic parts of a reactor (next slide): • Fuel element (generates heat by fission) • Moderator (slows neutrons to enhance fission) • Control rod (absorbs neutrons to avoid runaway) • Water (extracts heat,creates steam for turbines) • Shielding (shields the surroundings from radiation)

  6. Schematic of a reactor • Control rodsabsorb neutronsto suppress a runaway reaction. • Themoderatorslows neutronstofacilitate their capture by the nucleus. Thereby it enhances the fission reaction,contraryto its name.

  7. Many small reactors for safety? Self-contained, mass-produced, contains much less nuclear fuel. Butlesscontrolofnuclearwaste. Discover Magazine Nov. 2011

  8. Breeder reactors • A breeder reactorproduces morenuclearfuel than ituses. • Inert238U nuclei (see next slide) absorb neutrons and are transformed into plutonium (239P),which is a nuclear fuel. • This design is able to utilize 100% of the natural uranium. In standard reactors only 1% is used (topof Slide 27),and 99% becomes radioactive nuclear waste. • There is the risk that plutonium gets into the wrong hands. Therefore the US and many other countries don’t use such reactorscommercially.

  9. Uranium isotopes • Natural uranium consists mostly ofthe inert 238U isotope. • 235Uis required for fission,but makes up less than 1%. • Forareactorthefractionof 235U needs to be increasedto 3-5%by enrichment (90% for a nuclear bomb). • Enrichment is a difficult process, since two isotopes are chemically identical.The difficulty of enriching uranium is oneofthebest safeguards againstnuclearproliferation.

  10. A person Enrichment by gas diffusion UF6 gas diffuses (migrates) through a thin foil.The lighter 235UF6 diffuses slightly faster than the heavier 238UF6 . More than 1000 repeats areneededtoproduce UF6 for a reactor.

  11. The gas diffusion plant at Oak Ridge 12000 workers produced 50 kg of highly-enriched 235U.

  12. Electromagnetic isotope separation Ionized (= charged) UF6 moleculesaredeflected by an electromagnetic field.The lighter 235UF6 moleculesaredeflected somewhat more(dueto Newton’s Fel=ma). 10 kilograms of highly- enriched 235U were pro-duced this way for the Manhattan Project.

  13. Gas centrifuge enrichment Gaseous UF6 is placed in a centrifuge, where rapid rotation flings heavier 238U to the outer edge, leaving enriched 235UF6 closer to the center.Many centrifuges need to be cascaded for multiple repeats. This is the method ofchoice for producing enriched uranium thesedays(forexample by Iran).

  14. Enrichment for a reactor and a bomb 3-5% = enriched 235U, sufficient for a reactor. 90%= highly-enriched235U, needed fora bomb. Critical mass for a bomb 25kg 235U 8kg 239Pu Above the critical mass: Most neutrons cause fission. Below the critical mass: Most neutrons can escape.

  15. Chain reaction and critical mass • Neutrons are released during fission and capturedforadditional fission processes. • If each fissionprocess triggersmorethan one additional fission reaction,an expo-nential chain reaction ensues. • This defines the critical mass.

  16. Dropped onto Hiroshima Uranium fission bomb A uranium bullet is fired into a uranium target to exceed the critical mass andstartachainreaction. Easytobuildsuchabomb, but hard to produce 90% 235Ubyisotopeseparation.

  17. Plutonium fission bomb A chain reaction is started by compressing a 239Pu sphere by an implosion. The design is difficult, since the chain reaction is faster than in 235U and tends to break up the plutonium sphere into sub-critical pieces. But 239Pu can be extracted by standard chemical methods from spent reactor fuel, as done by North Korea. Dropped onto Nagasaki

  18. Hydrogen fusion bomb • Deuterium (2H) and tritium (3H) are fused into helium (4He), plus a neutron. A fusion bomb is typically 1000 times more powerful than a fissionbomb(10megatons versus 10kilotons of the conventional TNT). • The fissionbomb ontherightsidecreatestemperaturesof 300million degrees for initiating fusion. A fusion bombis rather difficult to build. X-raysfrom a fission bomb are used to heat thehydrogen fuel quickly, just before the blast from the trigger arrives.

  19. The nuclear arsenal A small hydrogen bomb (1 megaton) H bombs inject soot into the stratosphere,causing nuclear winter.

  20. Effect of a nuclear hit on San Francisco Hiroshima-size atomic bomb: Small hydrogen bomb: Downtown San Francisco is gone. All of San Francisco is gone. Blue: Destroyed by the blast.Red: Destroyed by fire. From Richard Muller, Physics and TechnologyforFuture Presidents

  21. Consequences of nuclear war: Nuclear winter The SORT treaty limited the number of nuclear bombs to 2000 each for the USA and Russia. India+Pakistan100 nuclear bombs (estimate). Physics Today Dec. 2008

  22. Consequences of nuclear war: Failing crops [%] Reduction of the crop growing season in the 2nd year after nuclear war. The 1st year is a total wipeout. And the reduction lasts several years.

  23. Consequences of nuclear war: Summary • The consequences of an all-out nuclear war go far beyond direct casualties (all major cities obliterated instantly). There is a worldwide breakdown of the infrastructure: • No crops for several years • No electricity, no fuel • No transportation for supplies • No factories to rebuild the infrastructure

  24. The nuclear arsenal During the Cold War the US and the Soviet Union produced more than 70000 nuclear bombs,eachofthem capableof destroying a large city. Nuclear disarmament treaties reduced the number to about 1600 each (compare the SORT war scenario), but thousands remainpartiallydisassembled. To reduce the risk of nuclear weapons getting into the wrong hands,US Senators Nunn and Lugar spear- headed a sweeping program that led to the destruc-tionof >7,000 bombs,>1000 missiles,27 submarines, and to peaceful jobs for former weapons experts. (Cooperative Threat Reduction Program = CTR)

  25. Global effect of nuclear testing The concentration of radioactive 14C in the atmosphere almost doubled. Test Ban Treaty 1963

  26. Nuclear proliferation / terrorism Could a rogue nation or a terrorist group obtain nuclear weapons? The main obstacle is either the enrichment of 235U or a contained implosionof plutonium. H-bombs are much more difficult to build. Possible scenarios: • Seizing a bomb: It would be hard to find out howto explodethe bomb. That would require inside information about the mechanism and code. • Seizing bomb material: The critical mass is about 25kg 235U,8kg 239Pu. There are recipes on the internet how to make a nuclear bomb,but the infrastructure forhandling these materials is difficult to assemble. • Sabotage of a nuclear power plant: The safety systems are designed such that a reactor automatically shuts down if anything goes wrong. It would require insider assistance to override the safety system.That happened at Chernobyl. • A dirty bomb (conventional explosive lacedwithradioactivematerial): It is a more likely but less damaging scenario. The damage would not spread far, but the psychological and economic fallout could be large.

  27. Nuclear Waste Fuel rods are usually discarded when about ¼ of the 4% enriched 235U is consumed.That leaves 99% of the uranium as nuclearwaste(mostly238U). Used fuel rods are kept in a cooling pool to allow the highly-radioactive nuclei to decay. At the Fukushima nuclear power plant, the pool was on top of each reactor. The storage pools exploded when hydrogen gas was formedbytheoverheating reactors underneath and reacted with oxygen. One canreprocess reactor fuel to extract the unused 99% uranium,whichisdone in France.Thatreducesthe nuclear waste.But thereisa risk that plutonium gets into the wrong hands. Nuclear waste storageis a problem.Itneedstobesafe foralongtime.Thehalf-life ofplutoniumis 24000years.

  28. Ch. 14.6 Radioactive exposure The biggest source of radiationis radon, aradioactivegasgeneratedbythedecay ofradiuminoldrockformations.Itdrifts up into basements emitting  particles. These donotpenetratethe skin but are dangerous when inhaled. This type of irradiation from within the body killed a former Russian spy in the UK in 2006. He was poisoned by a polonium isotope. Test kits for radonare readily available. Nucleartestingandaccidentshavespread various radioactive isotopes. Particularly dangerous are isotopes incorporatedinto the body, such as strontium (it replaces calcium in bones) and iodine(goestothe thyroid).They areintensebutshort-lived (8 days for 131I).

  29. Ch. 14.6 What happened at Chernobyl? Three major mistakes conspired to cause the nuclear accident at Chernobyl: • The reactor design was unsafe, allowing a runaway chain reaction. • The reactor lacked a containment dome. • The operators pushed the reactor beyond the allowed safety limits.

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