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?

Burning Coal:10,000 tons of coal per day.

Nuclear reactor:100 tons of uranium per year.

Hydroelectric power:60,000 tons of water per second.

Nuclear energy from fission• Adding a neutron to a heavy nucleus can

cause it to split into two mid-sized nuclei.• Energy is released,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.

Nuclear fuel

• Fission (= splitting) of a heavy 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, a runaway ‘chain reaction’ occurs and splits more and more nuclei, eventually cau- sing the meltdown of a nuclear reactor.

Nuclear reactor

• The reactor in a nuclear power plant produces heat, like a 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)

Schematic of a reactor

• Control rods absorb neutrons to suppress a runaway reaction.

• The moderator slows neutrons to facilitate their capture by the nucleus . Thereby it enhances the fission reaction, contrary to its name.

Many small reactors for safety?

Discover Magazine Nov. 2011

Self-contained , mass-produced, contains much less nuclear fuel. But less control of nuclear waste.

Breeder reactors

• A breeder reactor produces more nuclear fuel than it uses.

• Inert 238U 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 (top of 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 reactors commercially.

Uranium isotopes

• Natural uranium consists mostly of the inert 238U isotope.

• 235U is required for fission, but makes up less than 1% .

• For a reactor the fraction of 235U needs to be increased to

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 one of the best safeguards against nuclear proliferation.

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 are needed to produce UF6 for a reactor.

A person

The gas diffusion plant at Oak Ridge

12 000 workers produced 50 kg of highly-enriched 235U.

Electromagnetic isotope separation

Ionized (= charged) UF6

molecules are deflected by an electromagnetic field. The lighter 235UF6

molecules are deflected somewhat more ( due to Newton’s Fel= ma ).

10 kilograms of highly- enriched 235U were pro- duced this way for the Manhattan Project.

Gas centrifuge enrichmentGaseous 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 of choice for producing enriched uranium these days (for example by Iran).

Enrichment for a reactor and a bomb3-5 % = enriched 235U, sufficient for a reactor.

90 % = highly-enriched 235U, needed for a bomb.

Critical mass for a bomb

Below the critical mass:Most neutrons can escape.

Above the critical mass:Most neutrons cause fission.

25 kg 235U

8 kg 239Pu

Chain reaction and critical mass• Neutrons are released during fission and

captured for additional fission processes.• If each fission process triggers more than

one additional fission reaction, an expo- nential chain reaction ensues.

• This defines the critical mass.

Uranium fission bomb

A uranium bullet is fired into a uranium target to exceed the critical mass and start a chain reaction.

Easy to build such a bomb, but hard to produce 90% 235U by isotope separation.

Dropped onto Hiroshima

Plutonium fission bombA 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

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 fission bomb (10 megatons versus 10 kilotons of the conventional TNT).

• The fission bomb on the right side creates temperatures of 300 million degrees for initiating fusion. A fusion bomb is rather difficult to build.

X-rays from a fission bomb are used to heat the hydrogen fuel quickly , just before the blast from the trigger arrives.

The nuclear arsenal

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

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 Technology for Future Presidents

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

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.

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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

During the Cold War the US and the Soviet Union produced more than 70 000 nuclear bombs, each of them capable of destroying a large city.

Nuclear disarmament treaties reduced the number to about 1600 each (compare the SORT war scenario) , but thousands remain partially disassembled.

The nuclear arsenal

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- tion of >7,000 bombs, >1000 missiles, 27 submarines, and to peaceful jobs for former weapons experts. (Cooperative Threat Reduction Program = CTR)

The concentration of radioactive 14C in the atmosphere almost doubled.

Global effect of nuclear testing

Test Ban Treaty 1963

Could a rogue nation or a terrorist group obtain nuclear weapons ?

The main obstacle is either the enrichment of 235U or a contained implosion of plutonium. H-bombs are much more difficult to build.

Possible scenarios:

• Seizing a bomb: It would be hard to find out how to explode the bomb. That would require inside information about the mechanism and code.

• Seizing bomb material: The critical mass is about 25 kg 235U, 8 kg 239Pu. There are recipes on the internet how to make a nuclear bomb, but the infrastructure for handling 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 laced with radioactive material): It is a more likely but less damaging scenario. The damage would not spread far, but the psychological and economic fallout could be large.

Nuclear proliferation / terrorism

Nuclear WasteFuel rods are usually discarded when about ¼ of the 4% enriched 235U is consumed. That leaves 99% of the uranium as nuclear waste (mostly 238U). 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 formed by the overheating reactors underneath and reacted with oxygen.

One can reprocess reactor fuel to extract the unused 99% uranium, which is done in France. That reduces the nuclear waste. But there is a risk that plutonium gets into the wrong hands.

Nuclear waste storage is a problem. It needs to be safe for a long time. The half-life of plutonium is 24 000 years.

Radioactive exposureThe biggest source of radiation is radon, a radioactive gas generated by the decay of radium in old rock formations. It drifts up into basements emitting

particles. These do not penetrate the 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 radon are readily available.

Nuclear testing and accidents have spread various radioactive isotopes. Particularly dangerous are isotopes incorporated into the body, such as strontium ( it replaces calcium in bones) and iodine (goes to the thyroid). They are intense but short-lived (8 days for 131I).

Ch. 14.6

What happened at Chernobyl ?Three major mistakes conspired to cause the nuclear accident at Chernobyl:

1) The reactor design was unsafe, allowing a runaway chain reaction.

2) The reactor lacked a containment dome.

3) The operators pushed the reactor beyond the allowed safety limits.

Ch. 14.6