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2/22/19

Chapter 6, from Alternate Sources: nuclear and solar power

End of the chapter questions 3.5,9,12,13,16,17,20,22,23,27,31,40

Friday, March 25, 2011, 10AM CDT Breach in reactor suspected at Japanese nuke plant TOKYO – A suspected breach in the reactor at the stricken Fukushima nuclear plant could mean more serious radioactive contamination, Japanese officials revealed Friday, as the prime minister called the country's ongoing fight to stabilize the plant "very grave and serious."

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The Chernobyl disaster

On 26 April 1986, reactor # 4 at the Chernobyl Station, 60 mi north of Kiev, blew up during a routine daily operation. Nearly nine tons of radioactive material - 100 times as much as the Hiroshima bomb - were hurled into the sky. Winds over the following days, mostly blowing north and west, carried fallout into Belarus, Russia, Poland and the Baltic region.

The cause was traced to graphite control rods, which caught on fire.

Chernobyl-What Happened: April 26, 1986

• While performing a safety test on Reactor #4, technicians allowed a power surge that reached 120 times the rated capacity of the reactor.

• The surge (in fact not a ), ripped open the core, including cooling water pipes. The reaction of hot water with the graphite control rods produced gas, which combusted violently.

• The 4,000 ton concrete covering over the reactor was blown away. Fires broke out in many places in the site.

• Fifty different radioactive were released, with half-lives spanning from two hours to 24,000 years.

• These isotopes were shot 1.5 miles into the sky and were carried west by the prevailing winds.

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Chernobyl: Social and Environmental Consequences

Over 1,000 injuries and thirty-one deaths of firefighters and others who reported to scene of accident.

150,000 people evacuated from their Ukraine homes.

Radioactive cloud released over a large part of Europe. Health threatening levels of radioactive materials were found in at least twenty nations, and as far away as 2,000 km from Chernobyl. Estimated 250 million people were exposed to unhealthy amounts of .

Estimates of future from the accident range anywhere from 7,500 to 1 million.

Radioactive particles in the environment and in the food chain.

Large amount of uncertainty.

Chernobyl: Political Consequences

Distrust of government

Soviet Union cover up: Sweden and Poland were the first nations to bring attention to the accident.

Other nations attempted to downplay the health effects of the accident in their own nations.

Distrust of technology

Public opposition to building additional nuclear power plants increased significantly worldwide.

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Discovery of radioactivity In 1896, Antoine- unwittingly discovered radioactivity by wrapping together photographic film with some minerals and leaving them in the dark over a weekend.

Two years later, , as a doctoral student, began to study minerals that also emitted radiation. She found that the intensity of the radiation (1) was directly proportional to the concentration of the element in the mineral, and, (2) did not depend on pressure or temperature, unlike any other that was known until then.

For these discoveries she received the 1st Nobel prize in physics in 1903.

Marie Curie and husband set out to purify the radioactive elements in a uranium ore. Two fractions were obtained, one was precipitated with bismuth compounds and the other precipitated with KOH, NaOH, NaHCO3, or K2CO3.

Fraction 1 contained (M. Curie was born in Poland but could not train as a there). Fraction 2 contained , which was purified from several tons (!) of ore. 0.1g of RaCl was melted and pure radium was obtained by electrolysis.

For these studies she received the in 1911.

The Thompson and Rutherford cathode tube ray experiment, 1897

Beta particles have a negative charge. They are high speed electrons. 0 − symbol: −1β, or β, or β

Gamma particles have no €charge and 105 as much energy as visible light. These are high-energy photons. Symbol: 0 Alpha particles have a positive 0γ 4 charge. Symbols: 2He, α Gamma radiation was studied by P. Villard. In 1902 Rutherford suggested that different elements form as a result of radioactivity.€ For this he was called an alchemist€ and some other names, although it turns out he was right. Many nuclei are unstable, and unstable nuclei exhibits . Radioactive decay can result in the formation of a different element.

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Terms and notation of nuclear components + = nucleons. Nuclide = a specific composition of nucleons. For example, the C-13 nuclide has 6 protons and 7 neutrons. The C-12 nuclide has 6 protons and 6 neutrons.

The following nuclide notation is followed:

A Z X where X is the symbol of the element of which this nuclide is the nucleus; A is the , which is the number of neutrons and protons; Z is the charge of the particle. For nuclides, Z is the number of protons. For nuclides, N = number€ of neutrons = A - Z

0 −1e = ? 1 1p = ? 1 0n = ?

€ Two types of radioactive decay

1. results in emission of a nuclide.

2. results in the ejection of a high speed electron from the nucleus. A is converted into a and an electron; the electron is immediately expelled away.

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Estimating the magnitude of the forces required to hold the nucleus together

The mass of the nucleus is always slightly less than the sum of its unassociated components. This gives us a way of calculating the force that holds the nucleus together.

For example, the mass of a proton is 1.007825 g/mol, the mass of a neutron is 1.008665 g/mol, but the mass of the nuclide (of a hydrogen with one neutron) is only 2.01410 g/mol.

According to Einstein’s theory of relativity, E = mc2. If the “missing” mass has been converted into energy, this will give 2.15 x 1011 J/mol.

This has been measured to be the magnitude of the force that holds the deuterium together or the “binding energy” of a mole of protons and a mole of neutrons.

How much energy is trapped in the nucleus of a helium atom if the total mass of the atom is 4.02823 g/mol?

There is a significant amount of energy “trapped” or “stored” in the nucleus!

Nuclear fission of 235U

In the 1930s Enrico Fermi did experiments where he and co-workers bombarded uranium (Z=92) with neutrons. Strassman and Hahn proposed that was one of the products, but no one could explain these findings very well. In 1939, Lise Meitner proposed that the resulting barium was caused by the splitting of the uranium nuclei. The intermediate is 236U, which is highly unstable.

The amount of energy that is given off by this reaction is quite high. A mole of uranium-235 (about 0.5 lb) releases 2.1 x 1013 J. In comparison, the combustion of ~0.5 lb of coal burnt releases 2 x 104 J - about 109 times less energy! 2.1x 1013 J ≈ 7700 ton of TNT, enough to vaporize all the water in 9 olympic size swimming pools

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Chain reaction of 235U

2.1 x 1013 J is produced by the explosion of 7700 tons of TNT or the burning of 770 ton of coal, enough energy to push 160,000 cars 6 miles high.

When a sample exceeds its critical mass, the neutrons released by the first event will collide with additional nuclei, and additional fission reactions will result. At sub-critical mass amounts, most neutrons will leave the sample without causing further fission reactions.

Uncontrolled nuclear fission

If an uncontrolled nuclear fission is allowed to proceed, an extremely powerful explosive can be built.

In August 1939 Albert Einstein wrote President F.D. Roosevelt and warned him about the danger posed by the possibility of Nazi Germany having such a weapon.

President Roosevelt presently initiated the Manhattan project, at the end of which at least 2 nuclear weapons were built and detonated above Japan in August 1945. These weapons unleashed horrible destruction, and Japan surrendered shortly thereafter.

For an account of the Manhattan project see R. Rhodes, The making of the atomic bomb, 1986. Fizeau test shot, yield 11 ktons, Nevada test site, 1957.

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Controlled - nuclear energy reactors

Heat is generated to produce steam, and a turbine is spun to produce electricity. The fuel rods contain 235U that has been enriched from 0.7% to 3-4% abundance. The control rods are sandwiched between the fuel rods. The control rods are made of cadmium or (hafnium in nuclear subs). These are movable neutron “absorbers” that control the rate of the fission reaction. A reflector made of beryllium alloy is used to speed the reaction.

A coolant carries the heat to the turbine. D2O can be used because it absorbs very few neutrons.

Radioactive decay is a first order 10.00 9.00 reaction 8.00 7.00 A first order reaction can be described by 6.00 5.00 −kt [A] 4.00 [A] = [A]0e 3.00 This equation describes an exponential 2.00 1.00 decay, where [A] is the concentration of the 0.00 radioactive€ material at time t, [A]0 is the 0 2 4 6 8 10 12 14 16 18 20 starting concentration of the material, and k time (min) is the kinetic rate constant.

Taking the natural log of both sides of the equation above yields:

ln[A] = ln[A]0 − kt

This is a linear equation. Plotting ln[A] versus time should give a straight line for a first order reaction. €The slope of this line is the negative of the rate constant.

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Half life of first order reactions The half life of a reaction is the time at which half the original amount of reactant has been converted to product. Substituting A0/2 for A and t0.5 for t, we get [A ] ln( 0 ) = ln[A ]− kt 2 0 0.5

Rearranging gives

€ [A ] [A0] ln[A ] ln( 0 ) ln( ) 0 − [A ] / 2 ln2 0.693 t = 2 = 0 = = 0.5 k k k k

So the half life in a first order reaction is independent of substrate concentration. The half-life also is inversely proportional to the rate constant. €

Radioactivity units

Radioactivity is measured as disintegrations per second (d/s), which is defined as one Becquerel (Bq). 1 Bq = 1 d/s.

An older unit that is in common use is the curie (Ci), which is defined as the number of nuclei disintegrating each second in 1 g of radium-226.

1 Ci = 3.70 x 1010 d/s = 3.70 x 1010 Bq 1 mCi = 3.70 x 107 d/s 1 µCi = 3.70 x 104 d/s

The specific activity of a radioactive sample the decay rate per gram of radioactive material.

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Radioisotopic dating methods

In 1960, the American Willard F. Libby won the Nobel prize for his development of radioisotopic dating. The method relies on the fact that 14C is formed continuously through the bombardment of 14N with neutrons: 14 1 14 1 7N+ 0n→ 6 C+1p The amount of C-14 remains relatively constant because the formation rate and decay rate are almost exactly the same. One also can take into account fluctuations in C-14 levels. For example, a slight but sudden decline in C-14 levels has occurred about 3000 tears ago. € 14 14 - C-14 enters the pool of total as CO2 and H CO3 , and thus mixes with C-12 present in those compounds. Living organisms digest these compounds (through and consumption of plants). The C-14/C-12 ratio in living systems is equal to the ratio in the environment.

When an organism dies no more 14C is taken up, and the 14C already present decays steadily with a half-life of 5715 years. Thus the ratio 14C/12C decreases with time due to the decay of C-14:

14 14 0 6C→ 7 N+ -1β You discover a fancy-looking gown in an archeological dig. You submit a sample for analysis and find that the C-14 in the sample is 78% of the value in modern clothing. How old is the gown? €

How exposure to radioactivity is assessed

rad = “radiation absorbed dose” – absorption of 0.01 J of radiant energy/kg tissue rem = “roentgen equivalent man” = Q x (number of rads) where Q is a relative biological effectiveness factor 1 Sv = 100 rem

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How do we dispose of nuclear waste?

Radioactivity samples collected in spent fuel cooling chambers

What do you conclude about the integrity of the containment chambers in the following scenarios?

Scenario 1 Scenario 2 Radioactivity in water samples from radioactivity in water samples from cooling pools cooling pools 100 100 90 90 80 80 70 70 60 60 50 50 40 40 30 Radioactivity (mCi) Radioactivity

Radioactivity (mCi) Radioactivity 30 20 20 10 10 0 0 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 time (day) time (day)

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Nuclear Chemistry Review Questions

1. The half life of I-125 is 8 days. If a patient ingests 1 mCi of I-125, how much of it will be left after 60 days if no was secreted out?

2. The half life of C-14 is 5715 years. If a bone you discover in an archeological dig has 23% C-14 activity left, how old is the bone?

3. To deal with , a worker at a nuclear plant is suggesting to store the liquid waste in highly concentrated solutions in order to accelerate the decay. Is this a reasonable strategy?

Utilization of Solar energy: Solar-Thermal Vs. Photovoltaic cells Solar-thermal (CSP): uses solar heat as fuel for steam generation in power plants.

Photovoltaic cells (solar panels): use solar power to generate electricity directly.

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Electrons in solids: conductors vs. semiconductors

Metallic solids have too few electrons to fill their outermost atomic shells, so electrons are shared among the metallic nuclei.

Electrons in the resulting “sea” of are easy to move, and replace with incoming electrons, resulting in the typical metallic property of good electricity conductivity.

Metalloids display non- metallic and metallic behavior. Semiconductors have a limited capacity to conduct electricity because their outermost electron shells can be filled by sharing electrons.

Semiconductors: doping, n-types, and p-types

• Silicon-based semiconductors can be supplemented (doped) with elements from adjacent groups.

• Elements that have more electrons than Si (group 5A) form n-type semiconductors; the extra electron will move readily when a current is applied.

• Elements that have fewer electrons than Si (group 3A) form p-type semiconductors, the electron “hole” allows electrons to move in when current is applied. n-type semiconductor p-type semiconductor

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Schematics of a photovoltaic cell Light with sufficient energy (not intensity!) must be absorbed in order to cause electrons to flow away from the n-type semiconductor

Efficiency of the photovoltaic cell is a central consideration in economic viability. How is efficiency measured?

Photovoltaic cells must reach an efficiency of ~15% to be marketable. The highest efficiency to date is around 21%.

Increasing the commercial viability of photovoltaic cells • Lower resistance to current flow within the cell by using ultra thin layers constructed with amorphous SI, not crystalline Si (using CIGS). • Utilize a diversity of semiconductors to increase the range of photons that can be absorbed.

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