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Nuclear Chemistry

Module by: Andrew R. Barron. E-mail the author

Introduction

Everything around us is made from the elements of the periodic table (Figure 1). Elements are defined as the fundamental substances of chemistry, composed of atoms, in which all the atoms are the same. In turn, atoms are defined as being the smallest units of matter and cannot be further divided by ordinary means. It is the phrase “ordinary means” that is key to our discussions of nuclear chemistry.

Figure 1: The Periodic Table of elements (Copyright, Art Branch Inc. 2008).
Figure 1 (graphics1.jpg)

Before we can study the elements it is worth asking two fundamental questions:

  • Why are there so many elements?
  • How did they come into existence?

To answer these questions we need to consider two areas.

  1. Dalton’s view of the atom.
  2. The study of Alchemy.

Although Democritus first suggested the existence of the atom, it took almost two millennia before the atom was placed on a solid foothold as a fundamental chemical object by the English chemist John Dalton (Figure 2). Although two centuries old, Dalton's atomic theory remains basically valid in modern chemical thought. Dalton’s atomic theory states that:

  • All matter is made of atoms. Atoms are indivisible and indestructible.
  • All atoms of a given element are identical in mass and properties.
  • Compounds are formed by a combination of two or more different kinds of atoms.
  • A chemical reaction is a rearrangement of atoms.
Figure 2: John Dalton (1766-1844) from an engraving by Worthington.
Figure 2 (graphics2.jpg)

We will try and understand these and along the way we will understand more about the atom.

Phosphorescence, X-rays and radiation

Our story starts with the French physicist Antoine Henri Becquerel (Figure 3) and his study at the Ecole Polytechnique in Paris of phosphorescence. Phosphorescence is an effect whereby a material will glow (or emit light) without any perceptible heat. Phosphorescent paints have long been used on the dials of wristwatches to allow the time to be read in the dark (Figure 4).

Figure 3: Portrait of physicist Antoine Henri Becquerel (1852 - 1908).
Figure 3 (graphics3.jpg)
Figure 4: Photograph of the phosphorescent dial of a wristwatch.
Figure 4 (graphics4.jpg)

In 1896 Becquerel was studying phosphorescence, and he believed, incorrectly, that X-ray, which had been recently discovered, were connected to phosphorescence. He wrapped a uranium compound [potassium uranyl sulfate, K4(UO2)6(SO4)3OH10.4H2O] that was known to be phosphorescent in black paper next to a photographic plate. The idea being that if phosphorescence generated X-rays then the photographic paper would show the evidence since while visible light would not travel through the black paper, any X-rays would. However, during a particularly cloudy day in Paris he decided that the light was insufficient for his experiment; he placed the uranium compound and the film in a dark draw. To his surprise the photographic paper still showed spots! His conclusion was that the uranium compound gave off radiation that he thought to be X-rays. His student Marie Curie (Figure 5) gave the phenomenon a name: radioactivity. Curie continued her studies and found many elements gave off radiation. Based upon her studies it is known there are three forms of radiation (Table 1).

Figure 5: Marie Curie (1867-1934). Copyright Academy of Achievement.
Figure 5 (graphics5.jpg)
Table 1: Types of radiation
Name Source Comment
Alpha (α) Helium nucleus, He2+ Easily stopped by paper as well as skin and light clothing
Beta (β) High energy electrons, e- Stopped by a few mm of metal, dense wood or heavy clothing
Gamma (γ) Electromagnetic radiation Only stopped by cm of lead or thick concrete wall

So it was known that atoms emit radiation, i.e., radiation is an atomic not molecular effect. However, a big question remained. When uranium was seen to emit α and β all the particles travel at high speed with lots of energy. So where does this energy come from?

The Law of conservation of energy states, that the total amount of energy in an isolated system remains constant. But the emission of high-energy particles during radiation seems to break the law of conservation of energy. Scientists decided it was necessary to develop an explanation of this observation without the impossible situation of breaking a fundamental law of physics.

The study of radioactive compounds

The first experiment that gave an insight into the source of radiation was a study by Marie Curie of pitchblende, a uranium containing rock. During her study she noticed that pitchblende is radioactively hotter than uranium itself. To take this observation into account she reasoned that pitchblende contains other elements that are more radioactive than uranium. As a consequence she discovered polonium (Po, named after Poland), and radium (Ra, named after radioactivity). But this discovery opened up further questions.

If one wondered about the energy generated by uranium, then the nature of radium is even more of an issue. Radium appears to generate 3 million times more energy than uranium. Thus, 1 g of radium releases 140 cal/h for years. This still requires an answer to the question: where does the energy come from?

It could be suggested that the energy is absorbed from surroundings and converted to radiation, but this would break the 2nd law of thermodynamics, which states the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium. Therefore it is necessary to explain where the energy comes from without breaking this law.

In 1903 Ernest Rutherford (Figure 6) suggested that atoms posses a large amount of energy that is never tapped, and that the energy released by radioactivity was just a small amount of the total energy. He called this large amount of untapped energy atomic energy. It is interesting to note that the writer H. G. Wells (Figure 7) wrote of atomic bombs 40 years before one existed.

Figure 6: Chemist and physicist Ernest Rutherford (1871–1937).
Figure 6 (graphics6.jpg)
Figure 7: Author Herbert George (H. G.) Wells (1866 –1946).
Figure 7 (graphics7.jpg)

While it was possible to say that there is energy coming from the atom. The picture was still not complete until Albert Einstein (Figure 8) developed the concept that energy is mass, and mass is concentrated energy. Based upon Einstein’s concept, it is clear that in order for energy to be released from an atom it must loose mass. Once this statement is made then all the laws (conservation of energy and thermodynamics) are obeyed.

Figure 8: Albert Einstein (1879 – 1955).
Figure 8 (graphics8.jpg)

Radioactive decay

Once the picture of a nuclear atom was discovered it became clear that the energy released must come from the nucleus. It is for this reason that atomic energy became generally known as nuclear energy rather than atomic energy.

Note:

In the UK the government body, founded in 1954, that oversees nuclear energy is called the United Kingdom Atomic Energy Authority (UKAEA).

If mass is lost from the nucleus the following questions must be asked:

  • Where does it go?
  • What happens to the remaining mass?
  • What happens to the nucleus?

In 1900 William Crookes (Figure 9), working at what is now Imperial College, London, was investigating the chemistry of uranium. He found that if uranium was purified it showed very low (or no) radioactivity. This was counter to previous studies. However, the uranium sample was left for some time its radioactivity increased to the levels previously associated with samples of uranium! Soddy and Rutherford showed that thorium behaved the same as uranium, and they proposed that radioactive disintegration occurred. They proposed that uranium is converted to other more radioactive elements and eventually to lead, Equation 1.

Figure 9: William Crookes (1832 – 1919).
Figure 9 (graphics9.jpg)
graphics10.jpg
(1)

Based upon the periodic table (Figure 1) the difference between uranium and lead would suggest that sequential radioactive disintegration would involve 10 elements. However, by 1914 it was found that there were more than 30 steps between U and Pb. The only way to explain this is the concept of isotopes.

Note:

The word isotope is derived from the Greek, meaning “same place”. Isotopes are any of the different types of atoms (nuclides) of the same chemical element, each having a different atomic mass (mass number) but the same atomic number. Once the concept of isotopes are realized then the reason for all extra elements for the radioactive disintegration.

α-decay

Uranium-238 decomposes by the loss of an α particle, Equation 2.

graphics11.jpg
(2)

From this decay it should be noted that:

  1. (1) The sum of the mass numbers on the left and right of Equation 2 is the same.
  2. (2) The sum of the atomic number on the left and right of Equation 2 is the same.

Thus, the reaction does not create or destroys neither protons nor neutrons. Another example of α-decay is as follows:

graphics12.jpg
(3)

Exercise 1

Balance the following equation: graphics13.jpg

Solution

graphics14.jpg

β-decay

If we now understand that radioactive decay via the loss of an α particle results in the decrease of the atomic number by two. In contrast, thorium-234 decomposes to protactinium-234 by the loss of a β particle (high energy electron). Protactinium has an atomic number of 91, while thorium’s atomic number of 90. This observation initially caused consternation.

In the 1920’s people thought that the nucleus contained electrons, based on the concept that if a coin falls out of your pocket then your pocket must have contained a coin. But in the case of radioactive decay via the loss of a β particle this cannot be true since the nucleus only contains protons and neutrons. So, if there are no electrons in the nucleus where does the β particle come from?

If we consider the data in Table 2, note that, mass of a neutron > mass of a proton + mass of an electron.

Table 2: Properties of subatomic particles.
Particle Charge Mass (kg)
Proton +1 1.6726 x 10-27
Neutron 0 1.6749 x 10-27
Electron -1 0.00091 x 10-27

Therefore, we can consider the neutron to be comprised of a proton and an electron and energy:

graphics16.jpg
(4)

Thus, a neutron may decompose to a high energy β particle

graphics17.jpg
(5)

In this process there is no change in mass number, but there is an increase in one atomic number. This may be shown by the decomposition of thorium-234 to protactinium and a β particle.

graphics18.jpg
(6)

Radioactive decay series

With the knowledge that radioactive decay can occur by loss of either an α particle and the decrease in atomic number by two or the loss of a β particle and the increase in atomic number by one, then we can understand the complete radioactive decay series. For example, uranium-238 decomposes to lead-206 via fourteen decay processes (Figure 10), even though they are only ten atomic numbers apart.

Figure 10: Radioactive decay series for uranium-238.
Figure 10 (graphics19.jpg)

Half life

It is interesting to note that there are no stable atoms with an atomic number greater than 83 (bismuth). The more massive atoms only exist because certain isotopes are very long lived and therefore on a normal time scale of years appear stable. Thus, uranium-238, uranium-232 and thorium-232 are very long-lived isotopes, while technetium does not have any stable isotopes.

Since unstable isotopes decay (via either α or β decay) we need a way to express the relative stability of isotopes or the time for radioactive decay. This is expressed by an isotopes half life (t1/2). Empirical observation shows that the number of atoms of a radioactive element that disintegrates per unit time is a constant fraction of the total number of atoms (Figure 11).

Figure 11: Plot of the number of atoms of a radioactive element that disintegrates per unit time as a function of time.
Figure 11 (graphics20.jpg)

Based upon this the time required for 1/2 of the atoms of a radioactive element (X) to decay to a daughter element (Y) is defined as the half life. For example, polonium-216 decays to lead-206 with a half life of 0.16 seconds. Thus, as shown in Figure 12, a sample of eight polonium-216 decays in 0.16 s to mixture of four lead-206 atoms and a residual of four polonium-216. After a further 0.16 s two of the remaining polonium-216 decay to lead-206, then in another 0.16 s one of the two remaining polonium-216 atoms decay to a lead-206 atom (Figure 12). Table 3 shows selected isotopes and their half life.

Figure 12: Schematic representation of the half life in the radioactive decay of polonium-216.
Figure 12 (graphics21.jpg)
Table 3: Half life values for selected isotopes.
Isotope Half life
Ceriuium-142 5 x 1015 years
Radium-226 1590 years
Radon-222 3.82 days
Polonium-216 0.16 s

The rate of radioactive decay can be expressed as a rate constant (k):

graphics22.jpg
(7)

The relation of the initial concentration (C0) and the concentration at time t (Ct) can be expressed by the following:

graphics23.jpg
(8)

It should be noted that

graphics24.jpg
(9)

Using these equations the rate constant (k) can be calculated for a particular isotope. For example, colbolt-60 that is used in cancer therapy, decays to nickel-60 with loss of a β particle with a half life of 5.2 years:

graphics25.jpg
(10)

Using Equation 7,

New eq.jpg
(11)

As an alternative knowing the rate constant we can calculate the fraction or percentage of colbalt-60 isotope that will remain in 15 years. From Equation 8,

graphics28.jpg
(12)
graphics29.jpg
(13)
graphics30.jpg
(14)

The fraction remaining after 15 years is therefore determined as follows:

graphics31.jpg
(15)

Thus, 14% of a sample of colbalt-60 isotope remains after 15 years.

Carbon dating

Since we can determine the amount of an isotopic element that remains after a specific period of time, we can conversely determine the time over which a specific sample has been decaying once we know the rate constant for that element. This is the basis of carbon dating. Carbon dating is a process whereby the age of a material that contains carbon can be determined by comparing the decay rate of that material with that of living material.

Carbon-14 has a half life (t1/2) of 5.73 x 103 years for its decay to nitrogen-14 by the loss of a β particle.

graphics32.jpg
(16)

Using Equation 7,

graphics33.jpg
(17)

In 1947 samples of the Dead Sea Scrolls were analyzed by carbon dating. It was found that the carbon-14 present had an activity of d/min.g (where d = disintegration); by contrast in living material the activity is 14 d/min.g. Thus, using Equation 8,

graphics34.jpg
(18)

Thus,

graphics35.jpg
(19)

From the measurement performed in 1947 the Dead Sea Scrolls were determined to be 2000 years old giving them a date of 53 BC, and confirming their authenticity. This discovery is in contrast to the carbon dating results for the Turin Shroud that was supposed to have wrapped Jesus’ body. Carbon dating has shown that the cloth was made between 1260 and 1390 AD. Thus, the Turin Shroud is clearly a fake having been made over a thousand years after its supposed manufacture.

Transmutation of the elements

Although the conversion of one element to another is the basis of natural radioactive decay, it is also possible to convert one element to another experimentally. The conversion of one element to another is the process of transmutation.

In 1919 Rutherford (Figure 6) converted a stable isotope to an unstable one. By bombarding nitrogen-14 with α particles he created an unstable isotope of oxygen, Equation 20. Transmutation may also be accomplished by bombardment with neutrons.

graphics36.jpg
(20)

Historically, part of Alchemy was the study of methods of creating gold from base metals, such lead. Where the Alchemists failed in this quest, we can now succeed. Thus, bombardment of platinum-198 with a neutron creates an unstable isotope of platinum that undergoes decay to gold-199, Equation 21. Unfortunately, while we may succeed in making gold, the platinum we make it from is actually worth more than the gold making this particular transmutation economically non-viable!

graphics37.jpg
(21)

The mass defect

Given the concept of transmutation one may assume that the addition of a neutron to an element would allow for the synthesis of the element with a atomic number one higher than the starting element. However, it was soon found that sometimes neutrons do not add in to the nucleus. Italian physicist Enrico Fermi (Figure 13) found one such case. In 1934 he thought that the addition of a neutron into uranium should result in the formation of neptunium. However, not only could neptunium not be isolated, but there was a complete mess of products. It was the German chemist Ida Tacke (Figure 14) who suggested that what actually happened was that the interaction of the uranium atom and the neutron resulted in the splitting of the uranium atom. Her proposal was published in 1934 and in it she said, “it is conceivable that when heavy nuclei are bombarded with neutrons these nuclei could break down into several fairly large fragments, which are certainly isotopes of known elements, but not neighbors of the irradiated elements." At first nobody believed her proposal, but they could not explain why neptunium did not appear to be formed, and if it was not what was formed.

Figure 13: Enrico Fermi (1901 –1954).
Figure 13 (graphics38.jpg)
Figure 14: Ida Tacke (1896-1978).
Figure 14 (graphics39.png)

In 1938 Otto Hahn (Figure 15) proposed that the reaction of uranium-238 with a neutron radium and two α particles, however, they found that when the experiment was conducted they isolated barium and that the barium was radioactive. This was a great surprise and was the discovery of nuclear fission. What Hahn had observed was due to traces of uranium-235 that were present in the sample, whose reaction with the neutrons resulted in the splitting of the uranium atom into barium-140 and krypton-93:

graphics40.jpg
(22)

It was later shown that an additional reaction also occurred:

graphics41.jpg
(23)

This discovery shook the scientific world, however, it was something else that was missing that ended up shaking the rest of the world.

Figure 15: Otto Hahn (1879 – 1968).
Figure 15 (graphics42.jpg)

If we consider the law of conservation of mass, which states that the mass of a closed system will remain constant over time, regardless of the processes acting inside the system, and look calculate the mass of each side of Equation 22, we notice that we loose 0.184 amu per reaction. In other words the sum of the masses of the left hand side of Equation 22 is greater than the sum of the masses on the right and side. Where has the mass gone?

The so-called mass defect is defined the difference in the observed atomic mass of an atom and the sum of the masses of the protons, neutrons, and electrons that make up the atom. This mass difference is accounted for by assuming that the mass defect is a measure of the total binding energy (and, hence, the stability) of the nucleus, i.e., the energy that holds the atom together. In other words the mass defect in Equation 22 is accounted for by the release of energy. Taking Einstein relationship between mass and energy, Equation 24, where m is mass, and c is the speed of light.

graphics43.jpg
(24)

and applying it to the mass loss in Equation 22, we can calculate the energy liberated by the reaction of uranium-235 with a neutron.

graphics44.jpg
(25)

An energy of 1.656 x 1016 may not seem much, but consider that 1 g of uranium-235 undergoing the reaction in Equation 22 would provide enough energy to light a typical light bulb for approximately 28,000 years!

Nuclear fission

While the liberation of large amounts of energy was found to result from the splitting of the atom as a consequence of the mass defect, it is another observation that is the key to the successful harnessing of nuclear fission. It should be noted from Equation 22 and Equation 23 that in addition to the splitting of the uranium-235 by a single neutron and the formation of two new atoms, there is also formed additional neutrons. Clearly if these have sufficient energy they can continue and react with another uranium-235 atom. Thus, the reaction of a single uranium-235 atom with a single neutron can generate three additional neutrons. Each of these three neutrons can react with three uranium-235 atoms, from each of which will come a total of nine neutrons. It is this process whereby more neutrons are generated than used in the original reaction that is the basis of a chain reaction (Figure 16).

Figure 16: Representation of a chain reaction and the increase in total number of neutron with each reaction step.
Figure 16 (graphics45.jpg)

Despite the view of a chain reaction in Figure 16, there needs to be a minimum amount of mass to ensure that each neutron is absorbed and hence allow for the chain reaction to continue. This is known as the critical mass, which is defined as the smallest amount of fissile material needed for a sustained nuclear chain reaction.

Once the idea of uranium fission was reported several people started to think about the atomic bomb proposed by H. G. Wells. Hungarian physicist Leó Szilárd (Figure 17) had filed for a patent on the concept of nuclear fission, but his attempt to create a chain reaction using beryllium and indium, was unsuccessful. In 1936, he assigned the chain-reaction patent to the British Admiralty to ensure its secrecy (GB Patent 630726). Szilárd was also the co-holder, with Enrico Fermi, of the patent on the nuclear reactor, and possibly most importantly he drafted a confidential letter to the US President Franklin D. Roosevelt explaining the possibility of nuclear weapons. In order to receive the weight the letter deserved, Szilárd persuaded his friend Albert Einstein to sign the letter. It was this letter that initiated the Manhattan project that led to the creation of the first atomic bombs.

Figure 17: Leó Szilárd (1898 – 1964).
Figure 17 (graphics46.jpg)

Note:

Roosevelt signed the executive order to start the Manhattan project on a Saturday before listening to a baseball game on the radio. If he had waited until Monday the order may have been forgotten since the day he signed the order was 6th December, 1941, and that Sunday was the day of the attack on Pearl Harbor by the Japanese that brought the US into World War II.

The Manhattan Project resulted in the manufacture of two atomic bombs after a test of a plutonium-239 device at Trinity Site, New Mexico, on July 16, 1945. Little Boy (Figure 18), was made from uranium-235, and was used on Hiroshima on 6th August 1945, and Fat Man (Figure 19) that was dropped on Nagasaki on 9th August 1945, were made primarily of plutonium-239. The physical effect of these bombs was dramatic. Temperatures of over 3000 °C were observed over 2 miles from ground zero. To put this into perspective, the boiling point of iron is 2750 °C, while the boiling point of the sand in concrete is 2350 °C.

Figure 18: The Little Boy atomic bomb.
Figure 18 (graphics47.jpg)
Figure 19: The Fat Man atomic bomb.
Figure 19 (graphics48.jpg)

Nuclear energy

Despite the horrific power of the fission bombs it was realized that if the fission reaction was controlled a vast amount of energy could be obtained from relatively small amounts of material.

In a nuclear power plant the chain reaction is controlled through the absorption of the neutrons by control rods made of chemical elements capable of absorbing neutrons without undergoing fission themselves. These elements include silver, indium, cadmium, boron, cobalt, hafnium, or their alloys and compounds. The nuclear material is present in fuel rods between which the control rods moderate the flux of neutrons. The heat generated from the fission process is not used directly to generate electricity, but via a suitable heat exchanger it heats water to steam that in turn drives a turbine-generator (Figure 20).

Figure 20: Schematic diagram of a typical nuclear fission reactor. Copyright Thomson – Brooks/Cole 2004.
Figure 20 (graphics49.jpg)

Despite the large amount of energy potentially generated by nuclear fission there are several potential problems.

Real and imagined danger

Given the similarity of nuclear chemical reactions in a fission reactor and an atom bomb there is a misguided perception that a nuclear power plant could blow up in an atom bomb type explosion. The low amount of uranium-235 used in the fuel rods is below the critical mass to allow for a nuclear explosion. In comparison, a real risk does exist in which a system or component failure causes the reactor core to cease being properly controlled and cooled to the extent that the sealed nuclear fuel assemblies (which contain the uranium or plutonium and highly radioactive fission products) begin to overheat and melt. The resulting meltdown is considered very serious because of the possibility that the reactor containment will be defeated, thus releasing the core's highly radioactive and toxic elements into the atmosphere and environment.

Although several potential meltdowns have been reported, there have only been two actual significant meltdown events. The first was in 1979 at the Three Mile Island Nuclear Generating Station in Dauphin County, Pennsylvania (USA) (Figure 21). It was the most significant accident in the history of the American commercial nuclear power generating industry, resulting in the release of up to 13 million curies of radioactive gases. A failure in the non-nuclear secondary system, followed by a stuck-open relief valve in the primary system, allowed large amounts of reactor coolant to escape. This resulted in the core reaching 2000 °C: very close to a meltdown, however, the reactor was brought under control.

Figure 21: An aerial view of the Three Mile Island Nuclear Generating Station.
Figure 21 (graphics50.jpg)

In contrast when the chain reaction grew out of control Chernobyl Nuclear Power Plant in Ukraine, then part of the Soviet Union, a steam explosion resulted. This was followed by a second chemical explosion from the ignition of generated hydrogen mixing with air, which tore the top from the reactor and its building (Figure 22), and exposed the reactor core. This sent large amounts of radioactive particulate and gaseous debris into the air. The open core also allowed air to contact the super-hot core containing 1,700 tonnes of combustible graphite moderator. The burning graphite moderator increased the emission of radioactive particles, carried by the smoke.

Note:
The initial evidence that a major exhaust of radioactive material had occurred came not from Soviet sources, but from Sweden, where on 27 April workers at the Forsmark Nuclear Power Plant were found to have radioactive particles on their clothes. It was Sweden's search for the source of radioactivity, after they had determined there was no leak at the Swedish plant, which led to the first hint of a serious nuclear problem in the western Soviet Union and, incidentally, triggered evacuation of the towns surrounding Chernobyl over 36 hours after the initial explosions.
Figure 22: The Chernobyl Nuclear Power Plant after the disaster.
Figure 22 (graphics51.jpg)

Available supply of fissionable materials

Uranium is present in the Earth's crust at a concentration of 2 - 4 parts per million, and as such it is more abundant than antimony, tin, cadmium, mercury, or silver, and it is about as abundant as arsenic or molybdenum. However, in nature uranium atoms exist as uranium-238 (99.284%), uranium-235 (0.711%), and a very small amount of uranium-234 (0.0058%). Thus, in order to obtain sufficient uranium-235 a large quantity of uranium ore must be mined, and while nuclear generated electricity may be considered relatively environmentally friendly as compared to the combustion of hydrocarbons, the mining process has a significant environmental cost. It is for this reason that so-called fast-breeder reactors were developed. By wrapping a plutonium-239 core with uranium-238 the neutrons generated could be used to covert abundant uranium-238 to the fissionable plutonium-239:

graphics52.jpg
(26)

Disposal of waste

While it has been estimated that a coal power plant releases 100 times as much radiation as a nuclear power plant of the same wattage, and in 1982 US coal burning released 155 times as much radioactivity into the atmosphere as the Three Mile Island accident, the high level radioactive waste resulting from nuclear power stations does represent a major concern. In particular the long storage times needed (on the order of 10,000 years) posses significant issues.

Cost

Irrespective of the reactor designs, nuclear is a more expensive method of electricity production than coal or natural gas. However, with the high environmental (CO2) impact and pollution concerns of coal burning the balance is swinging to nuclear energy.

Nuclear fusion

The process of nuclear fission and radioactive decay are both associated with the conversion of an atom with a large nucleus to an atom (or atoms) with a smaller nucleus. In the process, mass is lost and energy is produced. However, what happens if two atoms with small nuclei are combined to give a single atom with a larger nucleus? In such a process the nuclei would be fused together, and this process is called nuclear fusion.

One of the simplest fusion processes involves the fusing of two hydrogen-2 (deuterium) atoms:

graphics53.jpg
(27)

The mass of each deuterium atom is 2.0140 amu, while the mass of the resulting helium is 4.0026 amu. The mass defect of the reaction is 0.0254 amu or 0.63% of the original mass. While this percentage of the original mass may not seem much, it should be noted that the mass defect for the conversion of uranium-238 to lead-206 is only 0.026%, and that for splitting uranium-235 is 0.056%. Based upon these comparisons it is clear that fusion of hydrogen produces 24x the energy kg/kg than natural radioactivity and 11x that of nuclear fission.

In addition to being a plentiful source of energy, fusion is actually the most important process in the universe. Since the statement in 1847 of the Law of conservation of energy (the total amount of energy in an isolated system remains constant) scientists had wondered how the sun works. No source of energy was known in the 19th century that could explain the sun. Based upon the age of the earth it is known that the sun is 4,550,000,000 years old, and it was marveled at the continued source of energy over that span of time. By the 1920s nuclear energy was defined as being the most powerful source of energy, and British astrophysicist Arthur Eddington (Figure 23) suggested that the sun’s energy arises from the fusion of hydrogen into helium.

Figure 23: Arthur Stanley Eddington (1882 –1944).
Figure 23 (graphics54.jpg)

In 1929 Henry Norris Russell (Figure 24) studied the spectrum of the sun. Based upon his studies he calculated the composition of the sun to be 90% hydrogen, 9%helium, and 1% of all other elements up to iron (but nothing of higher atomic number). Given the composition it became clear that the only reaction possible to account for the sun’s energy was the fusion of hydrogen. In 1938 Hans Bethe (Figure 25) demonstrated a model for how the sun worked. The energy source is the fusion of hydrogen to give helium (Figure 26), while in massive starts the presence of heavier elements such as carbon, oxygen, nitrogen, neon, silicon, and iron are the result of the fusion of helium.

Figure 24: Henry Norris Russell (1877 - 1957).
Figure 24 (graphics55.jpg)
Figure 25: Hans Albrecht Bethe (1906 - 2005).
Figure 25 (graphics56.jpg)
Figure 26: The proton-proton chain dominates in stars the size of the Sun or smaller.
Figure 26 (graphics57.jpg)

It is now known that the fusion reaction present in stars requires a critical mass of hydrogen, when a star starts to run out of fuel, they cannot support themselves and all the hydrogen collapses to the center, resulting in a super nova explosion. Much of the material is spewed out while the remainder collapses to a tiny neutron star or black hole. The material thrown out by a super nova includes elements heavier than iron, i.e., uranium and heavier. Eventually new stars are formed out of the interstellar gas from the super nova. But these second generation stars have planets. Our sun is such a second-generation star. Earth and the contents of the planet are formed from the heavier elements formed in the first explosion.

Why is it that uranium, thorium and other radioactive elements undergo radioactive decay, but hydrogen does not undergo spontaneous fusion? The reason for this difference is that any change that uranium undergoes occurs inside a nucleus that is already formed, while fusion requires that two nuclei must come together. This process results in extremely large repulsive forces. So why does it happen in a star? The temperatures inside a star approach 15,000,000 °C, while the high pressure results in a density of approximately 160 g/cm3 which for comparison is 8x that of gold. Under these conditions the nuclei are free to move in a sea of electrons. Nuclear fusion takes place in the core of the sun where the density is at the highest but an explosion does not result due to the extreme gravity of the sun: 333,000x that of Earth.

Research into controlled fusion, with the aim of producing fusion power for the production of electricity, has been conducted for over 50 years. It has been accompanied by extreme scientific and technological difficulties, but has resulted in some limited progress. At present, break-even (self-sustaining) controlled fusion reactions have not been demonstrated, however, work continues since the fuel for such a fusion reaction is hydrogen (in its compounds including water) is the 3rd most abundant element on the Earth.

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'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own (?)

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks