Chemistry 1020—Lecture 20—Notes

We ended last lecture pointing out the process which scientists were using to create new elements heavier than Uranium:

This series of reactions produces neptunium and plutonium. In a search for other transuranium elements by this process in 1938, German scientists Otto Hahn and Fritz Strassman found what they identified as barium as a product, but they questioned the identification because it was so unexpected. They told a former colleague, Lisa Meitner, about the observation. She had fled Germany to Sweden, and she told her nephew, Otto Frisch, who repeated the experiment at the Niels Bohr institute in Copenhagen. They concluded that the unstable nucleus formed when a neutron was absorbed had split in two, a process designated nuclear fission.

It turns out it was not the uranium-239 undergoing fission, but the small amount of uranium-235 isotope that underwent reactions such as:

Notice the unstable "compound" nucleus breaks apart into two intermediate sized nuclei, with some extra neutrons produced. A large variety of "fission products" is produced, not just these two. Most of them do not have the most stable balance of neutrons and protons and hence are radioactive, undergoing further alpha or beta decay.

While the mass numbers balance in the above equation, accurate measurement of the masses indicate that the masses of neutrons and protons in nuclei are actually less than the sum of the individual neutrons and protons. In the formation of the nucleus, some of the mass of the particles has been converted into the energy, which corresponds to the energy required to separate them. This is analogous to the energy released when chemical bonds are formed, except that the magnitude is much, much greater.

The large elements such as uranium have a bit less binding energy per particle than do the intermediate-sized elements.

To understand how this works, consider the formation of the helium nucleus from its proton and neutron components (just a hypothetical reaction in this case):

This process is accompanied by a decrease in mass of:

4.03190-4.00150 = 0.03040 amu or g/mol

This lost mass is converted to the binding energy that holds the particles together in the nucleus. The relationship between the mass lost and energy produced is given by Einstein’s famous equations:

E = mc²

where m is expressed in kg and c is the velocity of light (3.00x10⁸ m/s).

To compare the size of the binding energies from one atom to the next, one should express the amount of mass lost as the change in mass per nucleon (per proton and neutron). In this case, that change is 0.03040/4 = .0076 g/mol of nucleons.

This mass loss corresponds to an energy change calculated as follows:

     E = 0.0076 g/mol x 10^-3 kg/g (3.00 x 10⁸ m/s)²

        = 6.84 x 10¹¹ kg-m²/s² (or Joules)

        = 6.84 x 10⁸ kJ per mole of nucleon.

This loss of mass is called the mass defect. The energy is the binding energy of the particles in the nucleus; i.e. it would take this much energy to break the particles apart.

If we plot the mass defect (i.e. the loss of mass per gram of nucleon) for each of the elements, we get the following curve:

Notice that the mass defect reaches a maximum at intermediate mass numbers equivalent to Fe, and then decreases somewhat as we go to heavier elements. Therefore the fission of a uranium nucleus (mass defect of about .0082 /g of nucleon) to intermediate sized nuclei (mass defects of about 0.0092/g of nucleon) represents a loss of mass of about 0.001 g/g of nucleon, or an energy change of:

         E = 0.001 g x 10^-3 g/kg x (3.00 x 10⁸ m/s)²

           = 9 x 10¹⁰ J or 9 x 10⁷ kJ per gram of uranium.

This would correspond to 9 x 10¹⁰ kJ/kg or 4 x 10¹⁰ kJ/pound. The book points out this is about the energy one would obtain from 33,000 tons of exploding TNT or 3300 tons of burning coal.

Once scientists recognized the fission process for what it was, they recognized the enormous potential for obtaining energy from the process.

Notice that each fission reaction produces more than one neutron. If at least one of these neutrons could produce another productive fission reaction, then a chain reaction can be sustained. If more than one neutron from each fission produces an additional fission reaction, then one can have a rapidly escalating chain reaction that can produce a nuclear explosion. (See Figure 8.3)

Let’s say a bit about the explosion possibility first. Since most of the neutrons produced by one fission reaction are likely to be dispersed without a productive collision with a second fissionable nucleus, it is necessary to confine in a small space a critical mass of fissionable material in order to get an explosive chain reaction. (The critical mass of U-235 is about 15 kg). One of the goals of the Manhattan Project during the war was to work out the details of getting this critical mass produced under the appropriate conditions.

Uranium-235, the fissionable material, constitutes only a small proportion of uranium-238 samples, so one of the first goals of the project was to purify the 235 isotope from the 238 one. But it was also recognized that the transmutation process cited earlier can lead to production of plutonium-239, which is also fissionable. It was being produced as a byproduct of the reactions taking place in nuclear reactors, which were designed to produce a sustained chain reaction. The first atomic weapon exploded over Hiroshima contained uranium-235. Subsequent ones were constructed with plutonium-239.

But at the same time weapons were being developed, work was underway on creating a controlled nuclear reaction. In this situation, sufficient fuel was brought in proximity to create a chain reaction, but the fuel was interspersed with rods of neutron absorbing material called control rods. The control rods could be adjusted to absorb just enough neutrons so that there is a balance between neutron production and neutron utilization for fission. In this case, the energy is released as heat, which is absorbed by a coolant passing over the rods.

Note the diagram of Figure 8.5 in which a primary coolant is used to transfer heat to a secondary coolant which is water in a steam generator. The steam is used to drive a turbine to generate electricity.

One consequence of some reactor designs is that in addition to producing energy from uranium-235 fuel, some plutonium-239 is generated. This plutonium can be purified from the by-products of the process and be used as fuel because it is fissionable. Such reactors are called breeder reactors.

So what are the problems with this seemingly magic source of energy? Let’s consider a list of the major concerns:

Will a reactor go critical and produce a nuclear explosion? This concern is probably unfounded. The design of the reactors is such that one would not get a critical mass enough to produce an explosion. The two major nuclear accidents that we recall were not cases of this almost happening, but cases of things getting out of control enough so that some radioactive products were released into the environment. The first of these was at Three Mile Island in Pennsylvania in 1979. Some coolant was lost, and a partial "meltdown" occurred. There was a little release of radioactive gases, but not a serious amount. The most serious accident to date occurred in Chernobyl in 1986. There the flow of cooling water was interrupted, things overheated, and the graphite control rods caught fire. There was a chemical, not nuclear, explosion, but it resulted in the release of enormous amounts of radioactive materials that spread not only in the immediate vicinity but in the atmosphere to a much broader area.

Proponents of nuclear energy maintain that the safety features built in to modern plants would prevent such a disaster as Chernobyl. Your book describes a proposed nuclear plant at Seabrook, on the coast of New Hampshire. Given the sophisticated safety features and oversight in construction of this plant, costs escalated from an initial estimate of $973 million in 1979, to $6.45 billion till its testing 10 years later. It was a very costly project in both time and money.

In considering risks of nuclear plants, one must keep in mind that all forms of energy generation carry some risk. Consider the deaths in coal mine explosions and from miners developing "black lung disease". Table 8.6 summarizes some of these comparative risks between coal and nuclear powered electricity generation. Even hydroelectric power pose the threat of drowning deaths associated with the water reservoirs. We will say a bit more about this source of energy in the next chapter.

While it is very unlikely that a reactor would go critical, and even if one could prevent the accidental release of radioactive products (burning coal also releases some radioactivity into the atmosphere), there is a problem of disposing of the waste byproducts of the fission reaction. You may have read in the paper a few days ago of the first movement of major stores of radioactive byproduct material into a reservoir dug into a salt mine, a region that is believed to be geologically very stable.

The problem, of course, is that many of these products decay very slowly and, once produce, will be with us for many years into the future.

The rate of decay of radioactive isotopes can be measured in terms of a half life. It decays in such a way that the rate depends on the amount of material. If the half life is one year, say, after one year one half of the original isotope is left. After a second year one-half of the one-half (or one-fourth) is left. After the third year, one-eighth is left, etc. I mentioned in the beginning that the half-life of uranium-238 is 4.5 x 10⁹ years, and since this is the age of the earth, about half of the uranium present when the earth solidified is still with us.

Hazards of radioactivity

The problem with radioactive elements is that the decay particles can cause damage to living cells. They collide with various molecules in cells and break chemical bonds. They can split water to form hydroxyl radicals, which are very reactive and also can cause damage. When damage occurs in our DNA, the chemical substance making up our genes, then permanent mutations can result in addition to cell death, and these mutations can cause some cells to lose their ability to control their growth, leading to cancerous cells. Damage to cells of our immune system can lower our defenses to infection.

The biological effects of radiation are measured in a unit called the rad, short for radiation absorbed dose. One rad is the absorption of 0.01 joule of radiant energy per kilogram of tissue. Some forms of radiation are more damaging to tissues than others, however, so another unit is used to take this difference into account, the rem or roentgen equivalent man.

number of rems = n x number of rads

In this relation, n is a factor characteristic of the type of radiation. Alpha particles are very damaging, and have an n value of 10, while beta particles are less harmful and have an n value of 1.

We are exposed to small levels of radiation every day from a variety of sources. Doses below 25 rem have no observable effect, while larger doses begin to effect our immune system and white blood cell count. Table 8.2 summarizes effects of single doses. Table 8.3 is an inventory to use to check your annual exposure to every day sources. And Table 8.4 shows a typical annual exposure in the United States.

The radioactive products from a nuclear reactor have different half-lives, from milliseconds to millions of years. Those with the shortest half lives give off the highest rate of particles, and are the most dangerous and hardest to work with. Current practice is to store the material on-site for a number of years until the more active isotopes have decayed, then move it to permanent storage for the longer lived isotopes. The permanent repository must be one that will be geologically stable for thousands of years and will not allow leakage of material into surrounding waters.