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Nuclear Chemistry and the Community
Dr. Frank Settle

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discovery_neutron
playing_neutron
fission
PU239
chain_react
Nuclear Fuel Cycle
Uranium Production
Uranium Enrichment
Nuclear Reactors
Reprocessing Spent Fuel

Nuclear Weapons
Effects of Radiation
radiological terrorism
Nuclear Proliferation
problems
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  Nuclear Science in:
A complete review and case study of nuclear chemistry,
Two 45 minute classes
, or
A quick review for interested learners (below)

  Goals: In this session you will be introduced to:

einstain
Energy and Matter:  Nuclear science began with Albert Einstein who recognized that matter and energy were equivalent.  We have all heard the equation:
E=mc2
This was Einstein's understanding at the beginning of the last century.  Energy - the ability to provide heat or do work, had an equivalency with matter - the mass of the physical universe.

Origins of Nuclear Energy:  We know that atoms - the fundamental particles of matter - consist of a small, dense nucleus surrounded by lighter, charged particles or waves called electrons.

You may remember that the transformations we see in the world; the burning of fuels, the growth of plants, the rusting of iron, are all results of the movement of electrons with a negative charge, that are the source of the making and breaking of chemical bonds.  But in this scheme, matter is conserved -- there is no loss or gain in the mass of the chemical species involved.

The atomic nucleus is seldom affected by the chemistry going on around it.  The nucleus is composed of protons - positively charged charged particles with a mass of 1 awu (atomic weight units), and neutrons - similar particles of the same mass but with no charge.

Soon after the discovery of the neutron by Chadwick in 1932, scientists began to use neutrons as chemical bullets - firing them at atoms of other elements.  The element uranium, with an atomic number of 92, is mined in various locations around the world (near Moab, UT, for example).  The naturally occurring U consists of 99.3% of the isotope with atomic weight 238 and 0.7% of the isotope weighing 235.

When neutrons are fired at Uranium, an unusual thing happened that was named nuclear fission:

    235U92 1n0    -->   fission products + (about 2.5)1n0   +   Energy

Careful observation of the fission products - a mixture of atomic nuclei of other, lower molecular weight elements and neutrons, showed that the mass of the fission products was LESS than the mass of the uranium!  Einstein's proposal that mass and energy are interconvertable was confirmed.  The loss of mass in the fission products resulted in the production of energy - energy that perhaps was useful.

Nuclear Energy for Power and Weapons:  The energy released by fission excited the European scientists who discovered the phenomenon.  And it troubled others who recognized from the simple equation, above, that a powerful and rapid conversion of matter to energy could result from the fission phenomenon. 

Look at the equation.  Every neutron produces about 2.5 neutrons by reaction with 235U92 in addition to the energy.  Theoretically, if these newly produced neutrons could be directed back to cause fission in other U nuclei, and release energy we might conceive of a controllable, sustaining source of energy.  Such a controllable "chain reaction" was first demonstrated in 1942 at the University of Chicago by the Italian scientist, Enrico Fermi.  The uranium "fuel" was moderated in the chain reaction by neutron absorbers that could be added and removed to make certain the reaction didn't "run away" and release huge amounts of energy from fission caused by too many neutrons.

The Hungarian scientist, Leo Szilard, an expatriate in the United States, alerted Albert Einstein just prior to World War II that the fission chemistry held the possibility of weapons of mass destruction, far beyond anything heretofore imagined.

Under some conditions, the chain reaction might be condensed into a millisecond burst of fission, unleashing enormous energy.  Einstein's communications with President Franklin Roosevelt led to the "Manhattan Project" that demonstrated weapons feasibility and led to the use of two weapons on Japan.

The science was critical.  Only the 235U92 is fissionable at the rates necessary for a weapon- the vastly more common 238U92 is not.  So complicated separation plants were built to make the separation.  (In the 21st century, it is the presence of these uranium "gas separation" plants that is one mark of the presence of nuclear capabilities.)

But the 238U92  plays a role in the fission process.  Although it does not itself fission, the isotope reacts with neutrons and undergoes a series of nuclear transformations that occur rapidly resulting in the production of a new element, plutonium (Pu).  In the equations, ß-1 is an electron, or in the language of the nuclear scientists, a "beta" particle:

    238U92  1n0    -->  239U92 

    239U92       -->  239Np93  +   ß-1       t1/2=23.5 min.

    239Np93    --->  239Pu94+  ß-1        t1/2=2.33 days

The plutonium thus produced, is itself fissionable and the scientists of the Manhattan Project isolated Pu from the neutron bombardment of  238U92  and demonstrated it as a source for nuclear weapons. 

In the decades since World War II, plutonium has been the source for most fissionable nuclear devices. 

Radiation:  If we study the periodic table of the elements we see that many types of nucleii of the heavier elements degrade over time periods that we can measure - they are unstable.  As the nucleus degrades, energy and matter is released in the form of radiation.  We have seen the properties of one of these species - the neutron; uncharged, with an atomic weight of one.  Another particle, the alpha particle, a 2+   , has a positive charge and a weight of 4 awu.  High energy electrons called beta particles ( b - ) are emitted in some radioactive decays.   Other transitions release massless energy in the form of gamma ( g ) radiation.  High energy radiation, particularly g radiation causes damaging chemical transformations in living organisms.

chernobyl


Most controlled uses of nuclear energy protect workers and the environment against the natural radioactive decay and the additional hazards caused in the handling of materials.  But the hazards of radiation are magnified many orders of magnitude in uncontrolled release of energy from weapons or uncontrolled nuclear reactions such as that at Chernobyl in Ukraine in April, 1986.

Spent Fuel and Nuclear Weapons:  The nations of the world use nuclear power derived from uranium enriched to about 4% 235U92 .  In the "fuel rods", as the uranium is fissioned and the energy is drawn from the fission reaction, some neutrons react with the bulk of the uranium, the nonfissionable 238U92 . This process produces a small amount of Pu in the spent fuel rods.  These rods, then, become a potential source for scavenging the minute amounts of Pu produced in an attempt to make weapons.  Control of spent fuel rods and their safe disposable thus becomes a worldwide concern in the efforts to limit the proliferation of nuclear weapons.

 

 

 

 

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