Nuclear power and weapons depend on the fission process. Nuclear fission is the splitting of certain of the heaviest nuclei after they absorb a neutron. Neutrons, protons and electrons are the fundamental building blocks of atoms. Atoms have a tiny, heavy nucleus containing protons and neutrons with the tiny, light electrons whirling about the nucleus in precisely prescribed orbits. Protons, neutrons and electrons are about the same size, but the electron is about 2,000 times lighter than a proton or a neutron, which are equally heavy.

The electron orbits are l0,000 times bigger than the size of the nucleus, which is only slightly larger than a proton or a neutron. The number of protons in a nucleus is equal to the number of electrons in orbit in an atom. The number of electrons in orbit determine what the chemical properties of each atom are. Atoms with equal numbers of electrons in orbit are atoms of the same element, e.g. hydrogen = l electron; helium = 2; lithium = 3; carbon = 6; nitrogen = 7; oxygen = 8; uranium = 92; plutonium = 94; etc. The number of electrons in an atom (or the number of protons) is called the "atomic number" of an element.

The number of protons and neutrons in a nucleus determine the nuclear properties of each nucleus. The electrons in orbit do not affect the nuclear properties. Nuclei with equal numbers of protons (i.e., nuclei of a given element) can have different numbers of neutrons, and each number of neutrons would make a different isotope of a given element. For example, most naturally occurring hydrogen has one electron in orbit around the nucleus with one proton; "heavy hydrogen" is called deuterium and has one electron in orbit around the nucleus with one neutron to go with the proton; Tritium, the heaviest "isotope" of hydrogen (hydrogen, deuterium, and tritium are all "isotopes" of hydrogen) has electron orbiting a nucleus with neutrons and proton.

All elements have several "isotopes" with the heavier isotopes radioactive. Tritium is radioactive with a half-life (the time it takes for half of the original amount of tritium to decay) of l4 years. The heavier elements have many isotopes, and all of the isotopes of the heaviest elements are radioactive.

Uranium has 92 electrons orbiting around a nucleus containing 92 protons and l46 neutrons in U-238 (by far the most abundant uranium isotope), and l43 neutrons in U-235. U-235 is extremely interesting because it undergoes "fission" when it absorbs a neutron. "Fission" means that the nucleus splits into two daughter nuclei like Strontium 90 (Sr-90) and ?-l44, plus two neutrons, or CS-l37 and ?-l08 plus one neutron, and lots of energy! If the fission process didn't yield any neutrons, there would be no "chain reaction." But each fission does yield neutrons, and those neutrons can go on and strike other U-235 nuclei, cause them to fission and split into two different daughters plus one to two neutrons, and lots of energy. This "chain reaction" will continue as long as an average of one neutron per fission is absorbed by a U-235 nucleus. This is "guaranteed" if there is a "critical mass" and nothing around that absorbs neutrons. "Critical mass" is about 45 lbs. of U-235. U-235 is about 0.7% (almost one per cent) of naturally-occurring uranium; the rest of the 99% is U-238.

In order to make a bomb, the U-235/U-238 ratio must be increased from 0.7% to about 80% or more; but a reactor can operate with natural mix uranium, as in the case of Chernobyl, or like most U.S. nuclear reactors there is 2% to 4% U-235 (called slightly enriched) so the reactor doesn't have to be so big.

One more background topic (plutonium), and we'll get to reactor operations, meltdowns, safety systems, radiation releases, etc. Plutonium is virtually undetectable in nature, but its most easily synthecized isotope (Pu-239) is readily fissionable (critical mass of Pu-239 is ll lbs!). Too, Pu-239 is readily made by bombarding U-238 with slow-moving (thermal) neutrons. A normally-operating l,000 Megawatt reactor (Chernobyl and most U.S. nuclear power plants) will produce about 430 lbs. Pu-239 during one year of operation.

One critical mass of U-235 or Pu-239 will produce a Hiroshima-sized explosion (about l5,000 tons of TNT equivalent). Since it takes about l0,000,000 degrees fahrenheit to ignite an H-bomb (l,000,000 tons TNT equivalent), A-bombs and high-power lasers are the only two ways that humans can do it. (The usual way is an A-bomb.)

Now all you have to do to make this chain reaction available to generate power is to slow down the reaction rate -- slow down the neutrons. This is usually done with graphite or heavy hydrogen. To get the critical mass to release its energy, it must be in a sphere one yard in radius. So, all that we have to do to have a power plant is to put one or more critical masses within a 6-foot diameter sphere with graphite everywhere and we have a power plant.

But that power plant would release all of its energy too fast for anything but a slow bomb. So we put some "control rods" into our core (the many critical masses) so we can absorb too many of the neutrons for the chain reaction to occur. Boron (an element) absorbs neutrons with gusto. So we make a "pile" of graphite or "heavy" water with uranium rods through it so that you have several critical masses in each six-foot sphere, and enough boron control rods to guarantee that the chain reaction can't go with the control rods in the "in" (STOP) position. We then withdraw the boron control rods slowly until the chain reaction causes increasing heating of the core.

Now we need some kind of heat transfer system so that we can remove the continually generated heat from the core and use it (generate electricity, drive a ship, whatever). We usually boil water and use the steam to drive a turbine connected to an electric generator. The big U.S. reactors (about l,000 megawatts) have about l00 tons of 2%-4% U-235/U-238 uranium in the core. That's about three tons of U-235, and that's about l20 critical masses of U-235 in a freshly-loaded reactor. Each l,000 Megawatt nuclear reactor generates three Hiroshima-size A-bombs worth of energy and radioactive daughters (nuclear wastes) per day!

The water that cools the reactor and drives the turbines is called primary cooling water. If the flow of primary coolant is ever slowed down or stopped, the reactor is "scrammed" (the boron control rods are put in immediately to stop the chain reactions from continuing the heating of the core), and an auxilliary cooling system is activated in a few seconds. Although the fission chain reaction is stopped, the residual radioactivity of the accumulated fission daughters continue to heat the core. If the core is not cooled quickly the metal sheath on the uranium rods reacts with the water in the core and generates a large bubble of hydrogen gas which explodes (Chernobyl and Three Mile Island), and wrecks all kinds of things.

Then if the reactor has been operating for more than six months and the core doesn't receive adequate cooling, the core will start to melt. Now the reactor is out of control. The melting rearranges the careful geometrical design of uranium, graphite and boron such that the chain reactions start again and further increase the uncontrolled heating. Again, unless this is stopped quickly, the entire core could become involved in the melting, meltdown, and subsequent "China Syndrome" consequences.

In addition to the required auxilliary cooling capacity (which was illegally non-operative in the TMI disaster), all U.S. nuclear reactors must have an emergency core cooling system (ECCS) to insure that a loss of coolant accident (LOCA) as described above does not get underway in the early phases of an accidental reactor shutdown. In a reactor that is on line for three years, the residual radioactivity is capable of melting the core for one month after a shutdown! None of the ECCS's has the water to keep a mature core cool for more than a few days, much less one month. The ECCS at Chernobyl lasted only three minutes.

We lucked out at TMI because that new reactor had just started up three months before the interruption in primary coolant flow (with no auxilliary cooling pumps available to pick up the cooling load), and there wasn't enough residual radioactivity in the core to melt the fuel. Too, one of the reactor engineers used a lawn-watering faucet and hose to keep the hot core cool and pre-empted further hydrogen explosions and full deterioration of the core. Several lucky things kept TMI from being as bad as Chernobyl.


The attached graph shows the total radiation-induced deaths (23) as a function of time from the Chernobyl disaster that have been reported by the USSR as of May 29, l986, with space for you to continue marking the graph.

Typically, after a large-scale release of radioactivity into the public such as Chernobyl, there are few, if any, immediate deaths. Then about two weeks later there is an abrupt increase in radiation-induced deaths that keeps increasing for about one week (see the graph). There were (about) l5 deaths reported in the first death pulse at Chernobyl, even after bone marrow transplants for 38 of the victims exposed to the highest levels of radiation.

Then, about one month after the severe exposure to radioactivity, the second death pulse starts (see graph), and this second pulse of death is much larger (usually three to ten times larger) because many more people are exposed to the lower (but still deadly) levels of radioactive exposure. Depending on how well the USSR continues to report the deaths from Chernobyl, we'll see the second death pulse start to rise any time between May 25 and May 3l (it started on May 25). Since the Russians didn't evacuate anyone for 36 hours, the death toll could be up to l,000 in this second pulse. In this second pulse, the deaths go on for one or two months, so we may not have the whole picture until late July, l986.

Then, three or four years after the deadly release of radioactivity, the radiation-induced cancer deaths start, and continue to kill people for the next 50 years or so. First leukemias, then pancreas and lung cancers and a sequence of stomach, liver, kidney, heart, etc. cancers kill many more people than died in the combined first and second death pulses. The preliminary estimates of the number of radiation-induced cancers resulting from the release of radioactivity from Chernobyl range from 50,000 to l00,000 during the coming decades. Even with improving medical technologies for dealing with cancers, conservative estimates of the number of deaths resulting from these dreaded cancers range from 5,000 to l0,000. What a price to pay for 4% of the electricity for the USSR! And the havoc doesn't stop there. Genetic defects will show up, not so much in the children, but in the grandchildren of the survivors of exposure.

Such are the horrors of nuclear technology. Until we know how to isolate radioactive wastes from humans, we have no business operating or building nuclear reactors. End of primer. Good luck. I'm available for interrogation.

Charles Hyder, Ph.D. Lafayette Park Mail: PO Box 272l7, DC 20038