A PRIMER ON HOW NUCLEAR REACTORS WORK AND KILL
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 COSTS OF RADIATION RELEASES INTO THE PUBLIC
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