Lecture XVI
Physics 367
Safety and Nuclear Energy
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Three Mile Island
Three Mile Island in 1999.
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Three Mile Island 
900 MWe Pressurized Water Reactor.
High-pressure and high-temperature Water isolated in a closed
reactor loop heats water in the secondary system to produce steam
to turn a turbine.
The apparatus used to transfer this energy is called a
heat exchanger. These devices use a lot of
water to circulate to carry thermal energy from one place
within the reactor to another.
In case of emergency shutdowns [SCRAMs], the reactors built
in the United States have an emergency core cooling system [ECCS],
which involves pumping large amounts of water into
a reactor core that is heating too fast.
designs of that era had
(1)lots of pumps, valves, and water pipes
in the nuclear reactor operated from a control room and
(2)vast arrays of control dials and other indicators showing
conditions in the plant in the control room.
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The sequence of the Three Mile Island accident on
28 March 1979 occurred this way [refer to the discussion in Ch. 18 of
Energy].
During routine maintenance of the
steam generator side of the heat exchanger two
weeks before the accident, 2 valves in auxiliary feedwater
pumps were manually closed and then
inadvertently left closed.
The incident
began when a
feedwater pump to the heat exchanger failed.
The loss of feedwater caused the primary
system to overheat causing the primary system pressure to increase.
The backup systems did not begin immediately [by design], and a
SCRAM occurred. The pressure relief valve opened to release
excess pressure buildup. All this happened as designed by
safety engineers.
The three backup feedwater pumps,
however, were disabled by the closed valves, and so
did not kick in after the built-in
15 second delay.
The pressure relief valve stayed
open long enough to lower the pressure; at that point the relief
valve should have closed. The valve stuck, and
failed to close. This was
not noticed by the operators for
2 1/2 hours.
The emergency core cooling system
turned on by design as the pressure
dropped too low, but the operators thought that the
ECCS had turned on by mistake and manually turned the ECCS
off. This leads to an uncovered core as water boils
away and escapes through the stuck relief valve.
The operators saw the core cooling
still as the steam escapes, so they mistakenly turned off
the water pumps to the core, thinking that would push the
temperature back up. This made the accident
much worse. The core was uncovered for 13.5 hours.
The liquid released by the stuck
valve ruptured a holding tank seal, ultimately spilling 400,000 gallons
of radioactive water onto containment vessel floor!.
A bubble of hydrogen from water
that endothermically broke up was in the core during the
accident. The hydrogen could have exploded--chemically--but
did not.
The final toll:
15-30% of the core was
uncovered
45% of the core melted
70% of the core was damaged
20 tonnes of debris fell to
the bottom of the reactor
large amount of
radioactivity released
[1.1 x 1012 Bq]
Summary: human error was the main
cause; mechanical failure was important.
Some important lessons about the design of control rooms were learned by
power plant managers.
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Chernobyl
Chernobyl in 1997.
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The Chernobyl disaster was much greater than at Three Mile
Island. Here, too, human error was the cause.
Part of the problem was in the design of the Russian RBMK reactors,
especially the effective lack of a containment vessel and the choice
that graphite was used as the moderator.
What started as a reasonable test
of the use of turbine generators as source of emergency power for the
computers essential to control turned into a nightmare as repeated
operator mistakes amplified the design flaws. No one will build this
design again. Rehearsing the scenario is unfruitful.
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The sequence of events during the accident on 25
April 1986 occurred this way [also refer to the discussion in
Ch. 18 of Energy].
The operators wanted to
test the reactor by turning the power down slowly. They
reduced power to 1/2.
The power grid was short of energy
relative to demand, so the shutdown was delayed for many hours.
When the shutdown was recommenced, the power level plunged. This
wouldn't do for the experiment, so the
engineers switched off the turbine
to stop power drain but this also turned off the ECCS! This
was in direct violation of regulations.
The shutdown was also stopped, a
violation of operating protocol.
The power still fell, so all
control rods were pulled. Recall that control
rods are inserted into the reactor to absorb neutrons.
The drop slowed, and the control
rods were pulled out farther.
The operator blocked the
emergency valves to prevent the water
from carrying away the heat. This was truly crazy.
A few seconds later...BOOM!
A graphite fire broke out, and
continued. It carried radioactivity upward by convection.
The final toll:
31 people died in the accident.
Total destruction of the
reactor.
Radioactivity released over
northern hemisphere
[2 x 1017 Bq].
Recommendations for change
The worst problems in nuclear power facilities are in the machinery,
structure, and piping. These can generally be dealt with before they will
cause problems by intelligent design, retrofitting when problems with a
design are discovered, and maintenance. Human errors and unforeseen
catastrophes cannot be planned for directly. The
role of human error is clear in both accidents.
We were our own worst enemy
What was done?
The control rooms of new
plants were redesigned to impart information and reduce the
information overload for the operators. This is one good consequence
of the accidents.
Computers will be used much more in
the control rooms, decreasing the number of dials the operators must
watch, and allowing them access to relevant information about a
problem.
The role of redundancy was clear
before the accidents, but the continued training of operators was
neglected. Also, there were no mechanisms to allow operators to
learn from accidents or near accidents in other reactors. Such an
information exchange is now in place.
Should we panic?
The important thing to do is to see what the consequences actually
are in comparison to other risks in life. No
option is without risk. People use cars despite the fact that
there are over
-50,000 deaths per year in the U.S. alone from auto accidents.
-Falls account for 18,000 deaths.
-Fires for 9,000.
-Drowning for 6,000.
-Other accidents for another 30,000.
-Cancer alone claims 300,000 per year.
Estimates of mortality from coal from all causes range around 6,000 to
10,000--about 30--50 deaths overall for each of the 200 coal plants in
the U.S. In nuclear, perhaps there are 200 to 800 or so deaths per year,
mostly in construction and mining. The net mortality from TMI was 0; from
Chernobyl 31.
Nuclear Fusion
The binding energy curve shows that nucleons can fall deeper into
the nuclear ``well'' in two circumstances--when a large nucleus
breaks up or fissions into two smaller nuclei, and when two nuclei
smaller in mass than 
combine into a larger nucleus with mass that is still smaller than that
of .
Fusion can only occur when the nucleons get close
enough together so that the attractive strong nuclear force
is stronger than the repulsive electric force between the protons
in the nuclei that combine. This means that the nucleons must get within
about 1 fm of each other, because the strong
nuclear force does not extend beyond this distance.
To get a nuclear fusion reaction to start it is necessary to
give the nuclei enough energy to come close
together. The energy provided to start the fusion reaction going
is called the nuclear activation energy.
There are several ways that have been considered to achieve this
necessary closeness. Thermonuclear fusion works by giving the
atoms of material to be fused a very high kinetic
energy by increasing the temperature. The sun and other
stars are examples of thermonuclear fusion. The temperature in
the sun's interior is about 10 MK. When temperatures are this
high, there is enough energy to rip off all the electrons from the
atoms and produce a plasma. Plasma is ionized matter containing
equal amounts of positive and negative charges. Most material in the
universe is plasma.
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A schematic of a Tokamak. |
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On Earth, plasma must be contained within a
small region to allow fusion to occur. It is necessary to keep
the plasma from touching any walls of a container. Even if the
container were made of cardboard rather than metal, it would not be
vaporized by contact. The plasma would lose all its energy instead.
Magnetic fields are used for this purpose in
Tokamaks.
Two other published methods involve focusing of energy on small
pellets called ``microballoons'' containing the material to be fused.
In laser fusion, the beam of a powerful laser is broken into
32 parts, and all are focused simultaneously on a pellet from all sides.
This causes an implosion of the microballoon that pushes the nuclei
close enough together to cause fusion. In inertial fusion,
accelerators make beams of particles. As in laser fusion, the beams
hit the microballoon from all sides and initiate fusion through the
collapse of the microballoon, which pushes the reactants close together.
To get net energy out from one of these methods,
the reaction must produce more energy than is
needed to supply the activation energy. This is needed to
create the plasma and bring the plasma to high temperature in the case of
thermonuclear fusion. It is the energy needed to run the laser in laser
fusion. It is the energy needed to produce the beams of particles in
inertial fusion.
Example - deuterium fusion.
In deuterium fusion, two deuterium nuclei combine to produce helium-3.
The reaction is
+
+ n
The average binding energy per nucleon is:
deuterium = 1.1 MeV
helium-3 = 2.8 MeV
neutron = 0 MeV
The total binding energy is:
deuterium = 2(1.1 MeV) = 2.2 MeV
deuterium = 2(1.1 MeV) = 2.2 MeV
helium-3 = 3(2.8 MeV) = 8.4 MeV
The total energy released from the fusion of two deuterium nuclei into
helium-3 is thus the difference between the amount of energy given up by
the nucleons to form helium-3 minus the amount they had already given up
to be part of the deuterium nuclei:
8.4 MeV - 2.2 MeV - 2.2 MeV = 4.0 MeV.
This is less than the energy from fission,
but it involves combination of much smaller
masses. However, the amount of energy per mass is much greater.
The amount of energy released per unit of mass of the fusion reactants
is
(4.0 MeV)(1.6 x 10-19 J/eV)/(4 amu)(1.66 x 10-27
kg/amu)
= 9.6 x 1012 J/kg.
The energy from fusion in general appears as kinetic energy of
the nucleus and the protons, neutrons, and/or electrons produced, as
well as as 
-radiation
and neutrinos. Because the forces holding the nuclei together are so
strong, this is a lot of energy. There is about 2.8 x 105
times as much energy available from the fission of a deuterium nucleus
as from the burning of a carbon atom [33.8 MJ/kg] and thus there is plenty
of deuterium available in the oceans to provide virtually
unlimited energy for generations to come.
A coal-fired plant [1000MW] uses 100 kg of coal/s or 8,640,000
kg/day.
A fission power reactor [1000MW] uses ~3 kg of uranium-235 per day.
A fusion power plant [1000MW] would use 1 kg of deuterium per day.
To produce this amount of energy almost 1019 fusions/sec
must take place.
Sustained Fusion
In general to sustain fusion the Lawson criterion must be
satisfied:
(density)x(confinement time) >
6x1019 m-3s
This criteria has been met in research reactors. However in order to
construct a working fusion power plant the energy out must exceed
the total energy put in. At present it is not understood how to
accomplish this. As a result engineers cannot yet design a working
fusion power plant. The figure below indicates the history of fusion
research and where the research stands relative to the expected
parameters of a working fusion power plant.
Nucleosynthesis
Since the nuclei formed up to iron-56 are more stable than lower-A
nuclei, we expect that these will be formed in stars. As we saw, fusion
occurs in stars' interiors. In our sun, only helium fusion is possible.
In more massive stars, elements up to iron-56 can be made.
How do we get more massive elements?
The answer is that they come from endothermic nuclear reactions
rather than from exothermic nuclear reactions. When a star has burned
all of its fuel into the various elements, the fire blanks suddenly. The
outer part of the star is no longer suspended by the fire inside. The
whole outer part of the star falls in because of gravity, and the energy
of that in falling mass causes a tremendous implosion, immediately
followed by a tremendous explosion. Such an event is a
supernova.
This ejects most of the star's matter. During the implosion phase,
energy can be transformed from gravitational energy to other forms of
energy. In particular, heavy elements are fused from lighter
constituents. This material was part of the cloud that eventually
formed our sun and the solar system. It is for this reason that one can
either say that we all contain a little
stardust, or that we are all made of
interstellar garbage.
] we
find it is much larger than that for helium-3
[
], or lithium-5
[
]. Helium-4 is
much more stable for some reason. In fact, it might even be possible that
]. These