NUCLEAR ENERGY NOTES
Both fusion and fission are
examples of NUCLEAR reactions, which are accompanied
by a net release of energy. The energy
comes from the difference in MASS between the starting materials known as the
REACTANTS and the final PRODUCTS. Einstein’s
equation, E = mc2 explains the mass – energy relationship. Most of the energy release is in the form of
heat. This resultant hat can be captured, in principle, and converted to electricity or
directly to mechanical energy.
FISSION
Fission reactions are inducted by the collision of a NEUTRON with a target
nucleus; if the neutron provides sufficient activation energy, the target
nucleus will fragment into two parts of unequal mass and release additional
neutrons plus an excess of energy. In principle,
most of the atomic nuclei toward the heavy mass side of the periodic table
undergo fission. Uranium 233, Uranium
235, and Plutonium 239 are relatively unstable and undergo fission upon
colliding with low energy or thermal, neutrons.
The role of the neutron in fission reactions resembles that of a
catalyst.
N + U 235 fission fragments (Nd 144 + Y 89) + 3 n + E
ACTUAL
MASSES OF REACTANTS AND PRODUCTS
U 235 = 235.04394 Nd 144 = 143.91013
n =
1.00866 Y
89 = 88.90587
________
3n = 3.02600
236.05260 235.84200
mass
difference = .21060
E = mc2 931.1 MeV = 1 amu/c2
c2 = 931.1 MeV/amu
.21060 x
931.1 MeV = 196.08 MeV
amu = atomic mass unit
MeV =
million electron volts
FUSION
By contrast,
fusion reactions require collisions between two nuclei, which subsequently
undergo rearrangements to produce two new nuclei and release an excess of
energy. Most of the nuclei on the light
mass side of the periodic table undergo fusion or nuclear rearrangement
reactions. The sun derives its energy
from a chain of fusion reactions by which ordinary hydrogen is
converted to helium.
564 million tons of H ions collide/sec. 560 million tons of Helium
4 million tons lost as radiation
Nuclear Fuels
Fissile or Fissionable Isotopes
U
235 (0.7% of natural uranium)
U
233 (bred from Th 232)
Pu 239 (bred
from U 238)
Fertile Isotopes
U
238 (99.3% of natural uranium)
Th 232
(naturally occurring)
UNITS OF RADIATION MEASUREMENT
Roentgen – unit of exposure dose (exposure/unit volume)
for x-rays or gamma rays
REM - roentgen Equivalent Man (unit of biological
dose or dose equivalence)
RAD
- relative biological effectiveness; RBE factor
is used to compare the biological
effectiveness of absorbed radiation dose (i.e. RAD) due to different
types of ionizing radiation
RAD & REM are roughly equivalent
for x-rays and gamma rays since the RBE is 1 for both, thus REM = RAD x RBE
A does of 100
to 200 rems at one time over the entire body would
cause nausea, fatigue, and blood changes.
NUCLEAR POWER
PLANTS
In 1984, there
were 76 reactors on line in the
There are at
least seven obstacles hindering the development of nuclear power as a major
energy source.
1. supplies and cost of uranium fuel
2. concern over plant accidents and sabotage
3.
highjacking of nuclear fuel shipments
4. waste storage problems
5. nuclear weapons proliferation
6. soaring costs
7. net useful energy for the entire system
*major public
considerations
Benefits of
nuclear energy
1. minimal air
pollution
2. less radiation
than coal-fired plants
3. smaller amounts
of fuel used – a conventional nuclear reactor using 130 tons of uranium/year
will produce the same amount of energy as a coal-fired plant using 2 million
tons of coal/year.
Detriments of
nuclear energy
1. high and low
level radioactive wastes
2. large volumes of
cooling water required
3. heat loss from plant leading to thermal
pollution 1000 cubic feet/second is the flow needed through the condensor; the temperature of a lake cannot be raised more
than 3 degrees Fahrenheit; a river must have a flow of at least 5x that needed
to condense
4. siting
constraints – geological considerations and water flow
5. useful operating
life – about 30 years
The environment
is exposed at three points in the nuclear fuel cycle:
at the
reactor
at the fuel
processing and reprocessing plants, and
at the waste
disposal site
Light Water
Reactors
BWR boiling water reactors; water is used
as the coolant and moderator and converted to steam
PWR pressurized water reactor; water is used as a coolant and moderator but kept under pressure to
prevent conversion to steam; water in a second circuit is converted to
steam by means of heat exchanger
HTGR high
temperature gas cooled reactor; heat from the reactor core is removed by a
closed loop of circulating helium gas; the heated gas is pumped through heat
exchangers and the heat is used to convert water into steam in a secondary
system which drives the turbine-generator producing electricity; the HTGR
converts Thorium into U 233 which is a fissionable fuel and thus uses less
uranium than water-cooled reactor systems