Transcript nuclear fusion

Chapter 13
Nuclear Reactions
Radioactivity
• The spontaneous emission of particles or energy from an
atomic nucleus as it disintegrates.
• The particles emitted are:
alpha particles (  ) 2 protons and 2 neutrons
beta particles (  ) a high energy electron
gamma rays (  ) electromagnetic energy only, the highest
possible energy.
• The amount of protection needed for nuclear radiation is:
gamma (piece of lead)> beta (thin layer of metal) > alpha
(sheet of paper).
• There is often gamma radiation emitted along with alpha
and/or beta particles.
• The disintegration of a radioactive nucleus is called
radioactive decay.
Radioactive Decay
U
238
92
4
2
He
Is the same as an
242
94

4
2
particle.
Pu  He 
4
2
Th  He
234
90
238
92
U
Worksheet
Radioactive Decay
14
6
C N e
14
7
0
-1
0
-1
e
is the same as a

particle.
Ba  e La
141
56
0
1
141
57
When a beta particle is emitted, a neutron gets converted to a proton.
As a result, the mass number doesn’t change, but the atomic number
increases by one and the next element in the periodic table is
obtained.
Nuclear Fission and Fusion
• Nuclear fission occurs when an unstable
massive nucleus splits into smaller, more stable
particles through the emission of alpha or beta
particles.
This occurs rapidly in an atomic bomb and slowly
in a nuclear reactor.
• Nuclear fusion occurs when less massive
unstable nuclei come together to form more
stable and more massive nuclei.
This occurs rapidly in hydrogen bombs and occurs
continually in the sun, releasing energy essential
for the continuation of life on earth.
Nuclear Fission
• Some nuclei are unstable because they are too large (atomic
number greater than 83), because they have an odd number of
protons or neutrons, or because they have an unstable neutron-toproton ratio (larger ratios in elements with more than 83 protons are
more stable in general).
• The unstable nuclei undergo radioactive decay, eventually forming
products or larger stability.
• When nuclear decay occurs a tiny amount of mass, called a mass
defect, is converted to energy, according to Einstein’s equation:
E = mc2.
• The mass of the unstable large nucleus is higher than the masses of
the resulting stable nuclei after nuclear fission occurs. This is the
mass which is converted to energy according to Einstein’s equation.
A little bit of mass produces a large amount of energy.
• This is the energy which is released when nuclear fission occurs. It
is the binding energy and it is equal to the mass defect.
Nuclear Fusion
• For smaller nuclei, with atomic number 20 or less, if the
proton : neutron ratio is 1:1 the isotope is more stable.
• When two small unstable nuclei are joined together the
mass of the unstable nuclei is slightly more than that of
the resulting more stable nucleus.
• This is called the mass defect and it is converted to
energy according to Einstein’s equation:
E = mc2
• The energy released when a nucleus is formed is called
the binding energy.
• This energy is released when nuclear fusion occurs.
The maximum amount of binding energy released during formation
of the nucleus occurs around mass number 56. It decreases in both
directions. Fission and fusion both release energy.
The fission of U-235 produces 2 or 3 neutrons along with the other products.
These neutrons can then each move to other U-235 nuclei where they
are absorbed.
Fission occurs again with the release of
More neutrons, which move on to
other U-235 nuclei to continue the
process.
A reaction where the products
Are able to produce
More. Reactions on a
self-sustaining series
is called a chain reaction.
Fission of U-235
• Natural occurring uranium is mostly U-238, an isotope
that does not fission easily.
• Only about 0.7% of the natural uranium is the highly
fissionable U-235.
• This low ratio of readily fissionable uranium 235 nuclei
makes it unlikely that a stray neutron would be able to
achieve a chain reaction in naturally occurring uranium.
This is a sub critical mass.
• A critical mass is a mass of sufficiently pure U-235 (or
Pu-239) that is large enough to produce a rapidly
accelerating chain reaction is called a supercritical mass.
• Atomic bombs use a small, conventional explosive to
push sub critical masses of U-235 or other fissionable
materials into a supercritical mass. Fission occurs almost
instantaneously in the supercritical mass and
tremendous energy is released in a violent explosion.
http://www.howstuffworks.com/nuclearbomb.htm
Nuclear Power Plant
Nuclear Power Plants
The composition
Of the nuclear fuel
Ina fuel rod:
A) before use
B) after use
Nuclear Power Plants
• After a period of time the production of fission products
in the fuel rods begins to interfere with effective neutron
transmission, so the reactor is shut down annually for
refueling.
• The spent fuel rods contain an appreciable amount of
usable uranium and plutonium.
• The spent fuel rods are stored in cooling pools at the
nuclear plant sites. In the future the spent fuel may be
reprocessed to recover the U and Pu through chemical
processing or put it in terminal storage.
• The spent fuel rods represent an energy source
equivalent to more than 25 billion barrels of petroleum.
Six other countries do reprocess the spent fuel.
Fission and Fusion
• Nuclear energy is released when:
1. massive nuclei such as U-235 undergo
fission
2. less massive nuclei such as hydrogen
come together to form more massive
nuclei through fusion.
Nuclear Fusion
• Nuclear fusion is responsible for the energy the
energy released by the sun an other stars.
• At the present half way point in the sun’s life,
with about 5 billion years to go-the core is now
35% hydrogen and 65% helium.
• Through fusion, the sun converts about 650
million tons of hydrogen to 645 million tons of
helium every second. The other roughly 5 million
tons of matter are converted into energy.
• Even at this rate the sun has enough hydrogen
to continue the process for an estimated 5 billion
years.
Nuclear Fusion
• The reactions which take place in the sun are to convert
H-1 to H-2 (deuterium) and H-3 (tritium), and to then
converted the tritium to He-4.
• Fusion appears to be a desirable source of energy on
earth because:
1. There are massive amounts of H-2
available from all the water of the oceans.
2. Enormous amounts of energy are released with no
radioactive by products.
• There are problems, however, since the temperatures
required are in the order of 100 million degrees celsius,
and the concentrations of H-2 need to be huge so that
many reactions occur in a short time, so the pressure
needed is huge.
Half-Lives
• A half-life is the length of time required for
one-half of the unstable nuclei to decay.
• Each isotope has its own characteristic
half-life, ranging from fractions of a second
to billions of years.
• The half-life is independent of the amount
of the radioactive isotope which is present.
•
A sample of Bi-210 has a half-life of 5
days. How much is left after 10 days?
a)
b)
c)
d)
100 %
50 %
25 %
0%
Other uses of radioactivity
• Food preservation: Radioactive Co-60 or
Cs-137 emit gamma radiation which kills
bacteria and other pathogens.
• Nuclear medicine: Radioactive iodine is
used to treat thyroid cancer patients by
diagnosing the disease. It is also used for
many other diagnostic tests.
• Cancer treatment: To destroy cancerous
cells. Unfortunately this has side effects.
Big Bang Theory
• A “big bang” of energy created a plasma of protons and neutrons.
This led to the formation of H nuclei.
• The H nuclei were pulled together by gravity into masses that would
become the stars. The H nuclei fused into He nuclei, releasing
enough energy that the star began to shine.
• The fusion process continued for billions of years, releasing energy
as heavier and heavier nuclei were formed. Eventually the star
materials were fused into nuclei around iron, the element with the
lowest amount of energy per nucleon and the star used up its
energy source.
• Larger, more massive dying stars explode into supernovas. Such an
explosion releases a flood of neutrons, which bombard medium
weight nuclei and build them up to more massive nuclei, all the way
from iron up to uranium. These elements are then spread into
space as dust which became the materials of which planets were
made.
• The force of gravitational attraction provided the initial energy for the
whole process.
Review Exercises
• Applying the Concepts p. 13-26 to 13-25:
# 1, 2, 3, 4, 19, 20, 22
• Parallel Exercises, Group A:
# 1, 2, 4, 5, 6.
New Book: p. 380-383 # 2, 3, 4, 5, 6, 8, 9,
16, 17, 18, 19, 24, 25, 26, 27, 29, 30, 31,
32, 33, 41, 42, 44, 45, 46, 49.
Summary
•
•
•
•
Alpha, beta and gamma particles-what each of them is.
Reactions where alpha and beta particles are emitted.
Distinction between nuclear fission and nuclear fusion.
Binding energy and mass defect- The shortage of mass
which is converted to energy when unstable isotopes
undergo fission or fusion.
• Advantages and disadvantages of nuclear fusion.
• Half lives: what they are and determining amounts based
on half lives.
• Uses of radioactivity.