Why are you trying so hard to fit in, when you were born to stand out? - PowerPoint PPT Presentation

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Why are you trying so hard to fit in, when you were born to stand out?

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Why are you trying so hard to fit in, when you were born to stand out? WHAT HAPPENS IN A NUCLEAR REACTOR? Most nuclear reactors are used to power steam turbines to ... – PowerPoint PPT presentation

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Title: Why are you trying so hard to fit in, when you were born to stand out?


1
Why are you trying so hard to fit in, when you
were born to stand out?
2
WHAT HAPPENS IN A NUCLEAR REACTOR? Most nuclear
reactors are used to power steam turbines to
generate electricity. They take the heat energy
from fission reactions to convert water into
steam for this purpose. A few small reactors are
used to generate radioactive isotopes for use in
research or medicine. The most common type or
reactor is a pressurized water reactor - a PWR.
3
Uranium provides the energy source for nuclear
reactors. 1 ton of uranium has the equivalent
energy of 20,000 tons of coal!
Typical fuel pellet
Fuel assembly in a representative boiling water
reactor (about 4.3 meters 14 feet) tall and
each weighing about 317.5 kilograms (700 pounds).
NFI type 9x9 Fuel.
4
Water under high pressure is heated by the
fission reaction. This water converts water to
steam in a heat exchanger, and the steam drives a
turbine to generate electricity. The advantage is
that the water that contacts the reactor does not
contact anything outside the containment
structure.
5
It is similar to a coal fired power plant. The
big difference is how the heat is produced. U235
is the fuel of choice in current nuclear
reactors. In nature, most of the uranium is U238,
with only 0.7 being U235. To use it in a
reactor, it must be enriched to around 3.5
U235. For a nuclear weapon, one needs at least
90 U235. The important point is that a nuclear
reactor will not explode. At the worst, the core
would melt.
6
When a U235 nucleus captures a neutron, the total
energy is distributed among the 236 nucleons in
the nucleus. The nucleus becomes unstable and can
fission into fragments.
Typically, 2 or 3 neutrons (av. 2.5) are also
released. This keeps the fission reaction going
(chain reaction).
7
At criticality, the chain reaction system is
exactly in balance so that the number of neutrons
produced in fissions remain constant. This
balance is achieved by using control rods. To
raise or lower the power, the number of neutrons
must be increased or decreased, and this is done
by raising or lowering the control rods. The
control rods are made of neutron absorbing
material.
8
The ability to control a reactor is due to the
presence of a certain number of delayed
neutrons. Otherwise, there would be an instant
rise and fall of neutron population. For neutrons
to be absorbed by U235 and cause fission, they
must be slowed down. Neutrons released from
fission are fast (109 cm/sec). U235 fission is
caused by slow neutrons (105 cm/sec). The water
in the reactor serves as a moderator to slow the
neutrons down.
9
The fission fragments of U235 are distributed
around masses 95 and 135.
10
U-235 n --gt Ba-144 Kr-90 2n energy U-235
n --gt Ba-141 Kr-92 3n energy U-235 n
--gt Te-139 Zr-94 3n energy In these
reactions, number of nucleons is conserved, but a
small loss in mass occurs equivalent to the
energy released. The fission products can decay
further with some beta and gamma ray emissions.
Many have short half lives. It is these emissions
that make the waste products highly radioactive.
11
About 6 of the heat generated in the reactor
core is due to decay of the fission
products. This has to be taken into consideration
when a reactor is shut down. It also has to be
taken into consideration in the storage of spent
fuel rods. They are usually stored under water
for a couple of years until the short lived
isotopes decay.
12
In addition to the fission decay products,
neutron capture by one of the uranium isotopes
can occur to produce transuranic elements
(elements beyond uranium in the periodic
table). Uranium 238 can absorb a neutron to
become U-239. U-239 quickly emits a beta particle
to become neptunium 239. Np-239 emits a beta
particle to become Pu-239. Pu-239 is relatively
stable. Pu-239 can absorb a neutron to become
Pu-240, and Pu-240 can absorb a neutron to become
Pu-241. Pu-241 undergoes beta decay to americium
241.
13
Many of these transuranics are alpha emitters and
have long half lives. The materials the reactor
is constructed from can also absorb
neutrons. Activation products include tritium,
carbon 14, cobalt 60, iron 55, and nickel
63. These can make demolition a problem.
14
  • Fast neutron reactors are another alternative.
  • They use the high speed neutrons produced
    directly from fission without a moderator.
  • They have to use more highly enriched fuel -
    either U235 or Pu239.
  • Advantages
  • More neutrons are produced per fission than with
    slower neutron fission, so extra neutrons are
    available for transmutation
  • Can be used as a breeder reactor by surrounding
    the core with U238
  • Can convert transuranics to fissible products, so
    total waste is reduced.

15
  • 4. The remaining waste has a maximum half life of
    27 years rather than thousands of years.
  • Disadvantages
  • Design is more demanding, as the power density is
    higher
  • Water cant be used as a coolant, as it is a
    moderator. Liquid metal coolants such as lead or
    a mixture of Na and K are used.

16
India has a program that will use FNBRs to
convert thorium to U233 for fuel. In the first
stage, they will use PWRs and natural uranium.
In the second stage, the plutonium produced will
go to a FNBR. The blanket around the core will
contain thorium as well as U238, so more
plutonium will also be produced. In the third
stage, the U233 and Pu239 produced in the second
stage will be used as fuel along with additonal
thorium in a PWR.
17
MODERN REACTORS ARE BUILT INSIDE A CONTAINMENT
STRUCTURE THAT IS MADE OF CONCRETE 3 FEET THICK
AND IS REINFORCED WITH STEEL RODS 2.5 INCHES IN
DIAMETER PLACED AT 1 FOOT INTERVALS. THIS COULD
SUSTAIN AN AIRLINER CRASH. THIS WAS NOT THE CASE
FOR CHERNOBYL.
18
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