DESIGN PROJECT - All that didn't fit into the project
PRESSURIZED WATER REACTOR
The first pressurized water reactor, the Mark I prototype for the Nautilus
submarine, began operation in May 1953 at the National Reactor Testing
Center in Idaho. Since that time the development of pressurized reactors
for military and civilian purposes has been intensively pursued specially
in the United States.
By definition, in a pressurized water reactor the fission heat
is removed from the fluid elements by the water coolant without bulk boiling
occurring. This implies a two circuit heat transfer system - a primary
loop containing the reactor and one side of steam generator containing
a steam side of the steam generator and the turbine generator.
The extensive use of water as a reactor coolant is related to
the relatively low pressure drops accompanying flow at significant rates,
and the relatively high heat transfer coefficients. (See figure below.)
Figure shows the relationship of heat flux to the temperature difference
between the fuel element surface and the water. The nucleate boiling stage
is attractive for reactor operation, because of the favorable heat fluxes
that can be obtained. However, as the temperature difference increases
to the point where bulk boiling and film boiling occurs, there is a drop
in heat flux and the danger that the fuel element surface temperature will
rise above their melting point (burn out).
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Fuel assembly
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Resilient mounting pin
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Head
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Central pipe
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Upper spacer grid
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Fuel element
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Fual assembly hexagonal wrapper
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Spacer grid
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Lower support grid
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End plug
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Centring pin
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Fuel element
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Uranium oxide pellet
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Fuel cladding tube
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Upper ending
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Lower ending
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Inserted distantion spring
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Compression plate
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Some properties of water:
Coolant Temp(F) Density(lb/cuft) Specific Heat(BTU/lb.F)
m.p. 32F 212 60 1.006
b.p. 212F 482 50 1.21
Thermal Conductivity Viscosity(lb/(hr)(ft))
0.395 0.70
0.35 0.45
Some nuclear properties of Light Water:
absorption cross section (barn) scattering cross section (barn)
0.66 44.4
fractional energy loss/collision moderating ratio
0.925 62
ADVANTAGES OF PRESSURIZED WATER REACTOR:
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Water technology well known.
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Water is cheap.
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Water is very effective moderator of neutron energy
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core is compact.
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Water has high heat capacity.
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Negative temperature coefficient.
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Ordinary leakage can be tolerated.
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Fission products are contained, not circulated.
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Radioactivity of coolant is short-lived if kept pure.
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Conversion ratio may be high.
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Superheating steam in separately fired superheater is possible.
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Appreciable fast fission effect attainable.
DISADVANTAGES OF PRESSURIZED WATER REACTOR:
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Water must be highly pressurized to achieve even reasonably high temperature
without boiling.
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Fuel element fabrication expensive.
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The temperature is limited in metallic fuel elements.
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Fission product activity in the core builds up to high a level.
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Pure hot water is highly corrosive, requires special materials for the
primary loop.
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Fuel must be at least slightly enriched.
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Heat exchanger and control rods required.
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Large excess reactivity at operating temperature.
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Heat transfer only moderately efficient.
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Fuel reprocessing a difficult task.
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Rector must be shut down to unload and reload core.
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Water would flash to steam in case of rupture of primary loop.
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Water reacts with uranium, thorium, and structural metals under certain
conditions.
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Low thermal heads make heat exchanger, pumps and pipins large.
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Hot-channel factors are significant.
