Transcript Nuclear Power

25.3 : Fission
• Nuclear Fission
• When nuclei of certain isotopes are
bombarded with n, they split into
smaller fragments.
– U-235 & Pu – 239
• Chain Reaction – some neutrons
produced in the reaction react with
more U-235.
• 1 kg of U-235 fission = energy of 20,000 tons
of dynamite
• Uncontrolled – takes seconds
(nuclear bomb)
• Controlled = generates energy
(nuclear power plant)
• Heat energy  heats water to steam  turns
turbine generates electricity
A Nuclear Power Plant
Click on picture for Nuclear Power Plant Tour
PWR Reactor
BWR Reactor
Nuclear Power Station
Turbine Building
Containment
Discharge
Canal
Intake Canal
Neutron Moderation
• Slows n so that U-235 captures it
to continue chain reaction.
Otherwise, they go to fast and pass
through material
• Use H2O or carbon (graphite)
– H2O is better because it will turn to
steam if it gets to hot, therefore
stopping the chain reaction.
– (US plants all use H2O)
Ways to Control the Reactor Core
• Neutron Absorption = Prevents chain
reaction from going to fast. Some slowed
neutrons are trapped (absorbed)
before they hit the fissionable
isotopes.
• Control Rod (usually Cadmium)
• Absorbing chemicals in the coolant
(e.g., boron)
• Reactor temperature and pressure
• If reaction is too fast  heat produced too
quickly to be cooled by coolant
– Reactor core would overheat
– Possibly causing:
• Mechanical failure
Critical Mass = Stable
K IS DEFINED AS THE “MULTIPLICATION FACTOR”
K = __
NEUTRONS BORN_____________
NEUTRONS ABSORBED + NEUTRONS LOST
K = 1: THE NEUTRON POPULATION AND SYSTEM POWER ARE
STABLE. “CRITICAL” CONDITION
K > 1: THE NEUTRON POPULATION AND SYSTEM POWER ARE
INCREASING. “SUPERCRITICAL” CONDITION
K< 1: THE NEUTRON POPULATION AND SYSTEM POWER ARE
DECREASING. “SUBCRITICAL” CONDITION
Note: The above neutron population observations are only true
with no external neutron sources present.
Fuel Assembly
• Fuel Pellet
– Uranium Oxide Pellet
– Stacked on top of
each other
• Fuel Rod
– 0.4 inches diameter
– 12 feet tall
• Fuel Assembly
– 8.5 inches wide
– ~200 rods / assembly
– 15x15 or 17x17 fuel
rod matrix
– ~10 grids / assembly
Reactor Core Assembly
On-Site Barriers
http://www.nrc.gov/about-nrc/emerg-preparedness/images/barriers.jpg
Nuclear Reactor Safety
• Need to have
– Culture of Safety
– Industry & NRC Inspection
– Safety Margins in design & operation
– Sharing of information within the industry.
– Redundant (multiple) safety systems.
– Need a good Emergency Plan
Defense in Depth
Concept is fundamental to reactor safety
1) Prevention
Sound, inherently stable, conservative design with
high degree of reliability & stringent quality control
2) Protection
Reactor protection systems
Control systems
3) Mitigation
Multiple barriers to prevent release of radioactivity;
Engineered safety features;
Emergency preparedness
Need to be secure
Nuclear power plants have
always had armed guards
and other measures to
protect the facility from
people who didn’t belong
there.
After the terrorist attacks of
Sept. 11, 2001, even more
security measures were put
in place.
These measures are very strict
because protecting the plant
from sabotage or attack is
important to protect people
and the environment.
Nuclear Waste
• Fuel Rods = Uranium in ceramic pellets
– 3 m X 0.5 cm diameter
– 300 rods are bundled in reactor assembly
• U-235 decreases w/ use  eventually
become depleted or spent
• Spent rods = high level waste
– Stored in holding tanks “swimming pools”
– H2O cools and acts as radiation shield
– Less expensive in the US to mine new uranium
than to recycle remaining U-235 from spent
rods
– Many pools are full
Not a Lot of Waste
• In 35 years, the amount of waste
produced by all nuclear power plants is
relatively small.
• It would cover a football field to a depth
of 4 yards –in 35 years
• Other nations are already building
underground facilities for their nuclear
waste
Nuclear Waste Storage
Spent Fuel Pool
Dry Cask Storage
• Some of the fission products in the spent fuel will take
many years to lose their radioactivity. A special
disposal site is needed for this type of spent fuel.
• The U.S. Department of Energy is supposed to take
possession of the nuclear waste from nuclear
reactors. They have been collecting taxes for this
purpose, but have not taken possession of any waste.
• There was a law passed in the 80s. The U.S.
Department of Energy will build a waste depository at
Yucca Mountain, Nevada. The spent fuel, in heavy
casks, would remain buried there for thousands of
years.
• Congress is not providing money for the DOE to
progress this project (Harry Reed).
Proposed site in Yucca Mountain,
Nevada (an unpopulated desert)
• For more information contact the
Nuclear Regulatory Commission
• http://www.nrc.gov/reading-rm/basicref/students/
What is the blue glow?
Cherenkov radiation is
electromagnetic radiation
emitted when a charged
particle passes through an
insulator at a speed greater
than the speed of light in that
medium.
The characteristic "blue glow" of nuclear reactors is due to
Cherenkov radiation. It is named after Soviet scientist Pavel
Alekseyevich Cherenkov, the 1958 Nobel Prize winner who
was the first to rigorously characterize it.
Major Reactor Accidents
• OK, so we…
– Employ defense-in-depth in the design…and
– Promote and maintain a Safety Culture….and
– Maintain an Emergency Response
Organization…and
– Perform detailed safety analyses, PRA and
SOARCA…
• Yet, 5 reactor core damage events have
occurred in commercial reactors since 1979.
– Three Mile Island Unit 2
Pennsylvania 1979