Transcript The Nuclear Fuel Cycle

The Nuclear Fuel Cycle
Presentation
• Components of the Fuel Cycle
•Front End
• Service Period (conversion of fuel to energy in a reactor)
• Back end
• Storage (open cycle)
• Reprocessing (closed cycle)
•Alternatives and Economics
•Proliferation Concerns
The Front End of the Cycle
For Light Water Reactor Fuel
Uranium
• URANIUM is a slightly radioactive metal that occurs throughout the earth's
crust.
• It is about 500 times more abundant than gold and about as common as
tin.
• It is present in most rocks and soils as well as in many rivers and in sea
water.
• Most of the radioactivity associated with uranium in nature is due to other
materials derived from it by radioactive decay processes, and which are
left behind in mining and milling.
• Economically feasible deposits of the ore, pitchblende, U3O8, range from
0.1% to 20% U3O8.
Uranium Mining
•
Open pit mining is used where deposits are close to the surface
•
Underground mining is used for deep deposits, typically greater than 120m deep.
•
In situ leaching (ISL), where oxygenated groundwater is circulated through a very
porous ore body to dissolve the uranium and bring it to the surface. ISL may use
slightly acidic or alkaline solutions to keep the uranium in solution. The uranium is
then recovered from the solution.
The decision as to which mining method to use for a particular deposit is governed
by the nature of the ore body, safety and economic considerations.
In the case of underground uranium mines, special precautions, consisting primarily
of increased ventilation, are required to protect against airborne radiation exposure.
Uranium Mine in Niger (Sahara Desert)
Uranium Metallurgy
“Yellowcake”
“Yellowcake”
Tailings from Uranium Mining and Milling
•More than 200 pounds of byproduct material, tailings, are typically produced for
each pound of uranium.
• After extraction of uranium from the ore, the tailings contain much of their
original radioactivity.
•Toxic heavy metals, including chromium, lead, molybdenum, and vanadium, are also
present in this byproduct material in low, but significant, concentrations
Uranium Global Resources
Enriching Uranium for Reactor Fuel
• Increase the concentration of fissionable U-235 isotope
• Enrichment requires a physical process since
U-235 and U-238 have the same chemical properties
• Physical processes require gases for separation
• Uranium and its oxides are solids
• Must convert uranium to UF6
• Enriched UF6 must be converted back to solid
uranium or uranium oxide
or centrifugation
COMURHEX – Pierrelatte, France
UF4 → UF6
Enrichment
The two method of uranium enrichment are:
•
Gaseous diffusion (older)
•
Centrifugation (newer)
Both use small differences in the masses (< 1%) of the U-235F6 and
U-238F6 molecules to increase the concentration of U-235.
F6
F6
Gaseous diffusion plant
Paducah, Kentucky
Loading uranium
hexafluoride containers
Centrifuge Enrichment
Depleted
exit
U238F6 is heavier and
collects on the outside
walls
(Depleted/Tails)
Feed
Enriched
exit
Feed to
Next Stage
U235F6is lighter and
collects in the center
(enriched)
The gas centrifuge process has three characteristics that make it economically
attractive for uranium enrichment:
Proven technology: Centrifuge is a proven enrichment process, currently used
in several countries.
Low operating costs: Its energy requirements are less than 5% of the requirements
of a comparably sized gaseous diffusion plant.
Modular architecture: The modularity of the centrifuge technology allows for flexible
deployment, enabling capacity to be added in increments as demand increases.
Fuel Fabrication
• Reactor fuel is generally in the form of ceramic pellets.
• These are formed from pressed uranium oxide which is sintered (baked)
at a high temperature (over 1400°C).
• The pellets are then encased in metal tubes to form fuel rods, which are
arranged into a fuel assembly ready for introduction into a reactor.
UF6 Gas to UO2 Powder to Pellets
Fuel Pellets
Nuclear Fuel Assembly
Fuel
Pellet
Basic Pressurized Water Reactor (PWR)
Fuel
Rods
Fuel Assemblies are Inserted in Reactor Vessel
Production of plutonium in a nuclear reactor
In addition to the fission of U-235 atoms, some U-238 atom
absorb neutrons and emit beta particles to become plutonium
238
92U
+ 0n1
=>
239
94Pu
+ 2(-1β0)
U-235
Pu-239
Amount
Pu-240
Removal of fuel elements
for making weapons
Time in reactor
Fission of Pu produces about
1/3 energy from the reactor
Back End of the Fuel Cycle
(Open vs. Closed Cycles)
Closed Cycle
Open Cycle
Composition of Spent fuel Rods
from a Light Water Reactor
Material
Transuranic elements
U-236
Pu isotopes
Fission products
U-235
U-238
Initial Fuel
0.000
0.000
0.000
0.000
3.3%
96.7%
Spent Fuel
0.065%
0.46%
0.89%
0.35%
0.08%
94.3%
Type of Waste
TRU
TRU
High Level
TUR = transuranic
Fission products have
shorter half-lives and
higher activities.
Actinides have longer
half-lives and lower
activities
The actinides are the fifteen
elements with atomic
numbers 89 to 103.
The spent fuel removed from the reactors continues to release heat and is still
radioactive. It is, for those reasons, that the fuel is initially stored under water in the
spent fuel storage pools.
Spent Fuel Storage Pools
Dry Cask Storage on
Reactor Sites
Transport of Spent Fuel
Solidifying high-level waste in borosilicate glass
for long term storage in a repository
Reprocessing – Closed Fuel Cycle
Recovers of uranium and plutonium from spent fuel
Reduces volume and radioactivity of waste
France, the UK, Japan, and Russia currently reprocess spent fuel
Mixed Oxide Fuel (MOX)
MOX is produced from the output of reprocessing plants and is a mixture of
plutonium and uranium oxides with a composition of 3% to 7% PuO2 and the rest
UO2. The MOX is then mixed with ordinary LEU uranium-oxide fuel for use in light
water reactors. Mixture is 1/3 MOX and 2/3 LEU.
By 2001, over 20 power reactors in France were using MOX for one third of their
fuel In the US, MOX fuel is being used as a means of disposing of Pu from
dismantled nuclear weapons in the US and Russia.
Fuel Reprocessing Plant, Marcoule, France
Relative Costs
Process
$/kg fuel
Uranium
$500
Conversion
$50
Enrichment
$600
Fabrication
$250
Wet storage
Included in capital & O%M
Dry cast storage
$200
Geological storage
$400
Total Cost
$2000
Proliferation
of
Nuclear Materials and Weapons
HEU
Pu-239
Iranian Nuclear
Complex
Yongbyon Site
Presentation
• Background
• Components of the Fuel Cycle
•Front End
• Service Period (conversion of fuel to energy)
• Back end
• Open (Storage)
• Closed (Reprocessing)
•Alternatives and Economics
•Proliferation Concerns
Three Useful Educational Resources
•
The Alsos Digital Library for Nuclear Issues
• Nuclear Chemistry in the Community
• Concept Map for Nuclear Power
Concept Map for Civilian and Military Uses of Nuclear Energy
http://www.chemcases.com/nuclear/index2.html#concept
http://alsos.wlu.edu/
http://alsosconceptmap.wlu.edu/nuclearpower/main/index.html