Title: Nuclear Technology Part II: The Nuclear Fuel Cycle and Nuclear Material Production
1Nuclear Technology Part IIThe Nuclear Fuel
Cycle and Nuclear Material Production
- Fred Wehling
- IP 574
- Spring 2006
With thanks to Adam Bernstein, Charles Ferguson,
Lance Lesher, and Mary Beth Ward
2World Uranium Resources
3Mining Milling
Mining Uranium is found in several types of
Minerals (pitchblende, uranite, carnotite,
autunite, uranophane, tobernite) Also found in,
phosphate rock, ignite, and monazite
sands Milling Extraction of uranium oxide from
ore in order to concentrate it Milling produces
yellowcake, Concentrated uranium oxide (U308)
4Uranium Conversion
- Yellowcake (U3O8) is converted to other chemical
compounds of U suitable for enrichment or for
fuel fabrication - Converted forms
- Uranium hexafluoride (UF6)
- Uranium metal (U)
- Uranium dioxide (UO2)
- Uranium tetrachloride (UCl4)
5Five Grades of Uranium
- Depleted uranium (DU) contains lt 0.7 U-235
- Natural uranium contains 0.7 U-235
- Low-enriched uranium (LEU) contains gt 0.7 but lt
20 U-235 - Highly enriched uranium (HEU) contains gt 20
U-235 - Weapons-grade uranium contains gt 90 U-235
- Weapons-usable uranium
6Why enrich uranium?
- Except for certain types of nuclear reactors,
such as heavy-water moderated reactors, most
commercial and research reactors, and all nuclear
weapons that use uranium for fission require
enriched uranium. - Only 0.72 of natural uranium is U-235 the
fissile isotope. A tiny fraction is U-234. - Over 99 is U-238.
- Without a very efficient moderator, such as heavy
water or very pure graphite, a chain reaction
cannot be sustained in natural uranium U-235 is
too sparsely distributed.
7The Problem with Enrichment
- The chemical properties of 235U and 238U are
essentially identical - To increase the percentage of 235U, enrichment
must exploit minute differences in the physical
properties of the two isotopes (mainly the fact
that 235U is slightly lighter in weight)
8Uranium Enrichment Methods
- Electromagnetic Isotope Separation (EMIS)
- Gaseous Diffusion
- Gas Centrifuge
- Aerodynamic Process
- Atomic Vapor Laser Isotope Separation (AVLIS) or
Molecular Laser Isotope Separation (MLIS) - Thermal Diffusion
9Electromagnetic Isotope Separation (EMIS)
10EMIS Process
- Uranium tetrachloride (UCl4) is electrically
heated to produce UCl4 vapor, which is bombarded
with electrons to ionize it. - An electric field accelerates the ions to high
speeds. - A perpendicular magnetic field causes the
accelerated ions to bend in a circular path. The
lighter U-235 ions will follow a path with a
shorter radius than the U-238 ions. - The two types of ions then go through apertures
leading to different collectors.
11EMIS Proliferation Potential
- Inefficient -- Typically less than half the feed
is converted to U ions and less than half are
actually collected. - The process is time consuming and requires
hundreds to thousands of units and large amounts
of energy. - Feed material is corrosive.
- The U.S. used calutrons at the Oak Ridge Y-12
plant in two stages. - Although all five recognized nuclear weapons
states had tested or used EMIS to some extent,
this method was thought to have been abandoned
for more efficient methods until it was revealed
that Iraq had pursued it.
12Iraqi EMIS machine
13Gaseous Diffusion
Relies on the molecular effusion (the flow of gas
through small holes) to separate U-235 from
U-238. The lighter gas travels faster than the
heavier gas. The difference in velocity is small
(about 0.4). So, it takes many cascade stages to
achieve even LEU.
U.S. first employed this enrichment technique
during W.W. II. Currently, only one U.S. plant
is operating to produce LEU for reactor
fuel. China and France also still have operating
diffusion plants.
Uranium hexafluoride UF6 Solid at room
temperature.
14Gaseous Diffusion Whats Needed for a Bomb a
Year 25 kilograms of HEU
- At least one acre of land
- 3.5 MW of electrical power
- Minimum of 3,500 stages, including
- Pumps, cooling units, control valves, flow
meters, monitors, and vacuum pumps - 10,000 square meters of diffusion barrier with
sub-micron-sized holes
15Gaseous Diffusion Proliferation Potential
- Unlikely to be the preferred technology of a
proliferator due to - large energy consumption,
- the specialized equipment, and
- the difficulty in hiding the operation because
the facility tends to be large.
16Gas Centrifuge
- Uses physical principle of centripetal force to
separate U-235 from U-238 (slight mass
difference) - Very high speed rotor generates centripetal force
- Heavier uranium hexafluoride gas 238UF6
concentrates closer to the rotor wall, while
lighter 235UF6 concentrates toward rotor axis - Separation increases with rotor speed and length.
17Gas Centrifuge Unit
18Gas Centrifuge Cascade
Enricher stages increase product assay Stripper
stages decrease waste assay.
19Gas Centrifuge Cascade and Components
Key Components Rotating components Thin-walled
cylinders (3- to 16-in. diameter), end caps,
baffles, and bellows Made of high-strength
materials Maraging steel Aluminum
alloys Composite materials (e.g., graphite
fiber) Other components Magnetic suspension
bearings, bearings/dampers, vacuum pumps, and
motor stators Auxiliary equipment High-frequency
motor power supplies Rotor balancing
20What Centrifuge Gear is Needed for a Bomb a Year?
- Minimum of 350 very high-efficiency units (P-2 or
better) - Alternatively, about a few thousand
low-efficiency units (P-1 type) - Most likely that a developing proliferant state
would have the most access to these units, for
example, A. Q. Khans nuclear black market - About 0.5 MW of electrical power to operate
low-efficiency system (compared to about 3.5 MW
for gaseous diffusion plant) for one bombs worth
of material
21Atomic Vapor Laser Isotope Separation (AVLIS) and
Molecular Laser Isotope Separation (MLIS)
- Uses lasers to separate U-235 from U-238
- Lasers are tuned to selectively excite one
isotope - Technology and equipment are highly specialized
- U.S. cancelled its AVLIS program in 1999 for
economic reasons
22Would Proliferators use AVLIS or MLIS?
- Conventional wisdom says no, but over 20
countries have experimented with AVLIS and/or
MLIS - Advantages
- Easy to conceal
- Energy costs low compared to centrifuge system
- Disadvantages
- Complex technology
- Hard to acquire or make proper lasers
- Can be significant material losses of U
23Alternative Enrichment Processes
- Aerodynamic
- Becker nozzle developed at the Karlsruhe
Nuclear Research Center in Germany. - Used by South Africa for producing both LEU for
reactor fuel and HEU for weapons. - Mixture of gases (UF6 and hydrogen or helium) is
compressed and directed along a curved wall at
high velocity. - Heavier U-238 moves closer to the wall.
- Thermal diffusion
- Uses difference in heating to separate light
particles from heavier ones. - Light particles preferentially move toward hotter
surface. - Not energy efficient compared to other methods.
- Used for limited time at Oak Ridge during WW II
24Need SWU to Enrich U
- Enrichment effort (energy expenditure) is
measured in terms of separative work units
(SWU) - About 4 SWU ? 1 kg of LEU (3) from about 6 kg
of natural uranium - About 200 SWU ? 1 kg weapons-grade HEU from about
200 kg of natural U - About 5,000 SWU ? 1 weapon from about 5,000 kg of
natural U - About 100,000 SWU ? fuel for 1,000 MW(e) LWR for
1 years operation (e.g. Irans Bushehr reactor) - Important to realize that a majority of the
SWU/kg work to produce weapons-grade HEU is
already done in producing LEU from natural U
starting from LEU would give a proliferator a
huge head start - Assumes a nominal waste assay of 0.25 U
25Plutonium Production
- Because of its relatively short half-life (about
22,000 years for 239Pu), plutonium exists in only
trace quantities in nature. - Therefore, it must be produced through manmade
processes, such as using 238U as fertile material
in a nuclear reactor. - U-238 n ? U-239 ? Np-239 ? Pu-239
- Relatively rapid decays of U-239 (23 min.) and
Neptunium-239 (Np-239) (2.35 days)
26(No Transcript)
27Grades of Plutonium
- Desirable for weapons purposes to have 239Pu
percentage to be as large as possible. - Weapon-grade contains lt 6 240Pu and other non-
239Pu isotopes. - Fuel-grade contains 6-18 240Pu etc
- Reactor-grade contains gt 18 240Pu
- Super-grade contains lt 97 240Pu
- Weapon-usable refers to plutonium that is in
separated form and therefore relatively easy to
fashion into weapons any of the above
28Extracting Plutonium from Spent Nuclear Fuel
- Plutonium content is typically about 1
- Chemical processing (reprocessing) extracts the
plutonium and remaining uranium from the
radioactive fission products in spent fuel - Highly radioactive material ? requires remote
handling
29PUREX Method
- PUREX (plutonium-uranium extraction) is the most
widely used method for plutonium extraction (only
method used commercially) - First used at Savannah River in U.S. in 1954
- Spread throughout the world, helped in part by
Atoms for Peace programs - Three main stages
- Spent fuel assemblies are dismantled and fuel
rods are chopped up. - 2. Extracted fuel is dissolved in hot nitric
acid. - 3. Solvent extraction Pu and U are separated
from other actinides and fission products, and
then from each other. - Most complex stage
- Tributyl phosphate (TBP) is the typical organic
solvent.
30Alternative Plutonium Separation Processes
- Bismuth phosphate
- REDOX
- BUTEX
- Ion Exchange
- Pyroprocessing
31Two Paths to Nuclear Weapons Material Enrich
Uranium or Produce Plutonium
32Assessing the Proliferation Potential of a
Reactor
- 1 Megawatt-day (thermal energy, not electricity
output) of operation produces 1 gram of plutonium
in any reactor using 20 or lower enriched
uranium. - So, a 100 MW(th) reactor produces 100 grams of Pu
per day and could produce roughly enough
plutonium for one weapon every 2 to 3 months
depending on hours per day of operation. - A 25 MW(th) reactor produces approximately 1
bombs worth of Pu per year. e.g. the North
Korean 5MW(e) reactor
33Reactors and Proliferation Potential
- Reactor types that pose the biggest proliferation
potential - Heavy water research reactor of 25 MW(th) or
greater power rating (e.g. Israel and Iran) - Graphite reactor of 25 MW(th) or greater power
rating (e.g. North Korea) - Light water research reactor of 25 MW(th) or
greater power rating
34Civil Nuclear Energy and Proliferation (Dual-Use
Dilemma)
- Almost all reactors produce plutonium (large
commercial reactors produce about 1 kg/day) - Research reactors are in dozens of countries, but
most are low-power (lt 1 MW) - Reprocessing of commercial fuel has not spread as
broadly as originally forecast - Usually not economical to develop U enrichment
capability for a modest nuclear power program
e.g., Iran
35Proliferation and Reactor Choices
- Reactor design info readily available
- Natural U-fueled reactors do not need enrichment
plants - Heavy water reactors are smaller and have greater
neutron flux than graphite reactors. - But nuclear-grade graphite is usually easier to
obtain than heavy water. Both can operate on
natural U. - Possible to divert fuel from HEU-fueled reactors
e.g., Iraq considered doing this. - No proliferator has built and operated a
clandestine reactor ? hard to hide an operating
reactor ? need plausible dual-use story such as
scientific research or isotope production
36Proliferation Choices
- Heavy water, natural U-fueled reactor
- India
- Israel
- Pakistan
- Graphite, natural U-fueled reactor
- North Korea
- Light water research reactor
- Iraq (destroyed by Israel)
- No reactor (HEU path)
- South Africa
- Pakistan (until recently)
- Iraq (1982-1991)
- Iran
37Nuclear Fuel Cycle Links
- World Nuclear Association
- http//www.world-nuclear.org/education/nfc.htm
- How Nuclear Power Works http//science.howstuffwo
rks.com/nuclear-power.htm - Wikipedia Nuclear Power http//en.wikipedia.org/
wiki/Nuclear_power