just enough about nuclear power jerry peterson professor of physics
TRANSCRIPT
Just Enough about Nuclear Power
Jerry PetersonProfessor of Physics
Premises
• Electricity is the most valuable form of energy, most directly connected to ‘quality of life’
• Fossil fuels and CO2 are a problem
• Energy from nuclear fission can be clean and effective, and has a good history—a known science and a known technology
• But- there are problems-cost, proliferation of weapons, radioactive waste
Globally--
• 437 nuclear fission reactors for electrical power
• In 30 countries• 11% of global electricity• 70 under construction• 33 more nations are considering, planning, or
starting nuclear fission plants
(Y. Amano, IAEA Director General, Sept. 2014)
It all starts with 235U
-the ‘interesting’ isotope of uranium—Q? Where did that 235U come from?A. From gravity- which drives nuclear reactions in stars
which have exploded because they ran out of fuel—supernovae. These reactions made all heavy elements, and the ejecta condensed into new stars and planets. The isotope 238U has a half-life of 4.5 billion years, and 235U has a half-life of 0.7 billion years, so naturalU holds only 0.7% 235U, 99.3% 238U.
Why is 235U interesting?
The ‘curve of binding energy’ shows us that the fission of a heavy nucleus into two lighter nuclei gives off energy. Most nuclei are solidly stuck on the edge of this curve, but 235U is very lightly stuck, and may be released to roll ‘downhill’ by capturing a free neutron.
That fission releases 2.4 neutrons, so a chain reaction may follow
n + 235 92U 143 236U * FF1 +FF2+2.4 n
Or-Plan BMake a new element that behaves just like
23592U143.
n + 238 92U146239U239Np239 94 Pu145
Plutonium, an isotope with a 24,000 year half-life, so we can use it- fissions just like 235U,with extra neutrons
So-once you have 235U to yield fission neutrons, you may breed fissionable/neutron-yielding 239Pu from abundant 238U, and you are in business for a long time. This is the “breeder” reactor.
Neutron population dynamics
Four things can happen to a neutron amid Uranium #n Probability Expected neutronsFission 2.4 0.30 0.72Scatter 1. 0.28 0.28Capture 0 0.21 0.Escape 0 0.21 0. 1.00 1.00- a stable population
But- each fission releases energy.( with 2 net neutrons, 280=10 kilotons of TNT in a few microseconds)
We now use an ‘open cycle’
How to get 235U?
The isotope 235U (0.7%) may be separated from naturalU by gaseous diffusion or centrifuges in large plants. The chemical compound used is UF6, a corrosive gas.
Gas centrifuge farm
U3O8=yellow cake
Recap--natU, marketed as “yellowcake”, U3O8, 0.7% 235U
UF6, a gas used in enrichment
LEU=Low Enriched Uranium, 3-5% 235U, for power reactors
HEU=Highly Enriched Uranium>80% 235U, for bombsDU=Depleted Uranium, ~0.2% 235U
239Pu, made from 238U in reactors, for fuel or bombs
BWR
PWR
Nuclear News June 2014, page 94
Status-•99 currently in operation, 20% of US
electricity34 BWR65 PWR
•All within containment vessels•5 under construction
NY Times 11 October, 2010Companies retracting plans, because1.Haggles with gov’t loan guarantees.
2.Cheap gas, due to ‘fracking’3.Failure to pass any carbon tax.
Excelon says nuclear will pay only if gas $8/MBTU (currently $7.45 in New England) and $25/ton
carbon tax.
Problem #1-- Proliferation
• The hardest part of getting a nuclear bomb is the material
• “Front End”- obtain 235U (HEU=at least 80%) by exactly the same methods used to make Low Enriched Uranium (LEU), typically 3-4%.
• “Back End”- obtain 239Pu from Spent Nuclear Fuel by chemical reprocessing
Front End
• In a centrifuge farm, re-arrange the pipes to produce 50 kg of HEU instead of the expected larger amount of LEU.
• Convert the UF6 gas to metal
• Not very radioactive• Machine two hemispheres• Collide the hemispheres in a gun barrel—
works for sure: Hiroshima.
Back end
• Obtain Spent Nuclear Fuel, after the shortest possible exposure
• Reprocess– via chemistry• Machine a hollow sphere• Implode, with very precise technology• Each step involves strong radioactivity• Needs to be tested, and failure is likely:
Nagasaki
The second problem—radioactive waste
• Fission products, including infamous 131I, 137Cs, 90Sr
• Transuranics, from neutron capture on 238U, including plutonium
• Stored on site, in water or casks• Open cycle- to be buried in their rods
What’s in spent fuel?
SNF stored in water
How long can this be stored on site?
“at least 60 years beyond the licensed operating life of the reactor”
NRC proposal, Sept. 2010 ‘Nuclear News’
Where to put the other waste?
• Recent Defense Waste (Rocky Flats plutonium)– Waste Isolation Pilot Plant (WIPP), a salt mine in New Mexico. By truck.
• Old Defense Waste (plutonium and fission products)—buried at national labs in Washington and South Carolina, some as glass
• Low level waste– licensed commercial land fills
Small to Medium reactors
• The current buzz– 25-300 MWe, not the 1 GWe systems on current order.
• Many potential designs, vendors• Cheaper to build, modular• Sealed• Match market sizes• Stackable• Example—Hyperion, Denver/Los Alamos 25 MWe,
UN fuel, Pb-Bi coolant.
Public opinions, living within 10 miles of an operating nuclear plant.1800 adults, near 60 sites.Details:83% gave a high safety rating73% more should be built/ 69% would accept more reactors at the site
Nuclear News August 2015 page 25
