Just Enough about Nuclear Power Jerry Peterson Professor of Physics

Premises Electricity is the most valuable form of energy, most directly connected to ‘quality of life’ Fossil fuels and CO 2 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 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 235 U -the ‘interesting’ isotope of uranium— Q? Where did that 235 U 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 238 U has a half-life of 4.5 billion years, and 235 U has a half-life of 0.7 billion years, so natural U holds only 0.7% 235 U, 99.3% 238 U.

Why is 235 U 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 235 U 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 U 143  236 U *  FF1 +FF2+2.4 n

Or-Plan B Make a new element that behaves just like U 143. n U 146  239 U  239 Np  Pu 145 Plutonium, an isotope with a 24,000 year half-life, so we can use it- fissions just like 235 U,with extra neutrons So-once you have 235 U to yield fission neutrons, you may breed fissionable/neutron-yielding 239 Pu from abundant 238 U, 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 neutrons Fission Scatter Capture Escape a stable population But- each fission releases energy. ( with 2 net neutrons, 2 80 =10 kilotons of TNT in a few microseconds )

We now use an ‘open cycle’

How to get 235 U? The isotope 235 U (0.7%) may be separated from natural U by gaseous diffusion or centrifuges in large plants. The chemical compound used is UF 6, a corrosive gas.

Gas centrifuge farm

U 3 O 8 =yellow cake

Recap-- nat U, marketed as “yellowcake”, U 3 O 8, 0.7% 235 U UF 6, a gas used in enrichment LEU=Low Enriched Uranium, 3-5% 235 U, for power reactors HEU=Highly Enriched Uranium>80% 235 U, for bombs DU=Depleted Uranium, ~0.2% 235 U 239 Pu, made from 238 U in reactors, for fuel or bombs

BWR

PWR

Nuclear News June 2014, page 94

Status- 99 currently in operation, 20% of US electricity 34 BWR 65 PWR All within containment vessels 5 under construction

NY Times 11 October, 2010 Companies retracting plans, because 1.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 235 U (HEU=at least 80%) by exactly the same methods used to make Low Enriched Uranium (LEU), typically 3-4%. “Back End”- obtain 239 Pu 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 UF 6 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 131 I, 137 Cs, 90 Sr Transuranics, from neutron capture on 238 U, 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 ‘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– 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 adults, near 60 sites. Details:83% gave a high safety rating 73% more should be built/ 69% would accept more reactors at the site Nuclear News August 2015 page 25