Nuclear Energy. How does a nuclear reactor work? Is it a major energy source worldwide? Is it Green? Problems – Waste Disposal – Accidents Future – Research.

Nuclear Energy

Nuclear Energy

• How does a nuclear reactor work?• Is it a major energy source worldwide?• Is it Green?• Problems– Waste Disposal– Accidents

• Future– Research– Generation IV

Nuclear Energy Plant• Nuclear Fission• 235U + n → 236U → 92Kr + 141Ba + + 3n • Chain Reaction• Controlled by control (graphite) rods and water

coolant• Heat from reactor is cooled by circulating pressurized

water• Heat exchange with secondary water loop produces

steam• Steam turns turbine generator to produce electricity

Present Nuclear Energy

• 100 plant produce about 20 % of the electricity in US

• 431 plants worldwide in 31 countries produce about 17 % of the world’s electricity

• Environmental Impact– No Greenhouse gases– Completely contained in normal operation– Spent fuel issue

Waste Disposal

• Waste kept at plant, but running out of room.• Site chosen in Nevada for nuclear waste.• Research on safe transportation• Nuclear proliferation; fuel is very dilute and

not easily converted to weapons grade• Stored in very heavy casings (difficult to steal)

Accidents

• Nuclear Meltdown• Fukushima Daiichi • Chernobyl• Three Mile Island• Environmentalist watch dogs note other near

misses in recent years

Fukushima - 2011

• 6 Power plants on site• 9.0 earthquake , followed by a 45 ft tsunami• Flooded power plant• 1/10 radiation that was released in Chernobyl

accident

More info

• Reactor 4 had been defueled at time of shutdown and Reactors 5 and 6 were in shutdown mode for routine maintenance

• The tsunami destroyed the connection to the grid

• The tsunami flooded the pumps, shorting them out

• Reactors 1-3 experience complete meltdown

more• Tokyo in 2008, an IAEA expert warned that a strong earthquake

with a magnitude above 7.0 could pose a "serious problem" for Japan's nuclear power stations.

• In the late 1990s to comply with new regulatory requirements, three additional backup generators for reactors Nos. 2 and 4 were placed in new buildings located higher on the hillside. All six reactors were given access to these generators, however the switching stations that sent power from these backup generators to the reactors' cooling systems for Units 1 through 5 were still in the poorly protected turbine buildings. All three of the generators added in the late 1990s were operational after the tsunami. If the switching stations had been moved to inside the reactor buildings or to other flood-proof locations, power would have been provided by these generators to the reactors' cooling systems.[

• Hydrogen Explosions: Zr + 2 H2O → ZrO2 + 2 H2 • Sea water

Chernobyl (1986)• A planned test gone horribly wrong• The test

– See if turbine generator could power the water pumps that cool the reactor in the event of a loss of power

– Crew shut off power too rapidly, producing a Xe isotopes that poisons the reactor

– In response the rods were lifted to stimulate reaction– The lower cooling rate of the pumps during the experiment led to

steam buildup that increase reactor power– Temperature increased so rapidly, that rod insertion could not be

performed in time to stop meltdown– Roof blew off, oxygen rushed in a caused fire that spread radioactive

material over a large area

BlameManagement communicationA bizarre series of operator mistakesPlant design, poor or no containment vesselsLarge positive void coefficient (steam bubbles in

coolant)Poor graphite control rod designPoorly trained operatorsShut off safety systemsHelicopter dropsCoverup

Consequences

• Deaths of plant and workers • Medical problems (short and large term)• Thyroid cancer• Contaminated soil as far as Great Britain• Billions of $

Comparison

• A key difference between the Fukushima accident and the Chernobyl accident was that the Chernobyl explosion shattered the fuel and flung it out of the reactor building, while at Fukushima there was no steam explosion driven by the release of fission energy.

Three Mile Island

• Partial meltdown• No radiation escaped• Caused fear of nuclear power and cost $ in

terms of clean up• Operator error and lack of safety backups in

design• In some ways the accident showed how the

kind of catastrophic disaster at Chernobyl is avoidable

types

• Generation I – retired; one of a kinds• In operation Gen II and Gen III• Gen II was a large design changes• Gen III and Gen II, upgraded with many safety

features along the way• Gen III plus (passive safety systems)• Gen IV, 30 yrs away

Gen IV

• Very High Temperature Reactor• Advance Nuclear Safety; • Address Nuclear Nonproliferation and

Physical Protection Issues; • Are Competitively Priced• Minimize Waste and Optimize Natural

Resource Utilization• Compatible with Hydrogen Generation

Gen IV Roadmap - 2002

• Solicited design models• Chose six design models to base future

research• Out of these six, the DOE has relatively

recently selected two for further investment– Very-High Temperature Reactor (VHTR) – Sodium-Cooled Fast Reactors (SFR)

Very-High Temperature Reactor

• Reach temperatures > 1000 C• Drive water splitting for hydrogen production

– 2 M m3

• 50% efficiency for producing electricity• Heat and power generation• Fuel recycling/reprocessing• Fuel coating requirements, absorbers, ceramic

rods, vessel materials, passive heat removal systems

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Actinide management

• To support effective actinide management a fast reactor must have a compact core with a minimum of materials which absorb or moderate fast neutrons. This places a significant heat transfer requirement on the coolant.

Sodium-Cooled Fast Reactors• Old technology• Management of waste• Low system pressure, high thermal conductivity, large safety

margins.• Burns almost all of the energy in uranium, as opposed to 1%

in today’s plants• Smaller core with higher power density, lower enrichment,

and lower heavy metal inventory.• Primary system operates at just above atmospheric pressure• Secondary sodium circulation that heats the water (if it leaks,

no radiation release)• Demonstrated capability for passive shutdown and decay

heat removal.