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 235 U + n → 236 U → 92 Kr Ba +  + 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 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.magnitude 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 H 2 O → ZrO H 2 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

Blame Management communication A bizarre series of operator mistakes Plant design, poor or no containment vessels Large positive void coefficient (steam bubbles in coolant) Poor graphite control rod design Poorly trained operators Shut off safety systems Helicopter drops Coverup

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 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 m 3 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.