 Fission- a nuclear reaction in which a neutron strikes a relatively large atomic nucleus, which then splits into two or more parts. Nuclear Energy

C. How a Nuclear Reactor Works Nuclear fission of Uranium-235 and Plutonium-239 releases energy that is converted into high-temperature heat. This rate of conversion is controlled. The heat generated can produce high-pressure steam that spins turbines that generate electricity. These are non-renewable resources that must be mined -> leads to environmental issues and miners exposed to radon. U is enriched to make the reactor fuel, and is then formed into pellets (1-inch long and width of pencil)

Pellets are placed in thin metal pipes called fuel rods, which are bundled into clusters of rods called assemblies. Small reactors may have 250 assemblies while large reactors may have up to 3,000 assemblies. Nuclear Fission – splitting of an atom to release energy and particles. – Unstable Uranium is struck by a neutron and splits into 2 smaller nuclei, which releases more energy and neutrons – this initiates a chain reaction. Control rods are inserted between fuel rods to slow down a reaction (they absorb neutrons).

Periodic removal and storage of radioactive wastes and spent fuel assemblies Periodic removal and storage of radioactive liquid wastes Pump Steam Small amounts of Radioactive gases Water Black Turbine Generator Waste heatElectrical power Hot water output Condenser Cool water input Pump Waste heat Useful energy 25 to 30% Waste heat Water source (river, lake, ocean) Heat exchanger Containment shell Uranium fuel input (reactor core) Emergency core Cooling system Control rods Moderator Pressure vessel Shielding Coolant passage Fig , p. 346 Coolant Hot coolant

Light-water reactors (LWR) *Named so because they use light water as the moderator and coolant. This is what we use in the U.S.* 1. Core containing 35,000-40,000 fuel rods containing pellets of uranium oxide fuel. Pellet is 97% uranium-238 (nonfissionable isotope) and 3% uranium-235 (fissionable). 2. Control rods - move in and out of the reactor to regulate the rate of fission 3. Moderator - slows down the neutrons so the chain reaction can be kept going [ liquid water in pressurized water reactors; solid graphite or heavy water (D2O) ]. 4. Coolant - water to remove heat from the reactor core and produce steam

LWR These work by harnessing the energy from the nuclear reactions and heating water -> which creates steam to turn a turbine -> which is connected to an electrical generator. The water used requires cooling to be reused, so surface water from a lake, ocean or cooling tower are used to remove excess heat. This often results in thermal pollution. Reactor vessels and steam generators are contained in stainless steel and concrete containment buildings. These are built to last and are always built on stable land. Also, the air overhead is always a “no-fly” zone (worry of terrorists)

Nuclear Reactors

AdvantagesDisadvantages No air pollution is producedPossibility of accidents Countries can limit their need for imported oil Disposal of the radioactive waste Advantages and Disadvantages of Nuclear Energy

Low risk of accidents because of multiple safety systems (except in 35 poorly designed and run reactors in former Soviet Union and Eastern Europe) Moderate land use Moderate land disruption and water pollution (without accidents) Emits 1/6 as much CO2 as coal Low environmental impact (without accidents) Large fuel supply Spreads knowledge and technology for building nuclear weapons No acceptable solution for long-term storage of radioactive wastes and decommissioning worn-out plants Catastrophic accidents can happen (Chernobyl) High environmental impact (with major accidents) Low net energy yield High cost (even with large subsidies) AdvantagesDisadvantages Fig , p. 349

 Nuclear power is one way to achieve energy indpendence  Generates 70% of electricity in France and widely used in Lituania, Germany, Spain, U.K., Japan, China, and South Korea  Concerns over accidents and where to put the expensive, radioactive power plants and even what to do with the WASTES (and could they fall into the wrong hands) are the big cons  No NEW nuclear plants in the past 20 years, but a recent interest due to the lack of air pollution!

 Three-Mile Island – Pennsylvania 1979  Lack of cooling water around core caused a partial meltdown, radiation emitted, suspected increased infant mortality and cancer rates  Chernobyl – Ukraine, 1986  Ran “test” with no water and control rods – caused runaway reactions which led to explosion and fire. Several died at the scene, and many more later due to radiation, cancers, birth defects, etc

 Used nuclear fuel can continue to emit radioactivity long after its use – must be stored in highly secure locations for long periods of time  Two different types:  High-level radioactive waste- the form used in fuel rods.  Low-level radioactive waste- the protective clothing, tools, rags, and other items used in routine plant maintenance.

 Presently, nuclear power plants are required to store spent fuel rods at the plant itself – currently more than 100 sites in US storing spent fuel rods, most of them using “pool storage” (store in 20-ft deep water to shield radiation)  Some locations have run out of pool space and use lead-lined containers on land.  Eventually, all of this material will need to be moved to a permanent radioactive waste disposal factility.

 Movement and storage of nuclear wastes can be tricky – some old ideas were to shoot in space, dump in ocean, bury in ocean trenches, but all of these are too risky for leaks  Therefore, all wastes must be permanently stored on earth indefinitely, making sure it’s in a site where it cannot leach into water or soil and is far away from human habitation (NIMBY)  Transportation to site should minimize risk of accidents or theft by terrorists.  Yucca Mountain in Nevada was being scouted as a good location since the 70s, but the plans were recently cancelled due to change in presidential admins.  Famous case study – located somewhat near Vegas but also out in the desert

 Nuclear fusion- the reaction that powers the Sun and other stars. This occurs when lighter nuclei are forced together to produce heavier nuclei and heat is released.  Fusion is a promising, unlimited source of energy in the future, but so far scientists have had difficulty cotaining the heat that is produced. Fusion