Chapter 16 Nuclear Energy

Atoms and radioactivity Nuclear energy is derived from the splitting of atoms. A very small amount is converted to a large amount of energy The question is: is it worth the tradeoff? Atoms: protons, neutrons, and electrons Nucleus: protons and neutrons are found here (electrons outside) Atomic mass = protons + neutrons Charges….

Atomic number: = the # of protons (NOT NEUTRONS) Electrons are not counted in the atomic number nor the atomic mass because they are so small. Isotopes: different number of neutrons (same number of protons) CAN’T CHANGE THE # OF PROTONS. Uranium: 92 protons (elemental uranium would have 92 neutrons but is rarely found) - mostly uranium is found as an isotope where it has 146 neutrons, with a mass # of 238. - we call this U-238. Radioactivity: when isotopes are unstable, they decay, emiting particles, called radioactive atoms. - Marie Curie was the first to use the word “radioactivity”.

Two types of particles are given off by the nuclei of radioactive atoms, alpha and beta particles. Alpha: made up of 2 protons and 2 neutrons. Beta: are high speed electrons. A third particle that can be given off is Gamma. Gamma rays are a form of electromagnetic radiation. As a whole, alpha, beta, and gamma rays are what we call radiation. ***an atom that emits alpha particles loses protons and neutrons, while an atom that emits beta particles converts neutrons to protons. These thus alter the atomic number and mass number, therefore, we get a new element.

U-238 and U-235 (both isotopes of uranium) decay into stable forms of lead. Half-life: the amount of time it takes for half of the atoms in a sample or radioactive element to decay. - can be a few seconds to billions of years - the half life of u-238 is about 4.5 billion years and U-235 is about 700 million years.

Reactions and Reactors nuclear fission: a reaction in which the nucleus of a large atom is split into smaller nuclei. (when broken it emits a large amount of energy) U-235 is most used in fission reactions When split, forms new nuclei called daughter nuclei, which are often barium or krypton (many possible though). Almost all are radioactive

The steps: A neutron is fired into the nucleus of the atom. It hits the nucleus, which splits, forming two daughter nuclei This releases energy, and several new nuetrons These new neutrons strike other nuclei (hence the “chain reaction” you might have heard of) Each new split release energy (it grows exponentially)

Nuclear reactors: Main purpose is for the generation of electricity. Heat is produced from the energy released Takes place in a nuclear reactor vessel (which is about 20m tall and reinforced with 15-30 cm of steel) Must also be a containment building (shield) to keep neutrons and radiation from escaping This is all housed inside a thick concrete building The “fuel” are rods filled with U-235 pellets Rods are positioned so water can circulate around them Water serves two functions: Keep things cooled off To slow the movement of neutrons

Nuclear reactors continued, Control rods regulate the speed of the reaction (made of cadmium, boron, and other stuff) they absorb neutrons. These rods are lowered as needed to control the speed of the reaction (if there is a problem with this, the reaction will grow to quickly and explode) Water temps may reach 275 degrees Celsius. This water is moved through pipes to a heat exchanger, where it heats water to steam, thus turning turbines to generate electricity. Breeder reactors 99% of U-238 (the most common form found) is NOT fissionable, so therefor must be changed By adding a neutron (becomes U-239) it gives rise to an atom of plutonium-239 which can be used. Killing two birds with one stone (energy is produced in this reaction, and a new fuel is being formed) This is not used in the US due to the excess fuel being made and the threat of nuclear terrorism. The plutonium being produced is what is used in atomic bombs.

Radioactive Waste Approx. 32 metric tons of spent fuel is produced (average reactor) per year. Why is radiation bad: Actively dividing cells are very sensitive to radiation (can cause mutations) Amount of exposure determines extent of damage Extended exposure can be immediate (burns, anemia, and death) Exposure can also mutate DNA (cancer, etc….) Exposure is measured in REMS. Most Americans received .2-.5 exposure per year.

Types of Waste High level wastes: lots of radiation emitted (LLW= small amounts) Examples of HLW: fuel rods, control rods, and the water used to cool these. MLW and LLW: not as radioactive, but much larger amounts are generated. There are many sources of these, LLW are typical in hospitals and labs.

Waste Disposal Difficult problem to deal with Have to be stored safely for thousands or millions of years Plutonium U-239 has a half life of 24,000 years Sites must be geologically stable (no earthquakes, or volcanoes) Very expensive Most wastes have not been disposed of permanently, many are sitting around waiting to be dealt with (contaminating as they sit).

Meltdown: process by which a nuclear chain reaction goes out of control and melts the reactor core. Example: Chernobyl. Yucca Mountain: is currently identified by Congressional law as the nation's spent nuclear waste storage facility. However, while licensure of the site through the Nuclear Regulatory Commission is ongoing, political maneuvering led to the site being de-funded in 2010. Chemnuclear: EnergySolutions designed, licensed and constructed this 235-acre site to accept all classes of low-level waste including very high activity waste forms and large components like reactor vessels. EnergySolutions has also operated this facility since it opened in 1971 – more than 40 years. In more than three decades of operation, we have disposed of 28 million cubic feet (800,000 m3) of low-level radioactive waste, 40 large components, including Reactor Pressure Vessels and Steam Generators, with no regulatory interruptions, and unparalleled support from the local community.