Energy and the Environment

Energy and the Environment Fall 2012 Instructor: Xiaodong Chu Email：chuxd@sdu.edu.cn Office Tel.: 81696127

Flashbacks of Last Lecture • Nuclear energy is derived from the binding force (结合力) that holds the nucleons (核子) of the atomic nucleus (原子核) together • The binding force per nucleon is greatest for elements in the middle (e.g., Fe) of the periodic table and is smallest for the lighter and heavier elements • When lighter nuclei fuse (聚变) together, energy is released; when heavier nuclei undergo fission(裂变) , energy is also released

Nuclear-Fueled Power Plants: Radioactivity • Radioactivity is the spontaneous decay of certain nuclei, usually the less stable isotopes of theelements, both natural and man-made, which is accompanied by the release of very energeticradiation • After the emission of radiation, an isotope of the element (or even a new element) isformed, which is usually more stable than the original element • It is important to note that radiationemanates directly from the nucleus, not the atom as a whole • In radioactive decay, there are three types of radiation: α, β, and γ • Only the latter is a formof electromagnetic radiation; the two former are emissions of very high energy particles • All threeare called ionizing radiation (电离辐射) because they create ions as their energy is absorbed by matter through which they travel

Nuclear-Fueled Power Plants: Radioactivity • In alpha radiation, a whole nucleus of a helium atom, containing two protons and two neutrons,is emitted • The daughterisotope moves two elements backward from the parent element in the periodic table • Because α particles are relatively heavy, their penetration depth into matter is very small, on theorder of a millimeter, which a sheet of paper or a layer of dead skin on a person can stop

Nuclear-Fueled Power Plants: Radioactivity • In beta radiation, an electron is emitted • This electron stems from the nucleus itself: a neutron converts into a proton with the emission of an electron • A parent isotope converts into a daughter isotope, which is an element forward in the periodic table because a proton has been added to the nucleus • Because 90Sr is one of the products of uranium fission, this isotope is a major source of radiation from the spent fuel of a nuclear reactor • The emitted electron is relatively light; therefore it can penetrate deeper into matter, on the order of centimeters, to shield against which, a plate of metal, such as lead, is necessary

Nuclear-Fueled Power Plants: Radioactivity • The emission of gamma radiation follows the emission of beta radiation • The number of protons or neutrons does not change in that radiation, the radiating isotope does not change its position in the periodic table • Gamma radiation is essentially the emission of very shortwave, and hence energetic, electromagnetic radiation • Because γ rays carry no mass, its penetration into matter is very deep, on the order of meters, and very heavy shielding is necessary to protect against it

Nuclear-Fueled Power Plants: Radioactivity • Radioactivity accompanies the whole nuclear power plant fuel cycle, from mining of uranium ore, through uranium extraction (铀提取), isotope enrichment (同位素浓缩), fuel preparation, fuel loading, reactor operations, accidents, plant decommissioning(退役), and spent fuel disposal

Nuclear-Fueled Power Plants: Radioactivity • The decay rate of an ensemble of radioactive nuclei is governed by the law of exponential decline • Integration form • The time after which the number of decaying nuclei is halved is called the half-life

Nuclear-Fueled Power Plants: Radioactivity • The level of radioactivity of a sample of substance is measured by the number of disintegrations per second

Nuclear-Fueled Power Plants: Radioactivity • An accumulation of absorbed α, β, and γ radiation over time is called a radiation dosage (辐射剂量) • The average person in the United States receives in one year a dosage of about 360 millirems (3.6 mSv), of which 200 is from radon-86, 27 from cosmic rays, 28 from rocks and soil, 40 from radioactive isotopes in the body, 39 from X rays, 14 from nuclear medicine, and 10 from consumer products and other minor sources • The average person on earth receives about 2.2 mSv y−1 • A short-term dose of 1 Sv causestemporary radiation sickness; 10 Sv is fatal

Nuclear-Fueled Power Plants: Radioactivity • The greatest risk to humans of nuclear power plant operations is associated with radioactivity, alsocalled ionizing radiation, because of the creation of ions left by the passage of α, β, and γ rays andneutrons • Radioactivity affects humans and animals, causing somatic (躯体的) and genetic (基因的) effects • Somaticeffects can be acute when an organism is subjected to large doses of radiation, or chronic when theexposure is at low levels, but over lengthy periods • Geneticeffects become apparent in later generations but not in the exposed persondue to mutations (变异) in the genetic material, e.g., chromosome (染色体) abnormalities or changes inthe individual’s genes that make up the chromosomes

Nuclear-Fueled Power Plants: Radioactivity • The prescription of radiation protection standards is an onerous and controversial task • The task is difficult because direct evidence on biological effects of radiation comes from high-level exposures • Lower-level exposure data can only be obtained from animal studies, then extrapolated to humans • In setting radiation standards, the following three assumptions are made: • There is no threshold dose below which radiation has no effect • The incidence of any delayed somatic effect is directly proportional to the total dose received • There is no dose-rate effect • The Nuclear Regulatory Commission in the United States set a standard of exposure for workers in nuclear power plants to 50 mSv y-1 ; for the general population—that is, any person in the region outside the plant boundary—the standard is 1 mSv y-1

Nuclear-Fueled Power Plants: Nuclear Reactors • A nuclear reactor in a nuclear fueled power plant is a pressure vessel enclosing the nuclear fuel that undergoes a chain reaction, generating heat which is transferred to a fluid, usually water, that is pumped through the vessel

Nuclear-Fueled Power Plants: Nuclear Reactors • In a nuclear reactor of a power plant, the splitting of the nucleus and sustaining of the ensuing chain reaction has to proceed in a controlled fashion • The basic ingredients of a nuclear reactor are fuel rods(燃料棒), , a moderator(慢化剂), control rods (控制棒), and a coolant(冷却剂)

Nuclear-Fueled Power Plants: Nuclear Reactors • The fuel rods contain the fissile isotopes 235U and/or 239Pu • Natural uranium contains about 99.3% 238U and 0.7% 235U • The concentration of the fissile isotope 235U in natural uranium is not enough to sustain a chain reaction in most power plant reactors; therefore, this isotope needs to be enriched to 3–4% • The fuel rods contain metallic uranium, solid uranium dioxide (UO2), or a mix of uranium dioxide and plutonium oxide, called MOX, fabricated into ceramic pellets (陶粒) • The pellets are loaded into zircalloy (锆合金) or stainless steel tubes, about 1-cm diameter and up to 4 m long • Once the remaining fuel in the rods cannot sustain the rated capacity of the plant, even with complete withdrawal of the control rods, the fuel rods need to be replaced

Nuclear-Fueled Power Plants: Nuclear Reactors • Moderators are used to slow the energetic neutrons that evolve from the fission reaction, yielding low-energy neutrons, also called thermal neutrons • This increases the probability for the neutrons to be absorbed in another fissile nucleus, so that the chain reaction can be propagated • Moderators contain atoms or molecules whose nuclei have high neutron scattering (中子散射) and low neutron absorption (中子吸收)characteristics • Typical moderators are light water (H2O), heavy water (D2O), graphite (C), and beryllium (Be) (铍) • The light or heavy water moderators circulate around the fuel rods. • Graphite or beryllium moderators constitute a block into which fuel rods are inserted, and e.g., the Chernobyl-type reactors also use graphite as a moderator

Nuclear-Fueled Power Plants: Nuclear Reactors • Control rods contain elements whose nuclei have a high probability of absorbing thermal neutrons, so that they are not available for further splitting of fissile nuclei • In the presence of control rods, the chain reaction is controlled or stopped altogether • Typical control rods are made of boron (B) (硼) or cadmium (Cd) (镉)

Nuclear-Fueled Power Plants: Nuclear Reactors • The chain reaction inside the reactor is governed by the neutron economy coefficient (中子有效利用系数) k • Under a steady state, the number of thermal neutrons is invariant with time, dn/dt = 0, and k = 1 • The reactor is then in a critical condition • When k < 1, the reactor is subcritical (亚临界); when k > 1, it is supercritical (超临界) • A nuclear reactor becomes critical when control rods are lifted out of the core of the reactor to a degree where more than one neutron released by the fission of a fissile nucleus survives without being absorbed by the control rods • The position of the control rods determines the power output of the reactor • Monitoring the critical condition in a nuclear reactor while varying the output is quite complicated • Nuclear power plants are run at full load, providing the base load of a grid

Nuclear-Fueled Power Plants: Nuclear Reactors • Heat must be constantly removed from the reactor • Heat is generated not only by the fission reaction, but also by the radioactive decay (放射性衰变) of the fission products • Heat is removed by a coolant, which can be boiling water (沸水), pressurized water (压水), a molten metal (金属液) (e.g., liquid sodium), or a gas (e.g., helium or CO2) • The accident at the Three Mile Island power plant near Harrisburg, Pennsylvania, in 1979 occurred because after shutdown (full insertion of the control rods), the reactor was completely drained of its coolant, so that the residual radioactivity in the fuel rods caused a meltdown of the reactor • The heat removed by the coolant, in the form of steam or pressurized hot water, is used in conventional thermodynamic cycles to produce mechanical and electrical energy

Energy and the Environment