Chapter 9
What reactions does the uranium go through in the refining process? Why does is need to be converted into a gaseous compound, UF6?
(1) uranium occurs as the ore uranium dioxide (UO2) (2) it is first oxidized to UO2+2 and U2O7-2, (3) then partially reduced by heating to yellowcake, U3O8, which can be thought of as a 1:2 combination of UO2 and UO3 (4) to enrich the uranium, the yellowcake is first oxidized completely to UO3 and then fluorinated to UF6 (5) finally, after reduction from +6 back to the +4 state, it is ready to be fabricated as UO2 into fuel pellets for the reactor enrichment is only possible in the gas phase (gas centrifugation = process to separate isotopes in which heavy atoms are split from the lighter atoms by centrifugal forces) enrichment processes use the slight mass difference of the U-235 and U-238 molecules of UF6 to separate these two components the product of the enrichment plant is UF6 with a U-235 share of approx. 3 to 5%
How far can b-particles travel through air and tissue? Why are b-particle emitters more dangerous when inhaled or ingested?
1 m in air 3 cm in water or biological tissue the radioactive nucleus is consequently close to the cell when it decays
What are the next four isotopes in the decay series after 222Rn? What types of particles are emitted when these isotopes decay? What are the half-lives of these isotopes? Why are these of more concern, when produced in air, than the radon itself?
222/86Radon ---> 218/84Polonium ---> 214/82Lead ---> 214/83Bismuth ---> 214/84Polonium alpha, alpha, beta, beta Radon = 3.8 d Polonium = 3m Lead = 27 m Bismiuth = 20m Polonium = <1s daughter isotopes have short half-lives and can decay while in lungs
What reaction leads to formation of 222Rn? Why is it significant that radon is a gas? Where does most radon in homes come from?
226/88Radium ---> 222/86Radon + 4/2Helium it is mobile and so can diffuse through soil rock or soil and escape into air top meter of soil below and around the foundation
What does it mean for a fuel rod to be "spent"?
235U content is too low for it to be useful anymore as a fuel since the rate at which the uranium produces neutrons has become too low, it is removed from the reactor and replaced
What are the main radioactive elements in spent fuel rods after 10 years? Why would they pose a serious environmental problem is released into the environment?
After 10 years, most of the radioactivity from spent fuel rods is due to strontium-90, 90Sr (half-life of 29 years), and to cesium-137, 137Cs (half-life of 30 years), both of which are -particle emitters Ions of both metals can be readily incorporated into the body because strontium and cesium readily replace chemically similar elements that are integral parts of animal bodies, including those of humans. strontium replaces calcium in bones and teeth ions of cesium replace potassium Once these elements are in place in the body, their radioactive disintegration produces particles that can damage the cells in which they are present or near.
What is depleted uranium used for?
Because uranium is so dense (70% denser than lead), it is useful for making armor-piercing weapons, especially projectiles used against tanks
What types of study have been done to estimate the risk of lung cancer caused by radon exposure in homes? What have been the results of these studies?
From the miner's statistical data relating their excess incidence of lung cancer to their cumulative level of exposure to radiation, a mathematical relationship between cancer incidence and radon exposure has been developed It is estimated that radon causes about 10% of all lung cancers
Define fission and fusion:
In fission, the collision of certain types of heavy nuclei with a neutron results in the splitting of the nucleus into two similarly sized fragments called fission products The combination of two very light nuclei to form one combined nucleus is called fusion.
Why are the ions produced by ionizing radiation problematic? What reaction will a water molecule ionized by radiation undergo?
Ionized DNA and protein molecules cannot carry out their normal functions, potentially resulting in radiation sickness and cancer H2O + radiation → e- + H2O+ H2O+ → H+ + OH forms free radical
How long is the half-life of Pu-239? What is one way of greatly reducing the long-term radioactivity of spent fuel rods?
The plutonium-239 isotope that is produced during uranium fission is an -particle emitter and has a long half-life of 24,000 years. After 1000 years, the main sources of radioactivity from spent fuel rods will be plutonium and other very heavy elements, since the medium-sized nuclei produced in fission, having much shorter half-lives than 1000 years, will have largely decayed by that time Thus the long-term radioactivity of the spent fuel rods can be greatly reduced by chemically removing the very heavy elements from it.
What is a bequerel? What is a Curie? In what units are exposure limits for radon set in the US?
The rate of radioactive disintegrations in a sample of matter is usually measured in bequerels, Bq, where 1 Bq corresponds to the disintegration of one atomic nucleus per second equals 3.7 x10^10 Bq and is the radioactivity produced by one gram of 226Ra picocuries, where 1 pCi = 10^-12 Ci
What type of radioactive decay occurs in the spent fuel rods? Why?
beta decay the spent fuel rods are very radioactive due to the actinides produced from intact uranium nuclei and from fission products generated when the actinides undergo fission the fission products are neutron rich and so the excess neutrons undergo beta decay until the product is no longer neutron rich
Describe the fission process of 235U. Why is it called a "chain reaction?"
collision of 235U nucleus with a neutron the combination of these two particles is unstable when it decomposes, the products vary but are typically a nucleus of barium, 142Ba, one of krypton, 91Kr, and three neutrons an average of about three neutrons are produced per 235U nucleus that reacts; one of these neutrons can produce the fission of another 235U nucleus, and so on, yielding a chain reaction
What hazards are associated with the mining of uranium ore? How does uranium naturally occur? What is its oxidation state?
contamination of the environment by radioactive substances as the ore uranium dioxide +4
What is a "dirty bomb"?
conventional, chemical-based explosive mixed with radioactive material that would be distributed over a wide area as a result of the bomb's explosion
What is one of the main benefits of nuclear power as opposed to conventional fossil fuel use for power?
does not generate CO2 or other greenhouse gases
What is a gamma ray? Why does gamma emission on its own not change the mass number of an isotope?
emission of a high energy photon produced along with other modes of decay after decay, daughter isotope frequently in excited state daughter returns to ground state by emitting γ ray has no effect on nuclear composition of parent/daughter it posses no particle mass
What practical use is there for americium-241?
emits a constant stream of alpha particles over many years, it is used as the source of ionizing radiation in conventional smoke detectors as long as the small electrical current comprised of charged particles passes between two electrodes in the ionization chamber of the detector, the alarm is not triggered however, any smoke that is present will absorb the alpha particles, thereby reducing the current and setting off the alarm
What happened at Three Mile Island? Why was this accident much less serious?
failures in the water-based cooling systems in the reactor although control rods were inserted into the core and the fission process stopped, the reactor kept heating the source of the heat was the radioactive decay by gamma-ray emission, not of uranium, but of fission products that had naturally built up over time in the fuel rods because of mechanical malfunction and operator error, the core became partially uncovered; it was therefore heated even further (2200°C), and half of it melted hydrogen and oxygen gases were produced from the decomposition of the superheated water there was no large explosion the containment building was not breached = contained within the facility radioactive gases escaped into air but were diluted into atmosphere
What possible fusion reaction would eliminate the neutron emission of the reaction?
fusion of deuterium (2H) and tritium (3H)
What are the issues with building fusion reactors?
fusion reactions all have extremely large activation energies due to the huge electrostatic repulsion that exists between the positively charged nuclei when they are brought very close together, which must happen before fusion can occur consequently, it is difficult to initiate and sustain a controlled fusion reaction that provides more energy than it consumes
What is a first order rate process? How does this relate to nuclear decay? What is a half-life? How can calculations relating to the rate of decay of a given isotope be performed?
half-life the same for a given isotope regardless of amount of isotope present nuclear decay is a first order rate process amount of time it takes for ½ of original isotope amount to decay see notes for calculation
What are the two important geological parameters for a long-term storage site?
high stability from possible disruption by volcanic activity or earthquakes; and low permeability to water, to insure minimal interactions with groundwater and the biosphere
How is electricity ultimately generated?
high-pressure/high-temperature steam is created in the process and is used to turn turbines and create electricity
Why is the disintegration of 210Pb to 210Bi of less concern than the previous four decays?
its half life is 22 yrs and most of the lead will have been cleared from the body before this process occurs
What disease is the biggest concern with respect to radon exposure? What population is at the greatest risk from radon exposure?
lung cancer miners who work in poorly ventilated underground uranium mines
How well do alpha particles penetrate materials? Why are a-particle emitters more dangerous when inhaled or ingested?
massive - cannot penetrate far into tissue, do significant damage to localized area they do penetrate can only travel a few thousandths of a cm within the body most dangerous when inhaled/ingested If an particle is emitted outside the body, it will usually be absorbed in the air or by the layer of dead skin, so it will do you no harm. However, inhaled or ingested radioactive atoms can cause serious internal damage when they emit particles The damage is particularly severe with particles since their energy is concentrated in a small area of absorption located within about 0.05 mm of the point of emission
Where does the 239-Pu in a fuel rod come from?
neutron produced by the fission of a 235U nucleus can be absorbed on collision by a 238U nucleus the resulting 239U nucleus is radioactive and emits a particle the heavy product 239Np is also radioactive and emits a particle resulting in 239Pu n + 238U ---> 239U ---> B + 239Np ---> B + 239Pu
Are all uranium isotopes fissile?
no only 235U
How much fuel rod waste has been deposited in long-term storage worldwide?
none
What happened at Chernobyl?
nonnuclear explosion During a routine test, engineers at the plant lost control of the reactor by overriding the plant's safety mechanisms and withdrew most of the control rods from the reactor core. As a consequence, the reactor overheated and a fire broke out in the graphite moderator. This produced a huge explosion, which blew off the heavy plate covering the building, and several hundred million curies of radioactivity were released into the air and spread over a wide area. leakage of highly radioactive fission products to the surrounding environment
What is a breeder reactor? What are its advantages and disadvantages?
nuclear power reactors that are designed specifically to maximize the production of by-product plutonium; such reactors actually produce more fissionable material than they consume fission can keep on going (don't need new starting material) fast breeder reactor core could melt if cooling system failed, since plutoniums melting point is 640C violent explosion that would occur if liquid sodium were to come into contact with water or air (cooled by liquid sodium)
What can enriched plutonium be used for? How much is required to make an atomic bomb?
nuclear weapons a few kilograms
How is radiation absorbed by the body measured? What are the typical sources of exposure for a person? What is a typical annual dose?
rad (radiation absorbed dose) units, where 1 rad is the quantity of radiation that deposits 0.01 joule of energy to 1 kilogram of body tissue rem (roentgen equivalent man) : incorporates type of radiation into dosage a particles more damaging than b or g Sv (sievert) : 1 Sv = 100 rem Average annual dose: 0.3 rem from: 55% radon in air 8% cosmic rays 8% rocks and soil 11% natural isotopes in body 18% anthropogenic (e.g. X-rays)
Why does food irradiation work to kill microorganisms? Why do sublethal doses of radiation affect rapidly dividing cells the most? What are the potential long-term effects of radiation exposure?
radiation damage is sufficient to kill cells of living organisms The earliest effects of this malady to be observed occur in tissues containing cells that divide rapidly, because damage to the cell's DNA or protein can affect cell division Long-term effects from radiation may show up in genetic damage, because chromosomes may have undergone damage or their DNA may have mutated. Such damage may lead to cancer in the person exposed or to effects in her or his offspring if the changes occurred in the ovaries or testes.
What types of soils contain 238U? What is a radioactive decay sequence? What is the final product of the radioactive decay sequence of 238U?
rocks and granite soils Daughter isotope of decay process may also be radioactive Radioactive daughter will then proceed to decay: according to its own characteristic half-life and decay mode Nuclei will continue this process of successive decays until a stable nucleus is formed lead (Pb)
What is the purpose of the "moderator", and what might be used as the moderator?
slows down the neutrons that leave the 235U nucleus so that they can be efficiently absorbed by other nuclei and cause further fission regular water heavy water (water enriched with deuterium) graphite
How does the CANDU reactor use non-enriched uranium? What potential radioactive contaminant is produced as a result of this process?
the advantage of using heavy water as a moderator is that it absorbs fewer neutrons than light water this allows the use of natural uranium (0.711% 235U) instead of enriched uranium (2-5% 235U) for fuel natural uranium fuel is less expensive than enriched fuel, although somewhat more fuel must be fed through the reactor to produce the same amount of energy Some of the neutrons produced by fission are absorbed by deuterium nuclei, producing tritium, 3H tritium is a human carcinogen
Where does the energy from a fission or fusion reaction come from?
the conversion of a tiny fraction, m, of the masses of the atoms and other particles involved E = mc^2
What happened at Fukushima? What type of explosions occurred there, and why?
the earthquake caused the shutdown of nuclear power stations along the coast (fission reactions stopped) also generated tsunami which overwhelmed defenses of the stations, destroyed power lines, and flooded stations disabled emergency generators; overall the result was lack of cooling water over the fuel rods led to nuclear meltdowns(core damaged due to overheating), hydrogen-air explosions, and release of radioactive materials fuel rods = zirconium tubes = came into contact with high temps and pressure = created hydrogen gas = spark was ignited = explosion occured
How is the chain reaction slowed down in a nuclear reactor as opposed to an atomic bomb?
the extra neutrons produced are absorbed by the control rods in a power reactor so that on average only one neutron released from each 235U fission event initiates the fission of another nucleus
What is the mass number of an isotope? How can the number of neutrons in an atom be deduced from the atomic number and the mass number?
the number of heavy particles (protons and neutrons) and not the actual mass of the nucleus atmoic number = number of protons number of neutrons = mass number - atmoic number
What sorts of containment technologies are being tested for fusion reactors?
the reactor design is based on the tokamak, a doughnut-shaped machine having a series of overlapping magnetic fields that can hold the plasma within reactor walls
What is immediately done with the spent fuel rods?
they are immediately cooled by placing them in large pools of circulating water for several years
Why are the particles emitted in radioactive decay processes dangerous?
they have high energy and when absorbed by materials they often ionize the atoms/molecules When absorbed by tissues, damage DNA/proteins/membranes ions produced are free radicals free radicals are very reactive and can do significant damage to biological molecules
Why are gamma rays the most dangerous radioactive emissions?
they readily pass through concrete walls, and our skin they are the most penetrating and therefore the most damaging of the three, traveling a few dozen centimeters into our bodies or even right through them they are generally the most dangerous type of radioactivity, since they can penetrate matter efficiently and do not have to be inhaled or ingested
What process does a radioactive nucleus undergo? What types of nuclei tend to undergo this process?
they spontaneously decompose by emitting a small particle that is very fast moving and therefore carries a great deal of energy with it the resulting isotope may or may not be radioactive Nuclei radioactive when they have an unstable ratio of protons:neutrons - stable ratio varies with size - most stable ratio apparent from average atomic mass - all elements have radioisotopes - Bi (Z=83) and above have no isotopes that aren't radioactive
What is nuclear power used for?
to produce electricity
What is the purpose of the coolant, and what might be used as the coolant?
used to carry off the heat energy produced by fission water gaseous CO2
How does plutonium become dangerous in terms of human health? What are two ways of disposing of plutonium?
when exposed to air, plutonium forms the oxide PuO2, a powdery dust that disperses readily and can be inhaled. Even microscopic amounts of plutonium oxide lodged in the lungs can induce lung cancer Mix it with other highly radioactive liquid waste and then vitrify the mixture into durable glass logs that would be buried far underground in metal canisters, as will be discussed. Vitrification chemically bonds liquid waste into a stable and durable borosilicate glass in which fission product oxides make up about 20% of the mass. Convert it to plutonium oxide and mix it with uranium oxide to produce mixed oxide fuel, as previously described, that could be used in existing power plants.
How are spent fuel rods re-processed? What makes the process particularly difficult? What are the possible end products? How are these used or stored?
Used fuel still contains about 96% of its original uranium, of which the fissionable U-235 content has been reduced to less than 1%. About 3% of the used fuel comprises waste products and the remaining 1% is plutonium (Pu) Reprocessing separates uranium and plutonium from waste products by chopping up the fuel rods and dissolving them in acid to separate the various materials the content of the fuel rods undergoes reprocessing to separate certain actinide elements and use some of them for nuclear fuel or for other purposes the chemical properties of all the actinides are quite similar, so subtle differences between their characteristics must be exploited uranium and plutonium The uranium is recovered from the organic phase as the dioxide. If desired, it can be converted back to UF6 and re-enriched in 235U. The plutonium can be directly made into mixed oxide (MOX) fuel, in which uranium and plutonium oxides are combined. In reactors that use MOX fuel, plutonium substitutes for the U-235 in normal uranium oxide fuel
What is an alpha particle? How can a balanced nuclear reaction be written showing emission of an alpha particle?
a radioactively emitted particle that has a charge of +2 and a mass number of 4 daugther isotope has 2 fewer neutrons and 2 fewer protons
What is a beta particle? How is it symbolized? What process occurring in the nucleus results in the formation of a beta particle? How can a balanced nuclear reaction be written showing emission of a beta particle?
an electron a neutron splits into a proton and an electron in the nucleus proton stays in the nucleus, electron is emitted
What is the linear-no-threshold assumption about radiation, and how does this relate to risk of lung cancer from radon exposure?
assumption—that the observed effects of high doses of radioactivity can be extrapolated linearly to very low doses and that there is no threshold below which radioactivity causes no harmful effects such as cancer Recent reviews of epidemiological studies carried out in North America and Europe find no threshold for increase in lung cancer with exposure to residential radon