Chapter 7

Learning Objectives

▪ What are radiation, radioactive isotopes, fission, radioactive decay, and half-life? ▪ How nuclear power plant generates electricity? ▪ Is nuclear power plant safe? ▪ How is nuclear power plant different from nuclear weapon? ▪ What dilemma we have with nuclear wastes?

control rods & spent fuels (5)

▪ With the control rods fully inserted, the fission is not self-sustaining. - But as the rods are withdrawn, the reactor can "go critical"; that is, the fission chain reaction can become self-sustaining. This condition will not last. ▪ Over time, fission products that absorb neutrons build up in the fuel pellets. ▪ To compensate, the control rods are pulled further out. ▪ Eventually, the reactor bundles of fuel rods must be replaced. These replaced nuclear fuels are called spent fuels.

Mass number, atomic number, isotopes (5)

*Mass number = # of protons + # of neutrons* *Atomic number = # of protons* ▪ Along with its symbol, each element is identified by atomic number. ▪ Every nucleus of the uranium atoms has the same number of protons as 92 because those atoms are all the same element called uranium. ▪ But there are two or more forms of the same element whose atoms have different mass number because they have different number of neutrons. These are called isotopes. For example, here are two isotopes of uranium.

Balancing Nuclear Equations

1) Conserve mass number (A) - The sum of protons plus neutrons in the products must equal the sum of protons plus neutrons in the reactants 2) Conserve atomic number (Z) or nuclear charge - The sum of nuclear charges in the products must equal the sum of nuclear charges in the reactants ************************************ ▪ in every nuclear reaction, there must be conservation of mass number and conservation of atomic number ▪ let's take a look at the conservation of mass number first: - on the reactant side/left-hand side you have 235 +1... that must be equal to 138 + 96 + 2x1 bc there are two neutrons - why 2x? bc there are 2 neutrons... so that must be equal to each other ▪ Second, the numbers in the subscript should also be conserved ▪ the atomic number should also be the same between the sum of those numbers in the left-hand/reactant side, and the sum of those subscript numbers on the product side/right hand side - so 92 +0 must be equal to 55 +37 + 2x0

He, chain reaction

Although a fission reaction, once started, can sustain itself by a chain reaction, neutrons are needed to induce the process. One means of generating neutrons is to use a combination of beryllium (Be) and plutonium (Pu). Pu releases alpha particles, He:

Atomic number (Z)

Atomic number (Z) = number of protons in nucleus

Chernobyl: Political Consequences

Distrust of government. Soviet Union cover up: Sweden and Poland were the first nations to bring attention to the accident. Other nations attempted to downplay the health effects of the accident in their own nations. Public opposition to building additional nuclear power plants increased significantly worldwide.

Half - life

Half-life is the time required for the level of radioactivity to fall to one half of its initial value. For example, plutonium (Pu)-239 has a half-life of 24,110 years. Accordingly, it will take 24,110 years for the radioactivity of a sample of Pu-239 to halve.

Ionizing radiation

Ionizing radiation has enough energy to ionize atoms or molecules while non-ionizing radiation does not.

Mass number (A)

Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons

Nuclear Fission

Nuclear fission is the splitting of a large nucleus into smaller ones with the release of energy.

What is Fission and How Does it Produce Energy? (3)

Nuclear fission is the splitting of a large nucleus into smaller ones with the release of energy. Energy is released because the sum of the masses of these fragments is less than the original mass. This 'missing' mass (about 0.1 percent of the original mass) has been converted into energy according to Einstein's E=mc2 equation.

gaseous diffusion

One way to separate molecules of different masses is by gaseous diffusion, a process used to separate gases with different molecular weight by forcing them through a series of permeable membranes. Lighter gas molecules diffuse more rapidly through the membrane than heavier ones.

Chernobyl: Social and Environmental Consequences

Over 1,000 injuries and thirty-one deaths of firefighters and others who reported to scene of accident. 150,000 people evacuated from their Ukraine homes. Radioactive cloud released over a large part of Europe. Health threatening levels of radioactive materials were found in at least twenty nations, and as far away as 2,000 km from Chernobyl. Estimated 250 million people were exposed to unhealthy amounts of radiation. Estimates of future cancers from the accident range anywhere from 7,500 to 1 million. Radioactive particles in the environment and in the food chain. Large amount of uncertainty and fear in the population.

Radioactive material Radioactivity

Radioactive material is a physical material that emits ionizing radiation. Radioactivity refers to the spontaneous emission of radiation by certain elements.

Radon (222Rn)

Radon (222Rn) is a radioactive gas that is produced midway in both the U-238 and U-235 radioactive decay series. Thus, wherever uranium is present, so is radon.

E = Δmc2

The amount of the energy released as a result of nuclear fission is calculated from the equation: E = Δmc2 where Δm is the difference in mass between the products and the reactants of a nuclear reaction, and c is the speed of light equal to 3x10^8 m/sec.

Where is the boric acid solution of primary coolant kept?

The boric acid solution of the primary coolant is kept isolated in a closed circulating system, which makes the transfer of radioactivity to the secondary coolant water in the steam generator highly unlikely.

Where is the heat from the primary coolant transferred

The heat from the primary coolant is transferred to what is referred to as the secondary coolant, the water in the steam generators that does not come in contact with the reactor.

What is the new element formed (previous slide)

The new element formed, neptunium-239 (Np-239), also is a beta-emitter and decays to form plutonium-239 (Pu-239).

Process of nuclear fission

The process of fission usually requires neutrons to initiate it and always releases neutrons as follows: (see picture)

An example of alpha (α) radiation:

The sum of the superscripts on the left equals the sum of the superscript on the right. Likewise, the sum of the subscripts on each side equals.

An example of beta (β) radiation:

The sum of the superscripts on the left equals the sum of the superscript on the right. Likewise, the sum of the subscripts on each side equals.

Comparing different Uraniums

U-238 has the same number of protons as U-235, but U-238 has bigger mass number by three because it has three more neutrons than U-235.

Chernobyl-What Happened: April 26, 1986

While performing a safety test on Reactor 4, technicians allowed a power surge that reached 120 times the rated capacity of the reactor. The surge, or "slow nuclear explosion", ripped open the core, including cooling water pipes, and caused a huge steam explosion. The 4,000 ton concrete covering over the reactor was blown away. Fires broke out in many places all over the site. Fifty different radioactive isotopes were released, with half-lives spanning from two hours to 24,000 years. These isotopes were shot 1.5 miles into the sky.

radioactive decay & radioactive decay series

▪ *Radioactive Decay* - Disintegration of a radioactive atom through emissions of alpha or beta particle ▪ *Radioactive decay series* is a characteristic pathway of radioactive decay that begins with a radioisotope and progress through a series of steps of radioactive decays to eventually produce a stable isotope.

PuBe ("Poo-Bee") (5)

▪ A neutron source constructed with Pu and Be is called "Poo-Bee" source or a PuBe. One fission event, like the previous equation, produces three neutrons. - The trick is to "sponge up" these extra neutrons, but still leave enough to sustain the fission reaction. ▪ With extra neutrons, the reactor will run at too high a temperature; with too few neutrons, the chain reaction will halt and the reactor will cool. A delicate balance must be maintained. ▪ This balancing job is done by metal rods called control rods. - Control rods, composed primarily of an excellent neutron absorber such as cadmium or boron, can be slid up or down fuel rods to absorb fewer or more neutrons.

II. How nuclear reactors produce electricity (5)

▪ A nuclear power station consists two parts: nuclear and non-nuclear. - The nuclear part is the reactor which is housed, together with one or more steam generators and the primary cooling system, in a special steel vessel within a dome-shaped containment building. - Reactor contains fuel rods which house uranium fuel in the form of uranium dioxide (UO2) pellets. ▪ The non-nuclear part contains the turbines that run the electrical generator. - It also contains secondary cooling system. It must be connected to some means of removing heat from the coolants.

Nuclear Fission (3)

▪ A nuclear reaction in which a large nucleus splits into smaller nuclei with the release of a large amount of energy ▪ Fission reactions are usually induced by the bombardment of a neutron ***************************** - as you can see in the bottom reaction, we have U 235 that has a large nucleus and when that nucleus is bombarded by a neutron (n) , then the U nucleus splits into smaller nuclei , Sr and Xe, and in that fission reaction, it releases large amounts of energy, along with 3 additional neutrons

nuclear fuel for atomic bomb (3)

▪ An another way of making nuclear fuel for atomic bomb is to use the plutonium-239 formed from U-238 in a conventional reactor. ▪ U-238 absorbs a neutron and forms the unstable U-239. ▪ Instead of undergoing fission, U-239 undergoes beta decay in matter of hours:

Atomic Bombs

▪ An atomic bomb is never assembled with the critical mass already present. The critical mass is formed by using a conventional explosive to force the fissionable sections together. ▪ The energy released quickly blows the critical mass apart, stopping the fission. ▪ Fission of 1 kg (2.2 lbs) of pure U-235 -> 1.0g mass lost -> Energy release Twice that of the atomic bombs dropped on Hiroshima and Nagasaki in 1945

Third Coolant (2)

▪ As a third coolant, river or ocean water comes in direct contact with the condenser. - The ocean water does not come in contact with the secondary cooling system, so the ocean water is well protected from radioactive contamination.

Nuclear Power Plants

▪ At the nuclear reactor core: nuclear fission of U-235 is initiated by the neutrons ▪ Too many neutrons: the reactor runs at too high temperature Too few neutrons: the chain reactions halts and the reactor cools down ▪ Control of extra neutrons: control rods (Cd or C) absorbs neutrons

Uranium ore is not gas... so how do we separate it?

▪ But uranium ore clearly is not a gas; rather, it is a mineral that contains UO3 and UO2. However, the compound uranium hexafluoride (UF6) has a notable property. Known as "hex," UF6 is a solid at room temperature but readily vaporizes when heated to 56 oC. To produce hex, the uranium ore is converted to UF4, which in turn is reacted with more fluorine gas: ▪ On average, a 235UF6 molecule travels about 0.4% faster than a 238UF6 molecule. If the gaseous diffusion process is allowed to occur repeatedly through a long series of permeable membranes, significant amounts of 235UF6 and 238UF6 can be separated.

III. Chernobyl — the worst nuclear power plant accident (3)

▪ During a safety test, operators deliberately interrupted the flow of cooling water to the reactor as part of test. The temperature of the reactor rose rapidly. ▪ In addition, the operators had left an insufficient number of control rods in the reactor, and the steam pressure was too low to provide coolant (due to both operator error and faulty design). A chain of event quickly produced a disaster. ▪ An overwhelming power surge produced heat, rupturing the fuel elements, and releasing hot reactor fuel particles. These, in turn, exploded on contact with the coolant water and the reactor was destroyed in seconds.

Enriched uranium (3)

▪ Enriched uranium is uranium that has a higher percent of U-235 than its natural abundance of about 0.7%. ▪ Since U-238 is naturally much more abundant (99.9%) than U-235 (0.7%) and since these isotopes behave essentially the same in all chemical reactions, the separation of these two isotopes is extremely difficult and relies on advanced technology that is not readily available. ▪ U-235 differs from U-238 only by three neutrons; nonetheless, this mass difference can be exploited to achieve a separation, and thus to obtain the highly enriched U-235 (as high as 90%), sometimes referred to as weapons-grade uranium.

IV. Nuclear waste (5)

▪ High-level radioactive waste (HLW) has high levels of radioactivity and, because of the long half-lives of the radioisotopes involved, requires essentially permanent isolation from the biosphere. HLW largely comes from nuclear power plants. ▪ Low-level radioactive waste (LLW) is waste contaminated with smaller quantities of radioactive materials than HLW and specifically excludes spent nuclear fuel. LLW includes a wide range of materials such as contaminated laboratory clothing, gloves, and cleaning tools from medical procedures using radioisotopes and even discarded smoke detectors. ▪ Spent nuclear fuel (SNF) is the radioactive material remaining in fuel rods after they have been used to generate power in a nuclear reactor and is regulated as HLW. ▪ Huge quantities of HLW also were created during the Cold War because reactor fuel was reprocessed to produce plutonium for military uses. ▪ Most plans for isolating HLW employ a method known as vitrification, in which the spent fuel elements or other mixed waste are encased in ceramic or glass.

Nuclear equation subscripts

▪ In a nuclear equation, the sum of the subscripts on the left must equal that of the subscripts on the right. Likewise, the sum of the superscripts on each side of the equation must be equal. ▪ Left superscripts: 1 + 235 = 236 ▪ Right superscripts: 141 + 92 + (3x1) = 236 ▪ Left subscripts: 0 + 92 = 92 Right subscript: 56 + 36 + (3x1) = 92

Nuclear chain reaction

▪ Nuclear chain reaction is a self-sustaining sequence of nuclear fission reactions. ▪ The minimum mass of fissionable material required to generate a self-sustaining nuclear chain reaction is the critical mass.

Nuclear fission (2)

▪ Nuclear fission is the splitting of a large nucleus into smaller ones with the release of energy. ▪ Energy is released from nuclear fission because the total mass of the products is slightly less than the total mass of the reactants.

Chernobyl

▪ On 26 April 1986, reactor # 4 at the Chernobyl Nuclear Power Station, 100 km north from Kiev, blew up during a routine daily operation. Nearly nine tons of radioactive material - 100 times as much as the Hiroshima bomb - were hurled into the sky. Winds over the following days, mostly blowing north and west, carried, fallout into Belarus, as well as Russia, Poland and the Baltic region. - Control rods were made of graphite (unlike those used in the U.S.) which caught on fire

Iodine 131 (3)

▪ One of the hazardous radioisotopes released was iodine-131. It decays by beta emission with an accompanying gamma ray: ▪ If ingested, iodine-131 (I-131) can cause thyroid cancer. - In the contaminated area near Chernobyl, the incidence of thyroid cancer increased sharply.

Conditions (5)

▪ Only the nuclei of certain elements undergo fission only under certain conditions. ▪ The factors to consider are (1) the size of the nucleus, (2) the numbers of protons and neutrons it contains, and (3) the energy of the neutrons used to initiate the fission.

Radiation (2)

▪ Radiation is energy in the form of waves or moving subatomic particles emitted by an atom. ▪ Radiation can be classified as ionizing or non-ionizing radiation, depending on its effect on atomic matter.

U-238 Radioactive Decay Series

▪ Radioactive isotopes undergo decay until they reach a stable species. ▪ All isotopes of all elements with atomic number 84 (Po) and higher are radioactive.

Splitting atoms (7)

▪ Relatively light and stable atoms such as oxygen, chlorine, and iron do not split. ▪ Heavy atoms, such as uranium and plutonium, will split if they are hit hard enough with a neutron. ▪ Some isotopes of uranium will even fission with a more gentle nudge, such as in the conditions of a nuclear plant. ▪ Extremely heavy atoms may fission spontaneously. ▪ In nature, uranium is found predominantly as two isotopes: U-238 and U-235. ▪ U-238 is much more abundant (99.3%) than U-235 (0.7%). ▪ Under the conditions present in a nuclear reactor where the neutrons are of relatively low energy, U-238 does not undergo fission, only U-235 does.

Atomic Bombs vs. Nuclear Power Plants

▪ Similarity: - Both derive their energy from nuclear fission ▪ Difference: Atomic Bombs: rapid and uncontrolled release of energy. Use a highly enriched U-235 as high as 90% U-235 ▪ Nuclear power plants: slow and controlled release of energy. Use a lowly enriched U-235 around 3~5% U-235

TNT (4)

▪ TNT, or trinitrotoluene (discovered in 1863 by Alfred Nobel) became the standard of explosive power as a result of the birth of nuclear weapons-they needed to be compared to some substance of known explosiveness. ▪ The TNT molecule is very unstable and when explodes, 2 moles of TNT rearrange to form 15 moles of hot gas (3 mol N2, 7 mol CO, 5 mol H2O) plus some carbon. ▪ About 1 g of TNT will produce about L of hot gas - a 1000 times increase in volume. ▪ 1 kg of U-235, where only about 0.1% mass is converted to energy is equivalent to 33,000 tons of TNT

Nagasaki

▪ The bomb dropped on Nagasaki in 1945 was fueled by plutonium. Plutonium-239 poses an international security problem because the plutonium produced in nuclear power reactors could possibly be incorporated into nuclear bombs. ▪ Nuclear weapons are not the only threat. Consider also the "dirty bomb," a device that employs a conventional explosive to disperse a radioactive substance. If used in a city, such a device would create havoc both from the explosion and from the dispersal of the radioactive substance with long enough half-lives to persist in the environment. Again no fission is involved with a dirty bomb; only a conventional explosive.

Primary coolant (3)

▪ The fuel bundles and control rods are bathed in the primary coolant, a liquid that comes in direct contact with the nuclear reactor to carry away heat. ▪ In many nuclear reactor, the primary coolant is an aqueous solution of boric acid, H3BO3. - The boron atoms absorb neutrons and thus control the rate of fission and the temperature.

Graphite in nuclear reactors

▪ The graphite used to slow neutrons in the reactor caught fire in the heat. When water was sprayed on the burning graphite, the water and graphite reacted chemically to produce hydrogen gas, which exploded when it chemically reacted with oxygen in the air. The explosion blasted off the 400-ton steel plate covering the reactor. ▪ Although a "nuclear" explosion never occurred, the fire and explosions of hydrogen blew vast quantities of radioactive material out of the reactor and into the atmosphere. As the reactor burned, it continued to spew large quantities of radioactive fission products into the atmosphere for 10 days. The release of radioactivity was estimated to be on the order of 100 of the atomic bombs dropped on Hiroshima and Nagasaki.

Nuclear Weapons

▪ The isotopes U-235 and U-238 behave essentially the same in all chemical reactions, so the separation of these two isotopes is extremely difficult and relies on advanced technology that is not readily available. - Building and deploying a nuclear weapon is a very difficult operation to carry out.

Net production (3)

▪ The net production of neutrons allows a chain reaction to occur in which the fission reaction becomes self-sustaining. ▪ Each neutron produced can in turn strike another U-235 nucleus, cause it to split, and release a few more neutrons. - The result is a rapidly branching chain reaction that spreads in a fraction of a second (the figure below).

Three Types of Radiation (5)

▪ There are three types of radiation: alpha (α), beta (β), and gamma (γ). ▪ Alpha radiation refers to the emission of an alpha particle (α) that is positively charged (2+) and consists of the nucleus of a helium atom, that is, two protons and two neutrons. ▪ Beta radiation refers to the emission of a beta particle (β) that is a high-speed electron emitted out of the nucleus. - A beta particle has a negative electrical charge (-1) and a negligible mass, about 1/2000 that of a proton or a neutron. ▪ Gamma radiation refers to the emission of a gamma ray (γ) that has no charge or mass and is made up of high-energy, short-wavelength photons of energy.

Graphic presentation of nuclear fission of U 235

▪ This is a graphic presentation of the nuclear fission of U 235 ▪ Let's take a look at this diagram from the left to the right: - on the far left hand side, there's an approaching green ball, that's a neutron - if that neutron hits the U 235 nucleus, then it becomes temporarily to U 236, this is just a really transient feature bc it quickly displaces to different smaller nuclei of Sr90 and Xe143, along with the production of three additional neutrons - this fission accompanies the release of large amounts of energy - the reason why this fission releases a large amount of energy is because there's a slight difference in mass between the product's mass and the reactant mass - if we plug in that mass difference into the famous einstein energy equation of E=mc^2, we'll see the speed of light, then we get the energy which = 2.0 x 10^13 J per mole of U235 ▪ Now how enormous that energy is , we can appreciate if we compare that energy with the combustion energy to 1 ton of coal which is only 5x10^7