Energy. on this world and elsewhere. Visiting today: Prof. Paschke

Energy on this world and elsewhere Visiting today: Prof. Paschke Instructor: Gordon D. Cates Office: Physics 106a, Phone: (434) 924-4792 email: cates@virginia.edu Course web site available at www.phys.virginia.edu, click on classes and find Physics 1110. or at http://people.virginia.edu/~gdc4k/phys111/fall17/home.html Lecture #21 November 7, 2017

Nuclear Power: isotopes

Isotopes So when we speak of isotopes, we are generally referring to a specific element.

The number of protons in the nucleus determines the element associated with the atom 1 proton: 2 protons: 3 protons: 4 protons: 5 protons: 6 protons: hydrogen helium lithium beryllium boron carbon Why is it that the number of protons determines the number of electrons? Because the number of protons also determines the number of electrons, and the number of electrons determines the atom s chemical properties.

Elemental assignment based on chemistry From paper by Glenn Seaborg, Chemistry and Engineering News, vol. 57, pg. 46 (1979) Mendeleev (and others) constructed periodic tables of elements well before the modern atom was discovered. Mendeleev s table shown above reproduced from Seaborg s article. Placement in the table corresponded to its chemical properties, and what we now recognize as the atomic number.

A single element can have more than one isotope by having differing numbers of neutrons Example: two isotopes of helium 4 He 3 He Proton Neutron Electron Both isotopes have two protons, as they must to be the same element. One isotope has two neutrons, the other isotope has one neutron.

The number of protons in the nucleus determines what element the atom is This atom has two protons and two neutrons in its nucleus, making it an isotope of helium (He-4). This atom has three protons and four neutrons in its nucleus, making it an isotope of of lithium (Li-7)

Each of the elements shown below has multiple isotopes

Isotopes If two nuclei have the same number of protons but different numbers of neutrons, we say that they are two isotopes of a particular element. A nucleus is always a particular isotope of an element. A nucleus is uniquely identified when you identify both the element and the mass number, the total number of neutrons plus protons. Two nuclei with the same mass number but different numbers of protons are not the same isotope, they are two particular isotopes of two different elements.

Identifying a particular isotope The mass number, equal to the total number of protons plus the total number of neutrons. 235 U 92 Sometimes one would also just refer to this as U-235 The atomic number, which is equal to the number of protons. This number is often left out since it is essentially redundant with the chemical symbol The chemical symbol (here U for uranium) identifying the element.

Table of Nuclides Neutron capture Fusion (stellar) Alpha emission Neutron emission Proton emission Beta decay (N Z) protons neutrons

Beta decay The neutron mass is slightly more than the proton mass. When a neutron turns into a proton, some energy is released, so the system settles into a lower energy state (as far as nuclear forces are concerned). The net result is that the mass number stays the same, but the atomic number (the number of protons) goes up by one.

Example of Beta Decay Total number of nucleons or the mass number 137 55 Cs 137m 56 Ba + + Number of protons or atomic number This is a beta particle, which is really just an electron. Thick cardboard or a small amount of metal will stop beta particles

How plutonium is made U-238 captures a neutron, becoming U-239 U-239 Beta decays (half-life of 23 minutes) into neptunium-239 Np-239 beta decays (half-life of 2.4 days) into plutonium-239 23 minutes 2.4 days

The two isotopes of natural uranium # neutrons = 238-92 = 146 238 92 U # neutrons = 235-92 = 143 235 92 U

Only one isotope of uranium is easily fissionable 238 92 U 99.28% 99.27% of all uranium is U-238 When hit by a neutron it will sometimes undergo fission, but most of the time the neutron is just absorbed. 235 92 U 0.72% 0.63% of all uranium is U-235 When hit by a neutron it will almost always undergo fission.

So how does fission work?

Fission reactions in uranium

Uranium-235 can undergo fission Here the liquid drop model illustrates how the addition of a neutron can make a nucleus unstable While both isotopes of uranium can in principle undergo fission, only 235 U will undergo fission after absorbing a slow neutron. Note that after fissioning, the resulting nuclei are in a size range where a lower neutron to proton ratio is favored.

Uranium-235 undergoing a chain reaction A chain reaction is similar to the phenomena of burning when considered in the context of chemistry.

Mousetrap chain reaction Notice that on average, every mousetrap must release more than one ball or the chain reaction will not grow in size.

Nuclear reactors Pellets of uranium oxide are combined into what are called fuel rods. The fuel rods are arranged into a matrix. The space between the fuel rods is filled with a moderator that slows down the neutrons after they emerge from a fission reaction. Control rods, that are very good at absorbing neutrons, are inserted and withdrawn to control the rate at which reactions take place. prompt criticality is unstable. Make use of delayed neutrons for criticality helps control process.

file:///users/gordon/gordon's%20files/ Energy/energy_2013/lectures/ Reactor6.webarchive

Nuclear reactors need moderators A 235 U nucleus is increasingly likely to absorb a neutron as the neutron s speed is reduced. To slow down the fast-moving neutrons emitted during fission, a moderator is used. Basically, some material is introduced between regions containing fuel. When the neutrons collide with nuclei in the moderator, they are slowed down. Three approaches are most common, and they have implications for whether or not the fuel needs to be isotopically enriched: The first reactor used very pure graphite as a moderator. In this case, natural uranium can be used. Another option is to use heavy water (D 2 H) as a moderator. In this case the water is isotopically enriched, but again natural uranium can be used. If normal water is used as a moderator, the uranium must be isotopically enriched.

Two paths to a bomb 235 92 U Nearly pure U-235 (isotope enrichment)

Two paths to a bomb 239 93 Pu Pu-239 that is made in a reactor

Three paths to nuclear weapons As we will discuss, when 238 U in a reactor absorbs a neutron, it is transformed into plutonium. Thus ANY reactor signals at least the possibility of developing nuclear weapons. A graphite based reactor makes it possible to use natural uranium as a fuel. There are many good reasons, unrelated to weapons, for such a reactor. Nevertheless, it can be used to produce plutonium. Heavy water is D 2 O. That is, it is water in which the two hydrogen atoms are the isotope 2 H instead of 1 H. If a country is accumulating heavy water, they are giving themselves the capability of using natural uranium. Finally, a light water reactor, that is, a reactor that uses normal water, can only be operated if it uses isotopically enriched uranium. If the country is enriching their own uranium, they have TWO paths open to nuclear weapons. One is simply producing highly enriched 235 U for a uranium-based bomb, the other is using the reactor to produce plutonium Only if a country uses fuel produced elsewhere, and gives the fuel back after using it, is there reasonable assurance that a country can have reactors without developing weapons capability.

Isotopic enrichment It is very difficult to separate out different isotopes of a given element. Chemical techniques cannot be used because different isotopes have the same chemical properties. It is necessary to take advantage of the slightly different mass of the two isotopes. Something called gas-diffusion separation, combined with mass spectrometers were used during WWII. The more modern technique involves vacuum ultracentrifuges, invented by Jesse Beams right here at UVa. 238 92 U 235 92 U

Enriching Uranium

The vacuum ultracentrifuge Invented by Jesse Beams at UVa, the ultracentrifuge spins incredibly fast, separating things according to their weight. For uranium, the compound uranium hexafluoride (UF 6 ) is used. Above, Jesse Beams receives the Nation Medal of Science from President Johnson in 1967. 238 92 U 235 92 U

Breeding plutonium 0.7% Plutonium 99.3% U-238 U-235 In a reactor, U-238 is slowly (or not so slowly) converted into plutonium. When done intentionally, the process is referred to as breeding.

Creating Pu-239 from U-235 Conventional reactors burn the U-235 almost exclusively. ALL reactors that burn U-235 make Pu-239 in the process (more on this shortly), one of the isotopes from which you can make weapons. - That is why the existence of ANY reactor in a country represents a potential hazard of proliferation of nuclear weapons. Separating the plutonium, however, from the highly radioactive nuclear fuel, is still very difficult.

What if we used nuclear energy at the rate it was being used in 2005? The time scales given above assume that nuclear energy is used solely to produce electricity, and furthermore, to produce electricity at the same rate at which nukes are used to produce electricity today (actually in 2005). It is also interesting to consider what would happen if we used nukes for ALL of our energy.

How long would the entire world s uranium last if it were the world s only source of energy? Here we assume the same total conventional resources as on the previous slide, and total global energy consumption of 411 Quads/year. If only conventional reactors were used for ALL of our energy needs, total conventional resources would last something like 23 years. If fast breeder reactors were used (and around half the the U-238 were converted into plutonium), around 1,400 years. Thorium resources would greatly extend this number.

How an FBR works Conventional reactors use water as a moderator. It slows down the neutrons produced during fission reactions. Slower neutrons are more likely to cause the fission of U-235. Conventional reactors typically use 3-5% 235 U (LEU) Fast Breeder reactors typically use liquid metal as a moderator. The neutrons are not slowed down nearly as much and are thus fast. The fast neutrons are readily captured by U-238, which converts to plutonium. To make up for the fast neutrons, the reactors use a richer mixture of fissionable fuel (HEU, maybe 20% 235 U). This also makes them less stable.

Example of a Generation IV reactor FBR design

France s Superphoenix - example of breeder reactor

Russia s BN-800 Fast Breeder Reactor

Thorium-based breeder reactors Turns (stable) thorium-232, which is nearly 100% of natural thorium, into uranium-233 You need to start with something other than pure thorium to get the cycle going, but eventually, you could just use the uranium-233 that you breed. India is building a fast thorium-based breeder right now, and has a thermal thorium-based breeder in the works.

Shippingport Power Station World s first utility-scale nuke in the U.S. used entirely for peace-time purposes, first went critical in 1957. Last of three cores, designed to breed U-233 from thorium, was operated from 1977-1982 and produced 2.5 billion kilowatt-hours of electricity. Analysis after decommissioning showed that there was 1.4% more fissile material in the last core at the end of its operation than was the case when it was installed.