Technological Issues Related to the Proliferation of Nuclear Weapons by Thomas B. Cochran, Ph.D. Presented at the Strategic Weapons Proliferation Teaching Seminar Sponsored by The Institute on Global Conflict and Cooperation & The Nonproliferation Policy Education Center August 23, 1998 Revised UC San Diego, CA Natural Resources Defense Council, Inc. 1200 New York Avenue, NW, Suite 400 Washington, D.C. 20005 Voice (Main): 202-289-6868 Voice (Cochran): 202-289-2372 FAX: 202-289-1060 Email: [email protected] i TABLE OF CONTENTS I. Introduction………………………………………………………………………… 1 II. Basic Terminology………………………………………………………………… 1 A. Atoms and Nuclei…………………………………………………………. 1 B. Energy and Power…………………………………………………………. 1 C. Nuclear Weapons………………………………………………………….. 3 III. Nuclear Fission…………………………………………………………………… 3 A. Energy Released by Nuclear Fission……………………………………… 3 B. Chain Reaction ……………………………………………………………. 5 C. Fissionable Materials……………………………………………………… 6 D. Critical Mass………………………………………………………………. 8 E. The Amount of Plutonium and/or Highly-Enriched Uranium Needed for a Pure Fission Nuclear Weapons……………………………………….. 12 F. The Amount of Plutonium Used in Boosted Fission Primaries…………….. 16 G. The Effect of Using “Civil Plutonium” of Widely Varying Isotopic Composition on the Efficiency of a Nuclear Explosive ⎯the Issue of Preinitiation…………………………………………………… 16 IV. Nuclear Fusion…………………………………………………………………… 19 A. Thermonuclear Reactions and Fuels……………………………………… 19 B. Energy Released in Nuclear Fusion Processes…………………………… 21 V. Nuclear Weapon Types…………………………………………………………… 21 A. Gun-assembly Pure Fission Designs……………………………………….. 22 B. “Solid-pack” Implosion Type Pure Fission Designs……………………….. 23 C. Levitated Pit and Hollow Core Designs……………………………………. 24 D. Boosted Fission and Other Single-stage Thermonuclear Designs………….. 24 E. Staged Thermonuclear Designs…………………………………………….. 25 VI. Nuclear Reactor Fuel Cycle……………………………………………………….. 27 A. Fuel Cycle Overview………………………………………………………. 27 B. Isotope Separation or Enrichment………………………………………….. 28 Material Balance………………………………………………………. 30 Separative Work……………………………………………………….. 30 C. Classification of Reactors………………………………………………….. 33 D. Reactor Characteristics Important from a Proliferation Prospective……….. 33 E. Fresh Reactor Fuel Composition…………………………………………… 34 F. Plutonium Production in Reactors…………………………………………… 36 Plutonium Production Reactors………………………………………… 38 Light Water Power Reactors…………………………………………… 39 Light Water Reactors Fueled with MOX………………………………. 40 Fast Breeder Reactors………………………………………………….. 42 ii LIST OF TABLES Table 1. Conversion factors for energy unit…………………………………………… 2 Table 2. Prefixes and symbols used to identify powers of ten………………………… 3 Table 3. Emitted and recoverable energies per uranium-235 nucleus fissioned………. 4 Table 4. Delayed neutron fractions……………………………………………………. 6 Table 5. Categories of plutonium according to the percentage of plutonium-240 contained therein…………………………………………………………. 8 Table 6. Critical mass of a bare sphere of plutonium…………………………………. 10 Table 7. Approximate fissile material requirements for pure fission nuclear weapons…. 15 Table 8. Thermonuclear reactions important to nuclear weapons and controlled nuclear fusion……………………………………………………………... 20 Table 9. Explosive energy available per unit mass of fissile and fusion materials……. 21 Table 10. Enrichment Services (Feed and Separative Work Requirements)………….. 32 Table 11. Various ways in which nuclear reactors are classified……………………….. 35 Table 12. Plutonium production and the percent concentration of plutonium as a function of fuel exposure for the Hanford B reactor……….………….. 37 Table 13. Plutonium production in India’s Cirus reactor……………………………….. 38 Table 14. Characteristics of spent fuel from a VVER-1000 fueled with 4.4%-enriched uranium……………………………………………………. 39 Table 15. Characteristics of VVER-1000 fresh MOX fuel made with plutonium recovered from VVER-1000 spent fuel…………………………………… 41 Table 16. Characteristics of VVER-1000 spent MOX fuel…………………………….. 41 iii LIST OF FIGURES Figure 1. Critical mass versus U235 enrichment of uranium metal……………………. 11 Figure 2. Nuclear weapon yield versus plutonium mass for pure fission weapons…….. 13 Figure 3. Nuclear weapon yield versus HEU mass for pure fission weapons………….. 14 iv PREFACE This report provides an introduction to selected technical issues related to nuclear weapons proliferation and the physical protection and material control, accounting and safeguards of nuclear materials. Prepared for faculty and students, it should be read in conjunction with the companion report, “Proliferation of Nuclear Weapons and Nuclear Safeguards.” This and the companion report are derived from selected works by the author and his colleagues, Christopher E. Paine and Matthew G. McKinzie, at NRDC. Technological Issues Related to the Page 1 Proliferation of Nuclear Weapons NRDC Nuclear Program I. Introduction We begin in the next section with a discussion of some basic terminology associated with nuclear weapons and nuclear reactor operations. This is followed in Sections III, IV and V with a review of the basic physics underlying nuclear weapons and reactors. Section VI differentiates several well known types of nuclear weapons, and Sections VII discusses some of the elements needed to understand plutonium production in several common types of nuclear reactors. II. Basic Terminology A. Atoms and Nuclei Chemical compounds are made up of elements, e.g., hydrogen, helium, oxygen, lead, zinc, uranium, plutonium. A hydrogen atom consists of a nucleus containing a single proton and an electron orbiting around it. The nucleus of a helium atom has two protons in the nucleus and two orbiting electrons. Heavier elements have an even greater number of protons. Elements are distinguished by their atomic number, that is, the number of protons in the nucleus of the atom. Uranium has 92 protons in the nucleus and plutonium has 94. Some elements with atomic numbers greater than uranium have been found in trace amounts in nature, for example, in uranium ores; otherwise these heavier elements must be created synthetically either in reactors, nuclear explosive devices, or by accelerators. Elements 110-112 were recently discovered, and scientists are trying to create and identify even heavier elements. With one exception the nuclei of atoms also contain neutrons, which are similar to protons but without an electrical charge. Atoms of the same element must have the same number of protons, but can have a different number of neutrons. We distinguish them by calling them “isotopes” of the element. Thus, an element can have several isotopes. We call the collection of isotopes of the various elements “nuclides.” We distinguish and identify the nuclides by the name of the element and a number that identifies the total number of protons and neutrons in the nucleus. For example, deuterium (symbol, D) is an isotope of hydrogen with one proton and one neutron in the nucleus, tritium (symbol, T) is an isotope of hydrogen with one proton and two neutrons in the nucleus, uranium-235 (abbreviated “U-235” or “U235”) has 92 protons and 143 neutrons (92+143=235) in the nucleus, and plutonium-239 (abbreviated Pu-239” or “Pu239”) has 94 protons and 145 neutrons (94+145=239). For our purpose we can use the terms “nuclides” and “isotopes” interchangeably. B. Energy and Power Energy. Just as one can measure length in centimeters, inches, feet, yards, meters, rods, furlongs, kilometers, miles, light years or parsecs, physicists measure energy using a wide variety of different units. Some of the more common energy units associated with nuclear weapons and nuclear power, ranked by the size of the unit, are the electron volt (ev), erg, joule (J), calorie (cal), kilowatt-hour (kWh), megawatt-day (MWd), and kiloton of TNT equivalent (Kt). Some conversion factors that can be used to convert from one set of units to another are Technological Issues Related to the Page 2 Proliferation of Nuclear Weapons NRDC Nuclear Program provided in Table 1. Electron volts are often used to measure the energy involved in individual chemical reactions; million electron volts (MeV) for nuclear reaction, and kilotons of TNT equivalent for nuclear weapon explosions. To Convert from To Multiply by electronvolt (ev) joule (J) 1.60219x10-19 million electronvolt (MeV) joule (J) 1.60219x10-13 erg Joule (J) 10-7 watt-second (Ws) joule (J) 1 (1 watt ≡ joule/sec) calorie (cal) joule (J) 4.1868 kilowatt-hour (kWh) joule (J) 3.6x106 megawatt-day (MWd) joule (J) 8.64x1010 kiloton of TNT equivalent (Kt) calories (cal) 1012 Table 1. Conversion factors for energy units. The amount of explosive energy released by a given mass of TNT depends on its constituents and density. In order to avoid confusion, since the 1940s the convention has been to assumed that the explosive energy released by detonating one kiloton of TNT is equivalent to 1012 calories. Power is energy per unit of time. The power of a nuclear reactor can be measured in watts, but since this is such a small unit, the reactor power is typically measured in units of kilowatts (kW), megawatts (MW), or gigawatts (GW). To distinguish whether the unit of power refers to the rate of production of electrical energy or thermal (heat) energy, “W, ”the symbol for watt, is often accompanied by a subscript “e” or “t,” e.g., “MWt” for “megawatt (thermal)” or MWe for “megawatt (electric).” The prefixes kilo-, mega-, and giga- in the above units are three of a series of prefixes used to indicate a decimal multiple or