Nuclear Fundamentals Harnessing the Power of the Atom

References Required Introduction to Naval Engineering – Chapter 7 Principles of Naval Engineering – Chapter 8

Objectives A. Comprehend the advantages, disadvantages, and capabilities of the nuclear propulsion system. B. Know the composite parts and basic arrangement of an atom. C. Know the three frequent types of radioactive decay and their associated emissions. D. Know the four types of neutron interactions and their relationship with the fission process. E. Comprehend the fission process and the definitions of the terms critical, subcritical, and supercritical. F. Comprehend the purpose and desirable qualities of the moderator. G. Comprehend the basic operation, key components, and safety considerations of a nuclear propulsion plant. H. Know what is meant by the word “SCRAM”.

World Power Plants (2005)

Why Nuclear Power? – Energy stored in the nucleus of the atom is orders of magnitude greater than energy stored between electrons and protons – 24 eV needed to remove first electron from He atom, but 28 MeV needed to separate the two protons and two neutrons 3.6 x 10 5 times greater

Why Nuclear Power? 28 MeV… So what. Fission of 1 atom of U-235 releases ~200 MeV 1 MeV is 1.602x Joules (J) 1 U-235 is 320x J 1 watt is 1 J/s => 1 U-235 per second is 320x W The critical mass of a sphere of U-235 is 52 kg which contains 1.33x10 26 atoms… all of which can fission in < 1 sec 52 kg U-235 yields 4.26 Gigawatts

Why Nuclear Power 1 lb (0.45 kg) of highly enriched uranium as used to power a nuclear submarine or nuclear aircraft carrier is equal to something on the order of a million gallons of gasoline.nuclear submarine aircraft carriergasoline 1 lb of uranium is smaller than a baseball 1 million gallons of gasoline = a cube 50 feet per side (50 feet is as tall as a five-story building)

Why Nuclear Power? Advantages: – Long core life => Virtually unlimited endurance/range – Reduced reliance on outside air sources => less risk of detection – Less logistical support => Carrier carries more weapons, aircraft, jet fuel, food/consumables – High speed operations => easier to generate winds needed for flight ops (CVN) and rapid repositioning/avoidance

Why Nuclear Power? Disadvantages: – Higher initial cost – Higher maintenance costs – More personnel required to operate – Politically unpopular

Basic Atomic Structure Nucleus: the core of an atom – Proton: positive (+) charge primary identifier of an element mass: amu – Neutron: no charge usually about the same number as protons mass: amu Electron: orbits about the nucleus – Negative (-) charge – Mass: amu (over 1000’s times smaller) – Help determine how element reacts chemically

Basic Atomic Structure

Atomic Structure Isotopes: atoms which have the same atomic number but a different atomic mass number (ie: different number of neutrons) Standard Notation: where: – X = element symbol (ie: H for hydrogen) – A = atomic mass number (p’s and n’s) – Z = atomic number (p’s only)

Radioactivity Alpha (  – positively charged particle w/ 2 p’s & 2 n’s – usually emitted from heavy unstable nuclei – Presents little threat when exposed to the outside of the human body. When taken internally, an alpha particle causes serious damage to internal organs and tissues.

Radioactivity Beta  – negatively or positively charged particle – emitted from nucleus when n -> p or vice versa – like an electron (p -> n) or positron (n -> p) – Minimal threat: can be absorbed by clothing – Ex:

Radioactivity Gamma (  – electromagnetic wave of high freq/ high energy – Not a particle: thus no charge – lowers energy level of parent nuclei (no change in A or Z) – Potential threat to operators (must be shielded) – Ex:

Binding Energy The nucleons give up some energy when they are combined in to a nucleus. This energy is the Binding Energy. – It is the energy that must be added to the nucleus to break it up into the component nucleons – Equivalently, it is the energy that would be released if the nucleus could be made from its component parts (fusion). This difference in mass is a result of the nucleons being “bound” in the nucleus. The mass defect (ΔM) is the mass equivalent of the binding energy.

Binding Energy per Nucleon(BE/A) A useful tool for analyzing the stability of a nucleus is Binding Energy per Nucleon. In the heavier elements (> Fe-56), the repulsive force exerted by the protons begins to overcome the attractive nuclear force. As a result of this struggle between forces, less energy is required be injected in order to break up the nucleus (fission) U-235 one of the heaviest naturally occurring element susceptible to fission; thus, the element of choice for nuclear power.

Binding Energy per Nucleon(BE/A) A useful tool for analyzing the stability of a nucleus is Binding Energy per Nucleon.

Neutrons Naval reactors rely upon thermal neutrons (KE ~0.025 eV) to induce fission All neutrons produced from fission events are fast neutrons (KE > 1MeV) This energy is attenuated by neutron interactions with the materials in the reactor – Scattering – Absorption Capture Fission

Neutron Life Cycle THERMALIZATION U FISSION FAST n’s THERMAL n’s Thermal Absorption in non fuel Fast Absorption Fast Fission Fast Leakage Lf Thermal Leakage K eff = ε Lf p Lt f η

Fission Definition: splitting of an atom U is fuel for reactor – Relatively stable – Likely to absorb a neutron (large  a ) – U fissions readily (large  f ) Basic Fission Equation

Energy from Fission ~ 200 MeV of energy is released per fission event. Majority of the energy is KE of FF. Remainder is gamma energy, neutron KE and FF decay

The Moderator A medium that reduces the speed of fast neutrons, making them thermal neutrons, to sustain a nuclear chain reaction. Common moderators are water and graphite (not used in Navy Reactors) Selection of a moderator is very important to reactor operation – Qualities of a Good Moderator: high  s (scattering cross-section) low  a (absorption cross-section) atomic mass close to neutron (ie: hydrogen) Abundant

Controlling Fission Controlling the fission rate is a matter of controlling the neutron population. This can be done by: – introducing a material that has a high cross section for neutron absorption (control rod) OR – Changing the moderators ability to slow the neutrons down

Condition of Reaction Rate k eff = # of neutrons in a given generation k eff = # of neutrons in a given generation # of neutrons in preceding generation # of neutrons in preceding generation Critical Critical: fission rate just sustained by the minimum number of thermal fissions (k eff = 1) Subcritical Subcritical: fission rate is decreasing since not enough thermal neutrons are produced to maintain fission reactions (k eff < 1) Supercritical Supercritical: fission rate increasing since more than necessary thermal neutrons created (k eff > 1)

Critical Condition Do you think a critical reactor is bad? Do you think a subcritical reactor is bad? Do you think a supercritical reactor is bad? If, on average, exactly one of the free neutrons from each fission hits another U-235 nucleus and causes it to split, then the mass of uranium is said to be critical. The mass will exist at a stable temperature. A nuclear reactor must be maintained in a critical state. If, on average, less than one of the free neutrons hits another U-235 atom, then the mass is subcritical. Eventually, induced fission will end in the mass. If, on average, more than one of the free neutrons hits another U- 235 atom, then the mass is supercritical. It will heat up.

The Moderator Naval reactors are designed such that temperature of water determines amount of interaction: – Temp water becomes more dense causes more collisions n’s travel shorter distance to get thermalized less chance of leakage more fission power Why is the particular design important?

Control Rods Rods of a material with high neutron absorption cross section – Hafnium – Cadmium – Boron Hafnium has many properties that make it ideal – Subsequent isotopes have high  a – Subsequent isotopes are not radioactive

Harnessing the Power of the Atom Now that we’ve figured out how to get energy from the atom and how to control it, how do we make it work for us?

Steam Generator Coolant Pump Pressurizer Reactor Vessel

Primary Loop

Core Assembly Fuel Assembly: stores, supports, and isolates fuel – Plate: UO 2 clad with Zr metal; very thin to allow for effective heat transfer – Sub-Assembly: group of edge-welded plates w/ fluid channels between – Cell: group of several sub-assemblies w/ control rod & fluid in center – Core: collection of cells

Core Assembly Primary Coolant: removes heat produced by fission in fuel – Naval reactors use water (effective, easily replaceable, does not radiate) – Typical outlet temp ~ 500 o F – Typical inlet temp ~ 450 o F Typical temp w/in core = ?

Pressure Vessel Purpose: provides structural support for Rx core & directs flow of coolant thru core Closure Head: removable cover on top of pressure vessel – Closure bolts hold down – Uses seal to prevent leakage – Houses Control Rod Drive Mechanisms (CRDM)

Pressurizer (Pzr) Purpose: maintains primary coolant in subcooled state (prevent boiling) and provides surge volume for power transients Operates at saturation conditions to allow for steam space (NO other part of primary at saturation conditions) Uses electric heaters/spray to maintain high temp & pressure

Pressurizer (Pzr) If Pzr not used: – Boiling in reactor core reduces ability to remove heat (mass flow rate and heat capacity reduced) – Boiling in pumps causes cavitation -> loss of flow through core

Rx Plant Components Pressurizer – Provide a surge volume – Keep the primary coolant pressurized

Rx Plant Components Coolant Pump – Moves the primary coolant through the rx and steam generator

Reactor Coolant Pumps (RCP) RCP: circulates primary coolant through the core Multiple RCP’s for redundancy Hermetically sealed (no leakage)

Rx Plant Components Steam Generator – Transfers heat from the primary coolant to the feedwater to generate dry, satuated steam

Steam Generator (S/G) S/G: acts as heat sink for reactor and produces steam for MS system – Shell and tube heat exchanger – Moisture Separators – Non-nuclear side called the “Secondary”

CRDM’s Electronically position control rods Supported by closure head Allow for remote positioning of the control rods

Control Rods Two purposes: – Provide a means of keeping the thermal-neutron flux at the desired level in a critical Rx core – Provide a means to S/D the Rx by stopping the fission process. Shutdown: with all rods lowered, Rx cannot go critical Startup: lift control rods to reduce “leakage” until Rx is critical; continue to lift until temp of moderator reacts to rod height changes -> let moderator control power SCRAM SCRAM SCRAM: quick shutdown of Rx; drop rods to bottom vice electronically lower (SuperCritical Reactor Ax Man)

Rx Plant Components Shielding – Reduces the intensity of radiation, generated by reactor operation and the decay of fission products and other radioactive materials, to acceptable levels Properly designed, shielding reduces radiation levels by a factor of 10,000.

Shielding Serves two purposes: – Reduce radiation outside reactor compartment to protect personnel – Reduce radiation inside reactor compartment to protect instruments/equipment

Shielding All contained within RC to minimize radiation: – Pressure Vessel & Core – Pressurizer (Pzr) – Reactor Coolant Pumps (RCP’s) – Steam Generators (S/G’s)

Rx Safety Safe operation of Naval Nuclear reactors is paramount Accidents have severe consequences – Risk of death for sailors and civilians alike – Significant environmental consequences – Diminished public image (Navy and nuclear power) In order to maximize reactor safety, the NNPP relies upon: – Highly trained operators – Following procedures – Tests operators yearly for NR, NRC and DOE requirements – Ensure Rx are designed with safety in mind

Rx Scram The word “SCRAM” is used to describe the rapid gravity assisted, spring insertion of control rods to shutdown the reactor.

Take Away List the advantages and disadvantages of nuclear propulsion systems In terms of radioactivity, describe half-life, the common modes of radioactive decay Give the general fission equation Describe critical, sub-critical and super-supercritical in terms of neutron population and the effective multiplication factor. Describe the properties of a “good” moderator Describe why hafnium is a desirable material for control rods Describe the purpose of the pressurizer, coolant pump and steam generator Describe the purpose of shielding What is a scram

Any Questions?