Lesson 13: Nuclear Propulsion Basics

Nuclear Propulsion BasicsDr. Andrew KetsdeverNuclear Propulsion IntroductionNuclear Thermal Propulsion (NTP)System that utilizes a nuclear fission reactorEnergy released from controlled fission of material is transferred to a propellant gasFissionAbsorption of neutrons in a fuel materialExcitation of nucleus causes fuel atoms to splitTwo new nulcei on average (Fission Fragments)High KE from release of nuclear binding energyUsually radioactive1 to 3 free neutronsNecessary to keep reaction goingCritical if each fission events leads to anotherCan be absorbed by reactor material or leak from reactorNuclear PropulsionADVANTAGESHigh Isp (2-10x that of chemical systems)Low Specific Mass (kg/kW)High Power Allows High ThrustHigh F/WUse of Any PropellantSafetyReduced Radiation for Some Missions

A Nuclear/Chemical ComparisonOne gram of U-235 can release enough energy during fission to raise the temperature of 66 million gallons of water from 25oC to 100oC. By contrast, to accomplish the same sort of feat by burning pure octane, it would require 1.65 million gallons of the fuel

Nuclear PropulsionDISADVANTAGES:Political IssuesSocial IssuesLow Technology Readiness Level (Maturity)Radiation issues (Shielding)High Inert Mass

Nuclear Propulsion SchematicPropellant Tank: Similar to tanks discussed for liquid propulsion systems. Tank can also be used as a radiation shield.Turbopump: Provides high pressure propellants to the heat exchange region of the propulsion system. Warm gas from regeneratively cooled nozzle drives the turbines.Radiation Shield: Protects the payload from radiation from the reactor by absorbing or reflecting neutrons and gamma rays.

Nuclear Reactor

Nuclear Reactor

Reactor Schematic

Nuclear ReactorReflectorReflects neutrons produced in the reaction back into the corePrevents neutron leakageMaintains reaction balanceCan be used to reduce the size of the reactorTypically made of BerylliumNuclear ReactorModeratorSlows down neutrons in the reactorTypically made of low atomic mass materialLiH, Graphite, D2OH2O absorbs neutrons (light water reactor)Slow (or Thermal) ReactorUses moderator to slow down neutrons for efficient fissioning of low activation energy fuelsFast ReactorNo moderator. Uses high kinetic energy neutrons for fissioning of high activation energy fuelsNuclear ReactorFuel ElementContains the fissile fuelUsually Uranium or PlutoniumContains the propellant flow channelsHigh thrust requires high contact surface area for the propellantsHeat exchange in the flow channels critical in determining efficiency and performance of the system

Nuclear ReactorControl RodsContains material that absorbs neutronsDecreases and controls neutron populationControls reaction rateWhen fully inserted, they can shut down the reactorConfiguration and placement is driven by the engine power level requirementsTypically made of BoronAxial RodsRaised and lowered into place. Depth of rods in the reactor controls the neutron populationDrum RodsRotated into place with reflecting and absorbing sidesNERVANuclear Engine for Rocket Vehicle ApplicationsPower: 300 200,000 MWThrust: 890 kNIsp: 835 secHydrogen propellantReactorUranium-Carbide fuelGraphite moderator12 drum-type control rodsBoron and BerylliumPBRParticle Bed ReactorCore consists of a number of fuel particles packed in a bed surrounded by moderatorMaximizes the fuels surface areaIncreases the propellant temperaturePropellant directly cools the fuel particlesAdvantages over the NERVAHigher IspHigher FHigher F/W (~20 compared to ~4 for NERVA)Disadvantages over the NERVAMaturityCostCERMETFast reactor uses high energy neutrons (>1 MeV)No moderatorUranium-Dioxide fuel in tungsten matrixAdvantagesLong lifetimeAbility to restartFuel compatability with hydrogen propellant

Nuclear Propulsion

Table 1: Mars Mission Comparison - Round Trip

SystemChemical (H2/O2)NTR - Solid CorePayload Mass100 tonnes100 tonnesTravel Time1 year 1 yearMission Delta-V7.7 km/s7.7 km/sIsp500 s1000 sMass Ratio 4.8062.192Structural Mass25 tonnes (e=0.05)15 tonnes (e=0.10)Propellant Mass475 tonnes137 tonnesTotal Initial Mass in LEO600 tonnes252 tonnesPayload Fraction 0.1670.397

Basic Atomic StructureAtoms are fundamental particles of matterComposed of three types of sub-atomic particlesProtonsNeutrons ElectronsThe nucleus contains protons and neutronsMost of the atoms massSmall part of the atoms volumeElectron CloudContains electronsMost of the atoms volume

Basic Atomic StructureAtomic NumberNumber of protons in the nucleusMass NumberNumber of protons AND neutrons in the nucleusMass of proton : 1.6726 x 10-27 kgMass of neutron: 1.6749 x 10-27 kg Mass of electron: 0.00091x10-27 kg

Basic Atomic StructureIsotopeSame element implies two atoms have the same atomic numberIsotopes of a given element have the same atomic number but a different mass numberSame number of protons in the nucleusDifferent number of neutrons in the nucleus HydrogenDeuteriumTritium

Basic Nuclear PhysicsAn atom consists of a small, positively charged nucleus surrounded by a negatively charged cloud of electronsNucleusPositive protonsNeutral neutronsBond together by the strong nuclear forceStronger than the electrostatic force binding electrons to the nucleus or repelling protons from one anotherLimited in range to a few x 10-15 mBecause neutrons are electrically neutral, they are unaffected by Coloumbic or nuclear forces until they reach within 10-15 m of an atomic nucleusBest particles to use for FISSIONFissionFission is a nuclear process in which a heavy nucleus splits into two smaller nucleiThe Fission Products (FP) can be in any combination (with a given probability) so long as the number of protons and neutrons in the products sum up to those in the initial fissioning nucleus The free neutrons produced go on to continue the fissioning cycle (chain reaction, criticality)A great amount of energy can be released in fission because for heavy nuclei, the summed masses of the lighter product nuclei is less than the mass of the fissioning nucleus

Fission Reaction EnergyThe binding energy of the nucleus is directly related to the amount of energy released in a fission reactionThe energy associated with the difference in mass of the products and the fissioning atom is the binding energy

Nuclear Binding Energy

Defect Mass and EnergyNuclear masses can change due to reactions because this "lost" mass is converted into energy. For example, combining a proton (p) and a neutron (n) will produce a deuteron (d). If we add up the masses of the proton and the neutron, we get mp + mn = 1.00728u + 1.00867u = 2.01595uThe mass of the deuteron is md = 2.01355uTherefore change in mass = (mp + mn) - md = (1.00728u+ 1.00867u) - (2.01355u) = 0.00240uAn atomic mass unit (u) is equal to one-twelfth of the mass of a C-12 atom which is about 1.66 X 10-27 kg. So, using E=mc2 gives an energy/u = (1.66 X 10-27 kg)(3.00 X 108 m/s)2(1eV/1.6 X 10-19 J) which is about 931 MeV/u. So, our final energy is 2.24 MeV.

The quantity 2.24MeV is the binding energy of the deuteron.Uranium 235 EnergeticsFission Products: 165 MeVPrimary Gamma Radiation: 7 MeVNeutrons: 5 MeVBeta and Gamma Decay of FP: 13 MeVNeutrinos: 10 MeV

TOTAL: 200 MeVRadioactivityIn 1899, Ernest Rutheford discovered Uranium produced three different kinds of radiation.Separated the radiation by penetrating abilityCalled them a, b, g a-Radiation stopped by paper (He nucleus, ) b-Radiation stopped by 6mm of Aluminum (Electrons produced in the nucleus) g-Radiation stopped by several mm of Lead (Photons with wavelength shortward of 124 pm or energies greater than 10 keV)

Half-LifeThe half life is the amount of time necessary for of a radioactive material to decayStarting with 100g of BismuthHalf life of 5 days50 g of bismuth after 5 days50 g of thallium

a-Particle DecayThe emission of an a particle, or 4He nucleus, is a process called a decay Since a particles contain protons and neutrons, they must come from the nucleus of an atom

b-Particle Decay b particles are negatively charged electrons emitted by the nucleus Since the mass of an electron is a small fraction of an atomic mass unit, the mass of a nucleus that undergoes b decay is changed by only a small amount. The mass number is unchanged. The nucleus contains no electrons. Rather, b decay occurs when a neutron is changed into a proton within the nucleus. An unseen neutrino, n, accompanies each b decay. The number of protons, and thus the atomic number, is increased by one.

g-Radiation DecayGamma rays are a type of electromagnetic radiation that results from a redistribution of electric charge within a nucleus. A g ray is a high energy photon. For complex nuclei there are many different possible ways in which the neutrons and protons can be arranged within the nucleus. Gamma rays can be emitted when a nucleus undergoes a transition from one quantum energy configuration to another. Neither the mass number nor the atomic number is changed when a nucleus emits a g ray in the reaction

152Dy* 152Dy + gFission

Fission ProbabilityWhen a neutron passes near to a heavy nucleus, for example uranium-235 (U-235), the neutron may be captured by the nucleus and this may or may not be followed by fission. Capture involves the addition of the neutron to the uranium nucleus to form a new compound nucleus. A simple example is U-238 + n U-239, which represents formation of the nucleus U-239. The new nucleus may decay into a different nuclide. In this example, U-239 becomes Np-239 after emission of a beta particle (electron). In certain cases the initial capture is rapidly followed by the fission of the new nucleus. Whether fission takes place, and indeed whether capture occurs at all, depends on the velocity of the passing neutron and on the particular heavy nucleus involved. Fission Probability

The probability that fission or any another neutron-induced reaction will occur is described by the cross-section for that reaction. The cross-section may be imagined as an area surrounding the target nucleus and within which the incoming neutron must pass if the reaction is to take place. The fission and other cross sections increase greatly as the neutron velocity reduces for slow reaction fuels.For fast reaction fuels, a large activation energy requires high energy neutrons for fissionFission Cross Sections

Fission FragmentsUsing U-235 in a thermal reactor as an example, when a neutron is captured the total energy is distributed amongst the 236 nucleons (protons & neutrons) now present in the compound nucleus. This nucleus is relatively unstable, and it is likely to break into two fragments of around half the mass. These fragments are nuclei found around the middle of the Periodic Table and the probabilistic nature of the break-up leads to several hundred possible combinations. Fission Fragments

Fission Fragments and the Chain Reaction

Neutron EmissionCreation of the fission fragments is followed almost instantaneously by emission of a number of neutrons (typically 2 or 3, average 2.5), which enable the chain reaction to be sustained

keff = 1 implies critical mass

Want keff > 1Fission FragmentsAbout 85% of the energy released is initially the kinetic energy of the fission fragments. However, in solid fuel they can only travel a microscopic distance, so their energy becomes converted into heat.The balance of the energy comes from gamma rays emitted during or immediately following the fission process and from the kinetic energy of the neutrons.Some of the latter are immediate (so-called prompt neutrons), but a small proportion (0.7% for U-235, 0.2% for Pu-239) is delayed, as these are associated with the radioactive decay of certain fission products. The longest delayed neutron group has a half-life of about 56 seconds Reactor FuelsU-235 is the only naturally occurring isotope which is thermally fissile, and it is present in natural uranium at a concentration of 0.7 percent. U-238 is the main naturally-occurring fertile isotope (99.3%).The most common types of commercial power reactor use water for both moderator and coolant. Criticality may only be achieved with a water moderator if the fuel is enriched. Enrichment increases the proportion of the fissile isotope U-235 about five- or six-fold from the 0.7% of U-235 found in natural uranium. Enrichment is a physical process, usually relying on the small mass difference between atoms of the two isotopes U-238 and U-235. The enrichment processes in commercial use today require the uranium to be in a gaseous form and hence use the compound uranium hexafluoride (UF6). The two main enrichment (or isotope separation) processes are diffusion (gas diffusing under pressure through a membrane containing microscopic pores) and centrifugation. In each case, a very small amount of isotope separation takes place in one pass through the process. Repeated separations are undertaken in successive stages, arranged in a cascade.Uranium Enrichment

Plutonium