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Introduction to Nuclear Energy > NUK Questions > Flashcards

NUK Questions Flashcards

1
Q

Who discovered radioactivity?

A

Henri Becquerel

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2
Q

Explain the role that nuclear energy could play in satisfying the future world energy needs, especially the need for clean electricity and its advantages compared to fossil fuels and renewables.

A
  • Nuclear energy as part of a well-balanced energy mix
  • Nuclear energy can provide reliable and base load electricity, contributing to the continuous supply of electricity. Other less reliable sources could supply the peak energy needed when the demand requires it
  • Well-operated nuclear power plants are clean sources of energy because they do not release polluting or green-house effect gases to the environment
  • The main advantages with respect to fossil fuels: very long reserves of fuel (U, Th, Breeding): no polluting gases, no green-house gases, very low fuel costs, very low transportation needs for the fuel because of its huge energy density (1KG of U-235 = 2700t of coal)
  • The main advantages with respect to renewable sources: very small surface needed for a given power output because of its enormously larger power density with respect to renewable sources; much higher reliability of supply (24/7/365)
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3
Q

Who discovered the electron?

A

JJ Thomson

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4
Q

Who discovered the neutron?

A

James Chadwick

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5
Q

Who proved fission?

A

Otto Hahn & Fritz Strassmann

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6
Q

What is the linear interaction coefficient and what does it depend on?

A
  • Its the probability of interaction of radiation with matter per unit length.
  • It depends on the radiation particle, type of interaction & density of material
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7
Q

Whats the mean free path?

A

average distance travelled of a particle before interacting

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8
Q

Whats the half thickness?

A

Thickness of a medium required for HALF the incident radiation to experience an interaction

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9
Q

What is the microscopic cross section?

A
  • the microscopic cross section represents a probability for interaction per atom
  • measured in barns
  • cross sections can be way bigger than the actual nuclei
  • The microscopic cross sections are a function of the kinetic energy of the particle (for fast neutrons the cross sections are getting closer to the actual dimensions of the nuclei)
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10
Q

How to calculate the power?

A

Microscopic cross sections * atomic number density to get macroscopic cross section = total probability for interaction (for photons its linear attenuation coefficient)

Macroscopic cross section * flux (beam intensity) = Reaction Rate Density

Integrate Reaction Rate Density over Volume and Energy to get R (interactions per second)

R * wf (200 MeV per fission) to get Energy per second = Power

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11
Q

what are the three photon interactions?

A
  • compton scattering
  • pholoelectric effect/absorption
  • pair production
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12
Q

How does compton scattering work?

A
  • Elastic scattering of a photon by an electron
  • Energy and momentum are conserved
  • Photon energy has to be higher than binding energy of the atom to move the electron
  • Scattering angle is dependent on how much energy the photon lost
  • The lower the energy of a photon, the lower the cross section = probability for reaction
  • The heavier the atom, the higher the probability for reaction
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13
Q

How does photoelectric absorption work?

A
  • A gamma-photon interacts with “all the atom”
  • Atom recoils
  • 1 atomic electron is ejected: photoelectron
  • In case of the photoelectric absorption the photon is absorbed and all the energy is taken up the electron, creating an ion and some low energy photons (x-ray)
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14
Q

How does pair production work?

A
  • For very energetic photons
  • Interaction with the electric field of the nucleus
  • The photon transforms its energy into MASS
  • An electron and a positron appear
  • The electron slows down and ionizes the medium
  • The positrons slows down and annihilates with antlers electron emitting 2 gamma-photons
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15
Q

Name a few neutron interactions

A
  • elastic scattering
  • inelastic scattering
  • radiative capture
  • charged particle reactions
  • neutron producing reactons
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16
Q

Whats elastic scattering?

A
  • The neutron is not absorbed
  • It loses energy because of transferring part of it to the target nucleus
  • Kinetic energy is conserved
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17
Q

Whats inelastic scattering?

A
  • Inelastic Scattering (n,n´)
  • The neutron is absorbed
  • The nucleus is left in an excited state
  • Decays by gamma-ray emission and remits a lower energy neutron n´
  • Kinetic energy is not conserved
  • Binding energy is released
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18
Q

Whats radiative capture?

A
  • Radiative Capture (n, gamma)
  • Neutron captured by nucleus
  • Excited nucleus decays with gamma-emission
  • Neutron is not reemitted
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19
Q

What types of ionizing radiation exist?

A

Directly Ionizing Radiation

  • Charged primary particles ionize the medium
  • The charged particle leaves a trail of excitation and ionization
  • Examples: electrons, protons, alpha-particles

Indirectly Ionizing Radiation

  • Primary particles are not charged: gamma, neutrons
  • Produce secondary charged ionizing particles (through Compton scattering for example)
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20
Q

What is the specific ionization?

A

Number of ion pairs produced per unit path travelled (ions/cm)

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21
Q

Whats the stopping power?

A

Total energy lost per path length (keV/cm)

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22
Q

Whats the LET?

A

Linear Energy Transfer

  • LET increases with MASS and CHARGE of the particle
  • High LET radiation: alpha particles (they are heavier, move slower and can affect more atoms and ionize more, therefore higher LET), fission products, heavy ions and neutrons
  • Low LET radiation: electrons, photons and positrons
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23
Q

How is the stopping power and the range related?

A

Stopping Power and Range

  • range the particle takes to exhaust its kinetic energy
  • The higher the stopping power, the lower the range -> high LET radiation has a lower range
  • Stopping power * density of material = LET
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24
Q

What is the binding energy?

A

The binding energy is the energy that’s required to break an Atom into its components

  • the binding energy is related to the stability of a nucleus
  • The BE per nucleon has a maximum A = 60
  • Nuclei “multiple” of alpha particles have high BE/A values
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25
Q

What does “fissile mean”?

A

fissile nuclides can fission with thermal neutrons with a high probabiltiy

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26
Q

Cross sections are a function of energy - explain

A

Absorption cross sections (fission, radiative capture, etc.) vary with En following a pattern:

  • Thermal Energy Region (1/v region)
  • Low-to-middle energy region (resonance region)
  • Middle-to-high Energy Region (Fast region)
  • Probability of neutron absorption in thermal region higher, therefore more efficient
  • When the reactor is increasing power, neutrons are mostly stopped in the resonance area, which stops the fissions and the neutron energy goes back to normal power
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27
Q

Why do heavy nuclei fission?

A

Fission increases the Binding Energy per Nucleon BE/A: Energetically favorable

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28
Q

How does the fission process work?

A
  1. Scission (Break-up) explained by the Liquid Drop Model
  • Compound Nucleus highly excited
  • Large oscillations of shape of “Nuclear Fluid”
  • Elongated shape breaks in two (10^-20s)
  1. Primary Fission Products YH, YL
  • Highly excited states
  • Neutrons “evaporate” from surface (10^-17s) -> Prompt Neutrons vp*
  • Reduce excitation by gamma-emission (10^-14s) -> Prompt Gammas yp*
  1. Fission fragments transfer kinetic energy to the surrounding medium in ca. 10^-12s
  • -> When the reactor is shut down, fission stops but decay heat is still there
  • -> If excess energy is not removed, the fuel will heat up and melt the reactor
29
Q

Which neutron reactions are the most important in nuclear reactors and why?

A

Radiative capture: is used to control the fission chain reaction in the reactor by absorbing neutrons with materials with a large absorption cross section, so that they cannot be absorbed by the fissile nuclei (or fuel)

Scattering: used in the moderation process to reduce the energy of the fission neutrons (fast) to the thermal energy range so that they can efficiently be absorbed by U235 or Pu239

Neutron induced fission: this reaction produces the power in the reactor and in the form of sustained, controlled chain reaction it maintains the reactor in operation

30
Q

What is neutron MODERATION? Why is it needed in reactors which use U235 as fuel? Which elements are the most efficient in moderating neutrons, those with high or those with low mass numbers A?

A

> Moderation: reduction of the energy of fission neutrons (fast) to the thermal energy range through neutron scattering collisions (mainly elastic) in the moderator material

  • > Need: Because U235 has the largest fission cross sections at the thermal energy ranges, so the fission chain reaction uses neutrons in the most efficient manner
  • > Best moderators: those with lowest mass numbers A. Best Hydrogen A=1
31
Q

Describe the chain reaction induced by neutrons on U235 fueled reactors

A

-> A thermal neutron (first generation) is absorbed by a nucleus of U235. The compound nucleus of U236 is formed, which because it is unstable, it decays by fissioning into two lighter nuclei (fission products) and emits a few neutrons and gamma-photons.

The neutrons can leak from the reactor, be absorbed by materials or by another U235 nucleus and, in this case, it triggers a further fission reaction. If at least one neutron is always left to be absorbed by a nucleus of U235, the reaction will sustain itself: sustained chain reaction. Controlling the leakage and absorption controls the number of neutrons available for U235 fission in the chain, and allows the control of the reaction

32
Q

Whats the function of a pressurizer in a PWR?

A

Function

  • The primary liquid of a PWR expands and contracts in a constant volume
  • Small increases in temperature would lead to large increases in pressure if the system were completely filled with liquid water
  • The pressurizer is a tank with liquid and steam that acts as a pressure regulator:
  • Maintains the pressure constant
  • Acts as a “damper” for pressure increases

Regulation

  • Pressure decreases: electrical heats create more steam and the pressure increases
  • Pressure increases: liquid is sprayed in the steam, which condenses and the pressure decreases
  • An automatic control system maintains constant the primary side pressure
  • -> It has safety pressure relief valve to prevent overpressure
33
Q

Whats the function of a steam generator in a PWR?

A

Function

  • Thermal energy from the reactor core is transferred to the secondary loop to produce steam for the turbo-generator
  • Separates mechanically the primary and secondary fluids
  • No transfer of radioactive substances
  • Primary and secondary sides are only thermally coupled
34
Q

Advantages and Disadvantages of a PWR?

A

Advantages

  • Very stable reactors due to their tendency to produce less Nuclear Power as temperatures increase (strong negative doppler reactivity coefficient)
  • Well known and tested technology: robust, highly efficient reactor operation
  • Can be operated with a core containing less excess reactivity, preventing serious reactivity induced accidents (RIAs)
  • PWR turbine cycle loop is separated from the primary loop, so the water in the secondary loop is not contaminated by short lived radioactive materials

Disadvantages

  • High operating pressure requires high strength piping and a heavy pressure vessel and, hence, increases construction costs. The higher pressure can worsen the consequences of a loss of coolant accident
  • High temperature water coolant and boric acid dissolved are corrosive to carbon steel (but not stainless steel): radioactive corrosion products circulate in the primary coolant loop
  • Because the reactor produces energy more slowly at higher temperatures, a sudden cooling of the reactor coolant could increase nuclear power production until safety systems shut down the reactor
35
Q

How does a BWR work?

A

Process

  • Steam is produced in the core, separated and dried in the pressure vessel
  • The steam flows along the main steam pipe to the turbine, expands and drives the electric generator
  • The condensed liquid is pumped back into the vessel by the feed water pumps, and flows down the downcomer into the lower plenum and up into the core
  • Nuclear heat heats up the liquid until it reaches saturated conditions and starts to boil
36
Q

Advantages & Disadvantages BWR

A

Advantages:

  • Lower Working Pressure: less steel needed for components
  • Less neutron irradiation to pressure vessel. It does not become as brittle with age
  • Operates at a lower nuclear fuel temperature: more efficient heat transfer mechanism and lower nuclear power density
  • Fewer components: no steam generators and no pressurizer
  • Lower risk of a rupture causing loss of coolant compared to a PWR, and lower risk of a severe accident should such a rupture occur: fewer pipes, fewer large diameter pipes, fewer welds and no steam generator tubes
  • Can operate at lower core nuclear power density levels using natural circulation without forced flow

Disadvantages:

  • Complex fuel management during Nuclear power production due to “two phase fluid flow” in the upper part of the core
  • Much larger pressure vessel than for a PWR of similar power, with correspondingly higher cost (However, the overall cost is reduced because a modern BWR has no main steam generators and associated piping)
  • Contamination of the turbine by short-lived activation products. Shielding and access control around the steam turbine are required during normal operations due to the radiation levels arising from the steam entering directly from the reactor core
  • Control rods are inserted from below for current BWR designs.
37
Q

How is LWR fuel composed?

A

Uranium dioxide (UO2)

  • Used in the form of pellets
  • Mixed in powder form with an organic binder
  • Pressed into pellets
  • Sintered at higher temperature
  • Pellets are stacked in fuel rods
  • A space is left for gaseous fission products
  • The rods are pressurized
  • Pellets are surrounded by the clad (usually Zircaloy-2 for BWRs and Zircaloy-4 for PWRs)
  • -> Zircaloy is used because it has a very low absorption cross section for thermal neutrons (=not stopping them is good)
  • Fresh Enrichment: 2.5 to 5% atoms U235, rest U238
38
Q

How do changes in temperature affect LWRs?

A
  • As temperature increases the density of most materials decreases
  • Fewer atoms per unit volume
  • Lower reaction rates: for moderation, absorption and fission

In LWRs

  • An increase of temperature decreases moderator density
  • Formation of bubbles decreases moderator density
  • This results in a negative reactivity feedback
39
Q

How does the Doppler Effect work?

A

DOPPLER EFFECT: Changes in Resonance Interactions

  • most important temperature feedback mechanism in thermal reactors
  • Changes the neutron interaction rates with materials having large cross-section resonances just above the thermal region
  • Mechanism:
  • Higher temperature increases the velocity of nuclei
  • Increases relative velocity thermal neutron-nucleus: “Doppler”
  • More thermal neutrons can interact with nuclei in the resonances
  • The effective cross section is increased (“doppler broadened”): more neutron absorption is possible
40
Q

How does Gamma decay work?

A

Gamma Decay

  • In many nuclear reactions, a nuclide is produced and left in an excited state
  • The excited nuclide decays very rapidly to the “ground” state by emitting a y-photon
41
Q

How does Alpha decay work?

A

Alpha Decay

  • Nucleons tend to group into sub-units of Helium nuclei, called alpha particles
  • For heavy nuclei with many protons, a possible way of decay to a more stable state is by emitting an alpha-particle
  • The fast-moving alpha-particle loses its energy quickly by ionization and becomes a neutral He atom
42
Q

How does beta - decay work?

A

Beta Particle Decay (ß-)

  • Decay mode of many neutron rich radioactive nuclides
  • A neutron is changed into a proton (+) and an electron (-) is emitted: Charge conserved
  • Conservation of energy and momentum is satisfied thanks to the emission of an antineutrino vdach
43
Q

How does beta + decay work?

A

Positron Decay (ß+)

  • Decay mode of nuclei with too many protons
  • One proton (+) changes into a neutron (0) and a positron (+): Charge is conserved
  • Similar energy and momentum behavior to ß
  • neutrino is released
44
Q

How does electron capture work?

A

Electron Capture

  • can happen when the nucleus has too many protons but not enough energy to decay through beta+ decay (some excess energy is needed to convert to neutrons because they have a higher mass)
  • Orbital electrons can be in the nucleus. K- electrons have the largest probability and they can bee captured by a proton, which turns into a neutron
  • A neutrino is also emitted to conserve energy and momentum
45
Q

How does Internal Conversion work?

A
  • The excitation energy of a daughter nucleus can be transferred to an atomic electron (K-Shell usually, the closest to the nucleus)
  • The electron is ejected from the atom, which ends up with an inner electron-shell vacancy (positive charge)
46
Q

What is lambda? can it be changed?

A

Exponential Decay Law

  • Radioactive Decay behaves exponentially
  • Lambda is the radioactive decay constant; it is the probability of a nuclei decaying in an interval x
  • It is a property of each radionuclide
  • It cannot be changed by any known chemical or physical process
  • The exponential form of the decay law is characteristic of all processes with a constant rate of change
47
Q

What is the half-life and how is it related to radioactivity?

A

The Half-Life

  • The time it takes for it to decay to one-half of the initial value is a constant
  • isotopes that have a shorter half-life emit more particles (=are more dangerous) than the ones with longer half-lives
  • highly radioactive nuclides have shorter half-lives
48
Q

What is the activity? What is its unit?

A

In radioactive protection one is interested in the number of decays per unit time

The SI unit for Activity is the Becquerel (Bq):

49
Q

How can we protect humans from radiation?

A
  • Isolating the radioactive material (put it away from humans)
  • Surrounding it with a “SHIELD”: any material or combination of them that will reduce the intensity of the radiation (Pb is good for protection against photons but not neutrons because it hardly absorbs neutrons. Hydrogen or Gd is good for neutron)
50
Q

What is radiation exposure?

A

Radiation Exposure

  • Describes quantitatively the incidence of radiation on living or non-living matter
  • The unit of radiation exposure is the Roentgen (R) based on the ionization produced by X-photons (or gamma-photons) in the air
51
Q

What is radiation dose?

A

Radiation Dose

  • Is a quantitative measure to the impact of radiation (killing cells, mutations, cancer, genetic mutations)
  • It is closely related to the energy deposited by incident radiation in any material
52
Q

What is absorbed dose and effective dose and how are they related? what are their units?

A

ABSORBED DOSE (D): amount of energy deposited per unit mass

  • S.I. Unit: Gray (Gy), the absorbed dose corresponding to the deposition of 1J/kg
  • Rad (rad, old unit): absorbed dose corresponding to the deposition of 100 erg/g

EQUIVALENT DOSE (H): Dose that takes into account the biological effect of radiation - Sievert

  • Biological damage is determined also by:
    • Linear Energy Transfer (LET) or density of ionization: larger density (LET) leads to more damage per unit energy
    • For instance, the deposition of 1 Gy of alpha-particles is more damaging than 1 Gy of ß-particles
    • Low LET: X-rays, gamma rays, electrons
    • High LET (Alpha particles, heavy ions, neutrons, fission products) radiation ionizes water into H+ and OH-radicals over a very short tracks. A pair of OH radicals can recombine to form peroxide, H2O2, which can produce oxidative damage in the cell (DNA breaks)

The Dose Equivalent takes into account the relative biological effectiveness of the radiation:

  • Equal dose equivalents have an equal chance of producing similar biological effects
  • The radiation quality factor wr relates absorbed dose to dose equivalent
53
Q

Which organs are especially sensitive to radiation?

A

The sensitivity of a tissue to radiation is:

  • Proportional to the rate of proliferation of its cells: how fast they reproduce
  • Inversely proportional to the degree of cell differentiation: how specialized cells are
54
Q

What is the ALARA principle?

A

ALARA Standard: As Low As Reasonably Achievable: ALARA tries to reduce the doses to a minimum compatible with economic, technical and social factors ALWAYS below the exposure limits

55
Q

What is the difference between safety and risk?

A

Safety: “Freedom from DANGER or HAZARD”

Risk: “CHANCE for Injury or Loss”

  • Risk has a probabilistic foundation: It can be quantified with mathematical models
  • Risk and Safety are related: a low-risk activity has a high level of safety
  • Psychologically, safety is better accepted than risk
56
Q

What is defense in depth?

A

Defense in Depth

  • It is the key design principle in nuclear reactor safety
  • The concept provides a series of successive barriers in order to contain the radioactive products
  • If one barrier is broken, the next one will contain the radioactive release to the environment
  • The first barrier is “philosophical”: A strong safety culture
57
Q

What are the technological barriers in nuclear safety?

A

Technological barriers

  • the crystalline solid structure of the fuel
  • The metallic cladding of the fuel rods
  • The closed-circuit primary coolant system
  • The reactor vessel
  • The containment building
  • The selection of the place to build the power plant: siteing of the nuclear power plant
  • The emergency and evacuation plans
58
Q

How are nuclear accidents prevented?

A

Accident Prevention

  • Design a robust and reliable system: tolerant to failures and human mistakes
  • Construct the system as designed and with the best practices and materials
  • Operate the systems following established procedures and within regulations
  • -> The plant must “self-heal”: fail towards a safe state if possible
59
Q

What are redundancy and diversity?

A

Redundancy uses two or more systems in parallel to achieve the same function, e.g. multiple parallel valves or pumps in emergency coolant systems

Diversity uses two or more independent and physically different methods to achieve the same result, e.g. different methods for emergency reactor shutdown

60
Q

What is the purpose of a reactor safety system?

A

Reactor Protection System

Why is it needed?

  • Reactors have an excess of cooling capability and can operate safely at higher than designed powers (PWRs up to 118% and BWRs 120%)
  • If the thermal power rises above the cooling capacity, damage may be produced in the core

When does it act?

  • If a transient puts the reactor in state within operating limits, the automatic control system will return the reactor to operating conditions
  • If the transient is severe, the control system will not be enough to bring the reactor back to operation conditions, and the reactor protection system will shut the reactor down automatically

Purpose: to shut the reactor down and maintain it in a safe condition in the event of a transient or malfunction that can cause damage to the core -> It consists of a series of signals that will TRIP the reactor: activation of the shutdown mechanisms

61
Q

What are the engineered safety features?

A

The main features are:

  • Emergency Core Cooling System (ECCS): to provide replacement coolant in the case of a Loss Of Coolant Accident (LOCA)
  • The Containment Building (or structure): to provide a barrier for the fission products that escape to the primary circuit
  • The Clean-up System for removing part of the radioactivity in the containment atmosphere
  • The Hydrogen Control System to prevent the formation of a hydrogen rich containment atmosphere that may detonate (Zirkonium can react with water, take away the oxygen and form hydrogen, which can accumulate, react with the oxygen in the air and explode)
62
Q

How does the containment system work?

A

The Containment System

  • The containment encloses the reactor primary system
  • It is the final barrier to prevent the release of radioactive products to the environment

It is designed:

  • To hold the pressure resulting from the release of the coolant in a Loss of Coolant Accident
  • To withstand the impact of missiles that can be produced during accidents
  • It has a large margin of safety for the maximum pressure and impact

-> Containments are kept at a lower than atmospheric pressure to prevent the air inside from flowing outside

63
Q

What are some methods of enrichment?

A
  • Thermal diffusion
  • Gaseous diffusion
  • centrifuge separation
  • electromagnetic separation
  • laser separation
64
Q

How does laser separation work?

A

Laser Isotope Separation

  • super efficient new tech
  • The technique uses lasers which are tuned to frequencies that ionize a U235 atom and no others
  • The positively charged U235 ions are then attracted to a negatively-charged plate and collected
  • Based on the fact that different isotopes of the the same element have different electronic energies and therefore absorb different colors of laser light
65
Q

What are the grades of enriched uranium?

A

Grades of enriched Uranium

  • Slightly enriched uranium (SEU): 0.9% to 2% -> heavy water reactors
  • Low-enriched uranium (LEU) <20%, for commercial LWRs: 3-5%; 12-20% research reactors
  • Highly enriched uranium (HEU) >20% U235/Pu233; 90% or more of U235 is known as weapons-grade
66
Q

How does fuel fabrication for PWRs work?

A

Fuel fabrication for LWRs:

  • Begins with receipt of low-enriched uranium from enrichment plant
  • The UF6, in solid form in containers, is heated to gaseous form, and the UF6 gas is chemically processed to form LEU uranium dioxide (UO2) powder
  • This powder is then pressed into pellets, sintered into ceramic form, loaded into Zircaloy tubes and constructed into fuel assemblies
67
Q

What are the types of nuclear waste?

A

Types of nuclear wastes (radwaste)

  • Low Level Waste (LLW)
    • 90% of volume, but 1% of total waste radioactivity
    • Buried in shallow landfill sites
  • Intermediate-level Waste (ILW)
    • 7% of volume and 4% of radioactivity
    • May require special shielding: typically comprises resins, chemical sludge and irradiated reactor components
    • May be solidified in concrete or bitumen for disposal
  • High-level Waste (HLW)
    • 3% of the volume, but it holds 95% of the radioactivity
    • It contains the highly-radioactive fission products and some heavy elements with long-lived radioactivity
    • Requires cooling and special shielding during handling and transport
68
Q

Name two differences between a thermal and a fast reactor. Which one could be used more efficiently as a Breeder reactor?

A

Fast reactors do not use moderators

Thermal reactors use thermal neutrons to induce fissions

Thermal reactors are not good breeders

Fast reactors have lass delayed neutrons

-> The most efficient breeder is the Fast Reactor

Introduction to Nuclear Energy (1 decks)

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