Nuclear Power

Question 1

What is a thermal neutron?

Answer 1

A thermal neutron is a free neutron with a kinetic energy of about 0.025 eV. After a number of collisions with nuclei (scattering) in a medium (neutron moderator) at this temperature, neutrons arrive at about this energy level, provided that they are not absorbed. Thermal neutrons have a different and sometimes much larger effective neutron absorption cross-section for a given nuclide than fast neutrons, and can therefore often be absorbed more easily by an atomic nucleus

Question 2

What is a fast neutron?

Answer 2

A fast neutron is a free neutron with a kinetic energy level close to 1 MeV. They are named fast neutrons to distinguish them from lower-energy thermal neutrons

Question 3

What does the moderator do?

Answer 3

In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235 or a similar fissile nuclide. Commonly used moderators include regular (light) water (roughly 75% of the world’s reactors), solid graphite (20% of reactors) and heavy water (5% of reactors) possibility.

Question 4

what is a fissionable material?

Answer 4

material that is capable of undergoing fission by n

atural or induced processes

Question 5

What is a fertile material?

Answer 5

A material, which is not itself fissile but can be converted into a fissile material by irradiation in a reactor. There are two basic fertile materials: uranium-238 and thorium-232. When these fertile materials capture neutrons, they are converted into fissile plutonium-239 and uranium-233, respectively.

Question 6

What is a fissile material?

Answer 6

fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of any energy.

Question 7

Why is U235 a fissile material and U238 a fissionable material (but not fissile)?

Answer 7

Uranium-235 fissions with low-energy thermal neutrons because the binding energy resulting from the absorption of a neutron is greater than the critical energy required for fission; therefore uranium-235 is a fissile material. By contrast, the binding energy released by uranium-238 absorbing a thermal neutron is less than the critical energy, so the neutron must possess additional energy for fission to be possible.

Question 8

How is U233 created?

Answer 8

Uranium-233 is a fissile isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. uranium-233 is produced by the neutron irradiation of thorium-232. When thorium-232 absorbs a neutron, it becomes thorium-233. Thorium-233 decays into protactinium-233, Protactinium-233 beta decays into uranium-233

n + 232Th -> 233Th -> 233Pa ->233Th

Question 9

What is a moderator?

Answer 9

a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235 or a similar fissile nuclide.

(light) water (roughly 75% of the world’s reactors), solid graphite (20% of reactors) and heavy water (5% of reactors). REDUCES ENERGY THROUGH ELASTIC COLLOSION

Question 10

What makes a good moderator? (characteristics)

Answer 10

Should remove lost of energy per collision, Should not absorb neutrons (High scattering cross-section, low absorption cross-section

Question 11

Why use ‘heavy water’ as a moderator? what are the concerns?

Answer 11

Light water also acts as a moderator, but because light water absorbs more neutrons than heavy water, reactors using light water for a reactor moderator must use enriched uranium rather than natural uranium, otherwise criticality is impossible.

Heavy water absorbs more energy than hydrogen

Question 12

Why is a low mass good for a moderator?

Answer 12

In a perfect elastic collision between two objects, momentum is conserved and the maximum amount of kinetic energy is carried off by the lowest mass object. Neutrons have lower mass than any other element (with the exception of hydrogen, which has the same atomic mass), hence it is important to use an element with low mass number, so that the neurton can loose a significant amount of kinetic energy per collision.

Question 13

How much uranium is present in natural U235 ?

Answer 13

0.0711%

Question 14

What assumptions can be made when considering the collision of neutrons and the moderator? (4)

Answer 14

• Elastic collisions
• In-line direct collisions (no glancing blows), to ensure maximum energy transfer
• Conservation of momentum and kinetic energy at each collision
• The velocity of the nucleus before collision is zero and that after collision is v3
  .

Question 15

Why in general, the amount of energy lost by a neutron to nucleus collision tends to be less than that computed?

Answer 15

Because in general, collision are not necessarily direct in-line one; any glancing collision would result in less energy being lost from the neutron.

Question 16

Three possible moderators are available, hydrogen, deuterium and carbon. State the he forms (compounds) would you recommend these moderators take, and why

Answer 16

The gaseous forms of hydrogen and deuterium have very low densities, and hence wopuld result in very large cores, hence recommend that a denser material containing H and D be 10 used. The simplest is water and heavy water. But hydrocarbon waxes may also be used. For carbon, graphite would be a good choice. (high density, solid, easy to machine

Question 17

Define both nuclear fission and fusion (3)

Answer 17

Fusion = bring together of lighter atoms to form a heavier one

Fission = break up of a heavy nucleus into two or more fragments#

In both cases there is a small loss of mass and an accompanying release of energy (E = ∆m c2)

Question 18

Name the two principle elements which will be used as the fuel in future commercial fusion reactors

Answer 18

Deuterium and Tritium (both hydrogen isotopes)

Question 19

Why do similar countries have very different nuclear energy policies? (Germany and UK (4))

Answer 19

• Political decisions rather than economic/engineerin/commercial
• Britain has a nuclear fleet which is coming to its end of life now, and so new nuclear is required now, if nuclear is to be maintained as an option.

-Germany has invested more heavily other power generation sources (e.g. in photovoltaic)
than U and therefore feels it is less dependent on nuclear

• UK has a longer nuclear legacy, originally based on Magnox and the weapons programme, which most other countries do not

Question 20

What is a burnable poison?

Answer 20

A material which strongly absorbs neutrons, reducing the effective reactivity of the fuel mix. As neutrons are absorbed by the “poison”, the poison is transmuted into a different material which usually has a much lower absorption cross section; the poison is thus removed or burnt

Question 21

What are delayed neutrons? (3)

Answer 21

• During a fission reaction a heavy atom breaks up into at least two smaller atoms, neutrons and radiation is also emitted. Normally most of this break-up and emission occurs in a very short time frame.
• However there are a small fraction of neutrons which are emitted sometime after the fission event. These are known as delayed neurton
• it is these delayed neutron which make control of the reactor possibl

Question 22

What is the Resonance escape probability?

Answer 22

is term used to describe the probability of neutron to slow down from high energy states to thermal states without being captured during the slowing down proces

Question 23

Define the thermal utilisation factor

Answer 23

the fraction of thermal neutrons which are absorbed by the fuel (rather than the moderator of structural component)

Question 24

Define the Fission cross-section

Answer 24

The probability that an atom will absorb a neutron and fission

Question 25

What is a Barn?

Answer 25

unit of cross section for a specific process, e.g. absorption, fission

Question 26

What is a Sievert?

Answer 26

unit of radiation which takes into account the ‘quaility’ or ability of the radiation to cause damage to tissue.Siever

Question 27

What’s the formula for neutron flux?

Answer 27

Phi = n x v

where n=neutrons per unit vol and v = neutron velocity

Question 28

What;’s the formula for probability of reaction per unit volume (fission or absorption)

Answer 28

Sum = N x sigma

where N = number of neutrons per unit volume and sigma = cross section probability that a neutron in the vicinity of the atom will interact

Question 29

What’s the formula for reaction rate per unit volume R?

Answer 29

For a successful reaction to take place a neurton is needed, the likelihood of a reaction actually occurring will be Σ times the number of neutrons passing through the volume containing the atoms in question

R = Σ * phi

Question 30

What is the forumla for power generated by fission?

Answer 30

POWER = reaction rate x heat producted per reaction

Question 31

How would you work out the number of atmos, N, in 1 tonne for uranium?

Answer 31

wrong

Question 32

How does binding energy per nucleon vary with mass number? draw graph

Answer 32

become more stable up to roughly - binding energy per nucleon reaches a maximum and then begins to reduce again

Question 33

what is the equation for the rate of decay of N nuclei

Answer 33

dn/dt = -lamdaN

Question 34

What is the cross section area defined as?

Answer 34

for a nuclear reaction it is deined as the number of reactions per second divided by the incident number of neutrons per m^2 per second for a single nucleus

Question 35

Why are thermal efficiencies of gas-fired units typically higher than that of a nuclear power plant?

Answer 35

• maximum efficiency is the Carnot cycle
• governed by the temperature difference
• limit on the maximum temperature in a nuclear power plant due to safety (integrity and longevity)
• less important because fuel cost is low, no carbon

Question 36

what makes the overall cost of nuclear generated electricity high?

Answer 36

• capital
• management
• maintenance
• securty
• decommissioning
• legacy costs 50 - 100 years from now

Question 37

What is the upper temperature limit to a PWR and why?

Answer 37

Keep water liquid
374C
221.2bar

Question 38

How do you calculate the number of atoms in a substance?

Answer 38

Number of atoms = mass (g)/molar mass * Avooooooooooo

Question 39

why does binding together or splitting atoms result in energy?

Answer 39

A nucleus consists of protons and neutrons bound together by short-range forces. The mass of the assembly is less than the individual mass of the constituent nucleons. The mass deficit, Δm, is equal to the amount of energy required to pull the nucleons apart via Einstein’s E = Δm c2 equation. The ‘tightness’ of the bonding of the nucleons appears to depend on the absolute number of neutrons and protons and their ratio. The tightest bonding appears to occur when the number of protons equals the number of neutron. However, protons tend to (electrostatically) repel each other and the number of neutron has to increase as the mass of the nucleus increases. The nucleons (now being adrift from the optimum number of protons = number of neutrons) have less binding energy per nucleon as the mass number get very large.

Question 40

why is the binding energy lower for light elements ?

Answer 40

For small mass number, although the optimum proton to neutron number tend to be observed, the number relative number of nucleons on the surface of the nucleus increases with reducing mass number. This means that they are less tightly bound because they ‘see’ fewer neighbours (remember the nucleons are bound by short-range forces), and so the binding energy per nucleon decreases with mass number.

Question 41

How is the fuel contained in fusion?

Answer 41

Most advanced and currently preferred containment is magnetic. The fuel is heated into charged ionised gas which can then be contained in magnetic fields.

Question 42

What is the coolant used for? (3)

Answer 42

1 to remove heat and keep the nuclear fuel at acceptable working temperatures.

2 to transport thermal energy from the core area to heat engines or to intermediate heat exchangers.

3 In some cases to provide neutron thermalisation.

Question 43

What does shielding do?

Answer 43

reduce radiation to acceptable levels

Question 44

Why is the power density of a gas cooled graphite moderated reactor core lower than that of a light water reactor?

Answer 44

• water is much better at heat transfer
• best moderators have the same mass as the neutrons (hydrogen) . therefore less water is needed compared to gas (better moderator/fuel ratio for water)

Question 45

Define the four factor forumla

Answer 45

k = e * efficiency * p * f

Question 46

describe each element of the four factor formula

Answer 46

η = fast neutron yeid factor (how many fast neutrons are generated per thermal neutron)

e = fast fission factor (account for fast neutrons causing fission directly)

p = resonance escape probability, (the chance of a fast neutron being thermalised)

f = thermal utilization factor (how many thermal neutrons are absrobed in fuel ready for fission)

k = is the multiplication factorforan infinitely large the system(noleakages)

Question 47

derive expressions for the neutron population, N, as a function of these quantities and time for a system with prompt neutrons only

Answer 47

dN/DT = (k -1) N/L

where k = rho +1

Question 48

what is the neutron time dependent population density equation? (adjusted from the one with no delayed neutrons)

Answer 48

dN/DT = (k -1) N/Betatd

where betatd = (1-beta)t_p + beta x td

Question 49

when analysing collisions between neutrons and moderators, why are two coordinate systems needed?

Answer 49

The C co-ordinate system is needed to make the computation of the probability of slanting collisions easier to compute; within the C system it becomes easier to integrate through the complete θ spectrum.

Question 50

what is the four factor formula?

Answer 50

The four-factor formula is used to determine the multiplication of a nuclear chain reaction in an infinite medium

Question 51

state the different conditions of the four factor formula

Answer 51

If k is greater than 1, the chain reaction is supercritical, and the neutron population will grow exponentially.

If k is less than 1, the chain reaction is subcritical, and the neutron population will exponentially decay.

If k = 1, the chain reaction is critical and the neutron population will remain constant.

Question 52

what is the containment?

Answer 52

Barries to isolate and contain materials - e.g. reactor vessel to contain the inside of the reactor from theoutside (also acts as a pressure vessel)

Question 53

What are absorbers?

Answer 53

materials with high cross sectional area for neutrons e.g. boron

Question 54

What are control rods?

Answer 54

made from absorbers and are positioned within the reactor core so as to maintain a desired neutron flux. moving the control rods control energy output from the core

Question 55

what are prompt neutrons?

Answer 55

prompt neutrons are released immediately following a fissile event. it would be impossible to have a sustained chain reaction resulting from prompt neutrons only - switch itself off or it would go bang

Question 56

how is tritium made>?

Answer 56

tritium is ‘made’ by bombarding lithium (a metal) with fast neutrons. There are ample lithium supplies on the crust of the earth. Lithium occurs in two isotopes, 6Li (7%) and 7Li (93%)

Question 57

what is the lawson criteria?

Answer 57

The D-T reaction most readily takes place at 10kev, which is very hot. In order to ensure that the fusion reaction delivers more power than it requires to heat it to the 10keV values, the fusion power must be greater than all the losses. The power losses arise from diffusion of plasma to the containment walls, from synchrotron and bremsstrahlung radiation emissions and the fact that the energy carried off by the neutrons many not be readily available for sustaining the reaction.

The Lawson condition encapsulates these various effects and states that for a fusion reactor to be self-sustaining and produce power, the nuclei density, the time the density is held and the temperature of the nuclei must be above certain values. By adopting a break even energy balance, the Lawson criterion gives:

nτET ≥3x1021 m-3skeV

Question 58

what are the two containment methods for fusion?

Answer 58

1. Use magnetic containment; the plasma consists of charged particles which can be ‘contained’ in a magnetic ‘bottle’
2. Inertial containment; bombard the fusion material so that it implodes on itself, as it implodes the density increases, enabling the material to reach and surpass the Lawson criterion. The fusion material does not see any physical engineering containment walls.