Nuclear Energy Flashcards
State the form in which energy released from a decaying nucleus (alpha decay) initially appears as
Kinetic energy
or electrostatic potential energy
Graph to show how the number of neutrons, N, varies with the number of protons, Z, for stable nuclei over the range Z=0 to Z=80
Draw a scale on the N axis
Indicate where alpha, beta minus and beta plus emission occurs
Straight line between (Z=0, N=0) to (Z=20, N=20) then curves upwards to Z=80, N=120
Alpha: any region below the line of stability but N>80 Z>60
Beta minus: any region above and close to the line of stability
Beta plus: any region below and close to the line of stability
Changes in proton and neutron number after electron capture
-1 proton
+1 neutron
State what is meant by the binding energy of a nucleus
Energy needed to separate a nucleus into its constituent nucleons
Describe what happens to the majority of the alpha particles in the Rutherford scattering experiment
pass through with no or very small deflection
Name two deductions that can be made from the results of alpha particle scattering
volume of nucleus «_space;volume of atom / nucleus small and atom mostly empty space
Most of mass in nucleus
Nucleus has positive charge
Size of nucleus «_space;separation
What is meant by the random nature of radioactive decay
Which atoms decays at what time is chance
Explain what is meant by electron capture
Inner orbiting electron captured by a proton in the nucleus and is converted into a neutron
Give one reason why EM radiation is emitted following electron capture
Daughter nuclide/nucleus/atom might be excited and energy given up as EM radiation upon de-excitation.
Radioactive source - short range, different lengths and angles
What type of radiation is emitted and what can be deduced about the energy of the emitted particlees
Alpha
Different track lengths, short range particles have lower energy than long range particles, particles in each range have the same energy
What has a longer half life, Pu-239 or U-235
U-235 because of the inverse relationship between half-life and alpha particle energy
Explain why 231-Th-90 is likely to be a beta minus emitter
Because it is neutron-rich compared with more stable elements
Graph of intensity (y) against diffraction angle (x) for electron diffraction
Starts high up y axis Curves down with positive gradient Minimum Curves up and peaks Then curves down with negative gradient
Doesn’t touch x axis
State why high energy electrons are used in determining nuclear size
The de Broglie wavelength of high energy electrons is comparable to nuclear radii - not subject to SNF
Give the main conclusions from electron diffraction experiments about:
a) nuclear density
b) average separation of particles
a) nuclear density is constant
b) separation of neighbouring nucleons is constant - nucleons are closely packed
Sketch a graph of nuclear radius (y) against nucleon number (x)
-1/x
from origin
r shape
Explain what is meant by induced fission
A neutron strikes the nucleus causing it to split into two fragments
Explain, using the charged liquid drop model, the energy changes in the fission of a U-235 nucleus
Some electrostatic potential energy is converted to kinetic energy of fragments
Some electrostatic potential energy used to overcome strong interaction
Some electrostatic potential energy used to increase surface energy
Describe and explain how the fission of the U-235 nuclei in a fuel rod causes the fuel rods and the moderator to become very hot
Fission fragments repel and collide with other atoms in fuel rod
High energy fission neutrons enter/collide with moderator
Atoms gain kinetic energy due to collisions and vibrate more
Temperature depends on the average kinetic energy of vibrating atoms
A chain reaction occurs
State the nuclear energy that occurs during positron emission hence explain why no positrons are emitted in this sequence
Proton changes to a neutron and a positron and electron neutrino
New element has a lower neutron to proton ratio
Alpha emission raises the neutron to proton ratio slightly
Beta minus emission lowers the ratio more
Beta plus emission increases neutron to proton ratio
Positron emission competes with alpha emission but is energetically less favourable
Explain what is meant by half-life
time for half of active nuclei of radioactive substance to decay
Suggest two ways in which the accuracy of the experiment (counter next to radioactive substance) could be improved
Count over longer period than half minute/repeat experiment
Use stronger source
Use background count correctly
One pro one con of using nuclear fuel to produce electricity
Pro:
less mass of fuel used because more energy per kilogram
less harm to the environment because does not generate green house gases
Con:
hazardous waste because fission products are radioactie
Long term responsibility because waste needs to be stored for many years
State one reason why methods other than alpha particle scattering are used to determine nuclear radii
SNF complicates process
Scattering caused by distribution of protons not whole nucleon distribution
Alpha particles are massive causing recoil of nucleus complicating results
Explain what is meant by enriched uranium
Proportion of U-235 is greater than in natural uranium
Why is enriched uranium rather than natural uranium used in many nuclear reactors
Induced fission more probable with U-235 than U-238
By considering the neutrons involved in the fission process, explain how the rate of production of heat in a nuclear reactor is controlled
For steady rate of fission, one neutron per fission required to go on to produce further fission
Each fission produces two or three neutrons on average
Some neutrons escape
Control rods absorb sufficient neutrons to maintain steady rate of fission
Explain why all the fuel in a nuclear reactor is not placed in a single fuel rod
Neutrons need to pass through a moderator to slow them in order to cause further fissions or prevent U-238 absorbing them.
Neutrons that leave the fuel rod are unlikely to re-enter the same fuel rod making it easier to replace the fuel ins tages
State two observations about the alpha particles detected coming from the foil of alpha scattering
Most alpha particles undeflected
Some deflected through small angles
Very small but significant number deflected through > 90 degrees
State two features of the structure of the atom which can be deduced from these observations
Atoms mostly empty space
Positive charge concentrated
In a volume much less than the total volume
Define and explain binding energy
Binding energy (MeV) is the energy needed to separate all of the nucleons in a nucleus. To separate the particles of the object, they must be pulled apart against the attractive SNF. They must therefore have potential energy when separated. When the nucleons come together and form nuclei, their potential energy decreases and so because energy is always conserved, energy must be put in to separate the nucleons of a nucleus – this energy is the binding energy.
How does binding energy relate to gravitational potential energy of an object
Binding energy is like when an object is lifted up from the ground. It gains potential energy similarly to the nucleons because work is done separating the object from the ground when you are lifting it just like when lifting the nucleons from the nucleus.
Why does the fusion of small nuclei produce energy
as the two elements have a nucleon number < 56 they appear before the peak of the binding energy per nucleon curve implying that they are relatively unstable. Upon fusion, they create a more stable element higher up the curve. Work is done separating the nucleons and so by fusing nucleons together you decrease their potential energy – energy is always conserved so this reduction in potential energy means that energy is released to the surroundings. The a higher binding energy per nucleon is achieved and so there is a higher mass defect meaning a greater reduction in mass. This reduction in mass is equivalent to a reduction in energy (E=mc^2) meaning the new element has less energy thus energy is released to the surroundings
