Nuclear Energy Flashcards by L G

State the form in which energy released from a decaying nucleus (alpha decay) initially appears as

Kinetic energy

or electrostatic potential energy

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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

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Changes in proton and neutron number after electron capture

-1 proton

+1 neutron

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State what is meant by the binding energy of a nucleus

Energy needed to separate a nucleus into its constituent nucleons

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Describe what happens to the majority of the alpha particles in the Rutherford scattering experiment

pass through with no or very small deflection

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Name two deductions that can be made from the results of alpha particle scattering

volume of nucleus &laquo_space;volume of atom / nucleus small and atom mostly empty space

Most of mass in nucleus

Nucleus has positive charge

Size of nucleus &laquo_space;separation

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What is meant by the random nature of radioactive decay

Which atoms decays at what time is chance

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Explain what is meant by electron capture

Inner orbiting electron captured by a proton in the nucleus and is converted into a neutron

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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.

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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

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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

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Explain why 231-Th-90 is likely to be a beta minus emitter

Because it is neutron-rich compared with more stable elements

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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

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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

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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

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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