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Flashcards in Nuclear Energy Deck (24)
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1

What can be said about the mass for any change where energy is released?

The total mass after the change is always less than the total mass before the change because some of the mass is converted to energy which is released.

2

What is binding energy of a nucleus?

The work that must be done to separate a nucleus into its constituent neutrons and protons.
It is calculated by delta mc^2 where delta m is the mass defect

3

What is mass defect of a nucleus?

The difference between the mass of the separated nucleons and the nucleus.
It is calculated by the mass of all the nucleons it has when they are separate, take away the mass of the nucleus.

4

Why does mass defect of a nucleus exist?

When a nucleus forms from separate nucleons, energy is released as the strong nuclear force does work pulling the nucleons together. The energy released is equal to the binding energy. Because energy is released, the mass of the nucleus is less than the mass of the separated nucleons.

5

Binding energy per nucleon

The average work done per nucleon to remove all the nucleons from a nucleus. Measure of stability of nucleus (greater value means more stable). It is the binding energy of a nucleus divided by its mass number, A. Greatest for iron.

6

Binding energy per nucleon against mass number graph

Actual points are not on line of best fit. But line of best fit has steep positive gradient from hydrogen and levels off at about A=50. Then gentle negative gradient not getting near x-axis though. Max value is 8.7MeV per nucleon at about A=56.

7

Where are nuclei that do fusion and fission on the binding energy per nucleon graph?

Fission occurs to the right of A=56 because the large unstable nucleus splits into 2 daughter nuclei which are more stable than the original so the binding energy per nucleon increases. Fusion occurs to the left of A=56 because the two small nuclei fuse to form a larger nucleus which is more stable so has more binding energy per nucleon.

8

What is induced fission?

The splitting of a U-235 or Pu-235 nucleus, caused by an incoming neutron colliding with it, into two approximately equal fragments.

9

What is a chain reaction?

A series of reactions in which each reaction causes a further reaction. In a nuclear reactor this is due to fission neutrons from one fission event colliding with another nucleus causing it to split and release further neutrons that can go on to produce further fission.

10

Why is energy released in nuclear fission?

The fragments repel each other with sufficient force to overcome the strong nuclear force. The fragment nuclei and fission neutrons therefore gain kinetic energy. They are smaller and more tightly bound than the original nucleus so have more binding energy so are more stable. The energy released is equal to the change in binding energy.

11

Why does nuclear fusion release energy?

If two small nuclei are combined, the individual nucleons become more tightly bound. The binding energy per nucleon of the product is greater than that of the initial nuclei. As a result, energy is released equal to the increase in binding energy.

12

Why can nuclear fusion only take place if the two nuclei are at high speeds?

They need to overcome the electrostatic repulsion between the two nuclei so they can become close enough to interact through the strong nuclear force

13

How does the sun do fusion

The centre of the sun is thought to be at least 10^8K. At this temperature, atoms are stripped of their electrons. Matter in this state is referred to as plasma. The nuclei of the plasma move very fast due to the temperature. When two nuclei collide, they fuse together as they overcome the electrostatic repulsion and approach closely enough to interact through the strong nuclear force.

14

Coolant

Pumped through the reactor core to transfer thermal energy from the core to a heat exchanger so it doesn’t become dangerously hot.

15

Control rods

Rods made of a neutron-absorbing substance (eg cadmium or boron) that are moved in and out of the reactor core to control the rate of fission events in the reactor.

16

Heat exchanger

Steel vessel containing pipes through which hot coolant in a sealed circuit is pumped, causing water passing through the steel vessel in separate pipes to turn to steam which is used to drive turbines.

17

Moderator

Substance in a thermal nuclear reactor that slows down the fission neutrons so they can go on to produce further fission. Surrounds fuel rods.

18

Critical mass

Minimum mass of fissile isotope in a nuclear reactor necessary to produce a chain reaction. If mass is lower than this, too many of the fission neutrons escape without causing fission as the surface area to mass ratio is too high. Or they are absorbed without fission.

19

Differences between advanced gas-cooled reactors and pressurised water reactors.

AGR fuel is uranium oxide in stainless steel cans and PWR is uranium oxide in zirconium alloy cans. AGR moderator is graphite and PWR is water. AGR coolant is CO2 gas and PWR is water. AGR coolant temperature is 900K and PWR is 600. Typical AGR output power is 1300MW and PWR is 700MW.

20

Safety features of nuclear reactors

Reactor core is a thick steel vessel designed to withstand high pressure and temperature in core and absorbs beta, som gamma and neutrons. Core is in building with thick concrete walls which absorbs any escaping beta, gamma and neutrons from the reactor vessel. Emergency shut down system insert control rods completely. Fuel rods moved by remote handling devices.

21

What is high level radioactive waste and how is it stored?

E.g spent fuel rods. Removed by remote control and stored underwater for a year as they continue to release heat. Rods then transferred in large steel casks (crash resistant) to place where unused uranium and plutonium is removed and stored in sealed containers for further use. Rest (fission products and fuel cans) is stored in stainless steel containers in deep trenches.or deep underground.

22

What is intermediate level radioactive waste and how is it stored?

Radioactive materials with low activity and containers of radioactive materials. Sealed in drums that are encased in concrete and stored in specially constructed buildings with walls of reinforced concrete.

23

What is low level radioactive waste and how is it stored?

Laboratory equipment and protective clothing. Sealed in metal drums and buried in large trenches.

24

Problems with storing high level radioactive waste

It is initially very hot. It is initially very radioactive so can't have human contact. Liquid waste can leak so needs vitrifying (convert to glass). Will be radioactive for thousands of years so stored in geologically stable areas. Transportation could endanger public.