Chapter 27 - Nuclear energy Flashcards
What happens to the mass of an object when energy is transferred to or from it?
Changes too (due to E=mc2 relationship)
What is the energy of a photon produced in annhilation equal to?
E=mc2 where m=mass of the particle or antiparticle
An unstable nucleus that releases a 5MeV gamma photon would lose 8.0x10-13J of energy. What would it’s mass therefore decrease by?
Mass change = energy change/c2 = (8.0x10-13/(3.0x108)2 = 8.9x10-30Kg
What is the atomic mass unit (u)?
1u = 1.66x10-27Kg
How do you convert from atomic mass units to MeV (energy released)?
mass difference in u x 931.3
210Po84 emits alpha particles and decays to form 206Pb82. Write an equation for this and calculate the energy released when 210Po84 emits an alpha particle.
mass of Po-210 nucleus = 209.93667u
mass of Pb-206 nucleus = 205.92936u
mass of an alpha particle = 4.00150u
210Po84 -> 206Pb82 + 4He2 + energy released
mass difference = total initial mass - total final mass
= 209.93667 - (205.92936 + 4.00150)
= 5.81x10-3u
energy released (Q) = mass differnce x 931.3 = 5.41MeV
What is the binding energy of the nucleus?
Work that must be done to seperate a nucleus into its constituent neutrons and protons (overcoming the strong nuclear force and increasing the potential energy of each nucleon)
Why is energy released when a nucleus forms from seperate neutrons and protons? What is the energy released equal to?
- Strong nuclear force does work pulling the nucleons together
- binding energy
Since energy is released when a nucleus forms, what happens to the mass?
Mass of a nucleus is less than the mass of the seperated nucleons
What is the mass defect (change in m)?
Mass defect of a nucelus is defined as the difference between the mass of seperated nucleons and the mass of the nucleus
What’s the equation for binding energy of a nucleus?
mass defect x c2
(change in mass) x c2
What is the binding energy per nucleon of a nucleus?
Average work done per nucleon to remove all the nucleons from a nucleus
What is binding energy per nucleon also a measure of?
Stability of a nucleus
(greater binding energy per nucleon = more stable nuclei)
Draw graph of binding energy per nucleon against mass number (A) for all known nuclides
What happens to the binding energy per nucleon in nuclear fission where 2 fragments are formed from one large unstable nucleus?
Increases (fragments are more stable)
Change in binding energy per nucleon is around 0.5MeV
What happens to the binding energy per nucleon in nuclear fusion when small nuclei fuse together to form a large nucleus?
Increases (provided the nucleon number of the product nucleus is no greater than about 50)
Change of binding energy per nucleon is x10 that of in a fission reaction
What is induced fission?
When a nucleus splits into 2 approximately equal fragments. When 235U92 or 239Pu94 is bombarded with neutrons fission occurs
How is a fission chain reaction possible?
2/3 neutrons released in a fission event cause further fission events as a result of colliding with another 235U92 nucleus which releases further neutrons, inducing another fission and so on..
Why is energy released in a fission event? What is this energy equal to?
Fragments repel each other with sufficient force to overcome the strong nuclear force trying to hold them together. Fragment nuclei and fission neutrons therefore gain kinetic energy.
Changing in binding energy (and Q=(change in m) c2
Write equation for induced fission for 235U92 to 144Ba56 and 90Kr36
235U92 + 1n0 -> 144Ba56 + 90Kr36 + 2 1n0 + energy released
What is nuclear fusion?
2 nuclei combining at high speed to form a bigger nucleus. Electrostatic repulsion overcame and interact through the strong nuclear force
(binding energy increases and energy released is equalto change in binding energy)
Why is energy released in nuclear fusion?
Individual nucleons become more tightly bound together in a bigger nucleus. (binding energy per nucleon increases. Nucleons become more tightly bound together)
(energy released equal to increase in binding energy)
Write equation for nuclear fusion of a proton and a deuterium nucleus 2H1 producing a nucleus of 3He2
2H1 + 1P1 -> 3He2
Briefly describe a thermal nuclear reactor
Contains fuel rods spaced evenly in a steel vessel known as the reactor core. The reactor core also contains control rods, a coolant and fuel rods and is connected to a heat exchanger by steel pipes. A pump is used to force the coolant through the reactor core (where it’s heated) and through the heat exchanger where it is used to raise steam to drive turbines that turn the electricity generator.
Draw a simple diagram of a thermal nuclear reactor
What are the reactors called that use water as their coolant?
Pressurised water reactor (PWR)
What are fuel rods in a thermal nuclear reactor made out of?
Contain enriched uranium which consists mostly of U-238 - the non-fissionable uranium isotope, and aout 2-3% of U-235 which is fissionable
What is the function of the control rods?
Absorb neutrons
What does the depth of the control rods control?
Keeping rate of release of fission energy constant by keeping the number of neutrons in the core constant. (so that one fission neutron per fission event on average goes on to produce further fission)
What happens if the control rods are pushed in further?
They absorb more neutrons so that the number of of fission events per second and the rate release of fission energy is reduced
What does a moderator do?
Slows down fission neutrons by repeated collisions with the moderator atoms. (slowed to a kinetic energycomparible to that of the moderator molecules)
Why do fission neutrons need to be slowed down?
To cause further fission. Too fast otherwise
What acts as the moderator in PWR?
Water (both moderator + coolant)
What materials are usually used for moderators? And why?
Graphite (C-12 atoms) and water due to size of atom close to that of a neutron and transfer of kinetic energy is most effective if this is the case. This and chemical stability
What condition must be true in a nuclear reactor for a chain reaction to occur?
Mass of the fissile material (e.g. U-235) must be greater than a minimum mass (the critical mass)
Why does the mass of a fissile material need to be greater than the critical mass for a chain reaction to occur?
Some fission neutrons escape from the fissile material without causing fission and some are absorbed by other nuclei without fission (too many fission neutrons escape because the SA : mass ratio of the material is too high)
What does an advanced gas - cooled reactor use as it’s moderator and coolant?
Moderator - graphite
coolant - CO2 gas
What fuels are used for advanced gas-cooled reactors and pressurised water reactors?
AGR - Uranium oxide in stainless steel cans
PWR - uranium oxide in nirconium alloy cans
What is the coolant temperature and power output of an AGR and a PWR?
AGR - coolant temp = 900K power output = 1300W
PWR - coolant temp = 600K power output = 700W
What are the safety features of nuclear features of nuclear reactors?
- reactor core is a thick steel vessel
- Core is within thick concrete walls
- Reactor has an emergency shut down system
- Fuel rods are inserted and removed from the reactor by means of remote handling devices
How is having a thick steel reactor core a safety feature?
- Withstands high pressure and temperature in the core
- Absorbs beta radiation and some of the gamma radiation and neutrons from the core
How is thick concrete walls around the core a safety feature?
How can it become radioactive over time?
- Absorbs the neutrons (some) and gamma radiation that escape from the reactor used
neutrons which escape the reactor may enter the shielding nuclei causing them to become unstable/neutron rich and start experiencing beta-minus decay
Why is an emergency shut down system for the reactor a safety feature?
Designed to insert the control rids fully into the core to stop completely
Why are spent fuel rods so dangerous?
Contain Pu-239 as a result of absorption of neutrons by U-238 nuclei (plutonium isotope is very active alpha emitter and if inhaled causes lung cancer)
What are nuclear power stations hopefully able to provide in the UK?
Securing future electricty supply and reducing carbon emmissions
What 3 catagories is radioactive waste put in?
High level, intermediate level and low level
How must radioactive waste be stored?
In accordance with legal regulations and approved by disposal companies to ensure wate is stored safely in secure constainers until it’s activity is insignificant
What an example of high level radioactive waste?
Spent fuel rods (HLW is mostly from nuclear power stations or form specialist users in univerisities and industry)
How are spent fuel rods stored?
- Removed by remote control and stored in cooling ponds for up to a year as they release heat due to radioactivity
- Unused uranium and plutonium can be removed and stored in sealed containers for further use, with the rest (radioactive waste) vitrified (encased in glass) and stored in thick, sealed steel containers in deep trenches in geologically stable areas for centuries. (Long term storage is an issue as no one wants this in their own locality)
How is intermediate-level waste stored? Give examples
- Sealed in drums that are encased in concrete and stored in specially constructed buildings with walls of reinforced concrete.
e. g. radioactive materials with low activity or containers of radioactive materials
How is low level waste stored?
Sealed in metal drums and buried in large trenches
What are some examples of low-level waste?
Laboratory equipment and protective clothing
What 2 nuclei are most likely to be used in nuclear fusion
Deuterium (2H1) anf tritium (3H1)
What methods are used to raise the temperature needed for fusion?
Electrical heating, electrical discharge, em radiation, inducing a current in plasma
