1. Nuclear Fission and Fusion SACE Stage 2 Physics

2. Nuclear fission is the process in which a very heavy nucleus splits into two lighter nuclei. Spontaneous fission can occur with some massive nuclei which can split into two lighter parts called fission fragments. This process is not common and has no industrial importance. Induced fission occurs when a heavy nucleus absorbs a neutron to form a very excited nucleus which “oscillates” and splits into two positively charged parts and often one or more extra neutrons. The common process in nuclear reactors is when 235 U 92 absorbs a neutron. i.e. Spontaneous and Induced Nuclear Fission

3. The half life of 236 U 92 is <10 -12 seconds. The secondary decay occurs almost immediately. A particular case, Many other isotopes can be produced. In addition, the nuclei Y 1 & Y 2 are usually excited and emit  rays as they decay to their ground states.

4. Spontaneous and Induced Nuclear The mass of the Uranium nucleus plus the neutron is greater than the total mass of the product nuclei and neutrons so that energy is emitted in this reaction – it appears as kinetic energy of the product nuclei plus the energies of the emitted  ray photons. In a chemical reaction eg when carbon burns in oxygen to give CO and/or CO 2, the amount of energy released when the atoms of C and O combine is usually only a few electron volts (eV). When however nuclear fission occurs millions of eV of energy are produced.

5. Explanation of fission Can be explained assuming the nucleus behaves like a single drop. Fission of a 235 U 92 nucleus after capture of a neutron, according to the liquid drop model. Nucleus is now considered to be vibrating and begins to break apart. Nucleus undergoes a fission reaction forming 2 new nuclei and some free neutrons

6. Chain Reaction In nuclear fission processes, when averaged, more than one neutron is emitted in each fission process. If these neutrons can be used to induce further fissions then a nuclear chain reaction is possible.

7. Chain Reaction Most of the neutrons produced in fission have relatively high energies of 1 or 2 MeV. Probability of a 235 U 92 absorbing a neutron of this energy is very low. Need to slow neutrons down (10 eV or less). This can be achieved by using a moderator. The most effective moderating materials are those with low masses – approximating those of neutrons themselves. (Consider billiard balls of the same mass colliding.)

8. Chain Reaction

9. Expect that the best moderator would contain atoms 1 H 1 atoms (≈ neutron mass). Problem - 1 H 1 tends to absorb neutrons, therefore need to use deuteron (heavy hydrogen – in the form of heavy water)

10. Chain Reaction The Uranium ore can not be used directly to give energy from a chain reaction in a nuclear reactor. The fraction of U-235 in naturally occurring Uranium is about 0.8% the rest being U-238. It is with fission of U-235 that reactors usually operate. This means that before use, Uranium ore must be enriched. ie. The proportion of U-235 must be increased to several percent to get a power reactor, or about 90% for weapons! This process is both difficult and expensive. It is just possible to produce a reactor operating with natural Uranium but only if the moderator is heavy water – also expensive to produce!

11. Chain Reaction Many neutrons produced in fission are either absorbed by surrounding atoms or escape from the surface of the reactor – it follows that reactors can’t be made small, like tiny batteries (surface areas: volume ratio is too high). Nuclei with 30<Z<60 ie. in the range of most reactor fission products have N up to about 1.5Z, they are almost highly radioactive and can undergo many decay processes, emitting  - particles, before stable nuclei appear. This is why if fission fragments and hence energy are released into the surroundings when a reactor malfunctions, it is very dangerous indeed.

12. Chain Reaction When fuel rods in reactors needed replacing, the used rods are difficult to handle, very dangerous and hard to get rid of! Some of the fission isotopes eg Cs-137 are useful in industrial radiography, but most are not wanted.

13. Chain Reaction Notes: About 1% of the neutrons produced in nuclear fission are emitted after a delay of up to 10 seconds or more. There is no unique fission for any given nucleus. The many possible reactions result in the production of a range of fission products. The nuclei produced by fission reactions are likely to have an excess of neutrons, and hence are likely to be radioactive.

14. Example Calculate the energy released when the following nuclear reaction occurs.  Masses are: 1 n 0 = 1.675 x 10 -27 kg 235 U 92 = 3.9017 x 10 -25 kg 141 Ba 56 = 2.28922 x 10 -25 kg 92 Kr 36 = 1.57534 x 10 -25 kg

15. Example Total mass of the reactants = 0.01675 x 10 -25 + 3.9017 x 10 -25 = 3.91845 x 10 -25 kg Total mass of the products= 2.2892 x 10 -25 + 1.57534 x 10 -25 + 3(0.01675 x 10 -25 ) = 3.91481 x 10 -25 kg Mass defect (  m)= 3.91845 x 10 -25 – 3.91481 x 10 -25 = 3.64 x 10 -25 kg E=  mc 2 = 3.64 x 10 -25 kg x (3 x 10 8 ) 2 = 3.276 x 10 -11 J = 205 MeV

16. APPLICATION: Fission Nuclear Power A nuclear reactor. The heat generated by the fission process in the fuel rods is carried off by hot water or liquid sodium and is used to boil water to steam in the heat exchanger. The steam dives a turbine to generate electricity and is then cooled in the condenser.

17. APPLICATION: Fission Nuclear Power CORE: the core consists of the fuel rods (enriched uranium) and the moderator, which slows, down the neutrons. In this case the moderator is water which becomes very hot. The fuel rods are stationary but there are also control rods, which can be moved in and out to control the rate at which nuclear energy is produced.

18. APPLICATION: Fission Nuclear Power The control (or safety) rods are usually made of cadmium or boron; both are elements, which absorb neutrons strongly. The control rods are moved in to slow down the rate of reactions and up to speed it all up. The aim is to keep the reaction just critical so that the nuclear reactions are maintained at a steady rate by the released neutrons from each fission reactions. This would be impossible if some neutrons were not delayed ie emitted up to a few seconds after the reactions, not almost instantaneously. This gives the control rods enough time to react to the situation as required.

19. APPLICATION: Fission Nuclear Power Energy is transferred from the reaction in the form of heat energy - it is carried off either by water or liquid sodium. The rest of the power reactor is similar to that of a normal coal, oil or natural gas electricity generator. Nuclear reactors after they have been built can be used to produce electrical power relatively cheaply. They have the advantage that no greenhouse gases are produced continuously, as when oil, natural gas or coal are burnt. In addition we are not likely to run out of the basic fuel - the supply is not limited, as is the supply of fossil fuels.

20. APPLICATION: Fission Nuclear Power But, nuclear reactors are dangerous. Admittedly they have extra "shut- down" controls rods which, in case of emergency, can be used to stop the whole thing, but still! They also produce "thermal pollution" of the environment. Ordinary power stations do this also. In addition the materials used in the reactors are not "nice" to handle - liquid Sodium is not "nice" nor is Beryllium! Also the isotopes produced are highly radioactive. The other problem is that the ionising radiations produced also destroy (break-down the structure) of building materials - steel, concrete etc, - so they weaken and a reactor has, usually, a life-time of only about 30 years - we are then left with a dangerous lump of stuff.

21. APPLICATION: Fission Nuclear Power Handling and safely storing old fuel rods is also a problem - reprocessing is dangerous and storage - well, who wants them? Reactor accidents have occurred in Russia, USA, UK and Korea. These are often said to be due to "technical" problems but such problems are human produced - "human error", which can be never totally eliminated. Still, many countries now really do rely on nuclear power - they would be in a real mess economically if it were banned.

22. APPLICATION: Fission Nuclear Power Disadvantages of using nuclear reactors: (1) safety during transport of nuclear materials. (2) safety during enrichment. (3) disposal of nuclear waste. (4) possible leakage of radioactive material from the reactor. (5) risk of accidents in operating the reactor. Safe operating conditions for the workers and surrounding community is an expensive part of the reactor costs. (6) any reactor has a limited lifetime- then all parts of the core will show significant radioactivity and means a problem exists in safely disposing of this material.

23. APPLICATION: Fission Nuclear Power Advantages: (1) known technology. (2) relatively abundant uranium fuel resources. (3) does not produce as much green-house emissions as a coal burning power station. (4) cost wise it is competitive with conventional reactors. (5) can be utilised by countries without significant coal deposits.

24. Nuclear Fusion Nuclear fusion is the process in which two nuclei combine to form a single nucleus. Example 1 H 2 + 1 H 2  2 He 3 + 0 n 1 + 3.3 MeV (the D-D reaction) deuteriumhelium 3 OR 1 H 2 + 1 H 3  2 He 4 + 0 n 1 + 17.6 MeV (the D-T reaction) Deuterium tritium Total mass of Reactants is greater then the Products, ie energy is released, calculated by E =  mc 2.

25. Nuclear Fusion It is important to note that the single biggest difficulty in causing these reactions is the coulombic repulsion between the positive charges on each nucleus. Because the nuclear forces are of such short range the two nuclei have to be brought within a nucleon diameter range(10 -15 m) for interaction to take place. This means the electrostatic repulsive force of the protons has to be overcome.

26. Example Calculate the amount of energy emitted when the following fusion reaction occurs. 2 H 1 + 2 H 1  3 He 2 + 1 n 0

27. Example Calculate the amount of energy emitted when the following fusion reaction occurs. 2 H 1 + 2 H 1  3 He 2 + 1 n 0 Mass of the reactants = 2 x mass of 2 H 1 = 6.688 x 10 -27 kg Mass of products = mass of 3 He 2 + mass of neutron = 6.683 x 10 -27 kg Mass defect  m = 5 x 10 -29 kg E =  mc 2 = 5 x 10 -29 x (3 x 10 8 ) 2 = 4.5 x 10 -13 J = 2.8 MeV

28. Stars as Fusion Reactors The source of energy from the sun is produced by the nuclear fusion of hydrogen into helium. 1 H 1 + 1 H 1  2 He 2 1 H 1 + 2 H 1  3 He 2 3 He 2 + 3 He 2  4 He 2 + 1 H 1 + 1 H 1 4 He 2 (stable) is produced as well as 2 protons which can reinitiate the reaction series. The suns core is at about 20 million degrees. This is high enough to excite the atoms to release gamma ray photons and other radiation of low frequencies as they revert to lower states. The temperature is high enough to start the proton/proton reaction.

29. Advantages/Disadvantages of Nuclear Fusion Advantages of nuclear fusion: (1) Hydrogen is the most abundant element in the universe, and the fuel supply is therefore no a problem for the future. In the case of fission we have estimated uranium supply for a further 200 years. (2) Fusion is a "cleaner" process than fission. ie less radioactive by- products. (3) Safety precautions with the reactor can be less stringent than the nuclear reactor as the danger of venting radioactive material into the environment is less. (4) Should eventually cost less to produce electricity as the fuel is cheaper. (5) energy output of a fusion reaction is greater per nucleon than that of a fission.

30. Advantages/Disadvantages of Nuclear Fusion Dis-advantages: (1) The current technology (using toroidal doughnuts - pinch effect of a magnetic field) to contain then fusion reaction is not yet adequate to produce a significant energy output. (2) Expensive in terms of research & development at this stage. (3) production of high temperatures and containment of the high kinetic energy nuclei is a problem in fusion.