8.1 Nuclear power & misc. - November 28, 2017

Transcription

1 8.1 Nuclear power & misc. - November 28, [1 mark] The following are energy sources. I. a battery of rechargeable electric cells II. crude oil III. a pumped storage hydroelectric system Which of these are secondary energy sources? A. I and II only B. I and III only C. II and III only D. I, II and III B 2. [1 mark] The main role of a moderator in a nuclear fission reactor is to A. slow down neutrons. B. absorb neutrons. C. reflect neutrons back to the reactor. D. accelerate neutrons. A 1

2 3. [1 mark] What is the correct order of energy transformations in a coal power station? A. thermal chemical kinetic electrical B. chemical thermal kinetic electrical C. chemical kinetic thermal electrical D. kinetic chemical electrical thermal B 4a. [2 marks] This question is in two parts. Part 1 is about energy resources. Part 2 is about thermal physics. Part 1 Energy resources Electricity can be generated using nuclear fission, by burning fossil fuels or using pump storage hydroelectric schemes. Outline which of the three generation methods above is renewable. 2

3 pump storage; renewable as can be replaced in short time scale / storage water can be pumped back up to fall again / source will not run out; } (do not accept because water is used ) 4b. [1 mark] In a nuclear reactor, outline the purpose of the heat exchanger. (allows coolant to) transfer thermal/heat (energy) from the reactor/(nuclear) reaction to the water/steam; Must see reference to transfer cooling reactor/heating up water is not enough. 4c. [2 marks] moderator. 3

4 reduces speed/kinetic energy of neutrons; (do not allow particles ) improves likelihood of fission occurring/u-235 capturing neutrons; 4d. [3 marks] Fission of one uranium-235 nucleus releases 203 MeV. Determine the maximum amount of energy, in joule, released by 1.0 g of uranium-235 as a result of fission. 4

5 (203 MeV is equivalent to) (J); nuclei have a mass of 235 (g) / evaluates number of nuclei; ( nuclei produce) (J) / multiplies two previous answers; Award [3] for bald correct answer. Award [1] for correct conversion from ev to J even if rest is incorrect. 4e. [3 marks] Describe the main principles of the operation of a pump storage hydroelectric scheme. water flows between water masses/reservoirs at different levels; flow of water drives turbine/generator to produce electricity; at off peak times the electricity produced is used to raise water from lower to higher reservoir; 5

6 4f. [3 marks] A hydroelectric scheme has an efficiency of 92%. Water stored in the dam falls through an average height of 57 m. Determine the rate of flow of water, in power of 4.5 MW., required to generate an electrical output use of ; ; (t is usually ignored, assume 1 s if not seen) ; Award [3] for a bald correct answer. 4g. [2 marks] This question is in two parts. Part 1 is about energy resources. Part 2 is about thermal physics. Part 2 Thermal physics Distinguish between specific heat capacity and specific latent heat. 6

7 specific heat capacity is/refers to energy required to change the temperature (without changing state); specific latent heat is energy required to change the state/phase without changing the temperature; If definitions are given they must include salient points given above. The essential difference between specific heat capacity and specific latent heat is that the former refers to a change of temperature without changing state; whereas the latter refers to a change of state without changing temperature. Most candidates just wrote definitions which they had learnt by rote and omitted the constant temperature for a substance changing state. 4h. [2 marks] A mass of 0.22 kg of lead spheres is placed in a well-insulated tube. The tube is turned upside down several times so that the spheres fall through an average height of 0.45 m each time the tube is turned. The temperature of the spheres is found to increase by 8 C. 7

8 Discuss the changes to the energy of the lead spheres. gravitational potential energy kinetic energy; kinetic energy internal energy/thermal energy/heat energy; Do not allow heat. Two separate energy changes must be explicit. 8

9 This is a question specifically about energy changes so candidates are expected to use accurate language and spell out the changes one by one. Common mistakes were omitting the gravitational in gravitational potential energy; referring to heat rather than thermal energy; and saying that gravitational potential energy changed to thermal and kinetic energy as if it were a single process. 4i. [4 marks] The specific heat capacity of lead is turned upside down.. Deduce the number of times that the tube is use of ; use of ; equating ; 236 or 240; or use of ; 9

10 ; ; inversions) or 240; } (allow if answer is rounded up to give complete number of Award [4] for a bald correct answer. This was generally well done. There were four marks and the question asks the candidates to deduce so it is essential that the argument is transparent. The examiner cannot be expected to search through a mass of numbers in order to carry forward an error. 5. [1 mark] In nuclear power production, what is one advantage of a nuclear fusion reactor over a nuclear fission reactor? A. The operating temperature of the fusion reactor is lower. B. The nuclear reactants are more easily confined within the core of the fusion reactor. C. The disposal of the nuclear waste products from the fusion reactor is more straightforward. D. The nuclear fusion reaction is more easily sustained for long periods of time. C 6. [1 mark] Which type of power-production system is most suitable for responding to a sudden high increase in demand for electrical power? A. A wind generator B. A tidal water storage hydroelectric scheme 10

11 C. An ocean-wave energy converter D. A pump storage hydroelectric scheme D 7a. [8 marks] This question is in two parts. Part 1 is about renewable energy. Part 2 is about nuclear energy and radioactivity. Part 1 Renewable energy A small coastal community decides to use a wind farm consisting of five identical wind turbines to generate part of its energy. At the proposed site, the average wind speed is 8.5ms 1 and the density of air is 1.3kgm 3. The maximum power required from the wind farm is 0.75 MW. Each turbine has an efficiency of 30%. (i) Determine the diameter that will be required for the turbine blades to achieve the maximum power of 0.75 MW. (ii) State one reason why, in practice, a diameter larger than your answer to (a)(i) is required. (iii) Outline why the individual turbines should not be placed close to each other. (iv) Some members of the community propose that the wind farm should be located at sea rather than on land. Evaluate this proposal. 11

12 (i) total wind power required ; maximum wind power required per turbine, ; Award [1 max] for an answer of 48.9 (m) as it indicates 5 and 0.3 ignored. Award [2 max] for 22 (m) as it indicates 0.3 ignored. Award [2 max] for 89 (m) as it indicates 5 ignored. (ii) not all kinetic energy can be extracted from wind / losses in cables to community / turbine rotation may be cut off/ feathered at high or low wind speeds; Do not allow wind speed varies as question gives the average speed. (iii) less kinetic energy available / wind speed less for turbines behind; turbulence/wake effect; (do not allow turbines stacked too close ) (iv) implications: average wind speeds are greater / more space available; limitations: installation/maintenance cost / difficulty of access / wave damage; Must see one each for [2]. 12

13 7b. [7 marks] Currently, a nearby coal-fired power station generates energy for the community. Less coal will be burnt at the power station if the wind farm is constructed. (i) The energy density of coal is 35 MJ kg 1. Estimate the minimum mass of coal that can be saved every hour when the wind farm is producing its full output. (ii) One advantage of the reduction in coal consumption is that less carbon dioxide will be released into the atmosphere. State one other advantage and one disadvantage of constructing the wind farm. (iii) Suggest the likely effect on the Earth s temperature of a reduction in the concentration of atmospheric greenhouse gases. (i) mass of coal per second (= kg); 77.1 (kg); or energy saved per hour= (=2700MJh -1 ) ; mass of coal saved ; Award [2] for a bald correct answer. (ii) advantage: energy is free (apart from maintenance and start-up costs) / energy is renewable / sufficient for small 13

14 community with predominance of wind / supplies energy to remote community / independent of national grid / any other reasonable advantage; Answer must focus on wind farm not coal disadvantages. disadvantage: wind energy is variable/unpredictable / noise pollution / killing birds/bats / large open areas required / visual pollution / ecological issues / need to provide new infrastructure; (iii) greenhouse gas molecules are excited by/absorbed by/resonate as a result of infrared radiations; { (must refer to infrared not heat ) this radiation is re-emitted in all directions; less greenhouse gas means less infrared/heat returned to Earth; { (consideration of return direction is essential for mark) temperature falls (to reach new equilibrium); 7c. [1 mark] Part 2 Nuclear energy and radioactivity The graph shows the variation of binding energy per nucleon with nucleon number. The position for uranium-235 (U-235) is shown. 14

15 State what is meant by the binding energy of a nucleus. energy released when a nucleus forms from constituent nucleons / (minimum) energy needed/work done to break a nucleus up into its constituent nucleons; Award [0] for energy to assemble nucleus. Do not allow particles or components for nucleons. Do not accept energy that binds nucleons together OWTTE. 7d. [5 marks] (i) On the axes, sketch a graph showing the variation of nucleon number with the binding energy per nucleon. (ii) Explain, with reference to your graph, why energy is released during fission of U

16 (i) generally correct shape with maximum shown, trending down to U-235; maximum shown somewhere between 40 and 70; Award [0] for straight line with positive gradient from origin. Award [1] if maximum position correct but graph begins to rise or flatlines beyond or around U-235. (ii) identifies fission as occurring at high nucleon number / at right-hand side of graph; fission means that large nucleus splits into two (or more) smaller nuclei/nuclei to left of fissioning nucleus (on graph); (graph shows that) fission products have higher (average) binding energy per nucleon than U-235; energy released related to difference between initial and final binding energy; Award [2 max] if no reference to graph. 7e. [4 marks] U-235 can undergo alpha decay to form an isotope of thorium (Th). (i) State the nuclear equation for this decay. (ii) Define the term radioactive half-life. (iii) A sample of rock contains a mass of 5.6 mg of U-235 at the present day. The half-life of U-235 is years. Calculate the initial mass of the U-235 if the rock sample was formed years ago. 16

17 (i) ; (allow He for ; treat charge indications as neutral) (ii) time taken for number of unstable nuclei/(radio)activity to halve; Accept atom/isotope. Do not accept mass/molecule/amount/substance. (iii) three half-lives identified; 45 (mg); Award [2] for bald correct answer. 8a. [3 marks] This question is in two parts. Part 1 is about the oscillation of a mass. Part 2 is about nuclear fission. Part 1 Oscillation of a mass A mass of 0.80 kg rests on a frictionless surface and is connected to two identical springs both of which are fixed at their other ends. A force of N is required to extend or compress each spring by 1.0 mm. When the mass is at rest in the centre of the arrangement, the springs are not extended. The mass is displaced to the right by 60 mm and released. Determine the acceleration of the mass at the moment of release. 17

18 force of 1.8 N for each spring so total force is 3.6 N; acceleration ; (allow ECF from first marking point) to left/towards equilibrium position / negative sign seen in answer; This is a slightly different situation. Most candidates at SL did not use F and m to find acceleration. Very few added the force due to each spring and ECF was frequently applied. 8b. [2 marks] Outline why the mass subsequently performs simple harmonic motion (SHM). 18

19 force/acceleration is in opposite direction to displacement/towards equilibrium position; and is proportional to displacement; 8c. [2 marks] Calculate the period of oscillation of the mass. 19

20 ; ; Watch out for ECF from (a)(i) eg award [2] for for. 8d. [2 marks] The motion of an ion in a crystal lattice can be modelled using the mass spring arrangement. The interatomic forces may be modelled as forces due to springs as in the arrangement shown. The frequency of vibration of a particular ion is and the mass of the ion is. The amplitude of vibration of the ion is. Estimate the maximum kinetic energy of the ion. 20

21 ; ; Allow answers in the range of 4.8 to if 2 sig figs or more are used. 8e. [3 marks] On the axes, draw a graph to show the variation with time of the kinetic energy of mass and the elastic potential energy stored in the springs. You should add appropriate values to the axes, showing the variation over one period. 21

22 KE and PE curves labelled very roughly and shapes; } (allow reversal of curve labels) KE and PE curves in anti-phase and of equal amplitude; at least one period shown; either marked correctly on energy axis, or marked correctly on time axis; Care was needed in showing the constant and equal amplitudes. Many poor answers were seen. 8f. [1 mark] Calculate the wavelength of an infrared wave with a frequency equal to that of the model in (b). is equivalent to wavelength of ; 8g. [3 marks] Part 2 Nuclear fission A reaction that takes place in the core of a particular nuclear reactor is as shown. 22

23 In the nuclear reactor, fissions take place every second. Each fission gives rise to 200 MeV of energy that is available for conversion to electrical energy. The overall efficiency of the nuclear power station is 32%. Determine the mass of U-235 that undergoes fission in the reactor every day. number of fissions in one day ; mass of uranium atom ; mass of uranium in one day ; 8h. [2 marks] Calculate the power output of the nuclear power station. 23

24 energy per fission ; power output ; Award [1] for an answer of. 8i. [3 marks] In addition to the U-235, the nuclear reactor contains a moderator and control rods. Explain the function of the moderator. 24

25 neutrons have to be slowed down (before next fission); because the probability of fission is (much) greater (with neutrons of thermal energy); neutrons collide with/transfer energy to atoms/molecules (of the moderator); Mostly good answers although it was rare to find a candidate who stated that the probability of further fusion is increased with thermal neutrons. 8j. [2 marks] control rods. 25

26 have high neutron capture cross-section/good at absorbing neutrons; (remove neutrons from the reaction) thus controlling the rate of nuclear reaction; Too many answers lacked precision referring only to the use of control rods in avoiding an explosion or meltdown. 9a. [1 mark] This question is about nuclear reactions. A reaction that takes place in the core of a particular nuclear reactor is as shown. State the nature of X. neutron / ; X is a neutron and almost all answers were correct. G2 comments suggested that the term State the nature of X was rather vague. This is a helpful comment, but on this occasion candidates did not seem to be affected by it. 26

27 9b. [1 mark] State one form of energy that is instantaneously released in the reaction. kinetic energy / gamma radiation / binding energy; Instantaneous energy release: Binding energy, (particle) kinetic energy, gamma radiation were all accepted. Heat energy was not accepted. 9c. [3 marks] In the nuclear reactor, fissions take place every second. Each fission gives rise to 200 MeV of energy that is available for conversion to electrical energy. The overall efficiency of the nuclear power station is 32%. Determine the mass of U-235 that undergoes fission in the reactor every day. 27

28 number of fissions in one day ; mass of uranium atom ; mass of uranium in one day ; Quite a few candidates found the mass defect rather than the mass of U-235 fissioned. The mass of a Uranium atom was often incorrect candidates were expected to determine it from the mass number. A few stated the mass as 235g. Another common error was to find mass per second, rather than per day. However many correct answers were seen. 9d. [2 marks] Calculate the power output of the nuclear power station. energy per fission ; power output ; Award [1] for an answer of. 28

29 Working was often unclear. The power station efficiency of 0.32 was often overlooked. Whilst many candidates were eventually able to determine the power output of the power station in W, there were also answers giving the energy output per day. Quite a few candidates used ev/s and this was accepted for 1 mark. 9e. [3 marks] In addition to the U-235, the nuclear reactor contains graphite that acts as a moderator. Explain the function of the moderator. neutrons have to be slowed down (before next fission); because the probability of fission is (much) greater (with neutrons of thermal energy); neutrons collide with/transfer energy to atoms/molecules (of the moderator); There was some confusion between a moderator and control rods. However, most candidates knew that the graphite moderator slowed neutrons (due to inelastic collisions) to thermal energies to maximise the probability of further fission. 9f. [3 marks] Outline how energy released in the nuclear reactor is transformed to electrical energy. 29

30 kinetic energy of neutrons/thermal energy of core is transferred into thermal energy of the coolant (and elsewhere); (thermal energy) is converted into kinetic energy in moving steam; (kinetic energy of steam) is transferred into (rotational) kinetic energy of turbine; (kinetic energy of turbine) is transferred into electrical energy by dynamo/generator; Outlining the energy transfers occurring in a nuclear power plant is a frequent question, but answers were often poorly organised. A simple flow diagram would suffice. Thermal energy of core > thermal energy of coolant > KE of steam > KE of turbine > Electrical energy from generator, or similar. Far too many candidates barely mentioned energy; heat up water to make steam which turns a turbine gained no marks. 10. [1 mark] Which of the following best defines non-renewable fuels? A. They produce a lot of degraded energy in comparison with renewable fuels. B. They have very high energy density but produce greenhouse gases. C. They cannot be produced again. D. Their rate of consumption is much greater than the rate at which they are being produced. 30

31 D 11. [1 mark] In the production of energy from nuclear fission, fuel enrichment means increasing, in the fuel rods, the amount of A. uranium-238. B. plutonium-239. C. uranium-235. D. uranium-235 and plutonium-239. C 12. [2 marks] This question is about nuclear power production. State two advantages of power production using fossil fuels compared to using nuclear fuels. 31

32 no radioactive waste; no radiation risks to users; lower expense of decommissioning / easier to decommission / easier to install / lower set-up cost; transportation and storage less hazardous/safer; simpler technology; cannot be used for military purposes; fossil fuels can be extracted/found more easily; no chance of catastrophic accident/meltdown/chernobyl; 13a. [3 marks] This question is about electrical generation using nuclear power. Exposure to radiation is a safety risk both to miners of uranium ore and to workers in nuclear power plants. Outline why uranium ore needs to be enriched before it can be used successfully in a nuclear reactor. 32

33 U-238 is much more common than U-235 in ore; U-235 is more likely to undergo fission / critical amount of U-235 required to ensure fission / OWTTE; U-238 absorbs neutrons; U-238 reduces reaction rate in reactor; 13b. [6 marks] (i) One possible waste product of a nuclear reactor is the nuclide caesium-137 by the emission of a beta-minus (β ) particle to form a nuclide of barium (Ba). which decays State the nuclear reaction for this decay. (ii) The half-life of caesium-137 is 30 years. Determine the fraction of caesium-137 remaining in the waste after 100 years. 33

34 (i) ; ; anti-neutrino / ; (ii) ; or 9.9%; ; 13c. [2 marks] Some waste products in nuclear reactors are good absorbers of neutrons. Suggest why the formation of such waste products requires the removal of the uranium fuel rods well before the uranium is completely used up. proportion of waste builds up in fuel rod as uranium is consumed; increasing numbers of neutrons will be absorbed; this reduces the number available to sustain the chain reaction; build up of waste deforms fuel rod (which can then be difficult to remove); 34

35 14. [1 mark] In a nuclear power station, in order to increase the chances of a chain reaction A. kinetic energy is removed from the neutrons. B. kinetic energy is given to the neutrons. C. some neutrons are absorbed. D. extra neutrons are added. A 15. [1 mark] In a nuclear power station, in order to increase the chances of a chain reaction A. kinetic energy is removed from the neutrons. B. kinetic energy is given to the neutrons. C. some neutrons are absorbed. D. extra neutrons are added. A Printed for Jyvaskylan Lyseon lukio International Baccalaureate Organization 2017 International Baccalaureate - Baccalauréat International - Bachillerato Internacional 35