NUCLEAR TECHNOLOGIES

Transcription

1 Civilian & military dimensions of NUCLEAR TECHNOLOGIES Mirco Elena Isodarco, FOCC

2 TH OM PS O N 1896 D R O F R E H T U R BOHR Henri Becquerel

3 Electron (-) Proton (nucleus, +) HYDROGEN ATOM

4 Electron Electron Nucleus (2 protons, 2 neutrons) HELIUM ATOM

5 Periodic Table of the Elements The materials for Bombs and reactors

6 He 4 U 238 alpha particle: km/s > km/h! Th 234

7 Quantity Radioactive Decay & Half Life Time

8 1938: nuclear fission; 1 kg U-235 = 100 railroad carriages of TNT; 38-40: idea of The Bomb. Marie & Pierre Curie Leo Szilard Otto Hahn & Lise Meitner

9 Achieved for the first time in 1942 by E. Fermi (completed in millionths of a second)

10 (only artificially produced, e.g. in reactors) % of U isotopes Chain react. requires U or Pu 1942: 1st nuclear reactor; : production of enriched U and Plutonium. in nature for reactors U bombs: difficult to get U; easy to explode. Pu bombs: easy to get Pu; difficult to explode. for bombs

11 Strong Link btw Reactor and Bomb Technologies Both technologies use U enrichment or Pu separation

12 The Fuel Cycle (for nuclear reactors) It consists in a series of steps, in which ore is mined, milled, U separated, transformed in gas (UF6), enriched, put into a reactor, irradiated for some years, extracted and either reprocessed or stored/disposed forever. The entire cycle, from mine to repository, lasts about 8 years.

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14 The Fuel Cycle (ctd.) The fuel cycle is the most delicate phase, with regard to both nuclear proliferation (U enrichment and reprocessing for extraction of Pu) and to environmental impact (waste management).

15 Dual-use technologies

16 Essentially identical equipment and operations for: mining, refining, U enrichment, extraction of Pu. Ci vil ian M ilit ar y But for bombs: U enrichment up to 95%, or reprocessing of irradiated fuel to get Pu. Risk of terrorists getting HEU (90-95% in U235). Additional problem: radiological ( dirty ) weapons.

17 Plutonium Production fissions and fertilization Pu and unburnt U235 may be recovered by reprocessing. On average ~300 kg plutonium are produced per modern power station per year, enough for up to 40 nuclear bombs.

18 Varios Kinds of Pu % fissile isotopes (Pu239, Pu 241) Superpure Weapon grade Slightly irradiated reactor fuel Reactor fuel <80 In a LWR, 1 MW-day (thermal) produces 1 g of Pu. ==> a bomb requires about 2 months of operation of a 100 MWt reactor. Separation of enough Pu for one bomb requires an apparatus as big as a fridge, working for a couple of months.

19 U production by country

20 U conversion plants

21 U enrichment plants

22 Isotopic Enrichment Gas diffusion; Centrifugation; Mass spectrom.; To enrich U from 4% to 90%, not even 50% more separative work is needed, than to go from 0,7 to 4%! A plant of 600 sq meters surface and requiring only 100 kw of electricity could produce enough enriched U for a bomb in only 1 yr.

23 Energy Requirements for U Enrichment Gas diffusion is 140 less energy intensive than thermal diffusion; Gas centrifuges require only about 1/40 the energy of gas diffusion; Laser separation needs about 1/3 the energy of centrifuges. The more energy an enrichment plant uses, the easier it becomes to spot it, even remotely, and even if it is operated in a clandestine way.

24 Recipe: How to Make a Bomb

25 CRITICAL MASS (for a chain reaction) Little material is needed! (quantity can be reduced by using a tamper, or by increasing density) For Pu in aqueous solution, critical mass is only about 1 pound (0,5 kg)

26 Gun Mechanism Uranium Uranium Not indicated: neutron generator ( e.g. Po Be)

27 Im p Me losio ch a n nis m High explosive Neutron reflector ( tamper ) Empty Pu sphere ( pit )

28 Hiroshima: uranium bomb, 60 kg of HEU 16/7/45: Trinity test, Pu bomb; 6/8/45: Hiroshima (U bomb); 9/8/45: Nagasaki (second Pu bomb). Nagasaki: plutonium bomb, 6 kg WG of Pu

29 Bombs with RG Pu With RG Pu one can achieve explosive yields of nx100 t. Yield is uncertain: about 50% that of a bomb with = quantity of WG Pu. Possibility of predetonation. The weapon is very radioactive ==> risk for military personnel. Extra heat can damage the high explosive. In 1962 Usa achieved a successful nuclear test with RG Pu.

30 Horizontal Nuclear Proliferation The original five nuclear powers: USA, USSR/Russia, GB, F, China. We have today in the Nuclear Club also Israel, India, Pakistan, North Korea (and, potentially, all industrial nations with the right materials)

31 Proliferators That Work(ed) on U Enrichment Pakistan, S. Africa, Iran, Iraq, Argentina, Brasil Proliferators That Work(ed) on Pu Separation Israel, India, North Korea

32 NW proliferation: risks More civilian nuclear facilities increase potential for diversion of materials to military programs Determined states which have access to civilian nuclear programme are hard to stop going military. Terrorists interested in stealing fissile materials/radioactive substances. International Atomic Energy Agency (IAEA) complaints of lack of resources; also has the role of promoting nuclear power. Will the nuclear non-proliferation treaty (NPT) hold?

33 Contrasting Proliferation (1) Institutional Measures Treaties; verification (e.g.:iaea safeguards); controls on supply of nuclear materials & technologies; internationalization of fuel cycle; Technical Measures Avoid producing WG materials (no Pu separation and no enrichment of U); making access to WG materials difficult (e.g.: by radiation levels);

34 Contrasting Proliferation (2) More in detail Stay with once-through, high burn-up fuel cycles. (Need for U from seawater ). Employ breeder or particle accelerator-driven reactors that co-locate reprocessing with the reactor and do not separate Pu from other actinides. Restrict nuclear power to int.l energy parks, exporting electricity or H. Alternatively, export sealed reactor cores needing no refueling, to be returned to the energy park as spent fuel.

35 Cost of Bombs Cost of the Manhattan Project : for two bombs, 20 G$2000 (80% spent on U and Pu). Cost of the South African nuclear program: 6 gun barrel bombs for 200 M US$ over ten years. (Building the bomb is relatively cheap, when compared to developing rockets and other hightech delivery vehicles. It is then achievable also by poor countries )

36 Recipe: How to Make a Nuclear Reactor (How to boil water by a very, very complex mechanism)

37 Operation of Nuclear Reactors in a nutshell Fissions of U or Pu generate heat; A suitable coolant extracts heat; The heat drives a thermal motor; This motor produces electricity.

38 What you need Fuel to produce energy by nuclear fission (U or Pu); Cooling fluid (water, gas or liquid metal) that transports heat outside the reactor; For thermal reactors, a moderator (graphite, water or heavy water) to slow down neutrons; Standard turbines and electrical generators.

39 1954: 1st nuclear-propelled submarine, Nautilus ; 1st civilian reactor at Obninsk, USSR.

40 Control of a Nuclear Reactor (1) Critical mass Below the critical mass the number of fissions decreases with time. Above it the number of fissions increases with time. Immediate neutrons generation is very quick. Control of immediate neutrons emission is extremely difficult. Delayed fissions Some fissions produce short lived nuclei. These nuclei decay emitting delayed neutrons that contribute to the chain reaction.

41 Control of a Nuclear Reactor (2) There are two different critical masses: Immediate c.m.: prompt neutrons sustain the chain reaction. Delayed c.m.: in which both prompt and delayed neutrons are necessary. Reactors operate: Under the prompt critical mass Above the delayed c.m. Reaction rate controlled by control rods (and B in coolant)

42 Reactor fuel: U Pellet

43 Pellets Zr Tube to Form Fuel Rod; Many Fuel Rods Fuel Assembly

44 Fuel Assembly for PWR reactor

45 Burnt reactor fuel management Open cycle It s the most widespread form of nuclear fuel management at present: natural U has to be enriched (and depleted U is discarded). After irradiation, spent fuel is sent to long term storage repositories. Only 1% of U energy content is converted to electricity.

46 Irradiated Fuel Management Pu reprocessing Pu is extracted from irradiated fuel and mixed with U, to be used as fuel in other reactors. Efficiency in fuel use doubles (2%).

47 Military Reactors Bombarding U238 in the reactor cores, Pu239 is formed, which is ideal for bombs. Further bombardment changes Pu239 into Pu240, not the material of choice for military use. To get Pu239 one must then frequently extract the irradiated fuel elements from the reactor core. Reactors such as the Soviet RBMK and Canadian CANDU allow this operation to be performed without having to stop reactor operation. (For this reason, the reactor at Chernobyl lacked a robust containment )

48 Evolution of Civilian Nuclear Reactors

49 Evolution of Civilian Nuclear Reactors Nuclear reactors are classified as follows: 1st Generation: built in 50s- 60s. Still operating only in UK. 2nd Generation: derived from naval propulsion reactors, are the ones now operating in largest number (e.g. BWR, PWR). 3rd Generation III (& 3+): so called advanced reactors, first introduced in Japan in Others are under construction or planned. Olkiluoto, Flamanville, 4th Generation: still in the planning phase; not to be operative before 2020, at the very earliest.

50 Nuclear energy consumption by area Consumption by region Million tonnes oil equivalent

51 World Primary Energy Consumption Patterns World Consumption Million tonnes oil equivalent

52 3rd Generation Reactors (Advanced Nuclear Reactors) Promised Characteristics: Standardized design to speed up authorization procedures, reduce costs and building times; Easier and more robust concept to make them easier to operate and less prone to damage; More reliable and longer operating life, typically 60 years; Reduce chance of accidents due to overheating of the core; Reduced environmental impact; Increased fuel burn-up, thus diminishing also waste quantity; Neutron absorbers that self-destroy during operation; so as to extend time between fuel recharges.

53 Targets for 4th Generation Plants Sustainability Promote long term availability of fuel and its efficient use. Minimize waste production. Improve environmental protection. Economy Present cost advantages vs. all other energy sources. Present financial risks comparable to other energy projects. Safety and Reliability Very low probability of core accidents. In case of accidents, resulting damage must be very limited. Eliminate any emergency intervention outside the plant. Proliferation Resistance e Physical Protection Offer the least attractive path to obtain fissile materials. Have high protection level against terrorist attacks.

54 New Reactor Types (1) Thorium-based reactors Don t use U enrichment; they produce U233 (denatured with U238 and contaminated with γ-emitting U232 decay chain; potentially usable in nuclear weapons); they generate ~20% Pu/kWh with respect to LWR; this Pu is 6-12X richer in Pu238 high decay heat and higher spontaneous fission rate probab. fizzle yield. They need U or Pu for initial cycles; to increase efficiency, they would benefit from enriched U or Pu, in addition to U233.

55 New Reactor Types (2) Pebble bed reactors Graphite-moderated, passively gas cooled, very high operation temperature, passive safety. Pebbles are n x tennis ball size graphite shells (moderator! Flammable ), each containing n x 1000, one mm size spherules of U/Pu/Th fuel, each covered by ceramic protective cover (for fission products containment). Coolant is inert gas (He/N/CO2); no piping in the core. If core T, produced power due to Doppler broadening. No H embrittlement of metal structures. High operation T means higher efficiency, less fuel is used, w.r. to LWR. Some designs throttled down by T, not by control rods. Fuel replacement is continuous. Germany developed PBRs, but then abandoned them in South Africa also abandoned prototype in China has German license, 10MW prototype; plans 30x200MW PBRs by 2020.

56 New Reactor Types (3) Rubbiatron / ADS Subcritical Th-fuel reactor using liquid lead as coolant; high energy accelerator sends protons on heavy metal target; spallation; neutrons produced enable criticality. Accelerator as switch for the chain reaction. ADVANTAGES: Th is abundant; no enrichment needed; limited Pu production; could be used to burn excess Pu; shorter duration wastes. DISADVANTAGES: No prototype yet; economic viability=?

57 Fuel Cycle in 4th Generation Nuclear Systems From the present system (open cycle) to that of 4th generation (closed cycle with recycling of Pu)

58 Sustainability of 4th Generation Nuclear Systems

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