Download
1 / 52
520 likes | 715 Views
Transitions to a Low-Carbon Economy: Nuclear Power. Prof. Paddy Regan. Physics Department,University of Surrey & Radioactivity Group, National Physical Laboratory, Teddington. email: p.regan@surrey.ac.uk. 27-Feb-2014.
E N D
Transitions to a Low-Carbon Economy: Nuclear Power Prof. Paddy Regan Physics Department,University of Surrey & Radioactivity Group, National Physical Laboratory, Teddington. email: p.regan@surrey.ac.uk 27-Feb-2014
An overview of nuclear power, its current role and future directions ….in about one hour and a half….
Reactors Worldwide ~430 reactors make ~1/7 of the world’s electricity.
Source: IEA WEO. 2008 IEA Key Statistics give 2.3% of ‘Other’ (2006 data) Note that mixture of fuels used → electricity production is very different in different countries e.g. coal ~ 35% in UK, ~76% in China (where hydro ~ 18%)
From New Scientist, May 2013.
Some history / facts • UK Atomic Energy Authority (UKAEA) established 1954 to oversee UK nuclear energy programme. • Calder Hall connected to the grid in August 1956.
Current UK nuclear power stations (from http://world-nuclear.org/info/Country-Profiles) (Updated April 2013) The UK has 16 reactors normally generating about 19% of its electricity and all but one of these will be retired by 2023. The country has full fuel cycle facilities including major reprocessing plants. The UK has implemented a very thorough assessment process for new reactor designs and their siting. The first of some 19 GWe of new-generation plants are expected to be on line ~2018. The government aims to have 16 GWe of new nuclear capacity on line by 2030.
‘New’ sites for UK Nuclear Reactors (named June 2011).
Future reactors in the UK (as of 2013…) http://world-nuclear.org/info/Country-Profiles/Countries-T-Z/United-Kingdom
Nuclear Power Production in the U.K. Sizewell B
Asian Nuclear Reactors (~2013) • Taiwan 4 x BWR, 2 x PWR, 2 ABWR in Build • Japan 50 reactors in 2013, mix of PWR and BWR • India 20 reactors, mainly PHWR, 1 BWR • South Korea 23 reactors, mainly PWR some PHWR • PWR = Pressurised Water Reactor • BWR = Boiling Water Reactors • PHWR = Pressurised Heavy Water Reactor (- CANDU)
Japan? Japan needs to import ~5/6 of its energy requirement. Fukushima accident/earthquake/Tsunami on 11 March 2011. By May 2011, 17 out of Japan's 50 remaining nuclear power reactors were still operating. Now all reactors are ‘offline’.
China…. As of June 2013, • Mainland China has 17 nuclear power reactors in commercial operation, 28 under construction, and more about to start construction soon. • Plans to increase in nuclear capacity to approx. 60 GWe or possibly more by 2020, and then a further substantial increase, up to 200 GWe by 2030 and 400 GWe by 2050. • China is rapidly becoming self-sufficient in nuclear design and fuel cycle Source WANO http://www.world-nuclear.org/info/Country-Profiles/Countries-A-F/China--Nuclear-Power
Types of Reactors • Power reactors produce commercial electricity. • Research reactors are operated to produce high neutron fluxes for neutron-scattering experiments. • Heat production reactors supply heat in some cold countries. • Some reactors are designed to produce radioisotopes. • Several training reactors are located on college campuses.
Atoms – 10-8 m Z protons and Z electrons in neutral atom Nuclei – 10-14 m Z protons and N neutrons Nucleons – 10-15 m Three quarks Quarks
A Nuclear Chain Reaction • Each neutron inducing fission results in the production of several other neutrons. Each of these neutrons is capable of initiating fission in another nucleus with the emission of another 2.5 neutrons on average. The number of fissions and neutrons can increase very rapidly. This process is described as a chain reaction. A chain reaction is characterised by the neutron multiplication factor k, which is defined as the ratio of the number of neutrons in one generation to the number in the preceding generation. If k < 1 then the number of neutrons decreases with time and the process stops. In the context of a reactor it would be said to be sub-critical. If k > 1 then the number of neutrons increases with time and the chain reaction diverges. A reactor would be said to be super-critical. ( a nuclear bomb!) If k = 1 everything proceeds at a steady rate. A reactor in this state would be said to be critical.
The main elements of a reactor. • Fuel – pellets of UO2 (1cm diam.by 1.5 cm long) arranged in tubes to form fuel rods. • They are usually formed into fuel assemblies in the core. • 2. Moderator – usually water but may be graphite or heavy water. • 3.Control rods – Made with neutron absorbing material included so that inserting or • withdrawing the rod controls or halts the rate of reaction. • Note:-Secondary shutdown systems involve adding other absorbers of neutrons, • usually in the primary cooling system. • 4.Coolant- Liquid or gas circulating in the core to carry away heat. In light water • reactors the coolant acts as moderator and coolant. • 5.Pressurevessel – Usually a robust steel vessel containing the core and • moderator/coolant but it may be a series of tubes holding the fuel and • conveying the coolant through the moderator. • 6.Steamgenerator – Part of cooling system where the reactor heat is used to make • steam to drive the turbines. • 7.Containment –Structure round core to protect it from intrusion and protect the outside • from radiation in case of a major malfunction.
Boiling Water Reactor Energy Transfer • Most common method is to pass hot water heated by the reactor through some form of heat exchanger. • In boiling water reactors (BWRs) the moderating water turns into steam, which drives a turbine producing electricity. • In pressurised water reactors (PWRs) the moderating water is under high pressure and circulates from the reactor to an external heat exchanger where it produces steam, which drives a turbine. • Boiling water reactors are inherently simpler than pressurized water reactors. However, the possibility that the steam driving the turbine may become radioactive is greater with the BWR. • The two-step process of the PWR helps to isolate the power generation system from possible radioactive contamination. Pressurised Water Reactor
Where does the Uranium come from? • Uranium is relatively common – found in seawater and rocks. • Almost half of the World’s production is in Canada and Australia in open pit or relatively shallow mines It is then milled – the ore is crushed to form a fine slurry and it is leached with sulphuric acid to produce concentrated U3O8 – which is called yellowcake and generally has more than 80% U compared with the original 0.1% Underground mines cause less disturbance but one needs very good ventilation to protect against airborne radiation exposure. Tailings are radioactive with long-lived activities in low concentrations and also contain heavy metals. They have to be isolated. Increasingly the mining industry uses in-situ leaching. Here oxygenated groundwater is circulated through the U deposit underground to dissolve the U and bring it to the surface.
Uranium deposits. Source: Energy Visions 2030 for Finland, VTT Energy, Helsinki, 2003.
http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Mining-of-Uranium/http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Mining-of-Uranium/
Making Fuel rods Most reactors use enriched fuel- enriched in mass 235. The yellowcake is converted to UF6 – a gas- which is enriched either by gas diffusion or in a centrifuge. The former relies on the different diffusion rates of uranium isotopes with masses 235 (enriched) and 238 (depleted). In the latter the gas passes through spinning cylinders and the centrifugal force causes the mass 238 move to the outside leaving a higher mass 235 concentration on the inside. Uranium dioxide pellets are then made form the enriched material. The pellets are then encased in long metal tubes, usually made of zirconium alloy (zircalloy) or stainless steel to form fuel rods. The rods are sealed and assembled in clusters to form fuel assemblies for use in reactors.
Nuclear Fuel Production • 2. Yellow cake • 4. Fuel pellets • 1. Uranium ore • 3. Uranium hexafluoride Source: USDOE
Breeder Reactors • A more advanced kind of reactor is the breeder reactor, which produces more fissionable fuel than it consumes. • The chain reaction is: • The plutonium is easily separated from uranium by chemical means. • Fast breeder reactors have been built that convert 238U to 239Pu. • Breeder reactors could provide an almost unlimited supply of fissionable material. • One of the downsides of such reactors is the production of plutonium and its possible use in unauthorised nuclear weapons.
The Oklo reactor is interesting in itself but it is also highly relevant to the discussion of dealing with present day waste. Neither the fission fragments nor the Pu migrated from the site in 2 x 109 y.
Used Fuel • Uranium recovered can be used in MOX fuel or can be returned to conversion • plant to be included in new fuel. • 2. The Mixed Oxide (MOX) fuel is a blend of Pu and U. The Pu effectively • substitutes for the U in new fuel. • 3. Typically reactors use a one-third mixture of MOX and Uranium dioxide fuel • assemblies although 100% MOX is possible.
Used Fuel • Used fuel emits radiation and heat. • It is unloaded into a storage pond adjacent to the reactor to allow it to decay. • It can be stored there for long periods. It can also be stored in dry stores cooled • by air. • D. Both kinds of store are intended to be temporary. It will be reprocessed or sent to • final disposal. The longer it is stored the easier it is to handle. • E. Main options for long term – reprocessing to recover useful fuel • - storage and final disposal • F. Reprocessing – separates U and Pu from waste products by chopping up rods • and dissolving them in acid to separate the various materials. • G. Typically used fuel is 95% 238U, 1% 238U, 1% Pu and 3% fission products including • other transuranics. • H. Reprocessing enables recycling of fuel and produces a significantly reduced waste • volume.
Radioactive Waste • Classification is different in different countries and may be based on different • factors. • B High Level waste (HLW) – highly radioactive, generates a lot of heat. Mainly the • liquid waste from reprocessed fuel after U and Pu extracted. In UK it is concentrated, • mixed with molten glass and stored in 150 litre stainless steel drums. • C. Intermediate Level Waste (ILW) – less radioactive, much less heat. Mostly metal items • such as fuel cladding reactor components, graphite from reactor cores, sludges from • treatment of radioactive liquid effluents.It is stored in tanks,vaults and drums. It will be • repackaged following immobilisation in cement-based materials in 500 l stainless • steel drums. • D. Low Level Waste – largely consists of contaminated redundant equipment,protective • clothing and packaging. It is sent to Drigg and compacted, packaged in large metal • containers and placed in an engineered vault a few metres below the surface. Note:- At present the large amount of stored Pu is not included in the waste category because it may yet be used in MOX fuel.
Radioactive Waste and its Disposal • What to do with highly radioactive wastes? • Prevent dispersion • Shield • Present solution (“temporary”) • Stored in pools next to site • Long term • Store (bury) in deep stable geological formation • Treatment • Chemically processed • Vitrified • Packed in special canisters • Stored in disaffected mines (can be retrieved) • or specially constructed repository
CORWM’s Long List - Options for radioactive Waste Disposal 1.Interim or Indefinite storage on or below the surface 2.Near surface disposal – few metres to tens of metres down 3.Deep disposal with surrounding geology as a barrier 4.Phased deep disposal with storage and monitoring for a period 5.Direct injection of liquid wastes into rock strata 6.Disposal at sea 7.sub-seabed disposal 8.Disposal in ice sheets 9.Disposal in subduction zones 10.Disposal in space, into high orbit or propelled into Sun 11.Dilution and dispersal of radioactivity in the environment 12.Partitioning of wastes and transmutation of radionuclides 13.Burning of Pu and U in reactors 14.Incineration to reduce waste volumes 15 melting of metals in furnaces to reduce waste volumes
