energy and the environment

Energy and the Environment

Fall 2014Instructor: Xiaodong Chu

Email ： [email protected] Tel.: 81696127, 13573122659

mailto:[email protected]

Nuclear-Fueled Power Plants: Nuclear Reactors – Boiling Water Reactor

• Most boiling water reactors boiling water reactors (BWBWR) (沸水反应堆 ) use 3–4% enriched 235U as the fuel

• Light water serves as both the moderator and coolant• As the control rods are withdrawn, the chain reaction starts and the

moderator– coolant water boils– The saturated steam has a temperature of about 300 ◦C and pressure of 7 Mpa

• After separation of condensatecondensate (凝结水 ), in the steam separator steam separator (汽水分离器 ), the steam drives the turbine that drives the generator

• After expansion in the turbine, the steam is condensed in the condenser condenser (冷凝器 )and returned via the feed water pump feed water pump (给水泵 ) to the core core (堆芯 )


• The direct cycle has the advantage of simplicity and relatively high thermal efficiency, because the steam generated in the core directly drives a turbine without further heat exchangers heat exchangers (换热器 )

– The thermal efficiency of a BWR is on the order of 33%, calculated on the basis of the inherent energy of the nuclear fuel that has been consumed in power production

• In a BWR there is no need for an extra moderator, because the coolant light water slows down the fast neutrons to thermal velocities that can engage in further fission reactions

• Another advantage of a BWR is the fact that it is self-controlling– When the chain reaction becomes too intense, the coolant water boils faster and

since steam has little or no moderating propensity, the reduction of the liquid content of the coolant water automatically slows the chain reaction


• In a BWR, the core is enclosed in the primary containment vessel containment vessel (安全壳 ) made of steel and surrounded by reinforced concrete reinforced concrete (钢筋混凝土 )

• A secondary containment vessel made of reinforced concrete contains the steam separator and the spent fuel storage pool spent fuel storage pool (乏燃料储存池 )

• The steam turbine, condenser, and electric generator are located outside of the secondary containment vessel


• Some radioactive material may leachleach (渗透 ) from the core into the coolant water and be transferred by the steam to the steam turbines

• Furthermore, the coolant water may contain traces of mildly radioactive isotopes of hydrogen (tritium tritium ( 氚 ) [3H]) and nitrogen (16N and 17N)

• The steam remaining in contact with the turbine and other equipment loses its radioactivity quite fast, and with proper precaution no significant exposure is presented to workers in the power plant or the population outside the plant

Nuclear-Fueled Power Plants: Nuclear Reactors – Pressurized Water Reactor

• Pressurized water reactorsPressurized water reactors (PWRPWR) (压水反应堆 ) constitute a majority of all nuclear power plants

• In a PWR the primary coolant (water) is pumped under high pressure (15-16 MPa) to the reactor core where it is heated by the energy generated by the fission

• The heated water then flows to a steam generator steam generator (蒸汽发生器 ) where it transfers its thermal energy to a secondary system where steam is generated and flows to turbines which, in turn, spins an electric generator

• In contrast to a boiling water reactor, pressure in the primary primary coolant loop coolant loop (一次冷却剂回路 ) prevents the water from boiling within the reactor

• A PWR uses water as both coolant and neutron moderator


• Advantages of the PWR– Because of the single phase of the coolant water, the moderating capacity

of the water can be precisely adjusted, unlike in a BWR, where the coolant is in two phases, liquid and vapor

– BoronBoron ( 硼 )in the form of boric acid boric acid (硼酸 ) is added to the coolant to increase the moderating capacity so that fewer control rods are necessary to maintain the reactor at the design capacity

– The steam which is generated in the heat exchanger never comes into direct contact with the coolant water so that any radioactivity that may be present in the coolant is confined to the primary loop inside the containment vessel

• Disadvantages of the PWR– Because of the heat exchanger the overall thermal efficiency of a PWR is

somewhat lower than that of a BWR, on the order of 30%


• The CANDU (short for CANada Deuterium Uranium) reactor is a Canadian-invented, pressurized heavy water reactor

• CANDU uses natural uranium fuels (without enrichment) and heavy water (D2O) as the moderator

– Heavy water absorbs practically no neutrons; thus its neutron economy is superior to that of light water

– On the other hand, its moderating capacity is less than that of light water; therefore neutrons have to travel twice as far to be slowed down as in light water

• The fissile material is 235U as in enriched uranium reactors, but some of the fertile 238U is converted into fissile 239Pu, which can participate in the chain reaction or can be extracted for fuel recycling or for weapons production

• The CANDU-type reactor requires heavy water production stills stills (蒸馏器 )

• It is more advantageous to operate a natural uranium/heavy water reactor than to operate an enriched uranium/light water reactor since weapons grade plutonium could be produced during the process

Nuclear-Fueled Power Plants: Nuclear Reactors – Gas-Cooled Reactor

• In a gas-cooled reactor (GCR) (气冷反应堆 ) the moderator is graphite (石墨 ) and the coolant is gas, normally CO2 (helium can also be used)

• The gas-cooled reactors are fueled by natural and enriched uranium

• There were two main types of generation I GCR– The Magnox reactors developed by the UK– The UNGG reactors developed by France


• Magnox is a now obsolete type of nuclear power reactor which was designed and is still in use in the United Kingdom, and was exported to other countries, both as a power plant, and, when operated accordingly, as a producer of plutonium for nuclear weapons


• The UNGG (Uranium Naturel Graphite Gaz) is an obsolete design of nuclear power reactor developed by France, which was developed independently of and in parallel to the British Magnox design

• The main difference between the two designs – UNGG used a horizontal fuel rod orientation, rather than the vertical orientation

used in the Magnox reactor– The fuel cladding material was magnesium-zirconium alloy magnesium-zirconium alloy (镁锆合金 ) in the

UNGG, as opposed to magnesium-aluminium (镁铝合金 ) in Magnox

Cross section of UNGG fuel


• In the UK, the Magnox was replaced by the advanced gas-cooled reactor (AGR) (先进气冷反应堆 ), an improved Generation II gas-cooled reactors

• In France, the UNGG was replaced by the pressurized water reactor (PWR)


• AGR are the second generation of British gas-cooled reactors, operating at a higher gas temperature for improved thermal efficiency, requiring stainless steel fuel cladding to withstand the higher temperature– Because the stainless steel fuel cladding (燃料包壳 ) has a higher neutron

capture cross section than Magnox fuel cans, enriched uranium fuel is needed, requiring less frequent refuelling

– All AGR power stations are configured with two reactors in a single building and each reactor has a design thermal power output of 1500 MWt driving a 660 MWe turbine-alternator set

Nuclear-Fueled Power Plants: Nuclear Reactors – Breeder Reactor

• In a breeder reactor (BR) (增殖反应堆 ), fissile nuclei (可裂变核 ) are produced from fertile nuclei (可转换核 )

• A mechanism is the conversion of 238U to 239Pu ( 钚 )

– The formed 239Pu does not participate to a significant extent in the chain reaction, but accumulates in the spent fuel (乏燃料 ) from which it is later extracted and reused

238 239 239 239U+n U+ Np+ Pu+

Isotope Half-life239U 23 minutes

239Np ( 镎 ) 2.4 days239Pu 24 kilo years


• Another breeder mechanism is the conversion of 232Th ( 钍 ) to 233U

– Because worldwide thorium ( 钍 ) ores have about equal abundance as uranium ores, the use of thorium would extend the nuclear fuel resources almost doubly

232 233 233 233Th+n+ Th+ Pa+ U

Isotope Half-life233Th 22.3 minutes

233Pa ( 镤 ) 27 days233U 159 kilo years


• There are two type of breeder reactors corresponding to the two breeder mechanisms– Fast breeder reactor (FBR) (快中子增殖反应堆 ) : A fast breeder

reactor needs no moderator to slow down the neutrons since 238U captures efficiently fast neutrons so that light or heavy water is undesirable as the coolant

• Liquid sodium (液钠 ) is the preferred coolant and currently all FBR power stations are the liquid metal fast breeder reactors (LMFBR) (液态金属快中子增殖反应堆 ) cooled by liquid sodium

– Thermal breeder reactor (热中子增殖反应堆 ) : The excellent neutron capture characteristics of fissile uranium-233 make it possible to build a moderated reactor (慢化反应堆 )


Schematic of a liquid metal fast breeder reactor


• The efficiency of fuel utilization in a breeder reactor is expressed by the breeding ratio (BR) (增殖比 )

– When BR is greater than 1, breeding occurs– Fissile nuclei include not only 235U, but also 239Pu and 233U

Number of fissile nuclei producedBR

Number of fissile nuclei consumed


• Most breeder reactors were operated for the primary purpose of producing weapons-grade plutonium

• While more fissile fuel is produced than what is consumed in the process of power production, plutonium can be extracted to make atomic bombs

Nuclear-Fueled Power Plants: Nuclear Reactors

• Thirty countries operate nuclear power stations, and there are a considerable number of new reactors being built in China, South Korea, India, Pakistan, and Russia

• As of 2011, Germany and Switzerland are phasing-out nuclear power


• Of the thirty countries which operate nuclear power plants, only France uses them as its primary source of electricity, although many countries have a significant nuclear power generation capacity

• Some nations have plans to start a nuclear power program; these include OECD members, such as Poland, and developing countries, such as Bangladesh and Vietnam

• China, South Korea and India are pursuing an ambitious expansion of their nuclear power capacities

– China is aiming to increase nuclear power generation capacity to 40 GW by 2020– South Korea is constructing seven reactors with combined capacity of 8.6 GW, all of

which will be operationalised by 2017– India's Nuclear power expansion program is the third largest in the world next only

to China & South Korea


• Generation IV reactors (Gen IV) are a set of theoretical nuclear reactor designs currently being researched

• Most of these designs are generally not expected to be available for commercial construction before 2030, with the exception of the Next Generation Nuclear Plant (NGNP)


• Nuclear reactors in China


• Nuclear reactors under construction in China


• Nuclear reactor technology used in China: CPR-1000– The CPR-1000 (improved Chinese PWR) is a Generation II+

pressurized water reactor (PWR), based on the French 900 MWe three cooling loop design imported in the 1990s, improved to have a net power output of 1,000 MWe (1080 MWe gross) and a 60 year design life

– The CPR-1000 is built and operated by the China Guangdong Nuclear Power Company (CGNPC) (中广核 )

– Currently it is the most numerous reactor type under construction, with fifteen units under construction as of June 2010, and another 15 approved and proposed


• Nuclear reactor technologies used in China: AP1000– Westinghouse Electric Company's AP1000 reactor design is the first

Generation III+ reactor to receive final design approval from the U.S. Nuclear Regulatory Commission (NRC), which is two-loop pressurized water reactor (PWR) planned to produce a net 1154 MWe

– AP1000 is the main basis of China's move to Generation III technology, and involves a major technology transfer agreement

– The first four AP1000 reactors are being built at Sanmen and Haiyang, for the China National Nuclear Corporation (CNNC) (中核 ) and China Power Investment Corporation (CPI) (中电投 ) respectively while at least eight more at four sites are firmly planned after them, and about 30 more are proposed to follow


• Nuclear reactor technologies used in China: ERP– The EPR is a Generation III pressurized water reactor (PWR) design,

which has been designed and developed mainly by Framatome (now Areva NP), Electricité de France (EDF) in France, and Siemens AG in Germany

– In October 2008, Areva and CGNPC (中广核 ) announced establishment of an engineering joint venture as a technology transfer vehicle for development EPR and other PWR plants in China

– Two Areva EPR reactors are being built at Taishan, and at least two more are planned, with net power 1660 MWe

energy and the environment

Documents