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Small nuclea – safe power, very low p by Dr David Maddison S With the trend away from coal-fired thermal power stations, Australia still needs reliable base-load power stations. Apart from natural gas power stations, the only other alternative is to choose nuclear power stations but they don’t need to be the really large installations that would previously have been considered. Instead, they could be small modular nuclear reactors. ILICON CHIP discussed new developments in fossil fuel plants with supercritical steam plants in the December 2015 issue (www.siliconchip.com. au/Issue/2015/December/Super+%2526+Ultra-SuperCritical+Steam+Power+Stations) Now we take a look at new developments in nuclear reactor technology. Around the world, there is a trend away from centralised power generation, partly due to the proliferation of small solar and wind generation plants. This trend applies to nuclear reactors as well and a number of small size, transportable nuclear reactors are under development. “Small” nuclear power reactors are regarded as reactors with an electrical output of less than about 300MW, compared with large centralised power stations which might have an electrical output of as much as several gigawatts. The advantages of a small nuclear reactor, otherwise known as a small modular reactor (SMR), are as follows: • A small modular reactor can be built in a factory and then transported to the site where it will be used. Mass production should lead to economies of scale, standard designs, centralised quality assurance and lower cost 18  Silicon Chip compared to building a reactor on site. • A small reactor can be located where power is needed and avoids the need for large, high voltage power lines run over long distances. • Due to the small size of the reactor it can be buried in the ground for an extra level of containment and also the small size allows for passive safety systems that can work with no power whatsoever such as convective cooling. • If more power is eventually required, such as for a township growing in size, another reactor can be shipped in to supplement the first. • As the reactor unit is transportable it could be effectively operated as a sealed system and when it needed refuelling, returned to the factory and another unit installed in its place. • Relatively few staff would be required to operated the reactor. • Capital costs will be less so there are more financing options. • The cost of electricity might be a little greater compared to a large reactor but that might be offset by a lower acquisition cost per unit of power, due to the mass prosiliconchip.com.au ar reactors pollution, very low risk duction savings of the smaller reactor plus the fact that no long distance power lines are required if the reactor is built near where the power will be used. • Small reactors are not new, militaries have been using small powerful reactors in nuclear submarines, cruisers, icebreakers and aircraft carriers for many decades with few incidents. Even when the Russian submarine, the Kursk exploded with an explosive force that registered 1.5 on the Richter earthquake scale, the reactors automatically shut down without a problem. • In the USA an additional reason for interest is that they can be used to replace, on the same sites, a lot of small coal-fired plants which are currently being decommissioned due to age and environmental regulations. In 2010-12 the average size of coal-fired plants replaced was 97MWe (MWe means megawatts of electrical power) and in 2015-25 the size is expected to average 145MWe, so most plants that are to be retired are well within the sub300MWe size range covered by small modular reactors. • Due to their transportability they could be used to power remote mining sites or small towns in the outback. • Some SMR designs claim to response fast enough for “load following”, to stabilise the grid to compensate for the highly variable output of wind and solar power and could also be used for peak load and backup units. (Note: the most likely candidates to be suitable for load following are gas-cooled reactors like HTR-PM; see diagram overleaf.) • Apart from production of electricity, uses of SMRs include desalination, district heating, process heat for industry and production of hydrogen. The operating principles of small nuclear reactors are much the same as larger ones but can be somewhat simplified due to their smaller size which allows more simple cooling and control systems and the ability to mass produce them which will result in a standard, optimised design. Operating principles Nuclear reactors release energy due to a nuclear chain reaction. This process occurs when a single nuclear reaction such as the emission of a neutron from an atomic nucleus causes one or more nuclear reactions in other atoms, starting a self-propagating series of similar events. When a neutron from the chain reaction hits an atomic nucleus it will be either absorbed or it will cause the nucleus to split. In the event that the nucleus of an atom is split, a process known as fission, a large amount of energy is released. For fission to occur there must be a source of neutrons plus there must be atoms which are fissionable (capable of being split and sustaining a nuclear chain reaction). Note that fissionable materials are not necessarily fissile, ie, not siliconchip.com.au all fissionable material can be used for nuclear weapons. In addition, in most reactors the neutrons have to be slowed to a particular speed to be most effective and this is done with a moderator, which is usually normal water (H2O), graphite or heavy water (D2O). Nuclear fuels All nuclear reactors require specific fissionable isotopes of certain elements for their operation (see box describing what isotopes are). Three nuclear fuels and their particular isotopes which have been determined to be practical are as follows: 235U (uranium 235) that is enriched from mined uranium, which is mainly 238U. 235U can also be used to build nuclear weapons. Pure 238U is also known as depleted uranium or DU. It is not fissionable but can be converted to a fissionable fuel called 239Pu (plutonium 239) by the process of transmutation inside a reactor (the transformation of one element into another). 235U is the world’s most common nuclear fuel and is usually used in a “light water reactor” (LWR). 239Pu (plutonium 239) transmuted from natural mined 238U is used as either a by-product from the normal operation of a power reactor or is deliberately added into the fuel rods. 239Pu is also used in nuclear weapons. This 238U/239Pu fuel is less common and has been used in liquid sodium fast breeder reactors and so-called CANDU reactors (a Canadian pressurised heavy water reactor). One type of nuclear chain reaction involving 235U. First a neutron hits an atom of 235U which causes it to fission into two new atoms, three neutrons and a large amount of energy. One of the three neutrons is absorbed by an atom of 238U and no further reaction occurs. Another of the neutrons is not absorbed and l eaves the system. The middle neutron shown strikes an atom of 235U causing it to fission (split) just as at the first step, releasing energy and in this case, two more neutrons. The average numberof neutrons released from the thermal fission of uranium is just under 2.5 (slightly fewer when initiated by a fast neutron). June 2016  19 n U-238 92 protons 146 neutrons U-239 − 92 protons 147 neutrons 23.45 minutes (half life) Np-239 − 93 protons 146 neutrons 2.35 days (half life) Pu-239 94 protons 145 neutrons (Left): transmutation of 238U to 239Pu, an important reaction in the nuclear fuel cycle. The small circles represent neutrons or electrons and the arrows indicate whether they are arriving or being ejected. The intermediate isotopes that are created are relatively short-lived and soon decay into the desired isotope. 233U can be transmuted from 232Th (thorium 232). It is best utilised in molten salt reactors (MSRs), specifically Liquid Fluoride Thorium Reactors [LFTRs]. Note that of these three fuel types, the thorium-based fuel is the only one without military uses in nuclear weapons. Nuclear fission After an atom has been split, the total mass of particles involved in the fission process is less than before it because some mass has been converted into energy, according to Einstein’s famous equation, E=mc2. This is millions of times more energy than would be released if the same amount of mass was released in a chemical reaction such as the burning of coal. For example, a piece of 235U the size of a grain of rice contains as much energy as three tonnes of coal. This is the reason why nuclear power is so “energy dense” – a little fuel goes a long way. In a nuclear power plant the chain reaction is maintained at a constant rate and a runaway chain reaction that would cause a nuclear explosion is impossible due to the purity and physical arrangement of the fissionable material. In a nuclear explosive device, which contains highly pure fissionable material in close proximity, the design specifically allows a runaway chain reaction which is impossible to stop once started. In a nuclear power plant the energy that is produced by the fission process is mostly in the form of heat which is 20  Silicon Chip (Above): transmutation of thorium into uranium which takes around 27 days. When 233U absorbs a neutron, it fissions and releases energy and neutrons. Some of the neutrons it releases are absorbed by 232Th which continues the process of transmuting the thorium. typically used to convert water into steam to drive a turbine and alternator to produce electricity. As noted above, 238U and 232Th are not fissionable materials, ie, their atoms cannot be split. So how can they be used in a nuclear reactor? 238U and 232Th are known as fertile materials. That means they can be converted (transmuted) into a fissionable material by bombarding them with neutrons. Small reactors Electricity was first produced by a nuclear reactor in 1951 and the electrical output was just 45kW. Since then, commercial reactors for electricity production have tended to get larger and larger. There are currently 442 commercial power reactors in operation around the world producing a total of 383,513GW, giving an average output of 868MW per reactor. But now the trend is being reversed. The idea of a portable or modular nuclear reactor is not new. Two notable examples are as follows. The nuclear reactor near the South Pole There was once a small “portable” nuclear reactor at the US McMurdo Station in Antarctica. The rationale was to avoid shipping in of vast amounts of diesel for the generators plus steam from the reactor would be used in a desalination plant. The reactor could produce 1.8MW of electrical power and 56,000 litres of fresh water per day. The reactor went siliconchip.com.au The PM-3A reactor core being lowered into position at McMurdo Station, Antarctica. critical in March 1962 and after testing and debugging, was operational from 1964 until 1972. The model of the reactor was designated PM-3A. It was third in a series that were portable and deliverable with a ski-equipped version of the Hercules aircraft, the LC-130. Of the other two reactors in this series the PM-1 reactor was used to power a remote radar station in Wyoming while the PM-2A was used to power a remote US military base in northern Greenland. Each reactor had a power output of 1.25-2.0MW. Unusually, the 235U fuel was highly enriched at 93.1% which meant that it was weapons grade uranium which is classified as any uranium with greater than 90% 235U. Possibly for this reason, the reactor was under the control of the US Navy’s Naval Nuclear Propulsion Unit with a crew of 25. The fuel assembly itself was about the size of an oil drum. Unfortunately that reactor was not a great success, recording 438 malfunctions over its 8 years of use. It was available only 72% of the time. When it was decommissioned it was found to have leaked radioactive coolant through cracks in the reactor vessel into the soil beneath. That required the removal of 9,000 cubic meters of contaminated soil back to the US mainland along with the reactor itself. The US Army Nuclear Program started in 1954 and ran until 1977, to develop small portable nuclear reactors to produce electricity and heat at remote locations. Eight different reactor designs were built and the program made a number of significant technical achievements but ultimately it was thought to be a “solution in search of a problem”. The reactor mentioned at the McMurdo Station, the PM3A, as well as the related units PM-1 and PM-2A were part of this program. A video of the program can be watched at https://youtu.be/ HPWDMHH4rY4 (“Army Nuclear Power Program, 1969”). Various views of the CAREM-25 reactor. The EGP-6 reactor is a Russian 11MW design of which four units are in operation at one power plant, built between 1966 and 1976, to serve the gold mine operating in a remote area and are not connected to the national grid. SMRs under construction CAREM-25 (Central ARgentina de Elementos Modulares) is a reactor being built in Argentina to produce 25MW of electrical power. The design incorporates passive safety systems and is cooled by natural convection; no coolant pumps are required. Once the design is proven a larger version will be built of 100-200MW capacity. The HTR-PM reactor (high-temperature pebble bed modular nuclear reactor) is a Chinese design producing 100MW. It will be configured as a twin modular reactor design driving a single steam turbine to produce 200MW of electrical power. The unit uses helium as the coolant and the uranium fuel is in the form of 520,000 spheres. The total installation is known as the HTR-200. It is expected to be connected to the grid in 2017. Current small modular reactors The CNP-300 is China’s first commercial reactor design, of which two are in operation in China and commercial operation started in 1994. It produces 310MW of electricity and has a 40-year design life. Two units have also been exported to Pakistan. The PHWR-220 is an Indian design producing 220MW and 16 units are in operation. One reason that small reactors were chosen is that it was feared India’s electrical grid could not handle the distribution of power from large centralised reactors. Commercial operation of the first one began in 1973. siliconchip.com.au Steam is generated in the HTR-PM by the transport of heat by helium gas. The OTSG is the once-through steam generator. The reflector refers to the nuclear core’s neutron reflector. June 2016  21 Nuclear fuel cycles As nuclear fuels are consumed in a conventional uranium-plutonium reactor there comes a time when the depleted nuclear fuel has to be removed and new fuel added. Old fuel needs to be processed and prepared for disposal or it may be recycled in one of two ways. Spent nuclear fuel from a typical reactor still has most of its original potential energy within it, as only a few percent of the available energy is extracted. Usually, this nuclear material is considered “waste” and is buried. It has been estimated that if all the nuclear waste generated in the United States in the last 50 years was dug up and reused to extract the residual energy left within it the entire US electrical grid could be run for 93 years at present rates of consumption. Furthermore, the waste left from this recycling process would only be significantly radioactive for hundreds of years rather than tens of thousands. The process described above is termed the “once through cycle”. Typically uranium ore is mined, enriched, used in a reactor where 235U is gradually consumed and when that is sufficiently depleted the “waste”, which contains a variety of fission by-products is treated and buried in long term storage. An alternative to burying waste is to transmute it into shorter-lived radioactive materials in a “fast burner reactor” but while these exist, the are not yet widely used. The waste contains potentially useful components such as some unused 235U and some 239Pu. The closed fuel cycle The alternative to seemingly wasteful burial of nuclear waste as described above is to recycle it either within the “closed fuel cycle” or the “breeder fuel cycle”. In the closed fuel cycle, useful 235U and 239Pu is extracted from the waste and reintroduced to the reactor as fresh fuel. 239Pu acts much like 235U in a reactor and is used in much the same way. One downside of this process is cost and another is that it involves the extraction of pure plutonium which could be stolen and used to make a nuclear weapon which is why it is not done in most places. The main useful component of radioactive waste is 238U which is otherwise generally considered useless in a reactor as it is not fissile but it can be converted to something that is fissile which is 239Pu (plutonium). The conversion of 238U to fissile 239Pu can be done in a special type of reactor called a fast breeder. In this reaction 238U absorbs a neutron and converted to 239U which decays quickly to 239Np (neptunium) which decays quickly to 239Pu. In the breeder fuel cycle, breeder reactors are used to create new fissile material. They are designed to convert non fissile isotopes to fissile isotope materials like 239Pu from 238U or 233U from 232Th that can be used in a reactor. In this way the nuclear resources are greatly extended and the maximum amount of energy is extracted from the nuclear material. Downsides as with the closed fuel cycle are cost and proliferation issues. Thorium reactor designs are intrinsically breeders as they convert 232Th to 233U in their normal operation. 22  Silicon Chip Artist’s concept of the Russian floating nuclear cogeneration plant the Akademik Lomonosov, currently under construction. It can deliver onshore heat, electricity and fresh water. It will be returned to base for maintenance operations however it can run for 10-12 years before refuelling and has an expected service life of 40 years. See the video at https://youtu.be/VbSSjRS2CnU (“Russia Plans Floating Nuclear Power Plant”) Floating nuclear power plants – the KL-40TS A floating nuclear reactor is an effective way to deliver power to third world countries with no maintenance capability, deliver high levels of power capacity to regions on a temporary basis such as after a disaster or for a major construction project or to deliver power to otherwise inaccessible regions. Naturally the area to which power is to be delivered must be close to sea, a harbour or a major river. One example is the Russian Akademik Lomonosov. The vessel was launched in 2010 and it will begin operation in 2018. It is 144m long, 30m wide, has a displacement of 21,500 tonnes and a crew of 70. It has two model KLT-40C reactors of 150MW thermal and 38.5MW electric power each and an optional reverse osmosis desalination plant that can deliver 240 megalitres per day of fresh water (compare that with Victoria’s desalination plant that can deliver 410 megalitres) and can deliver onshore heat, electricity and desalinated water. Note that this vessel is expected to cost US$336 million (A$444 million) and Victoria’s desalination plant cost A$5.7 billion for only 1.7 times the capacity but nearly 13 times the cost. It is built within international regulatory guidelines. Such a design would be ideal for Africa because they are discouraged from developing fossil power due to international environmental opinion and are expected to develop using solar and wind power which is simply not going to provide their full energy needs at any reasonable cost. Planned SMRs The ACP100 is a Chinese design with an electrical output of 100-150MW. Two demonstration units are to be installed in the city of Zhangzhou and will provide electricity, heat and 12 megalitres per day of desalinated water. Construction was scheduled to start in 2015 year and commercial operation in 2017. In addition, China plans to build a floating nuclear power plant based upon this design to be put into commercial production by 2019. mPower is a design by Babcock and Wilcox for a reactor to produce 180MW. It will be bought to site by rail and combined modules could make a power station of any desired siliconchip.com.au ACP100 reactor. (Source IAEA). size. The reactor assembly is 4.5m in diameter and 22m tall and will be installed below ground level. Refuelling will be done every four years. A sixty year service life is expected and it has passive safety systems. The NuScale reactor is smaller than most others with a 50MW output. It is a factory built unit, 3m in diameter and 22m long. It incorporates convective cooling and the only moving parts are the reactor control rods. It is envisaged that a power plant would have 12 modules to give a 600MW power output. Refuelling would be at two year intervals. Design life is sixty years. This reactor has good load-following capabilities so can be used to back up solar and wind or cope with other rapid variations in grid production. The weight of a module is 700 tonnes and it can be shipped to site by barge, truck or train. Its cost is under US$5,100 per kW. Its reactor can automatically shut down with the complete absence of external power. South Korea is developing the SMART reactor or System-Integrated Modular Advanced ReacTor. Each unit will produce 90MWe and heat from the reactor will be used to boil salt water in a process to provide 40 megalitres per day of desalinated water. The unit is of the pressurised water design. Design life of the unit is 60 years and it uses 4.8% enriched fuel that needs to be replaced every three years. There is an agreement in place to build a unit in Saudi Arabia at a cost of US$1 billion. Future reactor concepts The General Atomics EM2 or Energy Multiplier Module is a novel modular reactor deThis gives a good idea of the size of small reactors, with a man shown at the bottom for comparison. In most cases, the vast majority of the reactor would be underground, with only a small building above ground. siliconchip.com.au What are isotopes? Chemical elements are comprised of a nucleus made of protons and neutrons (except the simplest form of hydrogen has no neutrons) and a shell of electrons, the number of which matches the number of protons. Isotopes are a variation of a particular element in which the nucleus has a different number of neutrons. The number of protons, which defines the atomic number of an element is always the same for any given element, no matter the number of neutrons it has. For a given element, certain isotopes may be stable and others may be radioactive and/or fissile. Specific isotopes of elements such as uranium and plutonium need to be selected for nuclear power applications while for thorium, no selection is necessary because nearly all the material that occurs in nature is of the one useful specific isotope. This fortuitous fact means that expensive enrichment to a particular isotope type is not needed, it is simply mined, purified, turned into the appropriate chemical form and used. Hydrogen, the simplest element and its two isotopes, deuterium and tritium. All have the same number of protons (one) and up to two neutrons. The chemical behaviour of different isotopes is similar. Protium is the name for the common isotope of hydrogen. signed to consume nuclear waste. As noted in the section on “Nuclear Fuel Cycles” in the conventional fuel cycle only a few percent or less of the potential energy of nuclear fuel has been extracted by the time it is buried as waste. This reactor extracts that remaining energy from what otherwise would be buried. Furthermore, once that waste has been through the reactor and its energy extracted, the storage requirements will only be hundreds of years for the waste rather than many thousands. The reactor is extremely versatile in the waste or fuel it can use. It is capable of consuming enriched uranium, weapons grade uranium, depleted uranium, thorium, used nuclear fuel and its own discharge. A low enriched uranium “starter” fuel is consumed in one part of the nuclear core to transmute used nuclear fuel (waste), 238U or 232Th to fissionable material and the residual of that is then used in a second generation of the cycle. The reactor is capable of operating for 30 years without refuelling and will also produce 240MW of electricity. (It should be noted that some have argued that this reactor is not as intrinsically safe as other designs). Thorium-fuelled reactors Thorium has many potential advantages over uranium and plutonium fuels. It is very common in nature, does not June 2016  23 require expensive enrichment and nor can it be used to make nuclear weapons. Thorium can be used in most current and foreseeable reactor designs. Today, thorium would typically be mixed with plutonium or enriched uranium. While it is feasible to use solid thorium in reactors, the real advantage is that it can be used in a liquid form, in particular as a molten fluoride-based salt. Such reactors are known as a Liquid Fluoride Thorium Reactors or LFTR (pronounced “lifter”). They are of a general class of reactors known as Molten Salt Reactors (MSRs). The liquid fluoride salt contains lithium and beryllium, mixed with 233U for the core salt and 232Th in the so-called blanket salt. As previously explained, it is the 233U which undergoes fission and this is the heart of the reactor. However 232Th is the source of the 233U via transmutation. A salt “blanket” containing 232Th is wrapped around the core where it absorbs neutrons to effect the transformation. In a LFTR reactor, the molten salt fuel would be continuously processed by chemical means to remove undesired nuclear by-products. Unlike solid fuels, this is relatively simple to do by pumping the molten salt through a treatment plant while the reactor is operating. The liquid salt mixture is chemically stable and not damaged by neutrons like conventional solid fuels. Being a liquid it is also the medium used to convey heat out of the reactor to a heat exchanger, to eventually make electricity. A further advantage is that a reactor based on molten salts is unpressurised, thus eliminating the possibility of failure due to over-pressurisation of the reactor core. Any notion of a meltdown as can happen with solid fuels is also irrelevant as the fuel is already in a molten state. In addition, if the liquid salt medium should overheat, the power produced automatically reduces, due to a reduction of density of the fuel salt and so the reactor is intrinsically self-regulating. At the bottom of the liquid salt bath there is a “freeze plug” of the salt solution and it is kept frozen by a fan blowing on it. In the event of a power failure, the fan stops blowing and the plug melts, enabling the liquid salt to drain into a tank which is passively cooled. Nuclear Energy in Australia Australia seems ideally placed to use nuclear energy and has abundant supplies of nuclear fuels including the largest reserves of uranium and the third largest reserves of thorium. In particular, Australia has a large number of remote towns and mining communities which rely on mainly diesel power generation at great cost due to the fact that diesel fuel has to be shipped in. These places would seem ideally suited to utilise small modular reactors. In addition, small modular reactors could be used to desalinate otherwise unusable saline bore water or sea water and vast expanses or the outback could be irrigated at relatively low cost. Australia has seriously considered Demonstration of the efficiency and energy density of thorium compared to uranium. 248 “MT” (metric tonnes) of uranium is eventually converted to 1000MW years of electricity (i.e. 1000MW continuous production for one year) compared to the same electricity production from thorium with just 0.9 metric tonnes (ie, 900kg). 500 metric tonnes of thorium could supply all of the United States energy requirements for one year. 24  Silicon Chip siliconchip.com.au nuclear power in the past. There was a 1969 proposal for a 500MW reactor to be built in Jervis Bay, NSW which was abandoned in 1971. There was also a proposal to build a reactor on French Island in Victoria. However, most Australian political parties are openly hostile to nuclear power. Most politicians do not even understand the inherent safety of thorium-based generation. The overall hostility to nuclear power in this country is unlikely to change anytime soon without a major shift in attitudes – and this is despite the recent (May) announcement of Australia’s first repository for nuclear waste. Small reactors currently in use Name CNP-300 PHWR-220 EGP-6 Capacity 300MWe 220MWe 11MWe Type PWR PHWR LWGR Developer CNNC, operational in Pakistan & China NPCIL, India at Bilibino, Siberia (co-generation) Small reactor designs under construction Name KLT-40S CAREM HTR-PM, HTR-200 Capacity 35MWe 27MWe 2x105MWe Type PWR integral PWR HTR Developer OKBM, Russia CNEA & INVAP, Argentina INET, CNEC & Huaneng, China Small (25MWe up) reactors for near-term deployment – development well advanced Name VBER-300 NuScale Westinghouse SMR mPower SMR-160 ACP100 SMART Prism BREST SVBR-100 Capacity 300MWe 50MWe 225MWe 180MWe 160MWe 100MWe 100MWe 311MWe 300MWe 100MWe Type PWR integral PWR integral PWR integral PWR PWR integral PWR integral PWR sodium FNR lead FNR lead-Bi FNR Developer OKBM, Russia NuScale Power + Fluor, USA Westinghouse, USA* Bechtel + BWXT, USA Holtec, USA NPIC/CNNC, China KAERI, South Korea GE-Hitachi, USA RDIPE, Russia AKME-engineering, Russia Small (25MWe up) reactor designs at earlier stages (or shelved) Name EM2 VK-300 AHWR-300 LEU CAP150 ACPR100 IMR PBMR SC-HTGR (Antares) Xe-100 Gen4 module Moltex SSR MCFR TMSR-SF PB-FHR Integral MSR Thorcon MSR Leadir-PS100 Capacity 240MWe 300MWe 300MWe 150MWe 140MWe 350MWe 165MWe 250MWe 48MWe 25MWe ~ 60MWe unknown 100MWt 100MWe 192MWe 250MWe 36MWe Abbreviation Key: PWR – pressurised water reactor LWGR – light water graphite reactor FNR – fast neutron reactor MWe – megawatts of electrical power Type HTR, FNR BWR PHWR integral PWR integral PWR integral PWR HTR HTR HTR FNR MSR/FNR MSR/FNR MSR MSR MSR MSR lead-cooled Developer General Atomics (USA) RDIPE, Russia BARC, India SNERDI, China CGN, China Mitsubishi Heavy Ind., Japan PBMR, South Africa* Areva, France X-energy, USA Gen4 (Hyperion), USA Moltex, UK Southern Co, USA SINAP, China UC Berkeley, USA Terrestrial Energy, Canada Martingale, USA Northern Nuclear, Canada PHWR – pressurised heavy water reactor HTR – high temperature reactor MSR – molten salt reactor MWt – megawatts of thermal output This table, from the World Nuclear Association, shows small reactors which are either in use, under construction, are in advanced stages of development or in early stages of development. Some of the latter are currently shelved. siliconchip.com.au June 2016  25 160504_AuthDB_SCHIP_AU_3rdVert.indd 1 5/3/16 10:32 AM Liquid Fluoride Thorium Reactors Molten Salt Reactor Experiment as run at Oak Ridge National Laboratory, USA from 1965 to 1969. 1) Reactor vessel 2) Heat exchanger 3) Fuel pump 4) Freeze flange 5) Thermal shield 6) Coolant pump 7) Radiator 8) Coolant drain tank 9) Fans 10) Fuel drain tank 11) Flush tank 12) Containment vessel 13) Freeze valve More than half a century ago, research into thorium reactors was conducted at the Oak Ridge National Laboratory, USA. From 1955 to 1972 Director Alvin Weinberg and his team envisaged liquid fluoride thorium reactors which would produce both electricity and desalinated water. But his research was stopped in 1974, as the US made a policy decision to discontinue research into thorium reactors. The experiment on the feasibility of liquid fluoride thorium reactors (LFTRs) at Oak Ridge ran from 1965-1969 and was known as the Molten Salt Reactor Experiment (MSRE). It used a lithium and beryllium salt mixture containing 233U fuel and ran at a temperature of 600-700°C at ambient pressure, producing around 7-8MW of power. The intrinsic passive safety of this reactor was demonstrated every weekend. When the staff wanted to shut down the reactor on Friday afternoons they simply let the freeze plug melt and the molten salt fuel drained out into tanks. On Monday morning, the salt was reheated and pumped back into reactor. The LFTR design of reactor has numerous advantages as follows: 26  Silicon Chip • Inherently safe and self-regulating. • Fuel meltdowns are impossible; the fuel is already in liquid form. • Unpressurised reactor core. • It is difficult (if not practically impossible) to use thorium to make nuclear weapons. • Thorium is abundant and cheap, unlike uranium. • In the event of an emergency, a LFTR reactor will shut down safely and permanently without any electrical power required or AS CRYSTALLISED SOLID AS LIQUID 7LiF – BeF2 – 233UF4 Fuel in the form of a molten salt used to fuel the Molten Salt Reactor Experiment. operator intervention. • If the reactor overheats it produces less power and cools; again, it is self regulating. • The LFTR has very high fuel “burn”, nearly all thorium is consumed and turned into useful energy compared with just 0.5% in light water reactors. • The high operating temperature of a LFTR reactor, around 700°C results in a high thermodynamic efficiency for steam production to drive a turbine. • The cost of producing electricity for a LFTR would be 25-50% less than for a light water reactor. Thorium is about as common as lead in nature and much more common than uranium. Thorium is a very energy dense fuel compared to natural uranium. One tonne of thorium costing US$300,000 could power a 1000MW reactor for one year. One tonne of thorium contains the same energy as 200 tonnes of natural uranium or 3,500,000 tonnes of coal. The molten salt solidifies at around 150C so if a spill occurs, the salt freezes and it can be scraped up. There is no possibility of radioactive liquid contaminating the ground or of dangerous radioactive aerosols being created. siliconchip.com.au Looking down into the containment vessel of the Molten Salt Reactor Experiment. The reactor vessel is the large cylinder just off the 12 o’clock position and you can identify some other components by comparison with the schematic. A thorium reactor produces about one hundredth the radioactive waste of conventional reactors and the levels of radioactivity drop to safe levels within a few hundred years (compared to thousands of years compared with conventional unprocessed waste). While established nuclear energy companies are unlikely to be interested in thorium energy due to their major investments in conventional nuclear infrastructure and resources, there are ample opportunities for entrepreneurial companies to become involved, including Australian companies if the appropriate legislative environment could be created. The US company Flibe Energy is developing a small modular reactor based on thorium. Their initial offerings will be in the 20-50MW (electric power) range followed by 100MW and more “utility class” units. They will be mass produced and will first be installed in remote US military bases. The liquid fuel thorium reactor design is highly scalable with power outputs possible from one megawatt up to over a gigawatt. Some thorium-related Australian web sites are http://thoriumaustralia.org/ and http://thoriumenergy.com.au/ siliconchip.com.au Proposed design for Generation IV Molten Salt Reactor (MSR). Generation IV reactors are a collection of advanced designs that could be demonstrated within the next decade and commercialised from 2030. The nuclear fuel is dissolved in a fluoride salt. Note the freeze plug and the emergency dump tanks. In the event of a power failure, fans that keep the freeze plug frozen will stop, the freeze plug will melt and the entire liquid fuel body will be dumped into the containment tanks under gravity. Image source: US Department of Energy Nuclear Energy SC Research Advisory Committee June 2016  27