How small can you make a nuclear reactor

A handful of microreactor designs are under development in the United States, and they could be ready to roll out within the next decade. These compact reactors will be small enough to transport by truck and could help solve energy challenges in a number of areas, ranging from remote commercial or residential locations to military bases.

Microreactors are not defined by their fuel form or coolant. Instead, they have three main features:. Microreactor designs vary, but most would be able to produce megawatts of thermal energy that could be used directly as heat or converted to electric power. They can be used to generate clean and reliable electricity for commercial use or for non-electric applications such as district heating, water desalination and hydrogen fuel production.

Nuscale is not alone in developing miniature reactors. China announced plans in to build its own state-funded floating SMR design. Three Canadian provinces — Ontario, New Brunswick, and Saskatchewan — have signed a memorandum to look into the development and deployment of small modular reactors.

Proponents say the time is ripe for this new wave of nuclear reactors for several reasons. First, they maintain that if the global community has any hope of slashing CO2 emissions by mid-century, new nuclear technologies must be in the mix. Second, traditional nuclear power is beset with problems.

Finally, advocates say that as supplies of renewable energy grow, small modular reactors can better handle the variable nature of wind and solar power as SMRs are easier to turn on and leave running.

Rolls-Royce is leading a group to develop smaller nuclear reactors on the sites of old power plants, like the one pictured in this artist's impression Credit Rolls-Royce. Critics of nuclear power, however, contend that small modular reactors suffer from many of the same problems as large reactors, most notably safety issues and the unresolved problem of what to do with long-lived radioactive waste.

Yet in September, the Los Angeles Department of Water and Power announced that it had accepted a bid of electricity coming from renewables with storage capacity that can deliver an energy supply at 2 cents per kWh, with a battery storage system for use during hours of darkness. Though, of course, solar power relies on sufficient sunshine and lithium-ion batteries have a limited capacity to provide power through dark hours and cloudy days — vagaries that do not affect nuclear power supply.

More than a third of US nuclear plants are now unprofitable or scheduled to close. Globally, nuclear energy now only supplied After the Fukushima disaster in Japan, Germany decided to close its nuclear industry altogether, and countries like Belgium, Switzerland and Italy have declined to replace existing reactors or move forward with plans for new ones.

But companies and scientists backing the development of small modular reactors say the technology offers a new way forward for nuclear power, one that overcomes many of the drawbacks of traditional, larger reactors. SMRs are much less likely to overheat, the proponents say, in part because their small cores produce far less heat than the cores in large reactors.

Innovative designs in SMR technology can also reduce other engineering risks, like coolant pumps failing. Nuscale says its SMR has far fewer moving parts than traditional reactors, lowering the likelihood of failures that could cause an accident. SMRs are generally designed to produce 50 to MW of electricity, compared to the typical 1,MW of traditional large-scale reactors. Perhaps most importantly, proponents argue that SMRs cost much less and can be built more quickly than large nuclear reactors, opening up new markets in the developing world.

Large traditional nuclear power plants are increasingly seen as an expensive way to produce power as the cost of renewables falls Credit: Getty Images. Since then, commercial reactor sizes have only grown. In , the Department of Energy funded a project at Oregon State University, among others, to study a multi-application small light water reactor.

In , the university granted Nuscale exclusive rights to the design of SMR, as well as the continued use of their test facility. In , Fluor Corporation, a multinational engineering firm, invested in the company. They describe a preliminary concept design study for a , dwt Suezmax tanker that is based on a conventional hull form with a 70 MW Gen4 Energy power module for propulsion. It pitched its design for remote sites having smaller power requirements.

The Westinghouse Lead-cooled Fast Reactor LFR programme originated from an investigation performed in aimed at identifying the technology that would best support addressing the challenges of nuclear power, for global deployment. It is at the conceptual design stage for up to MWe as a modular pool-type unit, simple, scalable and with passive safety. It will have flexible output to complement intermittent renewable feed to the grid. Westinghouse expects it to be very competitive, having low capital and construction costs with enhanced safety.

Because lead coolant operates at atmospheric pressure and does not exothermically react with air or with power conversion fluids such as supercritical carbon dioxide and water , LFR technology also eliminates the need and associated expense of extra components and redundant safety systems required by other plant designs for protection against coolant leakages.

In April an Ansaldo subsidiary was contracted to design, provide, install and test key components of the reactor at the Versatile Lead Loop Facility and Passive Heat Removal Facility, which are to be designed and installed at Ansaldo Nuclear's site in Wolverhampton in the UK.

Beyond base-load electricity generation, the high-temperature operation of the LFR will allow for effective load-following capability enabled by an innovative thermal energy storage system, as well as delivery of process heat for industrial applications and water desalination. A supercritical carbon dioxide power conversion system that uses air as the ultimate heat sink significantly reduces water utilization and eliminates the need for siting the plant near large water bodies.

The core is at the bottom of a metal-filled module sitting in a large pool of secondary molten metal coolant which also accommodates the eight separate and unconnected steam generators.

There is convection circulation of primary coolant within the module and of secondary coolant outside it. Outside the secondary pool the plant is air-cooled. Control rods would need to be adjusted every year or so and load-following would be automatic. The whole reactor sits in a 17 metre deep silo. After this the module is removed, stored on site until the primary lead or Pb-Bi coolant solidifies, and it would then be shipped as a self-contained and shielded item.

A new fuelled module would be supplied complete with primary coolant. The ENHS is designed for developing countries and is highly proliferation-resistant but is not yet close to commercialization.

The heatpipe ENHS has the heat removed by liquid-metal heatpipes. The core is oriented horizontally and has a square rather than cylindrical cross-section for effective heat transfer.

The heatpipes extend from the two axial reflectors in which the fission gas plena are embedded and transfer heat to an intermediate coolant that flows by natural circulation.

The SAFE space fission reactor — Safe Affordable Fission Engine — was a kWt heatpipe power system of kWe to power a space vehicle using two Brayton power systems gas turbines driven directly by the hot gas from the reactor. The STAR-LM is a factory-fabricated fast neutron modular reactor design cooled by lead-bismuth eutectic, with passive safety features.

Its MWt size means it can be shipped by rail. It uses uranium-transuranic nitride fuel in a 2. Decay heat removal is by external air circulation.

Its development is further off. After a or year operating lifetime without refuelling, the whole reactor unit is then returned for recycling the fuel.

The reactor vessel is 12 metres high and 3. SSTAR would eventually be coupled to a Brayton cycle turbine using supercritical carbon dioxide with natural circulation to four heat exchangers. A prototype was envisaged for , but development has apparently ceased. Fuelled units would be supplied from a factory and operate for 30 years, then be returned.

The concept is intended for developing countries. It has a subsidiary in Canada. The reactor vessel is designed to be small enough to permit transportation by aircraft. As the regulatory framework for licensing of small reactors in Canada is better established than in most other countries, Nunavut and the Northwest Territories are likely to become the first markets for SEALER units.

The Canadian Nuclear Safety Commission CNSC commenced phase 1 of a month pre-licensing vendor design review in January , but the review is now on hold at the vendor's request. In April the company began collaboration on safety analysis with Netherlands-based NRG, which operates the Petten high-flux research reactor.

The plutonium and minor actinides present in the spent fuel will then be separated and converted into nitride fuel for recycle in a 10 MWe SEALER reactor. The lead-cooled fast reactor would be able to generate 10 megawatts thermal, and is based on a Russian submarine reactor design. It is working on a lead-bismuth cooled design of 35 MW which would operate on pyro-processed fuel.

It is designed to be leased for 20 years and operated without refuelling, then returned to the supplier. It would then be refuelled at the pyro-processing plant and have a design life of 60 years. It would operate at atmospheric pressure, eliminating major concern regarding loss of coolant accidents.

These mostly use molten fluoride salts as primary coolant, at low pressure. Fast-spectrum MSRs use chloride salt coolant. In most designs the fuel is dissolved in the primary coolant, but in some the fuel is a pebble bed. A second campaign used U fuel, but the program did not progress to building a MSR breeder utilising thorium.

Much higher temperatures are possible but not yet tested. Heat is transferred to a secondary salt circuit and thence to steam o. The basic design is not a fast neutron reactor, but with some moderation by the graphite, may be epithermal intermediate neutron speed and breeding ratio is less than 1.

Thorium can be dissolved with the uranium in a single fluid MSR, known as a homogeneous design. Two-fluid, or heterogeneous MSRs would have fertile salt containing thorium in a second loop separate from the fuel salt containing fissile uranium and could operate as a breeder reactor MSBR. In each case secondary coolant salt circuits are used. The fission products dissolve in the fuel salt and may be removed continuously in an on-line reprocessing loop and replaced with fissile uranium or, potentially, Th or U Actinides remain in the reactor until they fission or are converted to higher actinides which do so.

The liquid fuel has a negative temperature coefficient of reactivity and a strong negative void coefficient of reactivity, giving passive safety. If the fuel temperature increases, the reactivity decreases.

The MSR thus has a significant load-following capability where reduced heat abstraction through the boiler tubes leads to increased coolant temperature, or greater heat removal reduces coolant temperature and increases reactivity.

Primary reactivity control is using the secondary coolant salt pump or circulation which changes the temperature of the fuel salt in the core, thus altering reactivity due to its strong negative reactivity coefficient. The MSR works at near atmospheric pressure, eliminating the risk of explosive release of volatile radioactive materials. Other attractive features of the MSR fuel cycle include: the high-level waste comprising fission products only, hence shorter-lived radioactivity actinides are less-readily formed from U than in fuel with atomic mass greater than ; small inventory of weapons-fissile material Pu being the dominant Pu isotope ; high temperature operation giving greater thermal efficiency; high burn-up of fuel and hence low fuel use the French self-breeding variant claims 50kg of thorium and 50kg U per billion kWh ; and safety due to passive cooling up to any size.

Several have freeze plugs so that the primary salt can be drained by gravity into dump tanks configured to prevent criticality.

Control rods are actually shut-down rods. Lithium used in the primary salt must be fairly pure Li-7, since Li-6 produces tritium when fissioned by neutrons. Li-7 has a very small neutron cross section. This means that natural lithium must be enriched, and is costly. Pure Li-7 is not generally used in secondary coolant salts.

But even with enriched Li-7, some tritium is produced and must be retained and recovered. The MSR concept is being pursued in the Generation IV programme with two variants: one a fast neutron reactor with fissile material dissolved in the circulation fuel salt, and with solid particle fuel in graphite and the salt functioning only as coolant.

Molten fluoride salts possibly simply cryolite — Na-Al fluoride are a preferred interface fluid in a secondary circuit between the nuclear heat source and any chemical plant.

The aluminium smelting industry provides substantial experience in managing them safely. One MSR developer, Moltex, has put forward a molten salt heat storage concept GridReserve to enable the reactor to supplement intermittent renewables. When electricity demand is low, the heat from a MWe Stable Salt Reactor SSR, see below can be transferred to a nitrate salt held in storage tanks for up to eight hours, and later used to drive a turbine when demand rises.

While MSR technology has been researched in many countries for decades, it is generally perceived that licensing MSRs is a major challenge and that in general there is so far very limited experience in design or operation of MSRs. See also Molten Salt Reactors information paper for more detail of the designs described below.

Some of the neutrons released during fission of the U salt in the reactor core are absorbed by the thorium in the blanket salt. The resulting U is separated from the blanket salt and in FLiBe becomes the liquid core fuel.

LFTRs can rapidly change their power output, and hence be used for load-following. Flibe Energy in the USA is studying a 40 MW two-fluid graphite-moderated thermal reactor concept based on the s-'70s US molten-salt reactor programme. Fuel is uranium bred from thorium in FLiBe blanket salt. Fuel salt circulates through graphite logs.

Secondary loop coolant salt is sodium-beryllium fluoride BeF 2 -NaF. It can consume plutonium and actinides, and be from to MWe. Several variants have been designed, including a 10 MWe mini Fuji. This simplified MSR integrates the primary reactor components, including primary heat exchangers to secondary clean salt circuit, in a sealed and replaceable core vessel that has a projected life of seven years.

The moderator is a hexagonal arrangement of graphite elements. Secondary loop coolant salt is ZrF 4 -KF at atmospheric pressure. Emergency cooling and residual heat removal are passive.

Each plant would have space for two reactors, allowing a seven-year changeover, with the used unit removed for offsite reprocessing when it has cooled and fission products have decayed.

Terrestrial Energy hopes to commission its first commercial reactor in the s. The total levelized cost of electricity from the largest is projected to be competitive with natural gas. The smallest is designed for off-grid, remote power applications, and as prototype. In February the project progressed to stage 2 of site evaluation by Canadian Nuclear Laboratories — a separate process to licensing — in relation to possibly siting a commercial plant at Chalk River by In August the company signed an agreement with Westinghouse in the UK for fuel development and supply.

The other three sites are located east of the Mississippi. This is a concept for a small nuclear fission source providing heat by molten salt with no pumps or valves to power a commercial gas turbine of MWe. No refuelling would be required for about ten years. The whole MsNB would be 3m diameter and 3m high. No other details. Idaho National Laboratory and Idaho University are involved. The revised TAP reactor design has a very compact core consisting of an efficient zirconium hydride moderator and lithium fluoride LiF based salt bearing uranium tetrafluoride UF 4 fuel as well as the actinides that are generated during operation.

The neutron flux is greater than with a graphite moderator, and therefore contributes strongly to burning of the generated actinides. Fission products would be continuously removed while small amounts of fresh fuel added, allowing the reactor to remain critical for decades.

Decay heat removal is by natural convection via a cooling stack. In September the company announced that it would cease operations and make its intellectual property freely available online. It is a single-fluid thorium converter reactor in the thermal spectrum, graphite moderated.

It uses a combination of U from thorium and low-enriched U Fuel salt is sodium-beryllium fluoride BeF 2 -NaF with dissolved uranium and thorium tetrafluorides Li-7 fluoride is avoided for cost reasons. Secondary loop coolant salt is also sodium-beryllium fluoride. There is no online processing — this takes place in a centralized plant at the end of the core life — with off-gassing of some fission products meanwhile.

Each module contains two replaceable reactors in sealed 'cans'. Each can is The cans sit in silos below grade 30 m down. Below each is a cylinder fuel salt drain tank, under a freeze valve. At any one time, just one of the cans of each module is producing power.

The other can is in cool-down mode. Every four years the can that has been cooling is removed and replaced with a new can. The fuel salt is transferred to the new can, and the can that has been operating goes into cool-down mode. Because the nuclear material is contained in fuel assemblies, standard industrial pumps can be used for the low radioactivity coolant salt. Decay heat is removed by natural air convection. Fuel tubes three-quarters filled with the molten fuel salt are grouped into fuel assemblies which are similar to those used in standard reactors, and use similar structural materials.

The individual fuel tubes are vented so that noble fission product gases escape into the coolant salt, which is a ZrF 4 -KF-NaF mixture, the radionuclide accumulation of which is managed. Iodine and caesium stay dissolved in the fuel salt. Other fission product gases condense on the upper fuel tube walls and fall back into the fuel mixture before they can escape into the coolant.

The fuel assemblies can be moved laterally without removing them. Refuelling is thus continuous online, and after the fuel is sufficiently burned up the depleted assemblies are stored at one side of the pool for a month to cool, then lifted out so that the salt freezes. Reprocessing is straightforward, and any level of lanthanides can be handled. SSR factory-produced modules are MWe containing fuel, pumps, primary heat exchanger, control blades and instrumentation.

Several, up to gigawatt-scale, can share a reactor tank, half-filled with the coolant salt which transfers heat away from the fuel assemblies to the peripheral steam generators, essentially by convection, at atmospheric pressure. The GridReserve version has heat storage. The SSR-W is the simplest and cheapest, due to compact core and no moderator. The primary fissile fuel in this original fast reactor version was to be plutonium chloride with minor actinides and lanthanides, recovered from LWR fuel or from an SSR-U reactor.

Secondary coolant is nitrate salt buffer. In April plans were confirmed for this plus a plant for recycling used Canadian nuclear fuel for it. The first operating reactor is envisaged after As well as electricity, hydrogen production is its purpose. It is designed to be compatible with thorium breeding to U It is seen as having a much larger potential market, and initial deployment in the UK in the s is anticipated, with potential for replacing CCGT and coal plants.

The SSR-Th is a thorium breeder version of the SSR-U, with thorium in the coolant salt and the U produced is progressively dissolved in bismuth at the bottom of the salt pool.

This contains U to denature it and ensure there is never a proliferation risk. If the fuel is used in a fast reactor, plutonium and actinides can be added. Moltex has also put forward its GridReserve molten nitrate salt heat storage concept to enable the reactor to supplement intermittent renewables. No details are available except that fuel is in the salt, and there is nothing in the core except the fuel salt. As a fast reactor it can burn U, actinides and thorium as well as used light water reactor fuel, requiring no enrichment apart from initial fuel load these details from TerraPower, not Southern.

It is reported to be large. In August Southern Nuclear Operating Company signed an agreement to work with X-energy to collaborate on development and commercialization of their respective small reactor designs. It will not require refuelling during its operational life. Core Power aims to partner with technology developers to enable deployment of the marine MSR, including amending maritime regulations for wide acceptance of m-MSR powered ships worldwide.

It operates below grade at near atmospheric pressure. It is designed to load-follow. Selected fission products are removed online. Passive safety includes a freeze plug.

It has negative temperature and void coefficients. See also information page on Molten Salt Reactors. Fuel salt is Li-7 fluoride initially with uranium as fluoride. Later, thorium, plutonium and minor actinides as fluorides are envisaged as fuel, hence the reactor being called a waste burner.

This is pumped through the graphite column core and heat exchanger. Fission products are extracted online. Spent LWR fuel would have the uranium extracted for recycle, leaving plutonium and minor actinides to become part of the MSR fuel, with thorium. The company claims very fast power ramp time. High temperature output will allow application to hydrogen production, synthetic fuels, etc. In March the public funding agency Innovation Fund Denmark made a grant to Seaborg to "build up central elements in its long-term strategy and position itself for additional investments required to progress towards commercial maturity.

Seaborg aims to deploy the first full-scale prototype power barge by This was a pre-conceptual US design completed in to evaluate the potential benefits of fluoride high-temperature reactor FHR technology.

It is designed for modular construction, and from MWe base-load it is able to deliver MWe with gas co-firing for peak loads. Fuel pebbles are 30 mm diameter, much less than gas-cooled HTRs. The project looked at how FHRs might be coupled to a Brayton combined-cycle turbine to generate power, design of a passive decay heat removal system, and the annular pebble bed core.

While similar to the gas-cooled HTR it operates at low pressure less than 1 atmosphere and higher temperature, and gives better heat transfer than helium. This could be used in thermochemical hydrogen manufacture. It is truck transportable, being 9m long and 3. Fuel is Refuelling interval is 2. Secondary coolant is FLiNaK to Brayton cycle, and for passive decay heat removal, separate auxiliary loops go to air-cooled radiators.

The reactor uses It has passive shutdown and decay heat removal. TVA holds an early site permit for the Clinch River site. This is also known as the fluoride salt-cooled high-temperature reactor FHR. A MWt demonstration pebble-bed plant with open fuel cycle is planned by about China claims to have the world's largest national effort on these and hopes to obtain full intellectual property rights on the technology.

The target date for TMSR deployment is Aqueous homogeneous reactors AHRs have the fuel mixed with the moderator as a liquid. Typically, low-enriched uranium nitrate is in aqueous solution. About 30 AHRs have been built as research reactors and have the advantage of being self-regulating and having the fission products continuously removed from the circulating fuel. Further detail is in the Research Reactors paper.

A theoretical exercise published in showed that the smallest possible thermal fission reactor would be a spherical aqueous homogenous one powered by a solution of Amm NO 3 3 in water. Its mass would be 4. Power output would be a few kilowatts. Possible applications are space program and portable high-intensity neutron source. The small size would make it easily shielded. Distinct from other small reactor designs, heatpipe reactors use a fluid in numerous sealed horizontal steel heatpipes to passively conduct heat from the hot fuel core where the fluid vapourises to the external condenser where the fluid releases latent heat of vapourisation with a heat exchanger.

The principle is well established on a small scale, but here a liquid metal is used as the fluid and reactor sizes up to several megawatts are envisaged.

There is a large negative temperature reactivity coefficient. There is very little decay heat after shutdown. Experimental work on heatpipe reactors for space has been with very small units about kWe , using sodium as the fluid.

They have been developed since at Los Alamos National Laboratory LANL as a robust and low technical risk system for space exploration with an emphasis on high reliability and safety, the Kilopower fast reactor being the best-known design.

Heatpipe microreactors may have thermal, epithermal or fast neutron spectrums, but above kWe they are generally fast reactors. It is generally perceived that licensing heatpipe reactors is a major challenge and that there is very limited or no experience in design or operation of them. Units would have a year lifetime with three-year refuelling interval. They would be transportable, with setup under 30 days.

The units would have 'walk-away' safety due to inherent feedback diminishing the nuclear reaction with excess heat, also effecting load-following. There are multiple fuel options for the eVinci, including uranium in oxide, metallic and silicide form. Westinghouse is aiming to complete the design, testing, analysis and licensing to build a demonstration unit by , test by , and have the eVinci ready for commercial deployment by From October an agreement with Bruce Power in Ontario will assess the potential for deployment in Canada, where it has been submitted for CNSC pre-licensing vendor design review.

Oklo Inc formerly UPower is a Californian company founded in It is developing a 1. It is a heatpipe reactor with sealed heatpipes to convey heat from the reactor core to a supercritical carbon dioxide power conversion system to generate electricity. It is designed to operate for up to 20 years before refuelling.

It is inherently safe, with a large temperature negative reactivity coefficient and does not require water cooling. It will be installed below grade. Idaho National Laboratory is working with the company on fuel and has agreed to host the prototype unit, for which the DOE has issued a site use permit. In April NuScale announced that it was developing a MWe "simple and inherently safe compact heat pipe cooled reactor" that "requires little site infrastructure, can be rapidly deployed, and is fully automated during power operation.

This is a new design from Northern Nuclear Industries in Canada, combining a number of features in unique combination. The coolant circulates by natural convection. The fuel pebbles are in four cells, each with graphite reflectors, and capacity can be increased by adding cells. Passive decay heat removal is by air convection. The company presents it as a Gen IV design. Westinghouse and IRIS partners have outlined the economic case for modular construction of their IRIS design about MWe , and the argument applies similarly to other similar or smaller units.

They pointed out that IRIS with its size and simple design is ideally suited for modular construction in the sense of progressively building a large power plant with multiple small operating units. The economy of scale is replaced here with the economy of serial production of many small and simple components and prefabricated sections.

They expected that construction of the first IRIS unit would be completed in three years, with subsequent reduction to only two years. Site layouts have been developed with multiple single units or multiple twin units. In each case, units will be constructed so that there is physical separation sufficient to allow construction of the next unit while the previous one is operating and generating revenue. In spite of this separation, the plant footprint can be very compact so that a site with, for instance, three IRIS single modules providing MWe capacity would be similar or smaller in size than one with a comparable total power single unit.

Many small reactors are designed with a view to serial construction and collective operation as modules of a large plant. In this sense they are 'small modular reactors' — SMRs — but not all small reactors are of this kind e. Eventually plants comprising a number of SMRs are expected to have a capital cost and production cost comparable with larger plants.

But any small unit such as this will potentially have a funding profile and flexibility otherwise impossible with larger plants. As one module is finished and starts producing electricity, it will generate positive cash flow for the next module to be built. Reactors built as neutron sources are not designed to produce heat or steam, and are less relevant here.

Traditional reactor safety systems are 'active' in the sense that they involve electrical or mechanical operation on command. Some engineered systems operate passively, e. Both require parallel redundant systems. Inherent or full passive safety depends only on physical phenomena such as convection, gravity or resistance to high temperatures, not on functioning of engineered components.

Because small reactors have a higher surface area to volume and core heat ratio compared with large units, a lot of the engineering for safety including heat removal in large reactors is not needed in the small ones. The committee had considerable involvement from SMR proponents, along with the Nuclear Regulatory Commission, Department of Energy laboratories and universities — a total of nearly 50 individuals.

The committee's interim report 1 includes the following two tables, which highlight some of the differences between the established US reactor fleet and SMRs.

Core heat removed by heat transfer through vessel. Spray systems are not required to reduce steam pressure or to remove radioiodine from containment. Complex systems require significant amount of online testing that contributes to plant unreliability and challenges of safety systems with inadvertent initiations. Emergency feedwater system, condensate storage tanks, and associated emergency cooling water supplies. Ability to remove core heat without an emergency feedwater system is a significant safety enhancement.

Reactor coolant pump seals. Leakage of seals has been a safety concern. Ultimate heat sink and associated interfacing systems. SMR designs are passive and reject heat by conduction and convection. Heat rejection to an external water heat sink is not required. Some of the early s small power reactors were developed so as to provide an autonomous power source ie not requiring continual fuel delivery in remote areas. The USA produced eight such experimental reactors 0.

The first two-unit VBER plant was planned to be built in Aktau city, western Kazakhstan, with completion of the first unit originally envisaged in , and for the second. See the Invap website www. Many experiments have been conducted on the NHR-5, such as heat-electricity cogeneration, air-conditioning and seawater desalination. The 69 fuel assemblies are identical to normal PWR ones, but at about 1. The fuel consisted of about , billiard ball-sized fuel elements.

It was used to demonstrate the inherent safety of the design due to negative temperature coefficient: reactor power fell rapidly when helium coolant flow was cut off. These were continuously recycled and on average the fuel passed six times through the core. Fuel fabrication was on an industrial scale. The reactor was shut down for sociopolitical reasons, not because of technical difficulties, and the basic concept with inherent safety features of HTRs was again proven.

It drove a steam turbine. It was licensed in , but was not constructed. This design was part of the technology bought by Eskom in and is a direct antecedent of the pebble bed modular reactor PBMR. In , Nukem reported that it had recovered the expertise for this and was making it available as industry support.

It is being developed in a French-led project, and its preparatory phase is planned to The release of hydrogen gas lowers the density of the UH3, which in turn decreases reactivity.

This process is reversed as the core temperature drops, leading to the reabsorption of hydrogen. The consequent increase in moderator density results in an increase in core reactivity All this is without much temperature change since the main energy gain or loss is involved in phase change. Another option is to have a secondary helium coolant in order to generate power via the Brayton cycle.

Most Air Cooled Condenser ACC technology has a limitation in that the tubes carrying the steam must be made of carbon steel which severely limits the service life of the ACC. Holtec has developed an ACC with stainless steel tubes bonded to aluminum fins and thus with much longer service life.

Russia plans deployment of small reactors, World Nuclear News 13 September 6. High hopes for hydride, Nuclear Engineering International January Vitali et al.

Big Book of Warfare Gabaraev, Yu. Kuznetzov, A. Romenkov and Yu. LaBar, A. Shenoy, W. Simon and E. Sienicki et al. Minato, Nuclear Engineering International October Appendix 6. Some of the developments described in this paper are fascinating and exciting. Nevertheless it is salutary to keep in mind the words of the main US pioneer in nuclear reactor development.

Admiral Hyman Rickover in — about the time his first test reactor in the USA started up — commented on the differences between an "academic reactor" and a "practical reactor". An academic reactor or reactor plant almost always has the following basic characteristics: 1 It is simple. It will use mostly 'off-the-shelf' components.

It is not being built now. On the other hand a practical reactor can be distinguished by the following characteristics: 1 It is being built now. The tools of the academic-reactor designer are a piece of paper and a pencil with an eraser.

If a mistake is made, it can always be erased and changed. If the practical-reactor designer errs, he wears the mistake around his neck; it cannot be erased. Everyone can see it. Small Nuclear Power Reactors Updated September There is strong interest in small and simpler units for generating electricity from nuclear power, and for process heat.

This interest in small and medium nuclear power reactors is driven both by a desire to reduce the impact of capital costs and to provide power away from large grid systems. The technologies involved are numerous and very diverse. A World Nuclear Association report on SMR standardization of licensing and harmonization of regulatory requirements 17 said that the enormous potential of SMRs rests on a number of factors: Because of their small size and modularity, SMRs could almost be completely built in a controlled factory setting and installed module by module, improving the level of construction quality and efficiency.

Their small size and passive safety features lend them to countries with smaller grids and less experience of nuclear power. Size, construction efficiency and passive safety systems requiring less redundancy can lead to easier financing compared to that for larger plants. The World Nuclear Association lists the features of an SMR, including: Small power and compact architecture and usually at least for nuclear steam supply system and associated safety systems employment of passive concepts.

Therefore there is less reliance on active safety systems and additional pumps, as well as AC power for accident mitigation. The compact architecture enables modularity of fabrication in-factory , which can also facilitate implementation of higher quality standards.

Lower power leading to reduction of the source term as well as smaller radioactive inventory in a reactor smaller reactors.