GBCN LFTR NUCLEAR DESALINATION STRATEGY

Background.

In many parts of the world, potable water is in short supply. Development is constrained by the lack of water in many areas as depicted in Figure 1 below.

Nuclear energy is already being used for desalination, and has the potential for much greater use. In addition, nuclear desalination is generally very cost-competitive with using fossil fuels.

About one fifth of the world's population does not have access to safe drinking water; this proportion will increase due to population growth relative to water resources. The most-affected areas are the arid and semiarid regions of Asia and North Africa. Wars over access to water, not simply energy and mineral resources, are conceivable.

Figure 1. – Projection 2025

Fresh water is one of sustainable development’s highest priorities. Where it cannot be obtained from streams and aquifers, desalination of seawater or mineralized groundwater is essential.

An IAEA study in 2006 provided evidence that 2.3 billion people live in water-stressed areas, 1.7 billion of them having access to less than 1000 m3 of potable water per year. With population growth, these figures will increase substantially. Further demand in the longer term will come from the need to make hydrogen from water, along with the need for increased agricultural activity to feed the new population.

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GBCN LFTR NUCLEAR DESALINATION STRATEGY

Desalination

Most desalination facilities today use fossil fuels. Total world capacity is approaching 40 million m³/day of potable water, in some 15,000 plants. Most of these are in the Middle East and North Africa, using distillation processes. The largest plant produces 454,000 m³/day. Two thirds of the world capacity is processing seawater, and one third uses brackish artesian water.

The major technology in use and being built today is reverse osmosis (RO) driven by electric pumps which pressurise water and force it through a membrane against its osmotic pressure (For a seawater osmotic pressure of 27 bar the minimal energy that must be overcome by RO is about 0.75 kW hour per cubic meter and it varies according to the water salinity.)

A thermal process, multi-stage flash (MSF) distillation using steam, was earlier prominent and it is capable of using waste heat from power plants. With brackish water, RO is much more cost-effective, though MSF gives purer water than RO. A minority of plants use multi-effect distillation (MED) or multi-effect vapor compression (MVC) or a combination of these. MSF-RO hybrid plants exploit the best features of each technology for different quality products.

Desalination is energy-intensive.

Reverse Osmosis needs up to 6 kWh of electricity per cubic meter of water (depending on its original salt content), hence 4 MWe (megawatt electrical) will produce about 16,000 to 24,000 m3 per day from seawater. MSF and MED require heat at 70-130°C and use 25-200 kWh/m³ (mostly heat plus a small amount of electricity), though a newer version of MED (MED-MVC) is reported at 10 kWh per cubic meter of water and is competitive with RO.

A variety of low-temperature and waste heat sources may be used, including solar energy, so the above kilowatt-hour figures are not properly comparable. For brackish water and reclamation of municipal wastewater RO requires only about 1 kWh/m3. The choice of process generally depends on the relative economic values of fresh water and particular fuels, and whether cogeneration is a possibility. (www.world-nuclear.org, 2010).

It is widely considered that there is a large potential for medium-capacity (50,000 – 100,000 m3/day) fixed / transportable desalination plants coupled to nuclear plants.

For example, in the North African IAEA nuclear desalination feasibility study, Small Modular Reactors (SMRs) have been indicated as the most suitable size for the majority of nuclear desalination applications. The feasibility of integrated nuclear desalination plants has been proven with over 150 reactor years of experience, chiefly in Kazakhstan, India and Japan. Large-scale deployment of solid-uranium-fueled nuclear reactor desalination on a commercial basis will depend primarily on economic factors. Indicative costs are US$ 70-90 cents per cubic meter of water, much the same as fossil-fuelled plants in the same areas. (2010.)

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GBCN LFTR NUCLEAR DESALINATION STRATEGY

Hybrid desalination systems integrating both thermal and membrane desalination processes and then combined with power generation systems are considered the best economic alternative.

Figure 2. – Steam Electricity

Hybrid (membrane/thermal/power) configurations are characterized by flexibility in operation, less specific energy consumption, low construction cost, high plant availability, better overall water quality, and better power and water matching. (IAEA GC, 1995.)

MSF (Multiple Stage Flash), the technology that has traditionally dominated continues to lose its share to RO (Reverse Osmosis) and MED (Multi-effect Distillation). This is due to the improvement of membrane technologies and the advantages of steady cost reductions and gains in efficiency.

Design Challenges

An energy and water plant is designed to answer a specific demand requirement that varies between winter and summer at that plant’s location. In South Africa summer electricity demand is much higher than winter demand, due to the air conditioning requirements, while water demand is almost stable all year long.

The plant designer will estimate what is the minimum heat flux permanently available at the outlet of the power system. Our desalination strategy will be closely coupled to the Small Modular Reactor – Liquid Fluoride Thorium Reactor (LFTR). In the LFTR’s case 700 degrees Celsius of heat will be permanently available. The designer will then specify a thermal desalination plant (MED / MSF) that can meet the water demand with this minimum heat flux. If water demand is too big to be met with this thermal plant alone, he will then complete the water plan with a reverse osmosis (RO) system and the result will be a hybrid desalination plant. This basic principle gives a good guideline of how to decide on the desalination architectural strategy.

Complex calculations must be made to determine the power and water production costs resulting from each technical combination in order to fine-tune the economical optimization.

MSF vs MED.

MED thermal desalination is the right solution because it has an inconsequential electrical consumption (1.5 Kwh per cubic meter, whereas MSF uses about 3 kWh per cubic meter MORE electrical consumption than does MED) and it is more flexible operationally than MSF. MED

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GBCN LFTR NUCLEAR DESALINATION STRATEGY

can be equipped with thermo-compressors (MED-TVC) that will adapt their efficiency to the quality of the heat recovered from the LFTR, thus boosting the plant’s overall economy.

After taking into account its lower operating power costs, the MED’s reliability is the determining factor, thus winning the battle of both OpEx and CapEx.

For example, an 800,000 cubic meter per day Saudi Arabian plant would require an additional power generation capacity of 100 MW if an MSF solution had been selected INSTEAD of the actual MED solution that was used. (1995.)

MED implementations can be improved by increasing the Top Brine Temperature (TBT) from 63 degrees C to 80 to 100 degrees C using nanofiltration techniques. MED efficiencies can also be raised by improving the heat transfer coefficient (HTC) using oval and corrugated plates. Furthermore, the hybridization with RO and Nanofiltration (NF) will have a favorable reduction on water costs as well – the integrated distillate and membrane permeate solution will reduce requirements on Boron removal by RO, and the RO and NF membrane life can be extended to last up to 12 years with this process combination.

The combination of RO and MED technologies in a hybridized desalination facility provides several important advantages: (Hamed, ND) Operational flexibility for seasonal water and energy demands – two alternative methodologies

enable solutions for water – power mismatches

The combination provides for higher RO product recovery

Using LFTR’s waste heat to pre-heat the seawater feed to RO’s effects reduces water cost up to 25%

Using a common seawater intake for both RO and MED with resulting lower capacity

Using a single stage versus a two stage RO operation saves over 45% of total energy consumption

Hybridization lowers RO’s chemical consumption and membrane replacement rate

Gas based head and electrical power from the LFTR can be used to power both RO and MED pumps

Hybrid RO + MED plants produce cheaper water than thermal desalination-only plants

Product cost comparison for typical desalination plants:

o RO - $0.50 to $0.80 per cubic meter

o MED - $0.75 to $0.85 per cubic meter

o MSF - $1.10 to $1.25 per cubic meter

Using water heated by a nuclear reactor to drive Rankine steam turbines and subsequently desalination facilities is problematic: (Ejjeh & Awerbuch, 2009) Tritium generated as a by-product of nuclear fission is deposited in the molten salt, and additional

tritium is produced by neutron irradiation of the fuel salt. This tritium can diffuse through high-temperature heat exchangers into the steam cycle – resulting in water contaminated with tritium that presents a high exposure risk for workers as well as a stubborn waste management problem to solve.

To obtain good salt physical properties for operations, the SMR-LFTR must operate around 650 – 700 degrees Celsius. At low temperatures, the salt freezes. At higher temperatures preferred by the

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GBCN LFTR NUCLEAR DESALINATION STRATEGY

LFTR system steam is highly corrosive. The resulting steam cycle developed for the LFTR is workable, but complex and the heat-to-electricity efficiency was less than expected for a reactor with coolant exit temperatures in excess of 700 degrees Celsius. (33%) High temperature heat was not efficiently utilized.

Salts also react with steam very slowly.

By using a closed gas Brayton power cycle turbine, the LFTR by-passes all of these issues. With the LFTR powering a Brayton cycle (2009) -- Tritium is easily removed from the helium-based closed Brayton cycle in the cold parts of the

cycle.

The 650 to 700 degree Celsius optimal operating temperatures of the LFTR match the temperatures that optimize operation of the Helium Brayton cycles.

No chemical reactions occur between Helium and molten salts.

The development of compact heat exchangers enabled reduction of the LFTR’s total fuel salt inventory to 60% or less than those of traditional molten salt reactors that relied upon Rankine steam turbines. The heat exchanger volume shrinks 5-fold, or more – making possible major improvements in reduced plant footprint. (2009)

Figure 3. – Size Matters (2009)

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GBCN LFTR NUCLEAR DESALINATION STRATEGY

This previous Figure 3 above compares the size of a high-temperature helium Brayton cycle (900°C turbine inlet temperature, 1300 MW(e) output) to a 1380 MW(e) steam Rankine cycle of a modern light water reactor. The Brayton system is clearly more compact, thus providing potential for major reductions in the turbine building volume and power conversion system capital cost for high-temperature MSRs. The much smaller size is primarily a consequence of two factors: (1) higher efficiency that reduces the size of heat rejection equipment as well as the size of many other components and (2) much higher power densities that are a consequence of the higher pressures in the Brayton cycle (minimum ~7 atmospheres) compared to steam turbines (which operate sub-atmospheric).A 2400 MW(t) reference point design of a multi-reheat Brayton cycle is illustrated in Figure 4. It was derived from the current Power Conversion Unit (PCU) design for the General Atomics gas-turbine modular helium reactor (GT-MHR).

Figure 4 - Closed Gas Brayton Cycle

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GBCN LFTR NUCLEAR DESALINATION STRATEGY

Figure 5. Coupling the Brayton gas cycle with seawater desalination (Peterson & Zhao, 2006)

The SMR-LFTR’s removal heat is transferred from molten salt to the helium and the pre-cooler and inter-cooler transfers it to the water in the isolation loop shown in Figure 5 above. The seawater heats up, a fraction of it evaporates while the rest is used to pre-heat the seawater feed, gets cooled itself and is pumped to the pre-cooler and inter-cooler again for the continuing circulation. The evaporation generated in the isolation loop is sent to the first effect of the low temperature MED, heats the seawater feed in the tubes of the first effect, which evaporates, and gets condensed. The condensate is first used to pre-heat the seawater feed and then is pumped to the pre-cooler and inter-cooler again for the continuing circulation. (Peterson & Zhao, 2006)

The cost of desalting water has been a significant constraint to the large scale adoption of desalination technology globally. Combined cycle based power stations are now proven to be more economic for co-generation plants producing power and water. The co-generation of electricity and desalted water by coujpling the closed gas Brayton cycle turbine of the SMR-LFTR with LT-MED seawater desalination has incomparable advantages over other alternatives.

The SMR-LFTR’s main advantage in this co-generation configuration is its much lower fuel cycle cost than a fossil fuel-fired power plant. The safety and environmental considerations of a nuclear desalination complex do not pose significant economic or health risks. The LFTR operates with greater safety features inherent to its architecture than any other reactor. Some provisions need to be made in order to ensure that when the desalination plant, which serves as a heat sink for the LFTR, is shut down or operated in partial load, there will be a backup heat sink available to accept rejected heat from the LFTR and prevent its shutdown. (Seneviratne, 2007)

Using an advanced Multi-Effect Distillation (AMED) system, waste heat from closed gas Brayton cycles can be deployed to desalinate seawater without affecting the power generation cycle’s thermal efficiency. GBCN-LFTR plans to combine Multi-Effect Distillation with closed gas Brayton cycles.

The cost of water from a MED plant varies significantly with design, size, location, brine water type and other factors. For a modern large MED plant, $0.70 per cubic meter water production is possible. The cost of water (COW) includes water plant installation, thermal energy cost, capital cost, maintenance cost, electricity cost, as well as other minor OpEx. The MED plant’s electricity cost represents only 1% of cost of water. For the tower type of MED plant, the water plant cost represents about 37% of COW and the thermal energy cost amounts to about 27%. By contrast, optimizing a closed gas Brayton cycle coupling with a MED configuration can reduce the thermal energy cost to zero. (Peterson & Zhao, 2006)

Solution to Environmental Issues of Desalination

Due to constraints and pressures for land use, the real-estate, environmental and social value of the shoreline in RSA is quite high. Coastal infrastructure plants should be sited in industrial development zones wherever possible.

The Ashkelon, Israel Desalination Plant, producing 274,000 cubic meters of water per day, consists of the following: (1) area of the plant, (2) the marine pumping station, (3) corridors for the pipe system and water pools. This covers approximately 17.3 acres of land. It is the intent of GBCN-LFTR to have a smaller output for the planned desalination plants but to be conservative, a 10-acre desalination footprint is estimated.

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GBCN LFTR NUCLEAR DESALINATION STRATEGY

Pumping installations should be located about 100 meters from the shoreline, comprising the suction pipe system and the brine outlet.

The 100 MW AMR-LFTR is projected to require a footprint of about 2 acres, which can include the closed gas Brayton cycle turbine assembly. Seawater suction can be carried out by a system of one-kilometer pipes, each 1.6 meters in diameter. A system of dissolved and suspended oil sensors will monitor the water quality. If a risk of pollutants is detected, then pumping water into the plant is halted. The duct for each pipe will be dup up to six meters deep. The dug up sand will be stored on the seabed at no less than 15 meters water depth. After laying the pipe(s), the original stored sand will be be refilled into the ducts over the pipe, thus restoring the seabed to its former state. It is anticipated that the returning concentrated brine will be pumped into the sea with the use of a multi-port diffuser whose purpose is to increase dilution of the concentrate to surrounding sea levels. (Einav & Lokiec, 2006)

Perth as a Discharge Model

Desalination brine, different from regular sewage outfalls, contains roughly twice the concentration of seawater. It is denser than seawater, and therefore naturally sinks towards the seabed and flows along deep ocean channels. The amount of dispersion and mixing is therefore severely restricted. Discharging this brine concentrate back into the sea can result in a ‘hypersaline’ layer of water on the seabed. The Perth, Australia RO desalination plant has been subject to more environmental consideration and care than any other plant anywhere. It should be taken as a model of environmental assessment and remediation. (Stuart Khan, 2007)

A series of models – a one-dimensional box model and several three-dimensional hydrodynamic models -- were structured. Tests using them were made to ensure the plant would meet strict mixing criteria set by the Australian environment agency. Various scenarios and different models were deployed. Tank tests were also undertaken during the diffuser design and expert review of the design was undertaken prior to its installation.

The Perth, Australia desalination plant outlet is 1.2 meters in diameter and has a 160-meter long, forty-port diffuser where the ports are spaced at five meter intervals with a 0.22 meter nominal port diameter, located 470 meters off share, at a depth of 10 meters, adjacent to the plant. The diffuser incorporates a discharge angle of 60 degrees. This design was adopted with the expectation that the plume would rise to a height of 8.5 meters before beginning to sink due to its elevated density.

It was designed to achieve a plume thickness at the edge of the mixing zone of 2.5 meters and, in the absence of ambient cross-flow, 40 meters laterally from the diffuser to the edge of the mixing zone.

The operating license of the Perth desalination plant requires that certain dissolved oxygen levels are met in order for the plant to operate. Furthermore, a minimum of 45 dilutions must be achieved at the edge of the mixing zone, defined in terms of a 50-meter distance from the diffuser.

Visual confirmation of the plume dispersion was achieved by the use of 52 liters of Rhodamine dye added to the plant discharge. The expulsion of the Rhodamine dye from one of the plant diffusers is shown below. The dye was reported to have billowed to within 3 meters of the water surface before falling to the seabed and spilling along a shallow sill of the Sound towards the ocean. The experiment showed that the dye had dispersed beyond what could be visually

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GBCN LFTR NUCLEAR DESALINATION STRATEGY

detected within a distance of around 1.5 kilometers, well short of a protected deeper region of Cockburn Sound and 5 kilometers from the diffuser. (Khan, 2007)

Pictures from The West Australian of this even are shown below:

Source: The West Australian newspaper.

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GBCN LFTR NUCLEAR DESALINATION STRATEGY

References.

“Nuclear Desalination,” updated February 2010, World Nuclear Association, http://www.world-nuclear.org/info/intt71.html

“Plan For Producing Potable Water Economically”, IAEA GC(39)/12, 8 Aug 1995

Hamed, ND. “Overview of Hybrid Desalination Systems – Current Status and Future Prospects,” Research & Development Center, Saline Water Conversion Corporation, Al-Jubail, Saudi Arabia, rdc@swcc.gov.sa

Ejjeh, G. & Awerbuch, L., 2009. “Challenges of Integration of Desalination, Power, Energy, Environment and Security”, Roundtable on Water and Energy of the World Energy Council 5 Feb 2009

Seneviratne, G., 2007. “Research Projects Show Nuclear Desalination Economical”, Vienna Correspondent for Nuclear News Magazine, April 2007.

Peterson, F. & Zhao, J., 2006, “Advanced Multiple Effective Distillation Processes for Nuclear Desalination,” NED, UC Berkeley, Idaho National Laboratory, American Nuclear Society Winter Meeting, 7 Nov 2006

Faibish, R.S., 2006. “Water and Power: Nuclear Desalination”, Argonne National Laboratory rfaibish@anl.gov; www.aiwrm.anl.gov - 14 Nov 2006

Einav, R & Lokiec, F., 2006. “Environmental Aspects of a Desalination Plant in Ashkelon,” Blue Ecosystems, Environmental Consulting, for IDE Technologies, Ltd., Israel, 2 Jan 2006.

Khan, S. 2007. “Desal Brine Disposal” Post in Water Recycling in Australia Blog, Principal Water Researcher in an Environmental Engineering Dept., Sydney, New South Wales, Australia, 8 July 2007.

Forsberg, C., Peterson, F. & Zhao, H. , 2004. “An Advanced Molten Salt Reactor Using High-Temperature Reactor Technology,” Oak Ridge National Laboratory, UC Berkeley, 28 Feb 2004

Zhu, S., Qi, W., Zhu, J., Yu, S. 2007. “Co-generation of Electricity and Desalted Water by Gas Turbine MHTGR,” Institute of Nuclear and new Energy Technology, Tsinghua university, Beijing, Transactions, SMiRT 19, Toronto, August 2007Sommariva, C., ND, “2.500 Desalination and Water Purification,”, MIT OpenCourseWare, retrieved 28 May 2010 from http://ocw.mit.edu

Semiat, R., 2000. “Desalination: Present and Future”, Water Research Institute, Haifa, Israel International Water Resources Assn., Water International Journal, Vol. 25 (1), Pp. 54-65, March 2000.

Uche, J., Serra, L, & Valero A., ND, “Hybrid Desalting Systems for Avoiding Water Shortage in Spain”, International Study Group for Water and Energy Systems, Centre of Research for Power Plant Efficiency, University of Zaragoza, Spain. – retrieved 28 May 2010 from javiuche@posta.unizar.es; serra@posta.unizar.esw; valero@posta.unizar.es

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