Freshwater from Floating Verenergie Thorium Molten Salt Reactor Desalination Plants
By Brian J. Donovan
March 27, 2017
Introduction
It is estimated that one-fifth of the world’s population does not have access to safe drinking water, and that this proportion will increase due to population growth relative to water resources. A UNESCO report in 2002 stated that the freshwater shortfall worldwide was then running at some 230 billion m3/yr and would rise to 2,000 billion m3/yr by 2025. An IAEA study in 2006 showed 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. Wars over access to water, not simply energy and mineral resources, are conceivable.
Where fresh water cannot be obtained from streams and aquifers, desalination of seawater or mineralized groundwater is required.
Most desalination today uses fossil fuels, and thus contributes to increased levels of CO2 emissions. Total world capacity in mid-2012 was 80 million m³/day (21.134 billion U.S. gallons/day) of potable water, produced in approximately 15,000 desalination plants. Two-thirds of today’s world capacity is processing seawater, and one-third uses brackish artesian water. Total world desalination capacity is projected to be 120 million m³/day by 2020.
Desalination processes include: (a) reverse osmosis (RO) driven by electric pumps which pressurize water and force it through a membrane against its osmotic pressure; and (b) thermal desalination which is a process that involves changing saline water, e.g. seawater, into vapor. This vapor, or steam, is generally free of the salt, minerals, and other contaminants that were in the saline water. When condensed, this vapor forms a high-purity distilled water. Thermal desalination processes include multi-stage flash (MSF), multiple-effect distillation (MED), and mechanical or thermal vapor compression (MVC, TVC).
The relatively less energy intensive technology at present is RO which is 30% lower than MSF and 15% lower than MED technologies. However, RO has potential problems of membrane blockages and needs periodic cleaning. Thermal desalination technology is proven, and there are no technological barriers.
Advantages of Verenergie Thorium Molten Salt Reactors
Among possible reactor coolants (water, gas, metal, and salt), only liquid salts offer the desirable combination of low pressure operation at high temperatures. Verenergie Thorium Molten Salt Reactors (VTMSRs) use liquid-fluoride salts as both a coolant and as a carrier for the thorium and thorium-derived fuels. VTMSRs operate at near atmospheric pressure offering unmatched safety and greatly simplified reactor designs. In particular, ambient or low reactor operating pressures means no risk of high-pressure atmospheric releases and no need for massive containment structures that can withstand high pressure. VTMSRs’ high temperatures (650°C) enable greater thermal-to-electric conversion efficiencies and use of more compact power conversion systems. Use of thorium fuel in a VTMSR generates orders-of-magnitude less mining waste and long-lived transuranic waste than existing light-water reactor (LWR) technology. Thorium is abundant and inexpensive and VTMSRs’ liquid thorium fuel is easily produced, compared to costly, complex fabrication of solid fuel rods used in legacy water-cooled reactors. The liquid fuel form allows fuel to be added and byproducts to be removed even while the reactor remains online. VTMSRs can consume the unused fissile material available in existing spent nuclear fuel waste and weapons stockpiles. Waste products from a VTMSR are predominantly fission products rather than actinides, and decay much more rapidly. VTMSRs will be produced as modules in a factory and can be made to be air-cooled or water-cooled, land-based or submersible, fixed or mobile, offering more flexible siting, installation and deployment options and with much less visual intrusion.
Only liquid salt coolants offer the ideal combination of increased efficiency through high-temperature operation and increased safety through low-pressure operation.
Liquid vs. Solid Fueled Thorium Reactors
Use of thorium in a liquid fuel form is VTMSR’s key differentiator from other efforts to employ thorium as a nuclear fuel. Several countries are researching the thorium fuel cycle for use in solid-fueled, water-cooled reactors. For example, LWRs can use thorium in solid fuel form to produce U233, however, they cannot provide the various advantages of the combination of the thorium cycle in liquid-fueled reactors that VTMSR technology achieves. Liquid-fluoride reactors use a chemically stable fuel form based on fluoride salts of lithium and beryllium. These salts have exceptional chemical stability, which gives them a remarkable heat storage capacity over a thousand-degree liquid range. Furthermore, the salts’ ionic bonds are unaffected by neutrons or radiation, making them a nearly ideal medium for sustaining a nuclear reaction. Into this liquid salt are mixed salts of uranium for the core salt and of thorium for the blanket salt. Thorium-232 in the blanket salt absorbs neutrons released by fission in the core and is ultimately converted into uranium-233. Uranium-233 is extracted from the blanket salts and is then fed back into the reactor core where fission of the uranium-233 produces high heat for power generation and more neutrons to convert thorium-232 into uranium-233, perpetuating the fuel production cycle. Once a VTMSR is operational, thorium is the only input required to perpetuate the thorium-to-uranium fuel cycle. The heat generated from nuclear fission in a VTMSR is transferred via heat exchangers to a clean coolant salt loop that exits the containment boundary and is then transferred to the working fluid of a gas turbine engine to generate power. The fuel salt, blanket salt and clean coolant salt circuits are each maintained at near atmospheric pressure.
Floating VTMSR Desalination Plants
Verenergie intends to design, build, own, and operate a fleet of floating VTMSR desalination plants. The VTMSRs will be coupled to MED units. The 40 years of experience in developing and operating desalination plants in Russia has shown that multistage distillation desalinating systems with horizontal tube film evaporators are the most suitable equipment for floating conventional nuclear desalination complexes. This is, in pertinent part, because MEDs are characterized by the absence of large moving water masses with a free surface, which would be sensitive to the ship's oscillating motions during operation.
Verenergie’s vessels will be either tankers or integrated tug-barge units (ITBs). In addition to thermal desalination, the VTMSRs will provide both propulsion and electricity for the vessels. The quality of water produced and the heat consumed in its production will both be defined when the system is designed. The total water cost is optimized when the tradeoff between operating costs (of which energy is a large portion) and capital costs is minimized. The type of vessel and selected efficiency is project-specific and reflects the increased capital cost for higher efficiency designs that is offset by a lower VTMSR operating (energy) cost.
Since the predominant cost for desalinated water is energy, reducing the cost of energy with VTMSRs will reduce the cost of the water. Replacing petroleum fueled desalination plants with VTMSRs will also reduce C02 emissions.
In addition to supplying fresh water to densely populated areas, floating VTMSR desalination facilities offer a particularly suitable choice for remote locations and island or coastal communities where the necessary manpower and infrastructure to support desalination plants are not available.
To avoid the usual environmental problems associated with desalination, floating VTMSR desalination facilities collect – and sell – the salt derived from the seawater, rather than discharging it back into the ocean.