Nuclear Propulsion Along Inland Waterways

Using mobile thermal storage batteries, small numbers of micro-scale nuclear reactors at fixed locations could provide propulsive energy for fleets of vessels that sail along inland waterways as well as railway locomotives used for shunting at terminals or short line railway service. Relevant developments have occurred in fixed location thermal storage, mobile thermal storage and engine technology that combined, could offer cost-competitive maritime propulsion.
 
Introduction

During his career, renowned management professor CK Prahalad illustrated that new possibilities develop from a convergence of evolving technologies, a more focused version of the lateral thinking techniques developed by Dr. Edward de Bono. 

The technologies that make nuclear propulsion along inland possible and feasible include advances in small-scale nuclear power technology and advances in thermal storage technology. While power utilities focus on developing large, grid-scale nuclear power stations, navies developed small-scale nuclear power conversion that could be applied to maritime propulsion, aboard ships and submarines. Micro-scale nuclear reactors of 9,000kW to 25,000kW evolved from maritime nuclear power. Navies of the world have accumulated some 5,600-reactor-years of experience involving small-scale technology. 

Energy Storage

Large-scale nuclear power installations deliver optimal reliability and lowest long-term operating cost when the reactor remains at constant high temperature and is continually cooled by water or by high-pressure gas. During overnight off-peak hours when market demand for electric power is minimal, power from nuclear installations is transferred into pumped hydraulic storage, where hydroelectric power dams pump water to reservoirs at higher elevation. A research team from MIT theorized that during off-peak hours, some heat from the nuclear reactor could be diverted into large-scale thermal storage such as high-temperature geothermal storage.

During such time, lower pressure steam at the same high maximum temperature would flow into turbines and deliver less electrical output, in turn allowing additional nuclear power stations to flow off-peak electrical power into existing pumped hydraulic storage installations. There is also scope to divert low-grade nuclear power station exhaust heat into geothermal storage and involving temperatures well below that of the boiling point of water. Energy at such temperature could sustain the operation of fleets of mobile and fixed-location Organic-Rankin-Cycle (ORC) engines such as the units offered by Electratherm of Nevada.

Thermal Energy Storage

Thermal storage batteries originally involved insulated tanks of hot water maintained at pressure of 250psia at 400^oF to sustain the operation of fireless steam railway shunting locomotives. Small versions of such tanks containing hot water were installed on compressed air powered locomotives used in mines, to preheat air prior to expansion in engines to raise efficiency. Early efforts aimed at applying heat-of-fusion molten materials proved problematic in railway operation. However, the solar thermal power industry recently developed reliable high-temperature thermal storage systems capable of raise steam to drive electrical generation technology for several hours every evening.

Thermal energy storage technology developed by the solar thermal power industry uses large amounts of low-cost, naturally-occurring salts and can be adapted for thermal storage operation at nuclear power stations. During off-peak hours while the nuclear reactor remains at constant temperature, some of the heat would be diverted into thermal storage to achieve multiple purposes that include peak period power generation and transportation vehicle recharge. Such thermal storage applied to micro-nuclear power installations would allow mini-reactors to operate at constant thermal output irrespective of the fluctuating demand for energy delivered from thermal storage.

Accessing Thermal Storage

The combination of a local power grid and the transportation sector could access stored thermal energy at coastal locations. In the mobile sector, tugs and railway shunting locomotives could utilize a variety of onboard thermal storage technologies to provide heat to operate any of low-temperature, high-temperature and combined-cycle mobile power conversion systems that could be applied to maritime propulsion. For low-temperature geothermal storage of power station exhaust heat at 70 to 80^oC, heat pumps could transfer heat into large thermal storage tanks aboard boats powered by ORC engines capable of operating on heat up to 120^oC (250^oF).

At geothermal storage temperatures of over 100^oC, water would function as a heat pump refrigerant to transfer heat into thermal storage aboard vessels. Even with thermal storage temperature at 300^oC, saturated water between 1,200psia and 2,000psia would still function as a refrigerant that could transfer heat from stationary thermal storage into mobile thermal storage aboard a vessel or railway vehicle. 

The useable service life of low-cost, naturally-occurring material used in stationary thermal storage installations would extend into decades, possibly beyond, as could some variations of mobile thermal energy storage, reducing overall long-term operating costs. 

Thermal Storage Materials

Stationary thermal storage installations such as the installation in southern Spain utilize low-cost, naturally occurring mixtures of materials such as 60 percent sodium nitrate and 40 percent potassium nitrate, with comparatively low latent heats of fusion and that melt at 400^oC. Some installations that operate at under 300oC using molten caustic soda or high-temperature geothermal storage would be compatible with nuclear reactors that operate at just over 300^oC. At that temperature water at 1,250psia would be in the saturated liquid state and suitable for mobile thermal storage, assisted by molten mixture of sodium hydroxide and sodium fluoride thermal storage.

While sodium hydroxide would melt at 300^oC, adding sodium fluoride could raise melting temperature to 310oC, making the mixture suitable for mobile application such as a large tug boat to convert water into steam, or to sustain the temperature inside a high-pressure water tank at 1,000psia. A mixture of 80 percent lithium hydroxide and 20 percent lithium fluoride can melt at 460^oC with a heat of fusion of over 1100 KJ/kg or 523 BTU/pound, making a small amount of it suitable to superheat steam in mobile maritime application, where an ORC engine could assist in maintaining energy efficiency.

Low Temperature ORC Engine

In maritime operation, an ORC engine could utilize steam engine exhaust heat at 100oC to generate power. Aboard a boat equipped with high-pressure, high-temperature tanks of saturated water at 1,200psia at 290^oC, the output of an ORC engine could drive a heat pump that uses high-pressure water as a refrigerant. It would transfer heat from a molten metal thermal storage tank into the saturated water, to maintain temperature in the high-pressure water tank as it releases steam to drive engines. The ORC engine and molten metal thermal tank would sustain steam engine efficiency and operating range.

Pressurized tanks of water at 140^oC that sourced heat from low-grade geothermal energy storage could sustain the operation of an ORC engine. The addition of water soluble salts such as sodium fluoride into the mobile thermal storage tanks could form hydrated molten salts that could increase thermal storage capacity at 140oC and allow for increased output or extended operating range. ORC engines would use river water for cooling and during winter in the northern hemisphere, river water temperatures drop to near the freezing point of water and would enhance the efficiency of (closed-cycle) ORC engines. 

Nuclear Fuel - Thorium

The present generation nuclear reactors use uranium as fuel and produce nuclear waste. However, much research has been underway over several years to develop thorium fuel nuclear reactors that produce less nuclear waste, with the possible option of reprocessing spent thorium to operate as nuclear fuel. Thorium occurs in greater abundance that uranium and especially in India and China where scientists have been actively working on developing thorium based nuclear power conversion. While uranium-based nuclear power conversion has elicited political opposition in some nations, some nuclear critics would likely be less opposed to thorium-based nuclear power conversion.

High-Temperature Nuclear Conversion

While present generation nuclear reactors operate at 300^oC, evolving new generation reactors and future reactors are expected to operate at temperatures at 500oC for liquid metal cooled reactors, 860oC for molten salt reactors and over 950^oC for reactors cooled either by high-pressure helium or high-pressure carbon dioxide. Naturally occurring sodium chloride salt involves low cost and melts at just over 800^oC with just over 200 BTU/pound heat of fusion. The solar thermal power industry has already developed material that can store heat at between 400^oC and 530^oC. 

As the nuclear power industry develops higher temperature reactors, some forms of suitable low-cost thermal storage materials are already available. Future nuclear power station could include thermal storage technology that could serve as an emergency thermal reservoir for the reactor, generate peak power for the grid and provide thermal energy to operate some forms of transportation technology, such as short-distance maritime propulsion and short-distance railway propulsion that include thermal batteries for energy storage. Such power generation and thermal storage technology would likely appear in China, including along some of their navigable inland waterways.

Comparative Costs – Transportation

While electrochemical storage batteries combine high cost with limited service life, thermal batteries often combine low cost with greatly extended useable service lives. When applied to propulsion as was the case with thermal rechargeable railway shunting locomotives, the service life of the thermal storage system was equal equal to that of the service life of the transportation vehicle. Hydrated salt mobile thermal storage installed aboard short-distance vessels powered by ORC engines would also offer greatly extended useable service life, allowing massive amounts of low-grade power station exhaust heat to be stored using low-cost geothermal methods.
 
The combination of insulated high-pressure tanks of saturated water combined with heat-of-fusion mobile thermal storage based on a mixture of 80 percent sodium hydroxide and 20 percent sodium fluoride offering some 350 BTU/pound at 305^oC would combine low material cost with greatly extended useable service life. Stationary thermal storage at 300oC would also be based on naturally occurring low-cost materials such as a mixture of potassium nitrate and potassium chloride (50 BTU/pound at 308^oC) would offer greatly extended useable service lives.  In maritime operation, ORC engines would operate on steam engine exhaust heat.

Propulsive Power

The low temperature storage tank (90 to 140^oC) would sustain the operation of a closed-cycle ORC engine that would operate at four to seven percent thermal efficiency. A vessel powered by an ORC engine would operate short-distance services such as tug operation or ferry service. An insulated high-pressure tank with corrosion-resistant lining could contain saturated water at 300^oC, supplying steam to a two or three stage engine driving propellers. Heat from a thermal storage tank of molten material would maintain constant temperature and constant pressure in the high-pressure storage tanks of saturated water.

Coastal Vessels

Coastal nuclear power stations that include stationary thermal storage capability would be able to provide energy recharge to ocean coastal vessels built with onboard thermal storage to sustain propulsive power. Such vessels would incur low long-term operating costs.

Conclusions

Many nuclear power stations are located along a coast line, a river or near to a lake where water is used to condense exhaust steam. The solar thermal power sector has developed stationary thermal energy storage that combines long service life with low cost. There is scope to combine existing nuclear power stations with stationary thermal energy storage, to be recharged during overnight off-peak hours and generate power during peak periods, also supply propulsive thermal energy to maritime and railway vehicles.