This chapter was published on “Inuitech – Intuitech Technologies for Sustainability”
on December 20, 2011
The scope of nuclear civil vessels includes the following three types of vessels: 1. Nuclear Merchant Ships, 2. Nuclear Icebreakers, and 3. Nuclear Cruise Ships. This chapter is dedicated to Nuclear Cruise Ships.
3. NUCLEAR CRUISE SHIPS
Lloyd’s Register, the international standards organization for the classification and design of ships, announced in November 2010 that it has begun a two-year project with a consortium of companies to look into the feasibility of nuclear-powered commercial ships. The primary application will be for cargo ships, but all large vessels, including cruise ships, could use the technology if Lloyd’s Register endorses it.
It is true as it was reported that the nuclear potential was never transpired in a true sense due to the traditional anti concerns associated with safety, radiation exposure, and the size of the reactors but nuclear propulsion is already widespread in the world’ oceans in nuclear submarines, aircraft carriers, and Russian nuclear icebreakers. The military grade naval vessels are good examples to see the impact nuclear power has on large ships. Nuclear marine propulsion has been around since the 1950’s, and by 1960, 26 nuclear submarines were operation with another 30 under construction. United States aircraft carriers use nuclear power to desalinate the necessary water on their ships. For large carriers this represents 400,000 gallons per day. The US military use of nuclear reactors for naval propulsion is a testimony to enormous benefit of nuclear power.
The benefits of nuclear ship propulsion are so robust and vigorous that this technology can neither be ignored nor disregarded. Furthermore, considering climate change priorities which are becoming urgent concern at a global level, companies and governments around the world are now dusting off some of those old dreams for carbon-free nuclear-and shipping, which accounts for roughly 5 percent of global greenhouse gas emissions, seemed to Lloyd’s Register like a logical place to start.
A new generation of small reactors appears to be addressing some of those concerns. Hyperion Power Generation, a spin-off from Los Alamos National Laboratory in the U.S. and a member of the Lloyd’s Register consortium, has developed a “Small Modular Reactor” that produces 25 MW of electricity (Traditional power plant reactors produce up to 1,500 MW) using low enriched uranium. The company has big plans for its little reactors, which called “Nuclear Batteries.” They hope their little atom splitters can be used to power everything from American subdivisions to plants in the developing world. The design of these reactors attracted Lloyd’s Register.
The other consortium members are ship designers BMT Nigel Gee and Greek shipping company Enterprises Shipping and Trading. In addition to the technical challenges associated with this technology, one of the primary obstacles will be how the ships can be used in countries that are currently unfriendly or have statutory prohibitions of nuclear power. BMT Nigel Gee will be looking at the feasibility of a physical separation of the ship, meaning that the portion of the ship with the nuclear propulsion would be used for deep-sea transit but then remain in international waters while a large module with the cargo (or passengers) enters port under battery power.
Unfortunately, these Small Modular Reactors do not have universal support simply because some environmentalists argue the size of these reactors make them vulnerable to terrorist sabotage or theft. Consequently, it is not clear we consider in how to manage the much larger risk of global climate change.”
Currently, there has only been three nuclear powered cruise vessels ever built. The N.S. Savannah was the world’s first nuclear powered cargo ship that was built by the New York Shipbuilding Corporation in New Jersey. The ship was launched in 1962. It boasted 9,400 tons of cargo and it was capable of traveling at 21 knots and 226,000 miles on a single fuel load. The N.S. savannah was not designed to be a competitive commercial vessel; rather it was built for Eisenhower’s “Atoms for Peace” initiative. It was designed to look more like a luxury yacht than a large commercial cruise ship. Many how investors will view a fleet of this kind of nuclear ships. Nuclear power requires political support, and another accident could at any time swing sentiment against the nuclear technology. But Nick Brown, Maritime Communications Manager at Lloyd’s Register, says that, like nations themselves, the shipping industry has been forced by climate change to look at all alternatives to fossil fuels. He suggested that “There is this perception that nuclear represents an increased risk but really it needs to be one of the options people were convinced that nuclear power is not viable for naval propulsion because of the N.S. Savannah, but this is not true. The ships planned mission was to prove that the U.S. was committed to using nuclear power for peace and not destruction. The objective of this project was to demonstrate nuclear power’s ability in fields that did not relate to the military. At the time, compared to oil powered ships, the N.S. Savannah was much faster and had a much larger range. The ship could circle the earth 14 times traveling at a speed of 20 knots without ever refueling. However, because the goal of the N.S. Savannah was not to be commercially viable, the ship was condemned to a short life that led many to believe that nuclear powered cruise ships were a failure.
As a background, the commercial shipping industry has been around since the early 1900’s when the first vessel built purely for tourism was completed. From that single ship, the industry has grown to a $27 billion dollar industry carrying over 18 million passengers to destinations all around the world.
The reality is that as an upshot of the size of modern cruise ships continues to increase, the requirement for fuel, power, water and crew boosts costs at an exponential rate. While the technical details of cruise ships vary slightly, the largest ships have very similar power, fuel, water and crew requirements. These ships are over 1,000 feet long with a height of over 230 feet above the water line and a depth of about 70 feet. They measure over 200,000 gross tons and displace about 100,000 tons. It is true that almost all large commercial cruise ships are powered by 16 cylinder diesel engines that each output 25,000 (18,642 KW). The number of engines per ship varies but the largest cruise ship have six, with each consuming over 1,300 gallons of fuel per hour when in operation. This huge fuel requirement amounts to 187,200 gallons of fuel per day of operation.
It is also true that each ship is built to hold over 5,000 passengers, which means that a massive amount of fresh water is needed for operation. The largest of cruise ships use over 260,000 gallons of fresh water every day. In order to meet this fresh water demand, a desalination process is used to convert the salt water into pure water. There are several different ways to desalinate water including reverse osmosis, ion exchange and multi-stage flash distillation.
The sizes of these cruise ships keep increasing to accommodate the increasing passengers demand and as a result, the pure volume of fuel that is consumed by these massive vessels results in huge costs for the cruise liner. Current prices of bunker fuel for the cruise ships are around $650 per ton of fuel. Assuming that the density of the marine fuel is around 970 kg/m3, this means that if a vessel consumes 187,200 gallons of fuel per day, the cost of just the fuel is $447,742 a day. This fact alone is enough to make the average person second guess the type of fuel used for commercial naval propulsion. Another problem is the amount of energy that is needed to desalinate enough ocean water to get 260,000 gallons of fresh water per day. The other major issue affecting desalination plants is corrosion of pipes because of the seawater. The Waterfields desalination plant in the Bahamas provides 2.64 million gallons of fresh water per day, but after 6 months of operation, the 316L stainless steel pipes began to show corrosion. The replacement was an AL-6XN alloy pipe, which has not corroded for over 10 years.
The most common propulsion system for current large cruise liners is a diesel-electric system. There are usually six main diesel engines that are attached to generators. Unlike older cruise ships, the diesel engines are not directly attached to the propeller shafts, instead they are attached to generators so the entire system is electric. The ships also have 4 bow thrusters, each of the bow thrusters generate about 7,500 hp (5,592 kW) which leads to a total of roughly 30,000 hp (22,370 kW) when combined.
Despite the current economic situation, construction of cruise ships is still going strong. Royal Caribbean just introduced a new Genesis Class of cruise ships that will cost over $1 billion to build; it is the first non-military vessel to be built with a price tag of over a billion dollars. The industry is only getting bigger, and with increased size, the desire for reduction in fuel and weight, as well as an improvement in speed distance and emissions will lead to the need for better technology.
Philosophy Paradise published the result of their study on 31 May 2010 that dealt with Nuclear Powered Cruise Ship – Engineering Analysis. The objective of this study was to determine the feasibility of a commercial cruise powered solely by nuclear power coupled with seawater desalination, utilizing the specifications for a nuclear Pressurized Water Reactor and deciding if the power and fresh water requirements of a medium size cruise ship can be met.
Currently, the two most popular methods are reverse osmosis and multi-stage flash distillation, and for the cruise ship design, the proposed design will be using multi-stage flash distillation. In Multi-Stage Flash (MSF), distillation seawater vaporization takes place in a vacuum at low temperature. The reason vaporization takes place in a vacuum is that the boiling point of water is lower which means less energy is required to complete the vaporization. Before going into the heater, the cold seawater passes through condensing coils in the vacuum flash chambers that serve two purposes. They preheat the cold seawater before entering the heater and condense the already flashed steam in the chambers to produce the fresh water. Then the seawater enters a brine heater that heats the seawater to a temperature between 90 °C and 110 °C to boil the water.
This process is performed in multiple chambers to increase the quantity of the water product. The desalination process takes a huge amount of energy to complete. Energy is needed in two stages, electrical energy to pump the water and steam energy to heat the brine. In order to produce the 260,000 gallons of fresh water needed per day, a vast amount of power is needed to complete the necessary desalination. One of the major design hurdles is to find the most efficient way to accomplish this desalination while using the minimum amount of energy. In the proposed design, it is suggested to couple its nuclear power cycle with a desalination plant.
In order to conduct a feasibility analysis of a nuclear powered cruise ship with desalination, two potential Rankine power cycles are proposed. Using Rankine cycles, can thermodynamically model both power generation as well as desalination using the laws of conservation of mass and energy. Energy is the combination of the internal energy (U) of a system with all other energetic contributions including kinetic energy (KE) due to inertial velocity effects and potential energy (PE) due to body force effects that include gravity effects. Entropy is a thermodynamic quantity that represents the amount of energy in a system that can no longer accomplish mechanical work. It also measures the disorder or randomness of a closed system. Enthalpy is a thermodynamic quantity equal to the internal energy of a system plus the product of its volume and pressure. More generally, it is the amount of energy in a system capable of doing mechanical work.
It was recognized by the team that before they begin the technical analysis must be explored the qualitative pros and cons of nuclear power in order to verify the possibilities in the commercial shipping sector.
In order to conclude the proposed design, it was considered viable to justify it against the disadvantages of nuclear power. To do this, it was necessary to acknowledge the shortcomings. The first and most obvious shortcoming is the large initial capital cost of purchasing a nuclear reactor. Nuclear reactors are very expensive to design, build and install. They require much more sophisticated and expensive operating and control systems when compared to the typical diesel engines that power cruise ships. Operating costs of a nuclear reactor will also be much larger than conventional marine propulsion systems. Nuclear reactors require individuals with specialized experience in the nuclear field for maintenance and operation. The reactors and turbines will also require more person-hours over their operational life that greatly increases the direct maintenance costs. Besides the pure monetary downside, there are also less tangible drawbacks to nuclear reactor propulsion systems which include security issues and political ramifications.
At the same time, in order to understand why analysis was performed, there was a need to recognize the huge possibilities nuclear fuel has for marine propulsion. The first and biggest advantage of nuclear propulsion is increased mobility of the marine vessel. For large marine ships that use diesel engines, there is a speed duration threshold, which limits the speed of a vessel for extended periods. Because of this, a commercial ship cannot travel far distances at a swift pace, which leads to larger fuel costs and a limit to where the vessel can travel. A nuclear powered cruise ship will not have any speed-radius limitation because of the nuclear fuel. This fuel also has many other advantages, which include cost, emissions and lifespan. At current prices, reactor fuel costs 85 percent less than bunker fuel and emits no greenhouse gases. Nuclear reactors can be built with fuel supplies that last for 10 to 20 years, which means that a vessel can go years without having to stop and refuel. Lastly, the huge difference in cargo-to-fuel ratios will lead to greater travel distances at faster speeds.
Because the advantages of nuclear power outweigh the disadvantages, the purpose of this analysis was to explore a nuclear powered cruise ship with desalination is feasible. Therefore, it was planned to develop a model using a general Rankine cycle with and without reheat then apply the cycle to the case of power generation with cogeneration capabilities of using the condensing stage for desalination. This allows the desalination process to replace condensation, and effectively use the waste heat. Here is a summary of assumptions:
- The efficiencies of the turbines and pumps are 85 percent;
- There is no pressure drop across the desalinator;
- The incoming mass flow rate of the seawater is 3500 kg/s;
- Total power output by the turbine(s) is 110 MW;
- Working fluid is water;
- Specs for nuclear power plant are obtained from a 500 MW pressurized water reactor, and mass flow rate was scaled down as necessary;
- Seawater properties are assumed to be equal to freshwater properties at the same pressure and temperature;
- No stray heat transfer from any component;
- Kinetic and potential energies are ignored;
- Each component operates at steady state;
- Water requirements for a cruise ship are 260,000 gallons per day, or 11.36 kg/s;
- Desalination replaces condensation;
- Pressure and temperature at state 1 are 60 bar and 320oC; and
- Inlet temperature of seawater is 30 oC. Outlet temperature of brine is 40 oC. The freshwater temperature is 100 oC.
In short, the basic and Rankine Power Cycle with heat were analyzed to determine the optimal desalination pressure for the required fresh water mass flow rate. The analysis showed that a pressure of 11.67 bar was optimal for the basic Rankine cycle, and a pressure of 15 bar was optimal for the Rankine cycle with reheat. Ultimately, it was concluded that a nuclear powered cruise ship with desalination is feasible.
The conclusion of this feasibility study is another testimony from an economical as well as environmental point of views that should encourage meaningful investments in nuclear cruise ship projects and convince politicians around the world not to ignore or disregard this technology.