Pressurized light water reactor plants have provided an effective, safe and reliable submarine propulsion system for over three decades. The next generation of nuclear submarines will continue to employ this successful and mature propulsion technology. High temperature gas-cooled reactors (HTGR) using a steam turbine (Rankine cycle) secondary loop have been developed and employed for electric power generation in this country and abroad. HTGR technology, when integrated in a closed gas turbine (Brayton) cycle offers an innovative option for small modular electric power plants and compact propulsion systems. Such a compact propulsion system integrated with electric drive could provide a highly effective submarine propulsion system in the 21st century. The compact singleprocess-loop of the high efficiency, closed Brayton cycle offers substantial promise for a smaller propulsion plant volume and attendant overall plant power density increases relative to light water reactor plants with steam turbine-based propulsion. This is possible even with the lower core power density of gas-cooled reactors relative to light water reactors because the reactor core volume is a small fraction of the overall nuclear propulsion plant volume. A simple schematic of the closed Brayton cycle is shown in the figure. Hot helium gas leaves the gas-cooled nuclear reactor and enters the closed gas turbine which is driving both a high efficiency, AC electrical generator and the compressor (there may be more than one turbine, one driving the compressor and the other the electric generator). The low pressure exhaust gas from the turbine enters a compact, high heat transfer recuperator where it heats cooler helium flowing on the other side of the heat transfer surface and is itself cooled. The helium leaving the recuperator goes to another heat exchanger called a precooler where it is further cooled before entering the compressor. The compressor raises the helium pressure to its highest value in the cycle and heats it somewhat. From the compressor the high pressure helium goes through the recuperator as discussed above where it is preheated before it enters the reactor.
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From the short description given above several potential advantages of the HTGR gas turbine power plant are obvious. The high cycle temperatures of 800-900 C (1470-1650 F) and simple closed cycle can provide cycle thermal efficiencies in the 40-45 % range. This high efficiency and the lack of a complex steam turbine-based secondary plant result in a compact, high power density propulsion plant with reduced thermal signatures due to lower waste cycle heat for a given shaft horsepower. The AC generator supplies power to a propulsion motor which is directly coupled to the shaft (no reduction gears); the generator can be located some distance from the propulsion motor and shaft. This provides a measure of flexibility in the internal arrangements of components inside the submarine. Another well known benefit of electric drive is that the substantial propulsion power can be made available for as yet undeveloped high power offensive and defensive systems. Another implicit advantage of the propulsion system is that it would probably require fewer plant operators which also frees up interior space. The helium gas at the turbine exhaust is still at a high temperature and could be used for auxiliary functions such as fresh water production.
A potential design is described below. A hypothetical nuclear attack submarine is assumed which is a body of revolution of length 100m (328ft.) and maximum diameter of 10m (32.8 ft.). From the unclassified submarine design course noted by Captain Harry Jackson, USN(Ret.), a straightforward calculation produces a hull wetted surface area of – 2700 m2 and a displacement of – 6300 long tons. With reasonable assumptions for surface area and drag coefficients of the sail and appendages, it is found that an effective power of about 26.3 MW (35,300 EHP) is required for a 35 knot flank speed. This corresponds to a shaft horsepower of about 32.9 MW ( 44,100 SHP), assuming a propulsive coefficient of 0.8. The propulsion plant parameters for these powering requirements are shown in the table. The numbers given in parenthesis in the location column indicate the location in the cycle as shown in the above figure. Account is taken for turbine and compressor efficiencies, nominal generator and propulsion motor losses and assumed ship electrical loads of – 3 MW.
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It can be seen that the cycle efficiencies are significantly higher than is feasible with a Rankine cycle (steam turbine plant). This results in a lower reactor power for a given shaft horsepower requirement and as explained above allows a smaller propulsion plant and less waste heal It is emphasized that the compact powerplant is due not so much to the increase in cycle efficiency but to the inherent simple, single-processloop nature of the closed Brayton cycle. Control of plant power on short times scales is an obvious requirement for a naval propulsion plant Two primary means of transient control are ellVBioned. They are:
1. Bypass control, where a portion of the helium flow is bypassed around the turbine(s), and
2. Inventory control, where the working pressure of the helium is adjusted t6 match a particular power level.
Inventory control has the advantage of maintaining high cycle efficiencies at modest power levels but is not fast enough, especially on a negative power transient, for naval maneuvering. Bypass control is faster but has the disadvantage of low cycle efficiencies at significant bypass flows. Inventory control requires high pressure gas compression and storage; the volume of these components has to be accounted for in the total plant volume. In a realistic control system both of these control methods would be used to maintain plant efficiency over a broad power level while providing a capability to handle fast transients without significantly perturbing the turbine and compressor.
In fairness, this proposed plant, while showing significant potential, is not off-the-shelf nuclear technology. Gas-cooled reactors have an extensive operating history and a closed Brayton cycle plant using a non-nuclear beat source has been operated in Germany at power levels up to SO MW. However, a closed Brayton cycle, nuclear-based power plant has never been operated at significant power levels. This is not so much a criticism of the concept as an indicator of the present state of innovative nuclear technologies. Several of the hardware and development issues associated with this concept are listed below:
1. An HTGR core design would need to be developed meeting the lifetime, power density and transient requirements for submarine applications.
2. Reliable, high power turbomachinery would have to be developed in the frequency range of this application which is of order 12,000-17,000 Hz. A concept like magnetic bearings may be required to maintain the purity of the helium cycle gas.
3. Reliable, compact heat exchangers (recuperator and precooler) are required which can operate at high pressure and at effectiveness factors of 90-95 % with reasonable pressure drops.
4. A reliable and effective control system needs to be designed which occupies modest volume, has a good transient response while minimizing perturbations to the turbomachinery and gas-cooled reactor.
5. A plant layout should be conceptualized which provides efficient and innovative use of the submarine interior volume while providing a rational approach to maintenance. A maintenance approach needs to be worked out based on anticipated impurities in the circulating helium gas which can support high plant reliability levels. For some items the best approach might be component replacement due to their small size.
6. The electric propulsion system needs to be developed which has reasonable motor and generator efficiencies and is well integrated into other submarin~ systems. The acoustic signatures of the propulsion plant need to be studied after suitable sound isolation and dampening concepts are developed. It is noted that the elimination of reduction gears with the all-electric drive removes a substantial acoustic source term.
The above items are substantial but are less challenging than those faced by the pioneers of navy nuclear power in the 1950’s in implementing the first naval nuclear propulsion plants. In conclusion, it is believed that HTGR’s coupled with a closed Brayton cycle, all-electric propulsion plant is an attractive option for high plant power density submarine propulsion in the next century. This option can become a reality if investment is made now in the enabling technologies which support this concept.[Acknowledgment: This research was sponsored by the U.S. Department of Energy under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.]