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| Fig. 1: Image of a U2540 lead acid battery. (Source: Wikimedia Commons)) |
US Naval submarines use three types of lead-acid battery cells: PDX-57, ASB-49, and LLL-69 Type cells. The reaction is
The nominal cell voltage is 2.0 V. [1] The PDX-57 cell designed for Ohio-class submarines weights 2,100 pounds with a capacity of more than 10,000 Amp-hours and stored energy of 2.6 MWh. [1] The ASB-49 cell designed for the Los Angeles-class submarine weights 1,300 pounds with a capacity of 7,200 Amp-hours and stored energy of 1.8 MWh. [1] The LLL-69 type cell weights 1,500 pounds with a capacity of 8,100 Amp- hours and stored energy of 2.0 MHh. [1] Fig. 1 shows the relative sizes of the batteries. Electricity generated from its nuclear rector is the main source of electrical and propulsion power for the submarine, but a battery, as a source of power, is required during emergency operations.
A not-so-intuitive advantage lead-acid batteries have over lithium-ion is that they have been around longer and were the initial batteries qualified to be used on submarines. The lead-acid batteries used in the Naval submarine force were designed in the early 1970s for the PDX-57 cell and in the mid 1980s for the LLL-69 cell. [1] Commercially, lithium-ion batteries were only available in 1991. [2] This is especially important because batteries used in submarines undergo a long and rigorous qualification period. [1] Before lithium-ion batteries could be used in service onboard submarines they would need to undergo a similar qualification which is costly.
Lead-acid batteries have a lower cost per unit of energy. This is due to the lower cost of materials used in lead-acid batteries. [1] Adding to the cost of lithium-ion batteries is that they require protection circuitry for safety and to prevent overcharge and overdischarge. [2] For example, lithium-ion sulfide cell have a thermal safety switch that limits the current by increasing resistance thus reducing current. [3] This feature helps prevent the battery from overheating. Lithium-ion batteries have a poor tolerance to overdischarging and overcharging.
Weight is a disadvantage of lead-acid batteries. Lead has an atomic mass of 207.19 while lithium is only 6.94. [3] This contributes to lithium-ion batteries having 3 times greater energy density per unit weight and 6 times greater energy density per unit volume than lead-acid batteries. The materials used in lead-acid batteries also pose safety and environmental concerns. Used batteries are potentially hazardous to the environment if they are not disposed of properly. Lead and sulfuric acid may seep out from batteries that are carelessly disposed of and pollute water sources, wildlife and humans.
Another disadvantage of lead-acid batteries compared to lithium-ion is that they require routine maintenance due to side reactions. During charging operation the electrolysis of water into hydrogen and oxygen gas may occur. [1] The lost water must be replaced to keep the cells covered. Hydrogen gas above certain levels is also explosive, this requires monitoring for hydrogen gas.
Lithium-ion batteries are also able to obtain a higher open-circuit voltage of 3.2 volts for LiMnO2 battery compared to only 2 volts for a lead-acid batteries. [3]
Lead-acid batteries are the heritage batteries used in nuclear powered naval submarines. Figure 1 shows a U-boat lead acid battery. Although, they have low energy density they are mature in technology and cost considerably less than lithium-ion alternatives. Lithium-ion batteries offer a lighter weight energy storage device with long retention time and relatively high energy density. Lithium-ion batteries do not require routine maintenance to replenish lost electrolyte or result in the production of explosive gases during charge or discharge processes. However, lithium-ion batteries are not tolerant to under or over discharging. Thus they require the need of electronic control for charge and discharge safety.
Radiation is all around us. Sources of radiation can be divided into two major categories: natural and man- made radiation sources. Natural sources of radiation include cosmic, terrestrial, and background radiation. The very ground we walk and build our homes on contain radioactive elements that decay in radon gas. Low levels of uranium, thorium, and their decay products are found everywhere. These are ingested with food and water, while others, such as radon, are inhaled. The sun is a source of cosmic radiation, but the earth's atmosphere helps absorb most of the gamma rays and protects us from high doses of cosmic radiation. [4] Internally, K-40, C-14, and tritium are found inside us from birth. We are also exposed to man-made sources of radiation. Man-made radiation sources are divided into those that the public is exposed to and those that are as a result of occupational exposure. [4] The public is exposed to radiation from tobacco, television, medical x-rays, smoke detectors, nuclear medicine, and building materials. Isotopes from these sources are I-131, Tc-99, Co-60, Ir-192, and Cs-137.
As you can see from the examples above there are many sources of radiation. Clothing or the buildings we work in shield some types of radiation, but there are certain types of radiation that are ore commonly considered when designing radiation shielding. These include neutron sources, gamma photons, and x-rays. Radiation shielding specialists control the many sources of radiation, protect people, and radiation sensitive systems from radiation. The first task to designing radiation shielding is to identify the type of radiation involved, the energy distribution, and strength of the source. [5] Another consideration in radiation shielding is the size of the space allotted for the shield. Due to lack of information about the source of the radiation field, shield designers often only use only estimates or approximations of the radiation field based on their knowledge.
Neutron sources include fission neutrons, photoneutrons, and neutrons from (α, n) reactions. Fission neutrons are important in shield design because they pose biological and radiation damage. [5] Most heavy nuclides will fission on the adsorption of a neutron producing energetic fission neutrons. The fission neutrons, in turn, may produce secondary radiation sources such as inelastic scattering, capture gamma protons, or turn stable isotopes into radioactive isotopes. [5] Fission species include U-235, U-233, U-238, Pu-239, and Th-232. A gamma photon with energy to overcome the neutron binding energy of about 7 MeV may cause (γ, n) reaction, however, most gamma photons have insufficient energy to overcome the binding energy. A few nuclides for photoneutron production are H-2, Li-6, Li-7, Be-9, and C-13. Capture gamma photons from neutron adsorption have the necessary energy (~7 MeV) and can cause production of energetic photoneutrons. [5] Laboratory neutron sources use energetic alpha particles from radioisotopes to induce (α, n) reactions in materials. Most commonly used alpha emitters are actinide elements that form intermetallic compounds with beryllium. This ensures that the emitted particles react quickly with the encounter converter and leakage is minimized. [5] X-rays may be produced from photoelectric adsorption that leaves the adsorbing atom in an ionized state or from the decay of a radionuclide.
© Paul Ditiangkin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. Szymborski, "Lead-Acid Batteries For Use In Submarine Applications," Proc. 2002 Workshop on Autonomous Underwater Vehicles, (IEEE, 2002), p. 11.
[2] R. Brodd ,"Synopsis of the Lithium-Ion Battery Markets," in Lithium-Ion Batteries: Science and Technology, ed. by M. Yoshio, R. Brodd and A. Kozaw (Springer, 2009), p. 1.
[3] R. M. Dell and D. Rand, Understanding Batteries (Roy. Soc. Chem., 2001), pp. 70-81 and 100-125.
[4] "Background Radiation Natural versus Man-Made," Washington State Department of Health, Fact Sheet 320-063, July 2002.
[5] A. B. Chilton, J. K. Shultis and R. E. Faw, Principles of Radiation Shielding (Prentice-Hall, 1983), pp. 78-120.
