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| Fig. 1: A photograph of the reactor vessel before it was lowered into place. - Library of Congress, Prints and Photographs Division, Historic American Engineering Record, HAER PA, 4-SHIP, 1-87. [Public domain image. HAER reprodution policy available at www.loc.gov/rr/print/res/114_habs.html.] |
The Shippingport Atomic Power Station was the first commercial nuclear power plant in the United States. Producing 60 MW of electric power, it was designed as an intermediate step between earlier small-scale test reactors and future large power stations. Engineers could develop technologies on an appropriate scale for commercial power production without dealing with the high costs associated with a larger reactor. The current widespread use of the light water reactor design is, in part, attributable to the success of Shippingport. [1] Details of the initial design of Shippingport provide an insight into the operation of nuclear power plants.
Shippingport had a single Pressurized Water Reactor (PWR). A PWR uses ordinary light water (in contrast to the heavy water used in some reactors) as a coolant in its two main coolant loops. The primary loop is maintained at a high pressure to prevent the water from boiling. Fission of nuclear fuel in the core heats the water in the primary loop, which is then pumped to a steam generator. The steam generator transfers the heat from the water in the primary loop to the water in the secondary loop. Since the secondary loop is not kept at the high pressure of the primary, the water can turn into steam and power a steam turbine. [2]
The pressurized light water design was chosen for Shippingport because it had previously been proven suitable for power generation. Light water acted both as coolant, transferring heat from the core to run the generator, and as neutron moderator, slowing down fast neutrons to sustain a fission reaction in the U-235 fuel. Light water reactors are designed to have a negative temperature coefficient of reactivity [3] – as the temperature in the core increases, the reaction rate slows down. Thus the reaction is stabilized around the normal temperature of operation. An additional advantage of using light water as both coolant and moderator is that, in the case of an accident where coolant is lost from the reactor, the moderator is lost also, and the reaction slows down. The coolant needed to be flushed out periodically to prevent corrosion. Any water potentially contaminated with radioactive products went through a decontamination system, after which it could be either recycled or released into the Ohio River. [4]
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| Fig. 2: Flow of the two coolant loops in a PWR. The primary (yellow) passes through the core, picks up heat, and transfers it to the secondary (blue), where it drives a turbine generator (gray). Also illustrated is the placement of the control rods (green). |
Shippingport used both enriched uranium and natural uranium as nuclear fuel. Natural uranium contains less than 1% of the fissile isotope U-235, with the rest being the non-fissile U-238. The enrichment process increases the proportion of U-235. Modern PWRs typically use uranium that is enriched to about 3% U-235. [5] Shippingport, however, used a unique “seed-and-blanket” design. The seed consisted of highly enriched uranium – about 90% U-235 – while the blanket was made of natural uranium. The excess neutrons produced by the highly enriched seed fuel would pass into the blanket, sustaining the fission reaction and breeding U-238 into fissile plutonium. About half of the power generated was from fission in the blanket [6]. The blanket was designed to remain in the reactor over a long period of time, so that only the seed would have to be refueled.
Held in place above the reactor at Shippingport were control rods made of the neutron-absorbing hafnium. The rods could be lowered into the core to reduce free neutron density and slow the rate of fission, or pulled out to do the opposite. The rods were present in all of the seed fuel assemblies. The blanket assemblies did not require control rods, as they did not contain enough fissile material to sustain a chain reaction without the neutrons provided by the fission of seed fuel. [6] In case of emergency, the reactor could undergo a “scram” procedure – the control rods would be released, falling under gravity to full insertion in the core. The nuclear chain reaction would quickly be halted.
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| Fig. 3: A top-down view of the core showing the arrangement of seed fuel assemblies (light gray) and blanket fuel assemblies (dark gray). The excess neutrons from the seed drive the nuclear reaction in the blanket. |
Shippingport produced a number of different radioactive wastes. Strict specifications were set so that release of liquid or gaseous material was not to exceed one tenth of the maximum allowable emissions. Liquids were typically treated by ion exchange, where the radioactive ions are captured by a bed of synthetic resins. These resins and other solid wastes were stored in underground tanks. Gases were stored until they underwent enough natural decay that they could be diluted and released to the atmosphere. The danger of radioactive materials is diminished when they are diluted. Solid wastes could be encased in concrete and either buried at the site or dumped at sea. [7]
© Aaron Coleman. 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] W. Beaver, Nuclear Power Goes On-Line (Greenwood Press, 1990), pp. 64-68.
[2] ibid., p. 30.
[3] S. S. Yu, S. C. Kim, and Y. W. Na, "Measurement of the Moderator Temperature Coefficient of Reactivity for Pressurized Water Reactors," J. Korean Nucl. Soc.y 29, 488 (1997).
[4] Ref. [1], p. 32.
[5] J. G. Collier and G.F. Hewitt, Introduction to Nuclear Power (Taylor and Francis, 2000), p. 43.
[6] J. C. Clayton, "The Shippingport Pressurized
Water Reactor and Light Water Breeder Reactor," Westinghouse Report
WAPD-T-3007, 1993
((PDF 0.54 MB).
[7] J. R. Lapointe and R. D. Brown, "Radioactive Material Control," Ind. Eng. Chem. 50, 980 (1958).