Reactor Types and Proliferation
By Dovile Vilkauskaite
Nuclear reactors are a proliferation concern because they can be used to produce both nuclear energy and the materials for a nuclear weapon. Some types of reactors are more vulnerable to proliferation than others because fissile materials are either utilized in the front end of a reactor as fuel or are produced as a byproduct. Fuel (uranium, plutonium, or a mixture of these) is placed in the reactor core, where neutrons fission some of the fuel’s atoms. A moderator (water, heavy water, or graphite) slows down neutrons so they have a better chance of fission, and control rods are added to the core to regulate fission and a coolant runs through a heat exchanger that spins a turbine to generate electricity.[1] The three most commonly seen reactors are outlined below.
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Light water reactors are the most abundant and the most “proliferation resistant.”[2] Fuel going into a LWR is low-enriched uranium (LEU) which must be enriched to a certain level, below 20% U-235. Since only a few countries have enrichment capabilities, the number of states that can supply LEU is limited. A limited amount of countries with enrichment capabilities decreases the proliferation concern. Agencies like the Nuclear Suppliers Group which implement guidelines for nuclear and nuclear-related exports.[3] Like all other reactors, LWRs make plutonium when U-238 is converted to Pu-239 in a nuclear reactor. Yet the plutonium lies in spent fuel with hazardous fission products and is therefore considered proliferation proof as long as it remains mixed with the fission products.
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Heavy water reactors are a concern for nonproliferation efforts when paired with a reprocessing plant.[4] These types of reactors can be fueled with natural uranium and therefore do not require enrichment capability to produce plutonium. Along with reprocessing technology, a heavy water rector can be used to produce plutonium from natural uranium.[5] Unlike regular water, heavy water (with a higher percentage of deuterium atoms), absorbs fewer neutrons and produces more plutonium from natural uranium. Heavy water can also be transformed into tritium, which is used to boost the explosive power of nuclear weapons.[6]
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Research reactors are small and can be found in universities and state labs to conduct research. Unlike power reactors, the core of a research reactor is small. Since the fuel going into a research reactor is smaller in quantity, so is the amount of plutonium that it can produce. Even though the quantity of plutonium is small, it was enough for North Korea to begin its nuclear weapons program.[7] A 25 MWt graphite-moderated, natural uranium fueled “Experimental Power Reactor” in Yongbyon produced plutonium for weapons.[8] The fact that North Korea’s graphite-moderated research reactor did not require enriched uranium made it easy for Pyongyang to receive the necessary materials for fuel input. Unenriched uranium is not difficult to obtain since it is not a fissile material. Using graphite as a moderator is what allowed North Korea to use natural uranium in its research reactor while most research reactors require enriched uranium (HEU) for fuel.
Diagrams with links to additional details for 2 types of Light Water Reactors used in South Korea and Japan are below.
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Pressurized Water Reactor (used in South Korea)
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Figure from U.S. NRC website [9]
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Brief description: This is a common reactor using regular water as a coolant that is kept at high pressure to prevent it from boiling. A heat exchanger transfers heat to a second loop that spins the turbine required to produce energy.[10]
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Boiling Water Reactor (used in Japan)
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Figure from U.S. NRC website [11]
Brief description: This is a common reactor using regular water as a coolant like the pressurized water reactor but in this case, the water boils.[12] A heat exchanger transfers heat to a second loop that spins the turbine required to produce energy.[13]
Endnotes
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[1]Gary T. Gardner, Nuclear Nonproliferation: A Primer (Boulder, CO: Lynne Rienner Publishers, 1994), Ch. 3: Nuclear Reactors, 25.
[2] Daniel Engber, “Can I Interest You in a Light-Water Reactor?” Slate, June 2, 2006. http://www.slate.com/articles/news_and_politics/explainer/2006/06/can_i_interest_you_in_a_lightwater_reactor.html.
[3]“About the NSG,” Nuclear Suppliers Group, http://www.nuclearsuppliersgroup.org/en/about-us.
[4]Gary T. Gardner, Nuclear Nonproliferation: A Primer (Boulder, CO: Lynne Rienner Publishers, 1994), Ch. 3: Nuclear Reactors, 33.
[5]Ibid, 28.
[6]Ibid, 29.
[7]“Nuclear Proliferation Case Studies,” World Nuclear Association, September 2016, http://www.world-nuclear.org/information-library/safety-and-security/non-proliferation/appendices/nuclear-proliferation-case-studies.aspx.
[8]Ibid.
[9]“The Pressurized Water Reactor (PWR),” United States Nuclear Regulatory Commission, June 16, 2016, http://www.nrc.gov/reactors/pwrs.html.
[10]“What is a nuclear reactor?” What is Nuclear, accessed November 26, 2016, https://whatisnuclear.com/.
[11]“The Boiling Water Reactor (BWR),” United States Nuclear Regulatory Commission, June 16, 2016, http://www.nrc.gov/reading-rm/basic-ref/students/animated-bwr.html.
[12]Ibid.
[13]Ibid and “What is a nuclear reactor?” What is Nuclear, accessed November 26, 2016, https://whatisnuclear.com/.
