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From Manhattan Project To Electricity Production

In December 1950, ORNL built its 2-megawatt Bulk Shielding Reactor by modifying its earlier Materials Testing Reactor mockup to create a swimming pool reactor. The reactor's enriched uranium core was submerged in water for both core cooling and neutron moderation.
In December 1950, ORNL built its 2-megawatt Bulk Shielding Reactor by modifying its earlier Materials Testing Reactor mockup to create a "swimming pool" reactor. The reactor's enriched uranium core was submerged in water for both core cooling and neutron moderation.

More than 430 nuclear power reactors are operating in the world, and 103 nuclear power plants produce 20% of the electricity used in the United States. Most of these reactors are cooled by ordinary water. Water also is used to slow, or moderate, the neutrons emitted by fissioning uranium fuel so that reactions are sustained and heat is produced to make steam for power generation.

A strong proponent for the use of water as a reactor coolant was Eugene Wigner, who won a Nobel Prize for physics. Wigner was a mentor for Alvin Weinberg, who calls Wigner the founder of nuclear engineering as well as a great theoretical physicist. Both became ORNL directors and coauthored The Physical Theory of Neutron Chain Reactors. Wigner's reactor design was used largely by DuPont for water-cooled, graphite-moderated reactors built at Hanford, Washington; in 1945 these reactors produced plutonium for the atomic bomb that ended World War II. Wigner designed a water-cooled, water-moderated converter that enabled neutrons from fissioning plutonium to convert thorium to fissionable uranium-233, making him the grandfather of today's research reactors, naval reactors, and nuclear power plants.

Oak Ridge experiments by Art Snell in 1944 showed that 10 tons of ordinary uranium slugs would not explode and that chain reactions would not occur in a water-moderated natural uranium lattice. Additional experiments at the air-cooled Graphite Reactor led by Henry Newson and calculations by Weinberg suggested that to achieve sustained reactions in such a lattice, the uranium must be slightly enriched in fissionable uranium-235. In a 1944 letter, Weinberg suggested using high-pressure water as a coolant and moderator for a reactor to produce useful power; he also described the idea in a 1946 paper coauthored with Forrest Murray. Weinberg became a progenitor of the pressurized-water reactor (PWR), the basis of many nuclear power plants.

In the mid-1940s many power reactor concepts were born, with some evolving into technologies still considered valid. Because uranium was thought to be quite rare, some scientists conceived fast reactors that produce, or breed, more plutonium than they consume. In 1945 Wigner and Harry Soodak published the first design of a sodium-cooled breeder reactor.

In 1947 Farrington Daniels conceived the pebble-bed gas-cooled reactor in which helium rises through fissioning uranium oxide or carbide pebbles and cools them by carrying away heat for power production. The "Daniels' pile" was a crude version of the later high-temperature gas-cooled reactor developed further at ORNL.

Wigner also predicted that radiation damage to materials used to build reactors could impair their safe operation. To determine which materials fare best under irradiation, he conceived the Materials Testing Reactor (MTR), the first high-powered, enriched-uranium reactor cooled and moderated with water. For the MTR, ORNL researchers developed fuel elements with uranium oxide sandwiched between curved aluminum plates to prevent buckling, as well as a beryllium reflector to scatter neutrons back into the core.

While designing the MTR, ORNL researchers built a small mockup to test controls and hydraulic systems. In 1950, the experiment produced the first visible blue Cerenkov glow of a nuclear reaction underwater.

Because it is corrosion-resistant, zirconium was considered a good candidate for reactor fuel rods to contain uranium pellets. But some samples absorbed too many neutrons, suggesting an impurity. ORNL's Herbert Pomerance discovered that hafnium, a common zirconium contaminant, absorbed many neutrons, and that pure zirconium soaked up very few. Oak Ridge researchers developed a separation process for producing pure zirconium, which was used in reactor fuel cladding for submarines and power plants.

Atomic Energy Commission officials, who had centered reactor development work at Argonne, realized that ORNL had much to contribute. The Laboratory was allowed to upgrade the mockup's shielding and cooling systems, increasing its power level to 10% of the MTR's. Labeled the "poor man's pile" by Wigner, the mockup was formally named the Low Intensity Test Reactor.

Experiments at the LITR established the feasibility of the boiling-water reactor, later a design prototype for commercial plants. ORNL researchers also developed principles of reactor control and protection systems that are used today.

Graduates of the Oak Ridge School of Reactor Technology (ORSORT), which was started by Wigner, designed a variety of reactors. One was the Aircraft Reactor for the ill-fated nuclear airplane project, whose aim was an aircraft that could fly indefinitely without refueling. The Bulk Shielding Reactor and Tower Shielding Facility were built to provide data on constructing lightweight radiation shielding for the aircraft. Another ORNL-developed reactor was built for display at the 1955 United Nations Conference on the Peaceful Uses of Atomic Energy. Yet another ORNL design was the Army Package Reactor, a transportable device built by a private contractor in 1957 for the Army Corps of Engineers to generate electric power at a remote base.

ORNL developed several homogeneous reactors that were fueled, cooled, and moderated by uranium-containing fluid, rather than the solid fuel and liquid coolant and moderator typical of most power plants today. A promising design was the molten-salt reactor, an outgrowth of the Aircraft Reactor project. Its fuel solution circulated continuously between the core and a processing plant that removed unwanted fissionable products. It was designed in the late 1960s to produce both electric power and new fuel, uranium-233. Today there is interest in Europe, Japan, and Russia in developing MSRs as actinide burners.

In the late 1950s there was international interest in gas-cooled reactors, and ORNL had a strong role in the U.S. commercial HTGR program. Responsibilities included developing an improved graphite moderator and developing and testing advanced coated-particle fuels that retain fission products at high temperatures. ORNL's experience in developing and testing spherical ceramic fuel has been sought to support development of the next-generation gas-turbine-modular-helium-cooled reactor and pebble-bed modular reactor.

Because of its researchers' extensive reactor experience and expertise, ORNL is the only national lab in Westinghouse Electric Company's international consortium to design the International Reactor Innovative and Secure power plant, a next-generation PWR. ORNL also is a co-leader of the Department of Energy's nuclear power programs.

ORSORT graduates also designed the many research reactors that have operated at ORNL (a half dozen in the 1960s). Among these were the Oak Ridge Research Reactor (1958-88) and the 37-year-old High Flux Isotope Reactor, still in operation for possibly another 30 years. In addition to producing isotopes for agricultural, industrial, and medical uses, the 85-megawatt HFIR is a valuable tool for materials testing and neutron-scattering research; it is one reason why ORNL will be the center of the neutron science universe at mid-decade. Thanks to neutrons, which make nuclear power possible, ORNL will soon become a world power in physical and biological materials research.

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