Editing Project Pluto (section)

== Development ==
[[File:L-63-6601 Pluto SLAM (LASV) Test 198 1963.jpg|thumb|right|Test of the aerodynamic characteristics of a Supersonic Low Altitude Missile (SLAM) or Low Altitude Supersonic Vehicle (LASV) configuration that was to be powered by nuclear ramjet engines developed in Project Pluto]]
The proposed use for nuclear-powered ramjets would be to power a [[cruise missile]], called SLAM, for [[Supersonic Low Altitude Missile]]. It would have many advantages over other nuclear weapons delivery systems. It was estimated that the reactor would weigh between {{convert|50000|and|200,000|lb|order=flip|sp=us}}, permitting a payload of over {{convert|50000|lb|order=flip|sp=us}}. Operating at [[Mach number|Mach]] 3, or around {{convert|2300|mph|order=flip|sp=us}} and flying as low as {{convert|500|ft|order=flip|sp=us}}, it would be invulnerable to interception by contemporary air defenses. It could carry more nuclear warheads than the sixteen aboard a [[George Washington-class submarine|Polaris]] ballistic missile submarine, they could be larger, with [[nuclear weapon yield]]s of up to {{convert|10|MtTNT|lk=on}}, and delivered with greater accuracy. Moreover, unlike an ICBM, it could be recalled.{{sfn|Butz|1964|pp=30–33}}

It was estimated that the [[unit cost]] of each missile would be less than $5&nbsp;million (equivalent to ${{inflation|US-GDP|5|1961}} million in {{Inflation/year|US-GDP}}), making them much cheaper than a [[Boeing B-52 Stratofortress]] bomber. Operating costs would also be low, as keeping them in readiness would be cheaper than a submarine or bomber, and comparable with a [[missile silo]]-based ICBM.{{sfn|Butz|1964|pp=30–33}} [[Range (aeronautics)|Range]] would not be unlimited, but would be determined by the fuel load. Merkle calculated that a MW-day of energy would burn about one gram of [[highly enriched uranium]]. A 490 MW reactor with 50 kilograms of uranium would therefore burn 1 percent of its fuel each day. Assuming that an accumulation of [[neutron poison]]s could be avoided, the missile could fly for several days.{{sfn|Merkle|1959|pp=10–11}} The success of the project depended upon a series of technological advances in [[metallurgy]] and [[materials science]]. [[Pneumatic motor]]s necessary to control the reactor in flight had to operate while red-hot and in the presence of intense [[ionizing radiation]]. The need to maintain [[supersonic]] speed at low altitude and in all kinds of weather meant that the missile would have to fly though much denser air. In turn, this meant that it would encounter much greater air resistance and have to generate more power to overcome it. The reactor, code-named "Tory", would therefore have to survive high temperatures that would melt the metals used in most [[jet engine|jet]] and [[rocket engine]]s.<ref name="fact sheet" /> 

[[File:Schematic cross-section of Tory reactor.png|thumb|left|upright=1.5|Schematic cross-section of Tory reactor]]
The solution arrived at was to use [[ceramic]] fuel elements. The [[nuclear reactor core|core]] of the reactor would be made of [[beryllium oxide]] ({{chem|Be|O}}),{{sfn|Rothman|1962|pp=1–3}} the only available neutron moderator material that could withstand the high temperatures required.{{sfn|Walter|1964|p=13}} Over 80 percent of the fueled tubes were {{convert|3.925|in|cm|order=flip|sp=us}} long; the rest varied in length so as to achieve the correct column length and arrangement.{{sfn|Walter|1962|p=6}} The tubes consisted of a BeO [[matrix (geology)|matrix]] with a grain size between {{convert|5|and|20|micron|sp=us}} in diameter containing a solid solution of [[uranium dioxide|urania]] ({{chem|U|O|2}}), [[zirconia]] ({{chem|Zr|O|2}}) and [[yttria]] ({{chem|Y|2|O|3}}).{{sfn|Rothman|1962|pp=1–3}} The Tory II-A reactor used a uranium-beryllia mixture, but by the time Tory II-C was built zirconia and yttria was added in a 1.06:1:1 ratio of urania:zirconia:yttria.{{sfn|Sandholtz|1965|p=3}} The zirconia and yttria stabilized the urania against [[phase transition]] to [[triuranium octoxide]] ({{chem|U|3|O|8}}) at temperatures around {{convert|1200|C|F}}. The fuel particles of the urania-zirconia-yttria mixture (known as "horseradish") were mostly from {{convert|0.5|to|1|micron|sp=us}} in size, although some were smaller or larger.{{sfn|Rothman|1962|p=1}} The uranium was in the form of oralloy: uranium enriched to 93.2 percent [[uranium-235]].{{sfn|Goldberg|1962|pp=2–3}}

The tubes had a hexagonal cross-section measuring {{convert|0.297|in|order=flip|sp=us}} from one flat side to the opposite, with a 7.5-millimeter diameter hole in the center.{{sfn|Walter|1962|pp=7–8}} They were closely packed to form a honeycomb pattern.{{sfn|Walter|1962|p=1}} The metal tie rods were made of [[René 41]] and [[Hastelloy]] R235 and were cooled so they did not exceed {{convert|1400|F|C|order=flip}}.{{sfn|Goldberg|1962|p=3}} The ceramic tubes surrounding the tie rods (known as guard tubes) were unfueled and had smaller {{convert|0.130|in|mm|order=flip|adj=on|sp=us}} diameter holes.{{sfn|Walter|1962|p=6}} The core was surrounded by [[neutron reflector]]s on all sides.The forward reflector was {{convert|9.7|in|order=flip|sp=us}} thick and the aft reflector {{convert|2.4|in|order=flip|sp=us}} thick. Both were composed of BeO tubes. The side reflector consisted of {{convert|2|in|order=flip|sp=us}} of BeO tubes around which was {{convert|1|in|order=flip|sp=us}} of [[nickel]] [[shim (spacer)|shim]]s.{{sfn|Walter|1962|p=5}} The reactor was controlled through the movement of [[hafnium]] [[control rod]]s that moved axially within the tie rods. Twelve of the rods, known as shim rods, were located about {{convert|9|in|order=flip|sp=us}} from the central axis of the core, while two were located closer to the reflector; one was a [[Vernier throttle|vernier]] rod and the other as a safety rod. Normally the movement of the rods was restricted to {{convert|3|in/s|order=flip|sp=us}} but in the event of a [[scram]] they could be moved in 1.5 seconds. The shim rods were moved by four [[actuator]]s, each of which handled three shim rods.{{sfn|Walter|1962|p=6}} The shim rods were {{convert|63.25|in|order=flip|sp=us}} long and {{convert|1.0|in|order=flip|sp=us}} in diameter, with a {{convert|40|in|cm|order=flip|sp=us|adj=on}} travel.{{sfn|Walter|1962|p=17}}

The contract to manufacture the fuel elements was awarded to the [[Coors Porcelain Company]].<ref name="fact sheet" /> The process of making horseradish involved mixing [[wikt:sinterable|sinterable]] BeO powder with oralloy [[uranyl nitrate]], [[yttrium(III) nitrate|yttrium nitrate]] and [[zirconium nitrate]] to form a [[slurry]] which was [[coprecipitated]] by adding [[ammonium nitrate]].{{sfn|Rothman|1962|pp=3–5}} Because the process involved oralloy, criticality safety required a long, narrow geometry for the mix tanks. The mixture was filtered, dried and [[calcined]] at {{convert|1000|F|C|order=flip}}. It was then blended with a binding mixture containing [[polyvinyl alcohol]], [[methyl cellulose]] and water and [[extruded]] through a [[die (manufacturing)|die]] at {{convert|8000|to|10,000|psi|order=flip|sp=us}} to form the tubes. The tubes were dried, the binder was burned out by heating to {{convert|1500|F|C|order=flip}}, and they were [[pottery#firing|fired]] in [[hydrogen]] at {{convert|1700|C|F}} to densify them.{{sfn|Rothman|1962|pp=3–5}}{{sfn|Sandholtz|1965|pp=4–9}} The maximum permissible effect on reactivity due to impurities in the tubes was 2 to 3 percent. In practice it was only 0.5 percent.{{sfn|Walter|1964|pp=15–16}}