Organic nuclear reactor
Some organics also have the advantage that they do not flash into gas in the same fashion as water, which may reduce or eliminate the need for a containment building.
These benefits are offset to a degree by the fact that organics also generally have a lower specific heat than water, and thus require higher flow rates to provide the same amount of cooling.
A more significant problem was found in experimental devices; the high-energy neutrons given off as part of the nuclear reactions have much greater energy than the chemical bonds in the coolant, and they break the hydrocarbons apart.
U-235 will undergo fission when struck by neutrons of this energy, so it is possible for U-235 to sustain a chain reaction, as is the case in a nuclear bomb.
However, there is too little U-235 in a mass of natural uranium, and the chance any given neutron will cause fission in these isolated atoms is not high enough to reach criticality.
The main disadvantage of water is that it has a relatively low boiling point, and the efficiency in extracting the energy using a turbine is a function of the operational temperature.
[3] In conventional water-cooled designs, a significant amount of effort is needed to ensure that the materials making up the reactor do not dissolve or corrode into the water.
Many common low-corrosion materials are not suitable for reactor use because they are not strong enough to withstand the high pressures being used, or are too easily weakened by exposure to neutron damage.
This includes the fuel assemblies, which in most water-cooled designs are cast into a ceramic form and clad in zirconium to avoid them dissolving into the coolant.
Greatly reducing corrosion allows the complexity of many of the reactor parts to be simplified, and fuel elements no longer require exotic formulations.
In most examples the fuel was refined uranium metal in pure form with a simple cladding of stainless steel or aluminum.
In this case the organic material is both the coolant and moderator, which places additional design limitations on the layout of the reactor.
However, this is also the simplest solution from a construction and operational point of view, and saw significant development in the US, where the PWR design was already common.
This limits the operational temperature, but is simpler mechanically as it eliminates the need for a separate steam generator and its associated piping and pumps.
Other potential explosion sources in water-cooled designs also include the buildup of hydrogen gas caused when the zirconium cladding heats; lacking such a cladding, or any similar material anywhere in the reactor, the only source of hydrogen gas in an oil-cooled design is from the chemical breakdown of the coolant.
Among these is their relatively low heat transfer capability, roughly half that of water, which requires increased flow rates to remove the same amount of energy.
"[13] It was polymerization of the coolant sticking to the fuel cladding that led to the shutdown of the Piqua reactor after only three years of operation.
[16] Atomic Energy of Canada Limited (AECL) began similar studies around the same time, with an eye to the design of a future test reactor.
[16][17][18] A similar but separate project began in Italy under the direction of the Comitato nazionale per l'energia nucleare, but their PRO design was never built.
This used biphenyl and Santowax (commercial name of an isomeric mixture of terphenyl) for coolant and moderation and operation was generally acceptable.
It began construction at Idaho in 1962, but was never loaded with fuel when the AEC shifted their focus mostly to light water reactors.
It ran only for a short time until 1966, when it was shut down due to films building up on the fuel cladding, formed from radiation degraded coolant.
It operated until 1985, by which time AECL had standardized on using heavy water for both the moderator and the coolant, and the organic cooled design was no longer being considered for development.
