Nugen Reactor
From the Third Age and past the Sixth Age, the NuGen Reactor was the preferred method of power generation throughout the Foundation. The name is short for "Nuclear Generation", but is often described as "New Generation" as they differ heavily from traditional reactor designs.
NuGen cores come in two types, called fixed and variable. Both work on similar principles. The underlying principle of the NuGen is variable decay: one material breaks down into another, which changes the interactions. The exact mix of fuels is a trade secret, but the function is as follows: the reactor core will continously generate heat for a known time period, at which point it will "go cold" and become inert.
NuGens are not "reactors" the traditional sense, in that the reaction(in a fixed core) cannot be externally changed. A fixed-core design will work for a set time frame(usually five years) before becoming inert. Variable-output reactors, which are not as common, have a number of "plugs"(not to be confused with control rodes) which can be inserted to increase pressure in the core, increasing heat output. Generally, ships bring additional steam turbines online, rather than increasing heat output, as doing so reduces the lifespan of the core.
Nugen cores are only marginally radioactive, with the lions share of the radiation being used up by the complex reactions going on inside. What little is not used is mostly contained by the cladding. Since coolant water never comes in contact with the fissile materials, once it cools it is technically safe to drink(though not advised, as it is highly purified and dangerous in it's own right).
Safety
A Nugen Core itself presents only marginal radiation concerns, and minimal shielding is required to protect personnel. Since the coolant does not come in to direct contact with any fissile materials, no protective gear is needed. Coolant is marginally radioactive but decays very quickly, heat is of greater concern than hard radiation. This changes in the event of a core breach, so radiation equipment is kept on standby and radiation protocol training is part of reactor operations.
Core Breach
The cladding on a nugen reactor core is comprised of zircon-alloy materials and is capable of withstanding enormous temperatures. The reactor's heat output is such that it cannot actually melt the cladding, except under extreme circumstances. Even in such an event, there is no risk of a complete "meltdown" of the core, as they are engineered without a high excess of reactivity. In the worst of all circumstances, a Nugen core is not likely to get much "hotter" than it already is(this applies to variable-output reactors as well).
The primary concern in a breach scenario is that the coolant water will become highly contaminated. Most smaller ships powered by Nugen Cores use only a single heat loop. That is, the same water that touches the core is also run through the turbines. If this water becomes contaminated, the whole steam system is affected, and any breach in the coolant will rapidly contaminate the entire ship.
Core breaches are rare. Since the reactor cannot be scrammed, the first safety measure is to "purge" the system, or to vent all coolant into space. With the reactor core in a vacuum, heat dispersion will be greatly reduced, and the crew can then assess the damage. Small cracks in the cladding can be patched, but larger breaks may nessestate blanketing the entire core in a hgih-temperature ressen to prevent the further spread of radiation. All Nugen systems have the resen injectors built in for such a contingnecy, but doing so effectively destroys the generator. If the breach contaminated the turbine systems, the entire ship is usually considered beyond economical salvage.
Utopia Gregaria actually maintains a special orbital storage area for ships with core breaches. Many of the vessels are in otherwise good condition, and are occasionally picked over for salvage. Some are badly contaminated, others are simply to contaminated for the entire ship to be cut up for scrap easily. In most cases, the ships are simply being stored until the radiation decays enough that they can be scrapped. Occasionally, a very badly contaminated ship may be scuttled in a gas giant.
In emergency situations, a ship with only a small crack in the core cladding may simply continue to operate the reactor. This is generally only considered under battle conditions, where a loss of main power would mean certain death, while the risk of radiation exposure only means probably death. In such circumstances, the ship is usually not salvagable after, and continued use may lead to a larger breach.
In small-scale breaches the danger to the crew is minimal, as only small amounts of fissile material leak out of the core. In full breach scenarios, the danger is considered extreme, and may call for the crew to immediately abandon ship.
Reactor Operations
Since a Nugen reactor is always "on", the system has to be made inert or rendered "safe" by draining the coolant and taking steps to ensure that dangerous amounts of heat do not dissipate through the ship and cause damage to other systems. For short-term work, such as a system re-fit, the core is isolated and exposed to the vacuum of space. The layer of hard-vacuum around the core effectively insulates it from the rest of the ship, as vacuum is a very poor heat conductor.
In the case of longer-term shutdown, inert gas fills the chamber, with a passive cooling system to vent heat out into space. These systems require routine checks, but no little maintenance is required.
Cooling Systems
The reactor core is cooled water. Nuetron moderation is handled internally to the core. On smaller ships, the reactor is a direct boiling-water reactor with a single heat loop. That is, the water surrounding the core is allowed to boil into steam. Larger vessels use a heat loop where one amount of coolant, kept at high pressure, surrounds the core, then transfers heat to another which boils. This method is more efficient for
Errata
Part of The Furthest Star states that the Nugen Reactor is chemical. Since this story has never been published, that is getting changed.
