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).
- 1 Safety
- 2 Cooling Systems
- 3 Variable Core Design
- 4 External Radiation Concerns
- 5 Core Installation and Replacement
- 6 Operational Lifespan
- 7 Core Production
- 8 Errata
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.
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.
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. If the passive system fails, heat from the core can easily spread. Most areas around the core are made from heat-safe components, but wires and workstation components can melt. In rare cases, materials outside the core can be heated to the point of auto-ignition, so fire-safety is strictly enforced in areas around the core.
The greatest incidence of core breach happens during improperly executed startup routines. The cladding on the core will not melt, but is highly susceptible to heat fracture. The core puts out a constant 932 degrees, so liquid water cannot be introduced directly. Instead, the start-up routine involves pumping high-pressure steam into the system until it reaches sufficiently levels that the water can then liquefy.
A "from vacuum" start-up can take days, and up to a week on larger ships, and requires a large amount of power. An emergency start-up procedure, called "re-profusing the system" exists that involves adding liquid water elsewhere in the heat loop and using the vacuum of space to boil it. Once some water vapor is in the system, more liquid water is added, and the core can transfer heat energy to this through the existing steam. However, if any liquid water comes into contact with the core, it can weaken or even breach the cladding.
Breaches on shutdown are extremely rare, as the standard shutdown procedure involves venting the core coolant into space. The sudden vacuum conducts very little heat, so the change in temperature is minimal. However, in the event of a coolant leak, this can allow cool air from the ship to rush past the core. In the event that the system is damaged, the standard procedure is to evacuate the reactor block and vent the entire section.
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 extracting maximum power from a larger core.
The reactor cooling system is also often linked to the plasma cooling system. That is, waste heat from the engines is introduced to the reactor so that some power can be generated from it. In the event of a core isolation event, this system can sometimes transfer enough heat to provide electrical power to the ship. It is also not uncommon to use the ship's engines to heat the core coolant during a restart procedure.
Variable Core Design
Nugen Cores come in two varieties: fixed and variable. A fixed core is sealed in cladding and has only one power output level. A variable core has a number of large gaps in the cladding, filled by large zircons. Mechanical pressure is applied to these, with increase the pressure at the reactor's core. This can increase the reactor's output by about 30%.
In a variable core, small amounts of fissile material leak through the gaps in the cladding, and so the core coolant becomes radioactive. A primary, isolated heat loop then becomes a requirement, and radiation protocols in the reactor block must be strictly observed. Very large vessels powered by variable cores often have a third heat loop for steam generation, as some radiative crossover between the first and second loops is possible.
Variable cores are at a higher risk of breach and require much more skill to operate. The old adage among the Crimson blade is "You need a plumber to operate a fixy, and a sir to operate a vari." ("fixy" is a colloquialism for a fixed core, "vari" is the same for a variable core, as well as being a double entendre: vari sounds like very as in "they are very complicated". The reference to a "sir" refers to the fact that nuclear-trained reactor technicians are officers, having attended an academy and earned the equivalant of an under-graduate degree in nuclear physics).
Another serious concern for variable reactors is the production of free hydrogen. The reactor cladding is inert at normal operating temperatures of around 932 degrees, and remains so at the higher temperatures achieved by "full compression" of the core. However, at around 1500 degrees, the amaterial begins to react with the water and produce free hyodrogen, the pressure build-up can breach the reactor housing. Venting coolant removes the hydrogen, but there is still a danger to the core itself. Even variable cores are not thought to be at risk for a full-on, catastrophic melt-down in this scenario, but core breaches and other dangerous situations are possible.
External Radiation Concerns
The greatest danger to a NuGen Reactor is external radiation, specifically external neutron bombardment. The core elements are carefully engineered around a precise reaction; the introduction of external neutron sources changes this reaction. This is, of course, especially a problem with warships, which tend to have nuclear weapons fired at them.
For this reason, the reactor core is very heavily shielded. Not to protect the crew from it, but to protect it from external radiation. The core itself is imersed in water already, with a second large, un-pressurized reservoir around it(that is, pressurized to the same air pressure as the ship's interior, not pressurized the same way as the core itself). Reactor operations then spiral out around the core, with steam tubes and cold-water returns used to add additional protection. Finally, thick plates of dense shielding are used.
Reactors are further placed as close to the middle of a ship as possible, so that the hull armor and other interior spaces provide protection.
Every warship and nearly all Nugen operators run active radiation sheilding. This is effectively a shield generator emitting a field within the ships hull, similar to a forcefield, but permeating bulkheads. These are especially important in variable-core designs where escaped fissile material already complicates the radiation scheme.
Transport ships and other non-combat vessels may rely on their external shields, turning them on in high radiation areas. Since they lack thick hull armor, these ships also have additional passive shielding.
Inside sources from the Foundation report that, even under the worst of all possible circumstances, the danger of an core melt-down is still low. If there was a hull breach, if the active shielding failed, and if multiple atomic weapons detonated, it still would not likely introduce enough neutrons to cause a runaway reaction.
External radiation impacts cause two concerns: short term overheating and long-term reactor function. The instroduction of external radiation changes the reaction coefficient within the core, and can increase it. In so doing, the core generates additional heat, which can cause all of the problems noted above. Long term, the major concern is that it can shorten the life of the core. When a Nugen core "goes cold" it happens almost immediately; the fuel burn-up is complete, and there is nothing left besides decay heat, which lasts, at most, a few days. If a core is bombarded by too much radiation, it may give out years early, which can have serious repercussions to a ship.
Variable cores are es[pecially vulnerable, as the use of "high power mode" in such a core also reduces the lifespan, though more predictably. To combat this, the Foundation instituted the "twenty/fifty" rule, which states that fixed cores should be replaced when twenty percent of their operational lifespan has been reached, and variable cores at fifty percent. Since Nugen Cores are entirely recyclable(albeit at high material and labor costs), this is not a serious issue. The numbers also offer a high margin for error, in the even a ship cannot be refit.
Core Installation and Replacement
Nugen Cores are installed as late in the construction process as possible to provide for maximum life span. On smaller ships, the vessel is usually designed to make removing the core as simple as possible. The core cannot be "ejected", and the ship usually has to be significantly dismantled in order to reach the core. This frequnetly involves cutting through external plating and armor, though most ships are designed with strategic passages through the super structure so that nothing important is compromised.
Most ship designs have either a "Core Module" or a "Reactor Block.
The block design is most common in smaller ships; in this case the entire power generator, the core, heat loops, steam turbines, etc, are contained in a single "block" or structure. Usually the condensers, which turn steam back into water, are separate. Cooling water usually involves several stages of heat-exchange which return excess energy to the core.
Comercial vessels usually use a heater exchanger that vents the excess heat into space.
Larger ships running larger cores use a Core Module desgin, where in most of the power generating capability is housed in seperate blocks, while the core itself, as well as the containment vessel and shielding are containted in a discreet element. The Core module can be isolated usually by closing a series of valves. Of course the high-presure pipes must also be cut to remove the module.
Every Nugen Core is designed to have a specific, set operational lifespan, after which it becomes inert. Once this happens, there is no way to reverse the rocess, the core must be replaced. Lifespan is limited by size and dictated by operational usage. In every care, the time span is a minimum of five years and a maximum of fifty.
Large capital ships such as carriers and battlecruisers use fifty-year fixed cores. Carriers, in particular, have very well-defined, predicable energy consumption needs. Smaller battleships can also operate fixed cores, and are preferred; however the inability to produce much additional power on demand makes them disliked by captains. Larger vessels can optionally fit additional steam turbines to produce more power. Fixed cores on larger ships are always fifty-year cores.
Dreadnaughts or often required to use variable cores, which means they must be changed more frequently. It has been concluded that the use of variable core output, even if operated at maximum, can only decrease the cores lifespan by at most 40%. However, additional factors, such as external radiation, can impact core-life, as such cores are replaced every twenty-five years, regardless of usage-history. Captains are discouraged from utilizing core-compression, except in times of war.
Smaller vessels have cor sizes and lengths determined by ship size and operational lifespan. Typically, the larger the ship, the longer the core. Cruisers prefer a twenty to twenty-five year span, destroyers are generally fifteen, firgates and cutters ten. In some special applications, such as fast attack ships, small, high-density, short lifespan cores are constructed, sacrificing longevity for output.
Part of The Furthest Star states that the Nugen Reactor is chemical. Since this story has never been published, that is getting changed.