Old Reactor Mechanics and Components

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This page goes into detail the Reactor Mechanics and Components of old IC2 (pre-experimental and/or before the 1.106 update).

Reactor Terms

Here are some of the terms often used when describing a reactor and its design.

Heat - The reactor itself and its components can all store heat. If heat levels gets too high, then components will melt, and there will be a risk of a reactor meltdown (see "Violent explosion").
Cooling - Cooling is provided by internal components like a Cooling Cell and the outside environment like water. Cooling is needed to counteract the effects of heat, preventing a meltdown.
Reactor Tick - A reactor 'ticks' once every second. This is when heat, EU generation, and cooling is calculated. (Not to be confused with EU ticks which is a completely different measurement)
Uranium Pulse - Pulses occur every reactor tick, each pulse produces heat and EU. Uranium Cells normally pulse once per tick, but will produce one 'bonus' pulse per neighboring cell (This does not cause the cell to deplete faster, essentially generating bonus energy, but also extra heat).
Breeder Reactor - A type of reactor design that re-enriches Depleted Isotope Cells into full Uranium Cells, but produces little power.
Reactor Design - The pattern in which components are placed within a reactor. A good design can give you nice, safe energy, and a bad design can spontaneously crater-ize your home and its contents (see "Violent explosion").
Full Cycle - The time it takes for a full Uranium Cell to be used up. 10,000 reactor ticks, or 2 hours 47 mins.
Cooldown Period - The time required for an inactive reactor to cool all the excess heat it has collected.
Reactor Hull - This is where heat goes when it's not stored in a component. The maximum heat storage is 10,000, but it can be increased with Reactor Chambers and Integrated Reactor Plating.
Reactor Class - All reactor designs can be a class like "Mark-I-O ED" or "Mark-III EB" which gives an indication of how well a design will perform.
Reactor Efficiency - The average number of pulses per Uranium Cell. (efficiency = pulses / cells)
The more Uranium Cells that are placed next to each other, the higher the efficiency, but also the higher the risk.
Violent explosion - Also known as "reactor meltdown" or "BOOM". What happens if a reactor overheats over it's limit due to overheating. Leaves behind a crater worth being given a name. Note that serious nuclear engineers always place signs next to such craters to remember how it was created. Like "First reactor setup that was almost working as intended" or "Probably should add explosion resistant walls to the next setup".

Reactor Components

A list of the various components that can be used within a reactor.

Main components

Grid Reactor Chamber.png Reactor Chamber
Placed adjacent to the reactor block, each additional chamber provides 6 additional cell slots, +1000 hull strength, and 2 cooling per reactor tick. Note that as a reactor only uses water blocks for cooling in the 3x3x3 grid around the central reactor, adding a Reactor Chamber will only add a total of +1 cooling to a water cooled reactor, as it occupies one of the possible water cooling spots.
Cables can be connected to a Reactor Chamber to transmit the power generated by the reactor setup. A Redstone signal applied directly to a Reactor Chamber (such as a redstone wire passing over/into the chamber, or a pulled lever supported by the chamber) will conduct through the chamber to the reactor itself, shutting it down.

Uranium components

Grid Uranium Cell.png Uranium Cell
The main fuel for the reactor. Every reactor tick, each cell produces a single generating pulse on its own and an additional generating pulse for each adjacent Uranium Cell, causing a "chain reaction" the more Uranium Cells are placed right next to each other and therefore increasing the efficiency of each single cell. Each such pulse produces a specific amount of heat (depending on the surrounding components) and 200 EU (10EU/t) as of 1.3.2 - Uranium Cells are reduced to 5EU/t. A cell lasts 10,000 reactor ticks (2 hours 47 minutes), and generates 2,000,000 EU total multiplied by the amount of other cells constantly around it, thus up to 10,000,000 EU can be generated from a single cell constantly surrounded by 4 others. When depleted, Uranium Cells may become Near-Depleted Uranium Cells or be lost.
Grid Dual Uranium Cell.png Dual Uranium Cell
As it says on the tin. 1.3.2 brings with it new, staged-upgrades for Nuclear Power. Along with it comes Dual and Quad Uranium Cells. These act as if they're 2 or 4 cells in a Grid pattern. 1 Dual cell acts as 2x Uranium Cells are side-by-side. The output from a single slot used is 20EU/t because of 5EU/t per cell, as well as the +5EU/t per cell adjacent to another.
Grid Quad Uranium Cell.png Quad Uranium Cell
4 Uranium Cells forged together to only take one space in the reactor slot; Output for these are 60EU/t. These new Uranium components allow for much more compact Nuclear Reactor designs - However you still have to manage that heat produced.

Grid Near-Depleted Uranium Cell.png Near-Depleted Uranium Cell
The 'empty' state of a Uranium Cell; these can be crafted manually, or have a chance of appearing when a Uranium Cell is depleted within a reactor. They produce 1 heat each reactor tick, but do not generate any EU.
Grid Depleted Isotope Cell.png Depleted Isotope Cell
A depleted Uranium Cell mixed with coal dust. When placed next to a Uranium Cell inside a reactor it recharges into a full cell after some time. The time it takes to recharge a depleted cell depends on the amount of surrounding Uranium Cells and the heat the reactor is operating at. The higher the reactor's temperature is, the faster the cells will replenish. Isotope cells produce 1 heat but do not generate any EU, the will however cause adjacent Uranium Cells to pulse an additional time. This additional pulse is used up to re-enrich the Depleted Isotope Cell and will therefore not generate any EU, but it will cause the pulsing Uranium Cell to generate additional heat as usual.
Grid Re-Enriched Uranium Cell.png Re-Enriched Uranium Cell
The fully charged state of an isotope cell, it will continue to produce only 1 heat and no EU but it will no longer react with adjacent Uranium Cells. Combined with another coal dust, it will become a brand new Uranium Cell.

Before starting a complete breakdown on how each cooling component works, here is an explanation of terms:

maxHeat - The maximum heat that can be stored before the component will melt.
selfCooling rate - How much heat a component can dissipate per tick.
reactorTransfer rate - How much heat the vent can receive from the reactor hull per tick.

Cooling components

Grid Cooling Cell.png Cooling Cell
Each cooling cell may absorb 10,000 heat before melting and will cool down itself by 1 point of heat each reactor tick. Unless overheated Cooling Cells are not used up, which makes them the main cooling component inside the reactor.
Grid Integrated Reactor Plating.png Integrated Reactor Plating
Plating will distribute heat from an adjacent Uranium Cells into surrounding cooling components, which allows to increase the amount of cooling that can be applied directly to an Uranium Cell. Heat distributed in this way can only travel a distance of one slot, possible further platings will only store the heat but not distribute it. Plating also increases the reactor's hull strength by 100 points and can store up to 10,000 heat itself before it melts, if it is unable to direct it into any other cooling component. Integrated Reactor Platings will cool down itself by 0.1 point of heat each reactor tick.
Grid Integrated Heat Disperser.png Integrated Heat Disperser
These components will attempt to balance out the levels of heat within the reactor hull, any adjacent component capable of storing heat and itself. During each tick a disperser can exchange up to 25 heat with the reactor hull and up to 6 heat with each of the surrounding components. Note that if all Uranium Cells in a reactor are next to at least one cooling component and therefore no longer emit heat to the reactor itself, an Integrated Heat Dispenser is required to utilize the reactor's own cooling. In addition, as this component is able to exchange heat with the reactor itself, it can as well be used to "move" heat from one place to another inside the reactor (using the reactor's own heat storage as a medium), if for example the space around an Uranium Cell is not sufficient to completely cool the heat it emits.

One time items

No longer work with 1.3.2

Grid Water Bucket.png Water Bucket
When a reactor's hull has more than 4000 heat, it will evaporate the water inside the bucket, reducing the heat instantly by 250 points and leaving the empty bucket in the slot. This can be used to manually adjust the reactor's temperature or as a hint that the reactor is overheating.
Ice Block
If a reactor's hull has more that 300 heat, it will evaporate ice blocks each tick, reducing the heat instantly by 300 points per block evaporated until it is below the limit of 300 heat. Note that only one ice block per reactor slot is used up per tick, therefore the total amount of heat that can be reduced per tick is limited by the slots occupied by Ice Blocks. Thus even if a reactor is filled up with stacks of ice, it still can blow up violently. Ice blocks can be used to manually cool down a reactor, as a short-time cool-down system in extreme reactor setups, or as an indicator that the reactor is building up heat.
Grid Lava Bucket.png Lava Bucket
Placing a lava bucket inside a reactor will instantly increase the reactor's hull heat by 2,000 points, leaving the empty bucket in the slot. This is useful for 'breeder' type reactors to heat-up the reactor to increase the speed at which Depleted Isotope Cell recharge.

Heating and Cooling

Almost every component and the reactor itself can store heat in an effort to stave off a disaster. It is up to the cooling systems (and you) to get rid of this accumulated heat before the reactor cannot take any more.

The reactor's own storage (known as the reactor hull) starts off at 10,000, but that can be increased by up to 6 extra chambers (+1000 each) or placing plating into the reactor (+100). If the reactor hull reaches 50% of its maximum heat storage, then nearby water will begin to evaporate, and at 85% the reactor has a chance of removing itself from existence... violently.

Heat stored in components will be safely tucked away from the hull, but it will need time for the cooling systems to quench it all.

The most common source of heat is uranium cells, which will produce heat for each pulse they perform. The amount of heat depends on how many cooling components (Cooling Cell, Integrated Reactor Plating, Integrated Heat Disperser) are adjacent to the cell:

No. of Components Heat Generated
0 10 per pulse into the reactor hull
1 10 per pulse into component
2 8 per pulse, 4 for each component
3 6 per pulse, 2 for each component
4 4 per pulse, 1 for each component

Pro Tip and Warning: Remember, though putting uranium cells next to each other doubles the energy output, it doubles the heat as well, meaning that two uranium cells next to each other will generate 12 heat each with the max possible 3 components around each.


H = Heat per Uranium cell
U = Number of adjacent Uranium cells
C = Number of adjacent Components
H = (U + 1) * (10 - (C - 1) * 2)

More cooling systems around a uranium cell mean less overall heat to deal with, making the reactor safer, but it also reduces the potential amount of EU a cell can produce. Risk vs. Reward.

Depleted Isotope cells only produce 1 heat per tick themselves, but they still react with adjacent Uranium Cells and make them pulse additional times.

Near-Depleted Uranium Cells and Re-Enriched Uranium Cells produce 1 heat per tick.

There are several ways to reduce a reactor's heat each tick:

Outside Source Cooling provided
The reactor itself 1 heat
Each reactor chamber added 2 heat per chamber
Water blocks within a 3x3x3 area* 1 heat per block
Air 'blocks' within a 3x3x3 area** 0.25 per block

* Both still and flowing water count.

** Torches, Redstone, and similar items won't count.

Internal Source Cooling provided
Cooling Cell 1 heat***
Integrated Reactor Plating 0.1 heat***
Ice blocks (single use) 300 heat per block
Water Buckets (single use) 250 heat per bucket

*** Cooling only occurs if the component in question has any heat stored.

The maximum outside cooling possible is 33 (reactor, 6 chambers and 20 water blocks).

If the amount of cooling available is less than the amount of heat produced then the reactor will gradually collect heat. There are various ways to deal with this:

  • Make a design that only has a slight amount of excess heat so that even when the Uranium Cells are used up the heat levels are still not dangerous.
  • Manually drop Ice blocks and/or Water buckets into the reactor.
  • Apply Redstone current to the reactor (or one of its chambers), causing it to stop generating heat and EU for as long as the Redstone current is active.
    • An overheating reactor can be detected by placing redstone dust on top of wood blocks close to the reactor and sending a redstone signal through it; when the reactor heats up past 40%, the wood will burn, breaking the circuit. If this circuit's output is inverted and routed back to the reactor (not on wood blocks, to ensure the segment after the inverter is not destroyed!), it can serve as an automated shutdown / alarm circuit.

Heat management for a 'Breeder' type reactor is different. Breeders work best when running hot, so it's best to make a design that has exactly the same heat and cooling amount, then manually boost the heat by adding lava buckets, removing cooling, or temporarily adding extra uranium cells.

Reactors will emit smoke particles when warm and fire particles when hot. Be careful when using lava buckets, as the 2000 heat goes directly into the hull, and the heat dispersers need time to pull it into the cooling systems.

Heat regulation

If you want your breeder reactor to operate efficiently (that means at high temperature), balancing heat can be very tricky. Fortunately Heating Cells are here to help. A stack of three of these components in your reactor will give three heat to each surrounding component if that component has less than 3000 heat. Five of them will give 5 heat to each adjacent component up to a maximum of 5000 heat, melting almost any component. This allows you to maintain a stable high temperature if used properly.

Click here for a comprehensive breeder reactor setup guide.

Environmental Effects of Reactor Heat

As reactors heat up, they will start having detrimental effects on their immediate surroundings. Each additional chamber increases the threshold by 1000 heat, to a maximum of +6000 with 6 chambers. Each piece of hull plating futher increases the heat threshold by 100.

The exact heat effects for reactors are:

 % of max hull heat Environmental effect
40% Flammable blocks within a 5x5x5 cube have a chance of burning.
50% Water blocks within a 5x5x5 cube (both sources and flowing) will have a chance of evaporating.
70% Entities within a 7x7x7 cube (instead of a 3x3x3 cube) will get hurt from the radiation exposure.
85% Blocks within a 5x5x5 cube have a chance of burning or turning into lava ('moving' lava only, no source blocks).
100% What environment? That hole in the ground?

Reactor Classification

All reactor designs fall into a set of pre-defined categories. This makes it easier to see, at a glance, how effective a design can be when either looking up designs on the IC forums or posting a design yourself.

Mark I

Mark I reactors generate no excess heat each reactor tick and thus are safe to use continuously for as long as you supply Uranium. Mark Is tend have a low efficiency, but that's the price of a completely safe reactor.

Mark Is have two sub-classes: Mark I-I for design that do not rely in outside cooling in anyway and Mark I-O for those that do.

Mark II

Mark II designs produce a small amount of excess heat and will need to be given a cool down period eventually to prevent the hull reaching 85% maximum heat or melting component. A Mark II must complete at least one full cycle before encountering heat problems.

The sub-class for Mark IIs denote how many cycles the design can run before reaching critical heat levels. For example Mark II-3 will need a cool down period after running 3 cycles in a row. Mark II s that can run 16 times or more get the special sub-class 'E' (Mark II-E) for almost being a Mark I.

Mark III

Mark III reactors tend to have an emphasis on efficiency at the cost of safety. Mark IIIs are unable to complete a full cycle without going into meltdown and thus need to be shutdown mid-cycle in order to deal with the high amount of excess heat. This can be done manually or by using Redstone.

Mark IIIs have the additional condition that they must run at least 10% of a cycle (16 mins 40 secs) before reaching critical heat or losing any components.

Mark IV

Mark IVs still have to run at least 10% of a cycle, just like Mark IIIs. The difference being that Mark IVs are allowed to lose components to overheating, and that must be replaced before the reactor goes critical.

Mark V

Mark Vs are for those who want to squeeze every last scrap of EU from their uranium cells; they cannot run long without needing a cool down period. You'd better have great Redstone timer skills, or you'll never be able to turn your back on these things.

Additional Suffixes

As well as being Mark I to V, reactor designs also have one or more suffixes to better inform people about their performance.

Single Use Coolants
A reactor that relies on a supply of ice and/or water buckets in order to maintain its classification should be suffixed with '-SUC'.
To calculate efficiency, take the number of uranium pulses a design makes per tick and divide it by the number of uranium cells it possesses.
The number provided will show the efficiency rating a design has:
Number Rating
Exactly 1 EE
Greater than 1 but less than 2 ED
2 or greater but less than 3 EC
3 or greater but less than 4 EB
4 or greater EA
This suffix is for designs that also recharges isotope cells. Isotope cells charge up faster when the reactor runs hot, so heat management is important. There are three breeder types:
  • Negative-Breeders slowly lose heat over time and will need heat to be added manually, or they can be left for a safe slow way to recharge isotopes.
  • Equal-Breeders have exactly the same heat generation as they do cooling ability and usually only require a user to boost the reactor's heat level manually at the beginning.
  • Positive-Breeders gain heat over time and will require more precise cool down management for the reactor to remain hot.
Reactors whose sole purpose is to recharge cells may not even have a 'Mark' classification and are simply called Breeders instead, with the efficiency/SUC suffix added.
Heat Ticks Required
0-2,999 40,000
3,000-5,999 20,000
6,000-8,999 10,000
OVER 9000!!! 5,000

Example Classifications

Mark I-O EE
A reactor design that can run continuously, but relies on outside cooling and only produces one pulse for each Uranium Cell.
Mark II-1 ED Positive-Breeder
A reactor with recharging capabilities that can only run one full cycle before needing a cool down.
Mark II-2 EC
A reactor design that can run two full cycles before needing a cool down period, producing at least 2 pulses per Uranium per reactor tick on average.
A reactor that can run at least 16 times before needing a cool down, relying on a supply of ice or water and has average efficiency.
Breeder EA
A heat-neutral reactor designed for the sole purpose of recharging Isotope Cells. Each Uranium Cell is capable of charging 3 or more Isotope Cells.

Reactor Security

As much as nuclear reactors are excellent terraforming devices - if you want to create huge craters - their main use is to create power for your other devices and machines. In order to avoid a sudden violent explosion, you need to take precaution and plan your reactor setup accordingly. In general there are two ways to ensure that a reactor does not unexpectedly react with its surrounding environment: you can either design a stable reactor that never overheats or, if you need more power, you can at least put the reactor into layers of material that - in the unlikely event of a nuclear meltdown - absorb the blast to keep the environment intact.

Stable reactors

The most simplest stable reactor is to put in a single Uranium Cell and surround it with adjacent 4 coolant cells. The Uranium Cell will now produce 4 heat each tick, which will be completely absorbed and cooled down by the coolant cells. This setup can never explode, no matter how many of these sets you manage to squeeze into your reactor chamber, it will always run safely.

This set however only produces 10 EU/t per Uranium Cell, which is very inefficient. If you would put two Uranium Cells adjacent to each other a chain reaction starts, and suddenly each cell produces two times their previous value, for a total of 40 EU/t. However those cells as well double their heat output. So if you still only surround them with Coolant Cells you would not only be restricted to 3 cells for each Uranium Cell (as the 4th spot is taken by the other Uranium Cell), but each of those Coolant Cells would now receive 2 heat per reactor tick, but can only cool down by 1 point per tick. Given some time (actually lots of time) those cooling cells would melt down, the heat emitted by each Uranium Cell increases as it lacks surrounding cooling components, till eventually they emit their combined 40 heat per tick into the reactor hull, which cannot dissipate all the heat on its own, and finally causes a massive explosion.

Adding more cooling components to the setup and using a smart layout of cooling cells, heat dispersers and platings, it is possible to run a chain reaction of 3 Uranium Cells while dissipating all the heat created by this chain reaction using only a reactor with two chambers, thus creating a Mark I (stable) reactor setup with an output 70 EU/t for a total of 14,000,000 EU over the life-time of those three Uranium Cells. It is recommended to insert ice blocks into the reactor chamber while experimenting, which will vaporize as soon as the reactor exceeds 300 heat, indicating that the setup is not stable.

It is not possible, no matter how smart the setup is, to run a stable chain reaction of 4 or more Uranium Cells, as even for the setup with the least heat emitted, you still would need more Coolant Cells than you can fit into the reactor chamber. However, if you have the buildcraft mod you can transport ice and water buckets to the reactor, thus cooling it down. It MAY be possible to use a bulidcraft pump and water proof pipes to pump water into the reactor WITHOUT the use of buckets, but it is not known if that will work. See the buildcraft wiki if you don't understand what I am talking about.

Reducing the Explosion Radius

If you require more than 70 EU/t from your reactor and you are not willing to use up the materials or the space to build a second reactor, you can put in more Uranium Cells into the reactor chamber than it can cool down for some time, to fulfill your need of EU. While extreme setups are capable of outputting over 2000 EU/t they require perfect timing to shut the reactor down just right before it explodes. During experimentation with such setups it is strongly recommended to take security precautions to reduce the explosion radius of an almost perfectly working setup.

In the unlikely event of a nuclear meltdown, the reactor will explode violently and force its internal pressure right into it's surrounding environment. Read: it will destroy everything in a large radius till the entire force of the explosion is used up. To avoid the destruction of your house or other valuable assets, you need to give the possible explosion material to react with that is stable enough to use up at least a good portion of the explosion force. You need Reinforced Stone.

While the reactor will turn everything into molten clumps of lava in a 5x5x5 cube centered around the central reactor given enough time and heat, everything beyond that will survive to absorb the explosion blast. Surrounding the reactor completely with a layer Reinforced Stone will reduce the explosion from a massive crater to a mere hole. Adding a second layer will reduce the explosion to a slight bump into the adjacent material and adding a third layer will guarantee that no matter how violent the explosion might be, only the Reinforced Stone will be consumed by it.

Now you might ask how to transport the generated energy into your storage if you completely surround the reactor with Reinforced Stone or how to apply a redstone signal to shut it down. Smart question! And the answer is: directed force. It is inevitable to have at least some holes in your secure layers, but the good news is, the explosion does not simply destroy a sphere around the reactor, the force that escapes the construction will keep its direction of travel. So if you attach your cables at the top of the reactor, the explosion's force escaping the shelter will only destroy things above the reactor, which under optimal conditions is just air. In the same way a redstone torch from below can be used to signal the reactor to run or shut down, which conveniently can be transmitted through one solid block per torch.

Building a proper shelter around your reactor allows you to experiment with nuclear energy and risk only the reactor itself, some reinforced stone blocks, and your life.

Breeder reactors

Obsolete: This information no longer applies to the current version of IC².

To get the most out of your ore, you'll want to produce a uranium cell from a Depleted Isotope Cell rather than directly from a Uranium Fuel Ingot . Not only can you craft eight depleted cells from one fuel ingot, but you will also receive a Near-Depleted Uranium Cell 25% of the time when a uranium cell is used up. This can be easily crafted into a depleted isotope cell.

However, creating uranium cells from depleted uranium cells requires a breeder reactor. A safe (low temperature) breeder reactor will change a Depleted Isotope Cell into a Re-Enriched Uranium Cell after it receives 10 000 neutron pulses. However, every time a uranium cell sends a pulse into an isotope cell, it generates heat as if the uranium cell was producing 5 EU/t additional energy. However, no additional energy is generated. Breeder reactors tend to generate a large amount of heat and little energy, but a lot of re-enriched uranium cells, which are easily crafted into uranium cells you can use to generate lots of energy.

It is possible to make your breeder much more efficient by operating it at higher temperature. At 3001 heat in the reactor, each neutron pulse will recharge isotope cells twice as quickly as at 0 heat (or 2999 heat). 6001 heat will operate three times more efficiently than 0 heat, and so on.

reactor heat relative recharge rate
0 - 2999 1
3001 - 5999 2
6001 - 8999 3
9001 - 11999 4

There is no hard limit to the efficiency of your breeder. The higher the operating temperature, the faster it will produce new cells. But be careful -- at higher reactor temperatures bad things begin to happen. Things that typically end with a crater where your base used to be.

Tips and Tricks

Here are a few tips and things to look out for when using a reactor.

  • There is a Thermometer add-on on the IC forum that can be useful for those who want to monitor heat levels closely.
  • Redstone timers can turn even Mark Vs into self regulating-reactors, but if you're not the Redstone equivalent of "The One", then you might want to make use of a Redstone enhancing mod, like RedPower, for example.
  • Heat Dispersers can only draw 6 heat out of a component per reactor tick, so look for components that seem to be holding more heat than the others and try to fix the problem.
  • While the reactor hull's maximum heat tolerance can be increased, all other components are fixed at 10,000. So even though a reactor can survive 10,000+ heat, Heat Dispersers will still pull that heat into components and melt them all.
  • A math trick to calculate the number of pulses (P) for complex (i.e. Mark IV or Mark V) reactors is to multiply the number of uranium cells (U) by 5 and subtract the number of sides of uranium cells not touching other uranium cells (S) so P = 5 * U - S. So a Mark V reactor (9x6 uranium cells) would make 5 * (9 * 6) - 2 * (9 + 6) pulses = 240 pulses (and 2400 EU/t which would explode even HV cables and HV transformers, and 2400 heat per second which would explode the reactor in less than a minute (i tested it yesterday).
  • It is possible to use buildcraft with a nuclear reactor by connecting pipe to either the main reactor block or its chambers. This can be used to pump ice into the reactor from a chest continuously. Ice can be produced with pumps over infinite water sources adjacent to compressors (creating snowballs) feeding into another compressor which turns snowballs to ice, but it requires about 1400 EU per ice block, and may not be practical.
  • As an additional precaution, consider adding an automatic cutoff switch. If the redstone current which turns the reactor on is wired over or through a flammable block near the reactor, then if the reactor gets hot enough to burn nearby blocks the reactor will probably turn off, which may stop the meltdown.