color, texture, light, line, shape, function
Do want a calm, restful room or an exciting, fun room? Do you want it to be formal or casual? What activities do you want to do in the room? It should compliment the other rooms, so that there is a natural flow from one room to another. This can be achieved by using a similar color or material and similar style as the other rooms in the house.

There are many different types of nuclear reactors, and each one has its own safety and control methods. Take a look at Wikipedia's Nuclear Reactor page.

You mention Chernobyl's reactor number 4. It was a graphite moderated thermal reactor, of the same basic design as had been used on the K-19, only larger (an RBMK). Graphite moderated reactors are the least "idiot proof" of all the nuclear reactor designs. Unfortunately, Chernobyl was in the Soviet Union. But you can rest easy(?), there are only 3 sites currently operating 11 RBMK reactors in Russia. Graphite moderated reactors were never used as power reactors outside of the USSR. However, the very first man-made reactor was graphite moderated.

After the Chernobyl accident, the RBMK design was modified with 5 additional safety features.
1) The fuel was enriched further to reduce reliance on cooling water as a moderator.
2) More manual control rods were added.
3) More neutron absorbers were added to prevent accidental operation at low-power.
4) The SCRAM was reduced from 18 to 12 seconds.
5) Precautions were put in place to prevent safety systems from being bypassed (which is what happened at Chernobyl).

In the US and other countries, light water moderated thermal reactors are commonly used in power plants. Particularly common are the pressurized water and boiling water types. In these reactors, the water is necessary as a moderator. In fact, the reactor won't operate if the water is removed (boiled out). Therefore, these reactors have a built-in negative feedback. If the reactor power increases too high, it will heat up the moderator, which will lower the density of the water and reduce its moderating efficiency, resulting in decreased power.

Common safety features of a water reactor are:
1a) Independently movable absorber rods which can be quickly and automatically inserted, even in the even of a power failure.
1b) In the event of a real emergency, liquid neutron absorbers can be injected into the reactor.
2) SCRAM times of 4 seconds or less.
3) They are loaded with less fuel than is necessary for criticality. Even if they lost all coolant, the reactor vessel would be hot and damaged, but not a meltdown. Putting the reactor vessel in a pool helps to avoid the "hot and damaged".
4) Bunker-like containment buildings to contain most of the radiation released in a catastrophe.
5) Lots of indicators and sensors so that the operators know what's happening.

The Three Mile Island accident is perhaps the best example of how safe these reactors are. Even though it lost coolant and had a partial "meltdown", the reactor was recovered, and the radiation was contained. Contrast that with Chernobyl, which had a full meltdown, and a hydrogen explosion and fire which destroyed the reactor building.