The answer is it depends on who is using the nuclear power. For the sake of discussion, we will define nuclear power as fission created power and not fusion. I’ll save fusion for another article.
Bear with me for a brief description of nuclear fission. Nuclear fission is the absorption of a neutron by an isotope (we’ll use uranium 235 for our discussion) which results in the instability and splitting of the uranium molecule into two new isotopes and the release of additional neutrons. The two new isotopes release a high kinetic energy imparting that energy to the surrounding structure creating heat. The new isotopes are radioactive (which is where nuclear waste comes from). The neutrons (usually two or more) released by fission and absorbed by other molecules creating a chain reaction. Such reactions occur at a fantastic rate. Nuclear bombs react with billions of such fissions virtually instantaneously. However, the controlled chain reactions produce heat relatively slowly; this controlled reaction is what we call nuclear power, that is, the creation of heat using the fission process. Nuclear power is a sophisticated way of boiling water to produce steam, spinning turbine generators, and generating electricity.
Is nuclear power dangerous? As stated above, it depends on who is using it. Playing with fire is dangerous if you don’t know what you are doing or if you have malicious intent. When you build a fire in a forest, you build containment around the fire keeping it from inadvertently spreading. The danger of fire is not only the spreading of the fire, but also releasing smoke that could result in smoke inhalation for anyone near the fire. Using nuclear power is pretty much the same idea, that is, you contain it and control the fuel supply, but in addition, you keep the waste products from escaping. The danger is the radiation from the radioactive isotopes. You may read reports of “releases of radiation”. It is important to understand the meaning of that phrase which is a very general statement. There are two types of radiation release: 1) electromagnetic radiation that passes through structure and irradiates everyone and everything around it; 2) radioactive isotopes escape and contaminate the surrounding area, subsequently irradiating everything with its emitting electromagnetic radiation. With the first type of release, we are concerned mainly with gamma rays, instead of easily shielded particles like neutrons, beta particles (electrons), and alpha particles (helium nuclei). However, radioactive isotopes can be like dust. Released radioactive particles escaping containment may be blown by the wind or escape into water supplies. Once you get the isotopes that emit the neutrons, beta particles, and alpha particles in your body, you are susceptible to cellular damage from the electromagnetic radiation. This is why nuclear power plant operators employ extremely clean standards and containment and treat any spill of potentially radioactive water or filter material as a major event. Note that the water itself is not radioactive, but may contain particles of radioactive isotopes.
Can a nuclear power plant explode like a nuclear bomb? No, with the nuclear fuel loaded dispersed throughout the reactor and therefore not susceptible to rapid chain reactions, a nuclear explosion cannot occur. In addition, other material that competes for the neutrons, poisoning or inhibiting the reactions, surrounds the fuel. The explosion of a nuclear power plant like a nuclear bomb is a physical impossibility. A nuclear power plant can have a steam explosion where the heat in the reactor core builds to a point where the steam over-pressurizes the containment. Interlocks preclude such an explosion, greatly reducing the likelihood of this event.
Can a nuclear power plant melt down? Yes; but it would take another highly unlikely set of circumstances to that end. The design that stops any runaway reactions is with moderators (material that controls the chain reaction), cooling systems (that cool the radioactive decay), and “poisons” that keeps the chain reaction controlled. Water-cooled nuclear reactors have an inherent stability, where the hotter the water becomes, the fewer chain reactions occur, and vice versa. It involves the energy of the neutrons since uranium 235 atoms do not readily absorb neutrons unless they are at the proper energy. Once a reactor shuts down, the fission dissipates but there is still decay heat from the radioactive isotopes created during fission. If the decay heat remains, there could be melting of the reactor core. The coolant system (usually water) continually cools the reactor for several hours or days until the high-energy isotopes decay away.
We have had disasters such as Chernobyl and Three Mile Island; doesn’t that show that nuclear power is unsafe? Not really, Chernobyl, designed differently than most commercial nuclear plants, uses graphite as a moderator, or control material bringing the neutrons to the proper speed for fission. Without going into too much detail, the accident at Chernobyl was a steam explosion followed by a fire of burning graphite, which destroyed the concrete containment and subsequently spreading radioactive contamination over the countryside. The improper operation of the plant was suspect, leading to the accident. Three Mile Island was an accident involving a number of improbable events leading to a partial melting and damaging the reactor. Eventually, a relief valve for the over pressurized system leaked radioactive material to the atmosphere, but not nearly the amount released by Chernobyl. (Wikipedia gives a good overview of these accidents).
Nuclear power is like any other high-energy device; in the wrong hands or with questionable operation, it is quite dangerous. However, with more than adequate controls, it can supply us with immense energy needs. We must keep any nuclear power system under control with high margins for error, which is not difficult to do. The only difficulty is maintaining the operational discipline and not becoming complacent with something so unforgiving.