Nuclear Reactors

mostly ocean currents carry the contamination

Nuclear reactors usually use uranium. U-235 is the isotope that undergoes fission, but it is usually only about 5% of the initial uranium in the fuel, the rest is U-238 which is not fissile itself but during reactor operation some turns into plutonium which is fissile and starts to contribute to the fuel output as the U-235 gets depleted.

Atom bombs usually use plutonium-239 but can also be made from pure U-235.

No, Enriched Uranium-235 is used in a nuclear reactor as the fuel in the fuel rods and boron is used in the control rods.

uranium is probably the source for radioactive isotopes derived from nuclear reactors

Electricity was generated for the first time by a nuclear reactor on December 20, 1951, at the EBR-I experimental station near Arco, Idaho, which initially produced about 100 kW.

Loss of reactor coolant results in lack of nuclear fuel cooling and hence melt down results.

Boron Carbide.

Conventional nuclear power plants use critical mass in a way that is similar to how a fission bomb uses it, in a chain reaction. There are some very important differences, however.

In a nuclear power plant, the reaction is controlled by a set of systems that moderate the amount and speed of neutrons, the temperature of the fuel, and so on. These prevent the reactor from getting too hot and melting down. This is not easy, but also not terribly hard.

In the bomb, there is no provision for ongoing control of the chain reaction. Instead, there is a design that causes the critical mass to undergo the chain reaction while staying at critical mass for the process. This is really hard to do because as soon as the whole heats up a bit, it wants to fly apart, destroying the integrity of the critical mass. The design problems associated with maintaining critical mass were difficult enough that they were secret for a long time.

In other words, in order for a critical mass to blow up as a bomb, it really has to be designed to be a bomb. And though a nuclear reactor can melt down and be very destructive in the process, and though the waste storage can be breached and be very destructive as a result, the nuclear explosion of a nuclear reactor is quite likely impossible. Explosions and meltdowns have happened, but the explosions are from steam or chemicals, rather than nuclear critical mass.

That would vary with reactor design, where the original problem occurred, operator training and response to the problem, and many other variables. An accident can vary from a tiny release of radioactive gasses that is stopped immediately, through steam ejection of a fuel rod whch impales a single worker onto the ceiling, through a massive steam explosion followed by a graphite fire (all three of these have happened), to a worst case complete core meltdown that melts down to the water table causing massive contamination of drinking water for hundreds of miles (this has not happened).

Mostly in Ontario, one in Quebec and one in New Brunswick. They are all CANDU type, ie heavy water moderated and cooled.

Because the world's worst nuclear reactor accident happened there. The reactor involved (a Soviet designed RBMK graphite moderated reactor) had many serious safety related design flaws, had been built in a rush to meet schedule deadlines without first having completed all safety related testing, and was undergoing a poorly planned and not properly reviewed test under the control of people with no knowledge of nuclear reactor principles (the man in charge of the test was only trained in hydroelectric dam principles).

This was a setting for disaster and disaster was what they got. Skipping over the details: the reactor became unstable and impossible to control during the test, it experienced a sudden power surge, a steam explosion resulted which blasted the roof off the reactor building (no containment building existed in RBMKs), with hot graphite exposed to air the graphite ignited, the smoke from the graphite fire carried radioactive debris from the damaged core across Europe and eventually around the world. Hundreds of firefighters died of radiation poisoning from the exposure they got in trying to extinguish the burning graphite and other fires nearby that had been started by chunks of burning graphite thrown from the reactor in the blast.

The surrounding land is slowly reverting to nature and has become a wildlife preserve, protected from human intrusion by the radioactivity.

This was at Shippingport in Pennsylvania, started operating 1957. See Wikipedia link given below. It was only rated at 60 MW so it was a demonstration plant, but gave birth to the PWR's of much larger size that are now the world wide leaders. Note the claim made that it was the first ever reactor devoted entirely to peaceful purposes. Since it was also the prototype for aircraft carrier reactors, one could argue with this! Actually the first power plant producing significant electric power was at Calder Hall in England, started 1956, but I must admit the additional purpose of this (perhaps its main purpose, though this was concealed from the public) was to produce weapons grade plutonium.

Alpha particle is composed of two protons and two neutrons (equivalent to He nucleus). So the parent decaying by alpha decay results in a daughter having atomic number less by two and mass number less by four.

The fuel itself is made from uranium dioxide UO2, which is in the form of small cylinders 10 mm in diameter. During manufacture these are sintered to make them physically stronger. The uranium content is enriched in uranium 235 to 4 to 5 percent, the rest is uranium 238 which is not fissile. These fuel cylinders are put into tubes made of a zirconium alloy ('zircaloy') which are then sealed so that gaseous fission products cannot escape. The tubes are assembled into fuel assemblies containing probably more than 100 tubes depending on the reactor design, and these assemblies are handled as units when fuelling or discharging old fuel.

No, Enrico Fermi constructed and operated the first nuclear reactor (CP-1) in 1942. Several larger nuclear reactors were needed first to make the plutonium for the MK-3 Fatman atomic bomb, which was not tested until 1945. Their principles of operation are quite different, except that both operate by nuclear chain reaction.

The reactor(s) at Chernobyl are fission reactors, and fission of fuel and fission products following the fire and the overheating of the core melted it down.

No, nothing happened like melting of people in Chernobyl nuclear reactor accident.

1986, the same year as the Challenger Space Shuttle explosion.

True. Cadmium (and boron plus some other elements) absorbs neutrons, thus limiting or slowing the fission chain in a nuclear reactor.

nuclear energy is fairly cheap for many reasons. Mainly, the life time of a nuclear power plants is 60 years and that the nuclear fuel contribution to the cost of generated kilowatt.hour is very low.

The kWh produced by nuclear energy is the cheapest. It is a clean energy that does not pollute the environment. It gives chance for people to work during its construction and operation. It is reliable source of energy.

You did not provide the list of "the following". However, the answer to the question is moderation. Moderation is the process whereby the neutron is slowed down in order to facilitate its subsequent capture by the nuclei of the fuel.

If the water coolant level drops enough to expose fuel rods, the surface of those rods can then get hotter than the boiling point of water. The fuel pellets inside the rods are clad in zirconium metal, which when it gets hot enough will decompose water (both as liquid and as steam) into hydrogen and oxygen. As the hydrogen is lighter than air it accumulates as a "bubble" in the top of the reactor vessel.

Note: early reactors clad the fuel pellets with aluminum, but it decomposes water at a much lower temperature than does zirconium.

Temperature sensors are installed in nuclear reactors as part of the design. An array of thermocouple-based sensors is not uncommon.