WHAT IS NEEDED TO START A NUCLEAR FUSION REACTION?

Answer 1

The usual source is simply an array of fuel elements containing sufficient U235 and a moderator which slows down the fast neutrons emitted by fission. When arranged in an optimum geometric array, there will be a surplus of neutrons formed in each successive generation, but this is controlled by neutron absorbing rods or soluble poison, so that a steady rate is achieved.

It is often thought that something is needed to start the reaction, but in fact U235 naturally emits a small number of neutrons (spontaneously), so all that is needed is to achieve criticality by withdrawing control rods (in a slow careful way) and the nuclear reaction just starts.

It is critical mass that is required to initiate a chain reaction. What is critical mass? Take a breath. Critical mass is the minimum amount of fissile material that is necessary for a given geometric arrangement of that material which will cause it to begin to fission at a rate sufficient to build a chain. Because fissile material is constantly undergoing radioactive decay by spontaneous fission, at least on some small level, there will always be a few neutrons being produced by the material. If enough material is assembled in a "workable" way (speaking to the geometry), critical mass will have been achieved, and the "natural" neutron flux will initiate the nuclear chain reaction and criticality will have been achieved.

There are basically a couple of applications where the implications make for a vastly different "look" to the machine that will employ the chain reaction. One is a nuclear reactor, and the other is a nuclear weapon. One will fit (approximately) in a Basketball court, the other will fit in a suitcase. Let's look at them beginning with the weapon.

In a bomb, we assemble subcritical masses of fissionable material in a specific geometry, then use conventional explosives to drive those subcritical elements together. This initiates the nuclear chain reaction, and it builds very quickly and we get the big blast. The shape and arrangement of things in the bomb, its geometry, are set up so that we use as little material as possible and get a maximum yield. (Set aside variable yield weapons for now.) Enough weapons grade fissile material is used to insure critical mass is reached, and a bit more. But not to much. It's different in a nuclear reactor.

In a reactor, which is inside the pressure vessel in a pressurized water reactor, we use lots and lots of fissile material. But we spread it out over a larger volume and put a bunch of things in there to inhibit or control the nuclear chain reaction. And that means dealing with those pesky neutrons. The chain will begin when the control rods are pulled to a certain height. The rods are neutron absorbers and act to "kill" the chain when they are in the reactor core. By pulling them out, neutrons released by spontaneous natural fission within the fuel are free to cause other fissions (after a bit of slowing down) instead of being absorbed by a control rod. The chain has been initiated and builds according to the physics of the reactor.

Answer 2

Heat should be applied.