How does fission reaction happen?

Answer 1

Fission is started in nuclear power plants by withdrawing the control rods. The rods are pulled in groups beginning around the perimeter of the reactor. These are extracted and pulled all the way out. The reactor design permits this to happen without starting the reactor up. Then the middle rods are pulled. These actually permit the reactor to start up. The rods being pulled last are the "control group" because they are going to set up the operating conditions. Rods pulled to control the reactor, those in the middle, permit a more uniform burn of the fuel. If it was done differently, the fuel in the middle would burn more quickly. And the fuel around the perimeter would not be used as efficiently. Here's the scoop. The control rods are a neutron absorbing material. They have to be because they must be able to absorb neutrons to control or shut down the reactor. Boron works really well because 1) boron has a fairly high neutron absorption cross section (it is a good or "big" target for a neutron), and 2) as boron is transmuted by neutron absorption, it (usually) becomes another boron isotope, and boron's isotopes all have good neutron absorption cross sections so they all continue to be pretty good neutron absorbers. The rods are pulled to a point where there is not enough of them in the reactor to absorb the neutrons that are spontaneously being generated by the fuel. (It always generates a few neutrons. Always. And that's the hinge for critical mass.) So the rods are pulled and the effective critical mass is reached and the chain begins. Monitoring instruments pick up the increase in neutron flux. Operators know the chain has begun and is building. Then by gradually heating things up and incrementally pulling the rods a bit more, the plant is brought to operating temperature and is able to provide heat to generate steam in a secondary system. The secondary system is gradually heated by bleeding steam when things in the primary are heated up. That way the secondary system can be brought on line efficiently and power production can begin. There are more subtle aspects to reactor operations, but this is a good start on a path to understand the workings of the reactor.

Answer 2

A fission reaction is started by neutrons being fired at uranium -235 atoms. When a neutron hits the nucleus of uranium-235 it becomes uranium-236, which is very unstable. The atom splits and gives off three neutrons and a large amount of energy. Those three neutrons will collide with three other uranium-235 atoms and the same reaction will happen. Each time it will get three times bigger! This is a chain reaction.

Answer 3

Neutron bombardment.

Nuclear reactors are started with a continuous neutron source, with the control rods all inserted to keep the reactor subcritical. While the reactor is subcritical it will just act as a neutron multiplier (the number of neutrons produced will be a constant multiple of the number injected by the neutron source). As control rods are removed, this multiplication factor will increase and the reactor will slowly approach critical. The neutron flux will increase briefly but then flatten out again at the new constant level. (If the neutron source were removed at this time the neutron flux would slowly drop back to zero.) When enough control rods have been removed and the reactor is critical, the neutron flux will keep rising without flattening out. The neutron source is now removed and the neutron flux in the reactor will flatten out and hold constant.

Use of a neutron source during startup is needed to prevent the accidental occurrence of prompt supercriticality, which could cause an energy surge and severely damage the reactor core.

Once started, control rods can be removed or inserted as needed to adjust the point of criticality to produce the desired amount of energy.

Answer 4

Radioactive materials emit particles as they decay. Sometimes that particle hits another atom and causes it to prematurely decay and emit particles. When there is a sufficient quantity of a radioactive material in a confined space, called a critical mass, the continuation of this process causes a chain reaction. If it is uncontrolled, an explosion of tremendous energy occurs. If it is controlled as in a fission reactor with rods of a material that block the decaying particles and keep the reaction from becoming uncontrolled, the resulting heat can be used to produce electricity. In short, a fission reaction starts when there is too much of a radioactive substance in too small an area.

Answer 5

Very carefully using a neutron source to keep fission going while subcritical, thus avoiding any sudden unexpected prompt supercriticality from occurring while removing control rods as the reactor is made critical then the power level is raised. Once instruments clearly show the reactor is exactly critical the neutron source is removed, the reactor has been started and further adjustment of control rod positions is only to raise or lower the operating power of the reactor.

Answer 6

Let's take for example Uranium 235. What happens is it gets hit by a fast neutron making it unstable and split up into two smaller atoms, producing more fast neutrons. A fission reaction is a nuclear reaction that splits up an atom into multiple ones.

Answer 7

by splitting of a radioactive atoms