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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.
What else can I help you with?
Which type of nuclear reaction is used in modern-day nuclear reactors?
Nuclear fission is used in nuclear weapons to create what some might call an atomic blast (nuclear blast). Nuclear fission used this way can also be applied in special complex designs to generate enough thermal energy (heat) to initiate a fusion reaction. This creates an even bigger nuclear blast.
What causes the process of nuclear fusion to start in stars?
It is generally thought that as the gasses needed to create a star collect, the gravity that is pulling them together will compress and heat that gas so much that at some point fusion will spontaneously begin.
What substance lowers the energy needed to start a chemical reaction?
A substance that lowers the energy needed to start a chemical reaction is a catalyst.
Is nuclear fission happening on the sun or nuclear fusion?
Fusion. Fusion requires very high temperature and pressure. So on earth we usea critical mass of weapons grade uranium to start a fission chain reaction whichenables a fusion reaction in hydrogen. Fusion is of course more powerful than fission
Nuclear energy is an alternate energy source produced from atomic reactions?
There are two: Nuclear Fission and Nuclear Fusion. Fission is when a neutron is fired at an element with a high atomic number (usually Uranium) which then splits, releasing energy and more neutrons. this produces a chain reaction, which continues until all nuclei have been split. Fusion occurs in stars and a few experimental reactors, and happens when two forms of Hydrogen nuclei (Deuterium and Tritium) fuse into an unstable nucleus, which in turn splits again into Helium and a spare neutron. Fission can start at any temperature, but Fusion only when Hydrogen is in a plasma state.
What is one obstacle that is used in nuclear fusion?
temperature/pressure needed to start reaction.
What layer of the sun does nuclear fusion occur?
It has to be at hundreds of millions of degrees kelvin, before a fusion reaction between deuterium and tritium will start
What are the technical problems related to nuclear fusion?
Nuclear fusion requires very high temperatures and immense pressures to start and continue. The problems with a nuclear fusion reactor would be:- 1) the high temperatures would melt the container: therefore, the reaction would have to be stored in a vacuum suspended by a magnetic field and the reactor would have to be continually cooled. 2) nuclear fusion occurs naturally in stars such as our sun: unless the fusion reaction was limited in size in some way, it would be likely that our planet is vapourised by the reaction.
Maximam dengrus bomb between nuclear bomb or hydrogen bomb?
If you are asking which has the maximum danger, a hydrogen bomb is one type of nuclear bomb. In general, nuclear bombs can be fission (called atomic) or fusion (hydrogen) A fusion bomb is larger than an fission bomb, and actually uses a fission bomb to start the fusion reaction.
Which type of nuclear reaction is used in modern-day nuclear reactors?
Nuclear fission is used in nuclear weapons to create what some might call an atomic blast (nuclear blast). Nuclear fission used this way can also be applied in special complex designs to generate enough thermal energy (heat) to initiate a fusion reaction. This creates an even bigger nuclear blast.
Why doesnt Jupiter make nuclear fusion?
Jupiter is not nearly massive enough or dense enough to hit "critical mass"; essentially, there's not enough pressure at the core of the planet to start the initial nuclear reaction and its not dense enough to maintain the reaction.
What function does Styrofoam perform in an hydrogen bomb?
Styrofoam is used in hydrogen bombs as a lightweight material to help trigger the fusion reaction. When compressed by the explosive detritus, styrofoam can create the high temperatures and pressures needed to start the fusion reaction in the hydrogen bomb.
What causes the process of nuclear fusion to start in stars?
It is generally thought that as the gasses needed to create a star collect, the gravity that is pulling them together will compress and heat that gas so much that at some point fusion will spontaneously begin.
What do chemist call the energy needed to start a reaction?
The energy needed to start a chemical reaction is called the activation energy :)
Why do nuclear reactions take place inside the core of the sun?
Because of the energy source and it can also provide the needed speed for fusion. +++ Not speed. The nuclear fusion of is hydrogen to helium, and it occurs because the core of the star as it developed gained sufficient pressure and temperature for the reaction to start and become self-sustaining. The outer regions of a star are neither hot enough nor compressed enough for fusion to occur.
What is used in the fusion process that triggers the reaction?
In nuclear fusion, high temperatures and pressure are used to trigger the reaction. These conditions are necessary to overcome the electrostatic repulsion between positively charged nuclei, allowing them to come close enough together to fuse. Additionally, fusion reactions are initiated by providing energy to the system to kick start the process.
What substance lowers the energy needed to start a chemical reaction?
A substance that lowers the energy needed to start a chemical reaction is a catalyst.
