Critical mass | physics | japancarnews.info
with a smaller mass than the original, is known as nuclear fission. chain reaction is controlled by restricting the number of neutrons available to . Be specific about the difference in the number of 'reactions' which occur during the trial. Section Critical Mass and Nuclear Reactors. Subcritical To slow down the rate of reaction, control rods are lowered between the fuel rods. The control rods . Critical mass, in nuclear physics, the minimum amount of a given fissile material necessary to achieve a self-sustaining fission chain reaction under stated.
The constant of proportionality increases as k increases. A supercritical mass is one in which, once fission has started, it will proceed at an increasing rate. The material may settle into equilibrium i.
Critical mass - Wikipedia
Due to spontaneous fission a supercritical mass will undergo a chain reaction. The probability that one such event will cause a chain reaction depends on how much the mass exceeds the critical mass. If there is uranium present, the rate of spontaneous fission will be much higher.Nuclear Fission, 1950's - Film 7414
Fission can also be initiated by neutrons produced by cosmic rays. Changing the point of criticality[ edit ] The mass where criticality occurs may be changed by modifying certain attributes such as fuel, shape, temperature, density and the installation of a neutron-reflective substance. These attributes have complex interactions and interdependencies.
These examples only outline the simplest ideal cases: Varying the amount of fuel[ edit ] It is possible for a fuel assembly to be critical at near zero power. If the perfect quantity of fuel were added to a slightly subcritical mass to create an "exactly critical mass", fission would be self-sustaining for only one neutron generation fuel consumption then makes the assembly subcritical again.
Critical mass | Restricted Data
If the perfect quantity of fuel were added to a slightly subcritical mass, to create a barely supercritical mass, the temperature of the assembly would increase to an initial maximum for example: Changing the shape[ edit ] A mass may be exactly critical without being a perfect homogeneous sphere. More closely refining the shape toward a perfect sphere will make the mass supercritical.
Conversely changing the shape to a less perfect sphere will decrease its reactivity and make it subcritical. Changing the temperature[ edit ] A mass may be exactly critical at a particular temperature. Fission and absorption cross-sections increase as the relative neutron velocity decreases. Neglecting the very important resonances, the total neutron cross-section of every material exhibits an inverse relationship with relative neutron velocity.
Thermal expansion associated with temperature increase also contributes a negative coefficient of reactivity since fuel atoms are moving farther apart. A mass that is exactly critical at room temperature would be sub-critical in an environment anywhere above room temperature due to thermal expansion alone.
Varying the density of the mass[ edit ] The higher the density, the lower the critical mass. The density of a material at a constant temperature can be changed by varying the pressure or tension or by changing crystal structure see allotropes of plutonium.
An ideal mass will become subcritical if allowed to expand or conversely the same mass will become supercritical if compressed. Changing the temperature may also change the density; however, the effect on critical mass is then complicated by temperature effects see "Changing the temperature" and by whether the material expands or contracts with increased temperature.
Figure A fission reaction can be slowed down to a consistent, stable rate; it doesn't have to happen all at once as in an explosion. This is what happens in a nuclear reactor; a controlled fission reaction generates electricity. The nuclear fuel is composed of uranium pellets inside metal tubes called "fuel rods. Fuel Rods and Control Rods Lowering the control rods in a reactor core slows the reaction.
Raising them allows the reaction to speed up and generate heat. If nothing intervened, the chain reaction could rapidly speed up and get out of control; the fuel rods would overheat and cause a meltdown.
To slow down the rate of reaction, control rods are lowered between the fuel rods. The control rods are made out of a material that can absorb neutrons such as graphite. When the control rods are between the fuel rods, the neutrons of one fuel rod cannot cause as much fission in a neighboring fuel rod, and the whole reaction slows down. Figure The fuel rods and the control rods make up the core of a nuclear power plant.
When the reactor is active, the fission chain reaction heats a liquid coolant surrounding the rods. The hot coolant is pumped through pipes to boil water in the steam generator; the pressurized steam flows through the steam line to the turbine, which powers the generator. The steam is then cooled and condensed by water from the cooling tower, and then pumped back to the steam generator. Figure Figure