Prompt critical

Criticality designated in nuclear engineering, both the neutron balance of a nuclear facility, as well as the critical state of a nuclear reactor or fissile material arrangement.

An arrangement is critical in per unit time the same number of free neutrons generated as disappear by absorption and leakage (ie, loss to the outside). The critical state is the normal operating condition of a nuclear reactor in which runs a self-sustaining chain reaction. The neutron flux and thus the generated power, so the per unit time released heat energy, it can be higher or lower; Criticality just means that these quantities remain equal time.

Neutron balance

The neutron balance is expressed numerically by the multiplication factor k This is the number of neutrons in the following "generation" per neutron of the present generation, or the number of new divisions per split core. In practice, instead of k most reactivity ρ = (k - 1 ) / k considered.

• Predominates in the neutron balance of the neutron loss (k <1), there is a subcritical assembly.
• A critical assembly is achieved with a balanced neutron balance (k = 1).
• If the neutron production is greater than the neutron loss ( k> 1), it is called a supercritical assembly.

Delayed neutron

About 99% of the neutrons produced during the nuclear fission in 10 femtoseconds after cleavage emitted ( prompt neutrons), while the rest is emitted after a few milliseconds to minutes (delayed neutrons). The delayed neutron fraction β to wear a multiplication factor k at which depends on the fissile material; in 235U, it is about 0.75%. The above-described critical condition with constant power, k = 1 refers to all of the delayed neutron including. It could therefore be more accurately referred to as delayed critical.

Delayed critical

An arrangement in the condition of 1 < K <1 β is delayed over a critical, i.e., the reactor power increases, but only by the effect of delayed neutrons, and therefore with the time constant ( in seconds ), so that the reactor is regulated with technical means remains. This area is used for "starting" the reactor and increasing the power levels up to nominal power.

Prompt critical

In the state with k = 1 β the prompt neutrons alone suffice to maintain the chain reaction. The state is uncertain, because the smallest random increase of k makes the arrangement prompt critical. Since such small random fluctuations always occur, the prompt criticality is the limit which, when reached a reactor " passes ".

Prompt critical

In state k> 1 β an arrangement is prompt critical, that is, the neutron flux and thus the power increases exponentially with a very short time constant which is determined by the prompt neutrons. This increase with external technical means no longer fast enough influence to prevent destruction of the device.

Nuclear weapons are between ignition and explosion very short, but far prompt critical. In most reactors prompt Überkritikalität must be avoided, however. The only exceptions are certain research reactors in which very short duration "Pulse" prompt Überkritikalität be (Prompt burst) produced and used. The Doppler spread, a physical effect, which occurs inevitably with the consequent temperature increase of the reactor automatically makes again under critical. One example is the research reactor TRIGA type.

Dollar

The difference β between the criticality critically delayed and prompt critical is in the English language literature than $ 1 ( divided into 100 cents) respectively. For example, the reactivity values ​​of control rods are conveniently expressed in cents. The reactivity values ​​are ( approximately) additive, that is, the retraction of two absorber rods of eg 5 per cent causes a reactivity of -10 cents.

Criticality

In reactors is an unintended positive reactivity insertion from the critical normal operation from - ie delayed or even prompt Überkritikalität - referred to as reactivity accident. For other nuclear installations, the far subcritical systems are in normal operation ( eg reprocessing plants or fuel fabrication ), the term reactivity is seldom used. The Incident by ( over-) criticality criticality means here, as he has, for example, in a 1999 plant near Tōkai -mura (Japan) occurred.