Prompt critical condition

The basic dynamic characteristics of the core under various reactivity insertion conditions are shown in Fig. 8. The power transient reflects the super prompt critical condition when a large reactivity insertion occurs. On flie oflier hand, the power transient is small for the 4S during potential reactivity insertion at the plant start up phase. 

Figure 4.i.d. The delayed neutron fraction is not actually zero, but can be viewed as being zero because delayed neutrons are not necessary to maintain reactor criticality and, in this prompt critical condition, they have no impact on reactor control. 

The prompt critical condition is a reactor that is critical on prompt neutrons alone. This is an unstable and uncontrollable condition,... 

Limits on the rate of reactivity insertion insure that a rod insertion or scram will be initiated prior to the prompt critical condition. These limits are imposed by ... 

A. In a critical reactor, the neutron population is exactly sustained by the contributions of fission neutrons, which appear promptly upon fission, plus the delayed neutrons which are emitted by the fission fragments and appear second or minutes after the fission event. This condition of neutron balance is called "delayed critical," denoting the fact that prompt and delayed neutrons are needed to balance neutron losses from absorption and leakage and thus to sustain the chain reaction. A "prompt critical" condition is one in which the neutron economy is balanced by the contribution of the prompt neutrons alone, without the need of the delayed neutrons. 

It is essential for the safety of the reactor to exclude the possibility of super prompt critical state at all times. This requires that the inserted reactivity at potential events should be below 1 under conservative conditions, neglecting reactivity feedback coefficients. 

The risk of a disturbance causing displacement of the SNF in the RPVs due to the corrosion-weakened or damaged condition of the fuel supporting structures. In this case, the possibility of a critical mass being formed at the bottom of an RPV, beneath the CCRs, cannot be ruled out. This could result in either slow or prompt criticality, similar to the situation discussed in the previous paragraph for displacement of the CCRs. 

Under these conditions, an increase of 10% in reactor power takes 19 s. Reactors must always be built in such a way that the construction makes it impossible to reach a criticality of A > 1 -I-( prompt criticality ). This is a basic law of reactor construction since Fermi s first test reactor. In fact, this is how western reactors are designed. In the reactor at Chernobyl, which was destroyed in an explosion, prompt criticality was not made physically impossible. It was only forbidden by regulations that were not followed on the day of the accident. As a result, the reactor exploded. 

An important advanta of the method is that only known or measured properties of the fts e system at the subcritical state of interest are required to interpret the measured quantities. Alternative methods fix inferring reactivity Usually require calibration at the delayed critical condition, which may not be possible or desirable in many situations. In addition to the neutron multiplication factor k, the -driven method inherently yields additional useful properties of the fissile system, such as the prompt neiutron decay constant and the inverse neutron count rate, which is related to the source neutron multiplication. 

Critical - The condition existi when t e effective multiplication constant for a reactor is unity (or l) so that a self-supporting fission chain reaction can be maintained is said to be "critical." As commonly used, this means "delayed critical" and the reaction is depezident upon both prompt and delayed neutrons. Prompt critical is capable of sustaining a chain reaction without the aid of delayed neutrons. 

Is prompt critical a desirable or undesirable condition Explain. 

In specifying the source term here, separate account has to be taken of the contributions of prompt and delayed neutrons. For accurate analysis of the time variance of the flux, the fractional yields and half-lives of the six delayed neutron groups have to be fed into the source term. The method of solution is given in many reactor physics texts, and will not be repeated here. To illustrate the time behavior predicted by the solution, we may consider a reactor where, starting from an initially critical condition, the reactivity is suddenly increased from zero to a small positive value, such as p = 0.001. Assuming six delayed neutron groups, the time dependence of the flux after the reactivity increase is given by an expression of the form... 

The lower margin to prompt critical leads to more stringent limitations on the possible reactivity changes that might occur under fault conditions. 

CARDIAC ARREST. Stoppage of the pumping of blood by the heart is usually followed promptly by cessation of breathing. These are the most critical conditions which may accompany a heart attack because cells in the brain begin to die within a few minutes after the delivery of blood and oxygen is halted. Hence, immediate actions must be taken in order to (1) get the blood back into circulation, and (2) force air into the lungs. 

Above condition is satisfied whenever p (5, i.e. prompt critical or subcntical. When it is prompt supercritical the pnmary requirement is that A < 2A/(p - i). 

This experiment is designed to determine the reactivity worth of the control rods by a pulsed neutron technique. A burst of neutrons is injected into the reactor, and the decay rate of the resultant neutron flux is measured. The decay rate measured is that of the prompt fission neutrons and is proportional to the prompt critical reactivity of the reactor. Measurements will be made with the reactor in subcritical conditions and at delayed critical. The decay rate at delayed critical yields the constant of proportionality between the decay rate of the neutron flux and the reactivity in dollars. This constant is equal to the ratio of the effective delayed-neutron fraction to the prompt-neutron lifetime. 

Compare the two reactor conditions "delayed critical" and "prompt critical."... 

A device described by Sawyer and Dixon [13] was used for the determination of alcohol and acid in beer and stout. Attempts to improve the reliability of this method and to improve the signaTto-noise characteristics of the measurements prompted a critical design described by Lidzey et al. [14]. This unit overcomes many of the fluctuations in results observed with use of the first unit in this a number of possible sources of surging were indicated and these were not controlled owing to the varying conditions in the coil. In addition, the separation of the waste involatile material from the volatile phase took place outside the heated flask distillation unit. Air bubbles present in the segmented stream were also responsible for considerable surging. 

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