Moderator temperature coefficient

Figure 4.2-6 shows the calculated temperature coefficient of reactivity for the BOC-IC condition. Curve A is the fuel prompt doppler coefficient due to heatup of the fuel compact matrix as a function of the assumed fuel temperature. Curve B is the active core isothermal temperature coefficient and is the Siam of the doppler coefficient and the moderator temperature coefficient of reactivity which is also strongly negative, due in large measure to the presence of LBP in the BOC condition. The moderator coefficient, not shown in Figure 4.2-6, would be the difference between Curve B and Curve A and would be -4.0 x 10" / C at 800 C (1472 F), for example. Curve C is the total reactor isothermal coefficient and includes the positive contribution of the reflector heatup to the estimated inner and outer reflector temperatures that would result when the fuel reaches the indicated temperature. 

As, however, PWRs adopt chemical shim, that is the control of reactivity through dissolution of boric acid in the reactor water, the presence of this neutron absorber decreases the safety effectiveness of the moderator temperature coefficient in fact, if the temperature increases, the amount of boron... 

B. Moderator temperature coefficient is zero. Analytical studies supported by core data show that the moderator temperature coefficient can vary between zero and -3.5 X 10 Ap/ F for various phases of core life. Therefore, a coefficient of zero is chose to maximize the power/pressure transient. 

There is a diverse emergency shut down system which is initiated manually if it is not possible to insert the control rods. It allows for injection of boron into the reactor. Since inadvertent operation of this system must be completely prevented, it requires insertion of a spool piece to connect the boron tank to the reactor. The negative moderator temperature coefficient is sufficient to maintain the reactor in a hot shut down condition for several days until the spool piece can be inserted. 

Negative Moderator Temperature Coefficient which will give subcriticality in the event... 

LOF 6 pumps limit consequences of a single pump failure Pump inertia coupled w ith moderator temperature coefficient cause shut down w ithout DNBR... 

The transient response of the reactor system is dependent on reactivity feedback effects, in particular, the moderator temperature coefficient and the Doppler power coefficient. These reactivity coefficients are discussed in subsection 6.3 of this PCSR. 

Faults in this category result in a cool-down of the RCS. As the AP1000 has a negative moderator temperature coefficient, a cool-down of the RCS will result in an increase in the reactivity and power of the core, which has the potential to threaten the integrity of the fuel cladding. If the... 

Power distributions within the core following the withdrawal of a single RCCA are calculated using ANC and APOLLO. The peaking factors are then used by VIPRE-01 to calculate the DNBR for the event. The case of the worst rod withdrawn from the mechanical shim or axial offset bank inserted at the insertion limit, with the reactor initially at full power, is analysed. This incident is assumed to occur at beginning of life because this results in the minimiun value of moderator temperature coefficient. This assumption maximises the power rise and minimises the tendency of increased moderator temperature to flatten ihe power distribution. 

The rod cluster control assembly banks provide sufficient reactivity to overcome the power defect in going from full power to zero power, and then to provide the specified shutdown margin. The ability to accomplish shutdown from hot conditions, at end of life when the moderator temperature coefficient is at its most negative value, is demonstrated in Sections 4.S.2.4 and 4.3.2.5 of Reference 6.1. 

To achieve an acceptable moderator temperature coefficient throughout core life. 

The burnable absorbers must have sufficient reactivity worth to prevent the moderator temperature coefficient from ever going positive rmder normal operating conditions. 

The burnable absorbers control the radial peaking factor and prevent the moderator temperature coefficient from ever going positive under normal operating conditions. They achieve this by reducing the requirement for soluble boron in the moderator at the beginning of the fuel cycle too high an initial concentration would result in a net reactivity injection as the moderator density reduces on heating up (Section 4.3.2.4.1.14 of Reference 6.1). 

Define excess reactivity, K x> shutdown margin, moderator temperature coefficient, void coefficient, overmoderation, and undermoderation. 

Sketch a typical moderator temperature coefficient versus moderator temperature curve and explain the effects of changes in moderator temperature,... 

At normal operating temperatures, a reactor has a negative moderator temperature coefficient which results from a decrease in moderator density in the core with increasing temperature. Since water molecules supply the principal nuclei for thermali2ing neutrons, as density decreases, less moderation occurs near fuel rods, more neutrons leak from fuel into the control rods and out of the core, and a net negative reactivity results. 

Moderator temperature coefficient is not constant, as can be seen in Figure 6.2, because one degree temperature change causes a greater water density change at higher... 

In a research reactor, moderator temperature remains almost constant, so moderator temperature coefficient is considered essentially constant. 

Figure 6.2 Moderator Temperature Coefficient Versus Temperature...

Moderator temperature coefficient decrease in moderator density per decrease per F is greater at high temperatures. 

The operating range of the moderator to fuel ratio is determined by the design of the reactor core. From Figure 6.3(b), it can be seen that at 70 F, an increase in temperature results in positive reactivity addition which means that the moderator temperature coefficient is positive. 

What fundamental processes in a reactor core account for a negative moderator temperature coefficient of reactivity Explain in detail. 

Explain how moderator temperature coefficient varies with moderator temperature. 

Moderator temperature coefficient (MTC) Fuel temperature coefficient Maximum peaking factor (HFP)... 

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