Moderator density reactivity coefficient

In a BWR, two reactivity coefficients are of primary importance the fuel Doppler coefficient and the moderator density reactivity coefficient. The moderator density reactivity coefficient may be broken into two components that due to temperature and that due to steam voids. 

During normal plant operations, the steam void component of the moderator density reactivity coefficients is of prime importance. The steam void component is large and negative at all power levels. At full rated power, the steam voids are equivalent to approximately 3% reactivity. 

The fuel assembly design is such that the moderator density reactivity coefficient of the water within the fuel channel is negative for all conditions of operation. The in-channel moderator coefficient is smallest at the cold, zero power condition. 

The inherent safety feature is the tendency of the system to fall to the safer side when a positive reactivity is inserted. The main contributions to the inherent safety features of the Super LWR are the positive coolant (and moderator) density reactivity coefficient (which is equivalent to the negative void reactivity coefficient of the BWR) and the negative Doppler reactivity coefficient. These inherent safeties should be maintained throughout the operation. 

The moderator temperature reactivity coefficient is also important for safety. In fact, when the moderator temperature increases, its density decreases and, as a consequence, the moderating effectiveness also decreases. This decrease causes an increase in the loss of neutrons from the core and an increase in the parasite captures, so that the reactivity tends to decrease. 

The branching bumup calculation modes of SRAC and ASMBURN allow the modeling of temporary changes in the coolant and moderator densities [14]. The branching bumup modes calculate the collapsed macro-cross sections when the coolant (moderator) density or fuel temperature is instantaneously changed from the base case. This concept is described in Fig. 2.22 [9]. The thick line in the figure represents the base case (coolant density po). For example, the dependence of the coolant density reactivity coefficient on the bumup can be evaluated as follows ... 

The water density reactivity coefficient corresponds to the void reactivity coefficient of BWRs or PWRs and it is an important index parameter when judging the inherent safety characteristics of the Super LWR. The density reactivity coefficient for a typical fuel is shown with respect to the water density (average of the coolant and moderator densities) in Fig. 2.60 [9]. The coefficients are derived from the change in the infinite multiplication factor of the fuel when the average density is instantaneously changed at a particular bumup using the branching bumup calculations (Sect. 2.3.1). 

Due to the negative reactivity coefficients, large heat capacity of the graphite moderator and reliable high temperature performance of the particle fuel, together with low power density and low operating temperature, any design basis accident is expected to be terminated in... 

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). 

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. 

The graphite structured and moderated core, having characteristics of negative reactivity coefficient, low power density, and high thermal conductivity. 

Figures D-2 and D-3. Furthermore, the relative temperature expansions have another interesting effect. Assuming the fuel has an average expansion coefficient of aluminum (2o x 10 K ), while aluminum, stainless steel, or a molybdenum alloy cladding has average thennal expansion coefficients of 26 x, 6 x, and 5 X 10 K respectively, then as the temperature rises, the axial linear fuel density varies differently inside each of the three cladding materials. Correspondingly, the axial linear moderator density also varies in each of the three clad materials. Both of these linear density changes affect the moderator-to-fuel ratio and, hence, the degree of reactivity control that can be obtained from the moderator temperature coefficient.

The time delay of the heat transfer to the coolant and moderator water is an important factor in the mechanism of coupled neutronic and thermal-hydraulic instability. The Super LWR is a reactor system with a positive density coefficient of reactivity and a large time delay constant. If there is no time delay, a decrease in density would cause a decrease in power generation, which suppresses any further decrease in density, stabilizing the system. However, if there is a large time delay, it causes a decrease in the gain of the density reactivity transfer function, and reduces the effect of density reactivity feedback, making the system less stable. The time delay of the heat transfer to the water rods is much larger than that to the coolant. Thus the reactor system becomes less stable when the water rod model is included than the case without it. 

This hydrogen density in e zirconium hydride is as high as in water at room temperature and is appreciably higher than in water at the 300°C used in power reactors. Another advantage of the uranium-zirconium hydride fuel-and-moderator mixture is its high prompt negative temperature coefficient of reactivity, a consequence of the intimate thermal contact between and hydrogen atoms. 

Less strict reactivity control requirements due to low power density (reduced power and Doppler reactivity effects) and the lower operating point parameters (reduction of the absolute value of moderator coefficient). 

Short-term negative reactivity response on the order of milliseconds, but larger than the mean neutron lifetime, can be provided by the moderator coefficient, which is dependent on the variation of the moderator (water) density and scattering characteristics. Table D-5 presents a summary of these response times. 

Canadian Deuterium-Uranium Reactor (CANDU). The CANDU reactor is interesting because it can mn continuously and be fueled online. With a capacity factor near 100%, the CANDU bums plutonium the quickest. Online fueling would also be very useful for putting the entire inventory through one reactor rapidly to self-protect the fuel. Because plutonium provides excess core reactivity, the core could be cooled and moderated with light water instead of heavy water. The large core also allows low power densities. However, this modified CANDU concept could have positive temperature coefficients of reactivity. Lack of experience with this concept is the main reason that it has not been studied further. 

Simple control system. Density changes in the moderator create a sensitive, negative temperature coefficient of reactivity which makes this system self-stabilizing. This eliminates the need for mechanically dri en regulating rods. In addition, shim control can be achieved by changing the fuel concentration. 

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