Coefficient prompt positive

If there is a prompt positive coefficient no sudden effect on reactor stability occurs as the coefficient changes sign, though the critical power is gradually reduced. We note from (23) that for sustained oscillations the frequency of oscillation is reduced as the prompt coefficient becomes more positive. We may thus reach a condition in which the reactor takes off rather than undergoing sustained or undamped oscillations. 

The first serious attempt to calculate the Doppler effect for a fast reactor was by Goertzel and Feshbach (4, 12j, 15), who developed a technique that is basically equivalent to that presented here for the high end of the Doppler effect energy region. Their work was directed toward obtaining an estimate for the effect in EBR-1, which exhibited what at that time was an unexplainable instability, which was at least in part due to the existence of a prompt positive temperature coefficient of reactivity (16). Since this reactor was fueled with fully enriched uranium, it was conceivable that the positive coefficient was due to Doppler broadening of... 

In Eq. 15, the coefficient of ZXS/ZXR has suggested a preference for an aromatic substituent for the R group. The positive regression coefficients of MPCN and PP/D are in favor of an increase in the partial charge and polarity in the compounds for better activity. This has prompted us to suggest that thiadiazole derivatives with aromatic substituents would enhance the affinity of the compounds for the adenosine Ai receptor. 

As a leading example for short-spaced dyads, a n-n stacked porphyrin-fullerene dyad (ZnP-Ceo) 21 should be mentioned, which was probed in light of their electron transfer and back electron transfer dynamics [361, 362], The close van der Waals contact ( 3.0A) is responsible for pronounced electronic interactions in the ground state between the two 7t-chromophores. For example, the ZnP Soret-and Q-bands in the n-n stacked dyad 21 show a bathochromic shift and lower extinction coefficients compared to free ZnP [361], In the n-n stacked dyad 21 the linkage of the two bridging units occurs in the trans-2 position at the fullerene. A charge-separated radical pair evolves from a rapid intramolecular electron transfer k 35 ps) between the photoexcited metalloporphyrin and the fullerene core in a variety of solvents (i.e., ranging from toluene to benzonitrile). Remarkably, the lifetimes in tetrahydrofuran (t = 385 ps) and DCM (t = 122 ps) are markedly increased relative to the more polar solvents dichloromethane (r = 61 ps) and benzonitrile (t = 38 ps) [362]. This dependency prompts to an important conclusion ... 

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. 

If both the prompt and delayed temperature coefficients are negative, then Cl, C2, and C3 are all positive, as is also 73. We consider first the condition for sustained oscillations. In this case si = 0, and we must have... 

The experiments that have been performed are standard. Critical configurations have been determined for many different compositions. Reactivity coefficients have been determined by period measurements or by use of control rods calibrated by period measurements. Reactivity worths have also been determined analysis of the response to oscillator and rod drop experiments. Rossl-o measurements have been made on a number of assemblies to determine the ratio (rf the effective delayed neutron fractlqn to the prompt neutron lifetime and thus indirect to give information on the neutron spectrum. Detector responses, both as a function of detector material and as a function of position, have been made to determine data relevant to power distributions, bucklings, reflector savings, and neutron spectra. Spectrum measurements have been made by use of ehiulslon plates. 

Volume 4 of this report concludes that a pure plutonium fuel type is not desirable in LWRs because of the low mass loading per fuel rod (yielding short fuel cycles), relatively small prompt Doppler temperature coefficients, and strong positive isothermal temperature coefficients. Any workable fuel composition must have a negative prompt temperature coefficient (reactor power decreases as temperature increases) for safety and control purposes. 

The fastest potential positive reactivity transient postulated for SRS cores is the startup accident. In this transient, groups of control rods are unintentionally withdrawn from the reactor at no power, or very low power conditions. This results in the creation of very short reactor periods before any temperature changes occur. WSRC examined the consequences of the startup accident with all but the least restrictive safety feature disabled and found that even with the limiting value of the prompt coefficient given in DPST-88-956, the transient would be safely terminated by safety rod scram. 

A positive reactivity ( 0.1) is inserted stepwise by withdrawing the CRs. The feedwater pump speed and the turbine control valve stroke are kept constant. The results are shown in Fig. 7.68 [31]. The reactor power increases about 10% almost stepwise due to the prompt jump and then gradually decreases due to the reactivity feedbacks from the fuel temperature and coolant density. This behavior implies that the Super FR also has inherent self controllability of the reactor power despite the much smaller density reactivity coefficient compared to that of the Super LWR. The main steam temperature increases, which leads to an increase in the main steam and core pressures because the specific volume of the main steam increases. The increase in the core pressure leads to a decrease in the feedwater and core flow rates, which increases the main steam temperature further. As a result, the maximum increase in the main steam temperature is nearly 40°C while that in the Super LWR is only 9°C (see Fig. 4.10). 

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