Power Transients

A.m blent Environment. The environment around the flow conduit must be considered in meter selection. Such factors as the ambient temperature and humidity, the pipe shock and vibration levels, the avadabiHty of electric power, and the corrosive and explosive characteristics of the environment may all influence flow meter selection. Special factors such as possible accidental flooding, the need for hosedown or steam cleaning, and the possibiHty of lightning or power transients may also need to be evaluated. 

Moujaes, S. F., and R. S. Dougall, 1990, Experimental Measurements of Local Axial Gas Velocity and Void Fraction in Simulated PWR Steam Generator Rod Bundles, Can. J. Chem. Eng 68 211.(3) Moxon, D., and P. A. Edwards, 1967, Dryout during Flow and Power Transients, UK Rep. AEEW-R-553, UK AEEW, Harwell, England. (5)... 

The system control must ensure correct working of the system, not only under steady-state conditions, but also during power transients. All the elementary cells must be controlled (the cell voltage of each elementary cell, if possible) and the purging system must be activated in the case of a technical hitch. 

In our circuit, turnoff power transients dominate the switching losses because the snubber circuits allow the voltage across the device to fall before the current... 

An interesting class of exact self-similar solutions (H2) can be deduced for the case where the newly formed phase density is a function of temperature only. The method involves a transformation to Lagrangian coordinates, based upon the principle of conservation of mass within the new phase. A similarity variable akin to that employed by Zener (Z2) is then introduced which immobilizes the moving boundary in the transformed space. A particular case which has been studied in detail is that of a column of liquid, initially at the saturation temperature T , in contact with a flat, horizontal plate whose temperature is suddenly increased to a large value, Tw T . Suppose that the density of nucleation sites is so great that individual bubbles coalesce immediately upon formation into a continuous vapor film of uniform thickness, which increases with time. Eventually the liquid-vapor interface becomes severely distorted, in part due to Taylor instability but the vapor film growth, before such effects become important, can be treated as a one-dimensional problem. This problem is closely related to reactor safety problems associated with fast power transients. The assumptions made are ... 

Scheme 51 presents an entirely different mode of catalyzed [4 + 2] cycloadditions. Thus irimediylsilyl triflate or triflic acid apparently transform vinyl orthoesters and acrolein acetals to powerful transient allyl cation dienophiles (214) and (219), respectively. The ionic Diels-Alder reactions (213) (216)... 

The Achilles heel of the AIMS system was the Redwood microvalves that were used for positive flow shut-off Failures generally occurred when the system was first powered up, which suggested that power transients were the cause of many of the problems. Specifically, these valves were thermally actuated, so opening one of the SOVs produced a large transient current draw of about one amp, but for less than % second. Adjustments were made in the LabVIEW control program to prevent the simultaneous actuation of multiple valves. In particular, the valves were not allowed to actuate when power was first applied to the chassis. 

The PMS should be designed to monitor the status of all the main circuits in the system, the generator currents and powers, and all the outgoing currents and powers. The monitoring should be a cycle updating process, with a cycle period of at least five cycles of fundamental frequency e.g. 100 to 250 millisec, to allow power transients to decay. 

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. 

Fig. 8 Reactor Power Transients for Various Reactivity Insertion with p =100 /s...

Indirect effects of lightning can cause power transients and anomalies that can damage sensitive/criti-cal LRUs or damage wire looms. EMI can be induced into wiring by the lightning currents flowing on the outside of the fuselage. 

The investigation of safety and more particularly of severe accident conditions is important for accelerator driven systems (ADS). Subcritical ADS could be of particular interest for the actinide transmutation from the safety point of view, because fast reactors with Neptunium, Americium and Curium have a much smaller fraction of delayed neutron emitters (compared to the common fuels and U), a small Doppler effect and possibly a positive coolant void coefficient. This poses a particular problem of control since the fraction of delayed neutrons is essential for the operation of a nuclear reactor in the critical state. In addition, the IRC presented in the past a review of accelerator-driven sub-critical systems with emphasis on safety related power transients followed by a survey of thorium specific problems of chemistry, metallurgy, fuel fabrication and proliferation resistance. 

Theory of power transients in the SPERT I reactor, AEC report TID-4500, 13th ed., Ramo-Wooldridge Company, January, 10, 1958. 

To overcome these problems various concepts of accelerator driven systems aiming at the transmutation of actinides and long lived fission products have been proposed in the recent past. The JRC presented a review of accelerator-drivai sub-oitical systems with emphasis on safety related power transients followed by a survey of thorium specific problems of chemistry, metallurgy, fuel fabrication and proliferation resistance. 

The negative reactivity effects of coolant density and fuel temperature attenuate the power transient following control system faults. 

The reactor core is designed so its nuclear characteristics do not contribute to a divergent power transient. The reactor is designed so there is no tendency for divergent oscillation of any operating characteristics considering the interaction of the reactor with other appropriate plant systems. 

Finally, inversion has already been used in robotics for sizing manipulators, by Potkonjak and Vukobratovic [39, 52] who introduced criteria based on power, transient power (first time derivative of power), and power jerk (second time derivative of power) and by Dequidt et al. [9, 10] who proposed a selection method of high-performance motion servomechanisms. 

Time-dependent neutron population data for these reactivity measurements are obtained from high sensitivity ion chambers located outside the reactor core. This Information is multiscaled into an on-line computer at a rate of up to 100 cps, and is used to determine reactivity as a flinction of time during the course of a. power transient. This technique allows a complete safety-rod scram-time and worth measurement to be made within one minute, as compared to several hours by other methods. 

W. R. STRATTON, L. B. ENGLE, and D. M. PETERSON, "Reactor Power Transient Studies, International Conf. on Engineering of Fast Reactors for Safe and Reliable Operation, Karlsruhe, Germany (Oct. 9-13, 1972). 

As in all light water-moderated and low-enrichment reactors, the fuel Doppler reactivity coefficient is negative and prompt in its effect, opposing reactor power transients. When reactor power increases, UO2 temperature increases with minimum time delay, resulting in higher neutron absorption by resonance capture in the U-238. 

Separation of coolant and moderator and the slow time response of moderator temperature eliminates moderator temperature feedback effects on power transients. The only way of diluting moderator poison (if present) is through an in-core break, which is small and hence would have an effect that is slow relative to SDS capability. 

As shown in Table 4.2, large break LOCA events involve the most physical phenomena and, therefore, require the most extensive analysis methods and tools. Typically, 3D reactor space-time kinetics physics calculation of the power transient is coupled with a system thermal hydraulics code to predict the response of the heat transport circuit, individual channel thermal-hydraulic behavior, and the transient power distribution in the fuel. Detailed analysis of fuel channel behavior is required to characterize fuel heat-up, thermochemical heat generation and hydrogen production, and possible pressure tube deformation by thermal creep strain mechanisms. Pressure tubes can deform into contact with the calandria tubes, in which case the heat transfer from the outside of the calandria tube is of interest. This analysis requires a calculation of moderator circulation and local temperatures, which are obtained from computational fluid dynamics (CFD) codes. A further level of analysis detail provides estimates of fuel sheath temperatures, fuel failures, and fission product releases. These are inputs to containment, thermal-hydraulic, and related fission product transport calculations to determine how much activity leaks outside containment. Finally, the dispersion and dilution of this material before it reaches the public is evaluated by an atmospheric dispersion/public dose calculation. The public dose is the end point of the calculation. 

Clearly, reforming CH4 is not a trivial task, and many materials, catalysts, and parameters must be properly selected and controlled. During the power transient process, a sudden change in the need for H2 leads to corresponding changes in the flows of CH4, H2O, and O2, which makes control more difficult. The transient response time of a reformer is normally on the order of 10 s or slightly longer. 

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