Transient heating events

To summarize, chondrules and CAIs formed by transient heating events that processed a large fraction of the matter in the accretion disk. These heating events appear to overprint the thermal processing that produced the volatile element depletions among chondrites. The exact nature of these events is unknown, although shock waves in the nebula and the X-wind model are currently receiving the most attention. 

Transient heating events were important in the formation of the Solar System and provided the energy to produce chondrules and refractory inclusions. These objects are not predicted in astrophysical models for the formation of planetary systems. They comprise 50-80% of the mass of many primitive meteorites. However, the mechanism that produced the transient heating events is still unknown. Future work must focus on putting the details of the petrographic and chemical analysis of these rocks into an astrophysical and cosmochemical framework of Solar System formation. 

Photometric surveys to search for and constrain the frequency of transient heating events in protoplanetary disks... 

A second interpretation of carbonaceous chondrites is as primary condensates of the solar nebula. By this view, their hydrolytic alteration is due to melting in cometary nuclei during close passes with the Sun, or due to transient heating events by shock waves or collisions (McSween, 1999). Other carbonaceous chondrites show metamorphic alteration with minerals similar to those in Earth formed during deep burial under elevated temperatures and pressures (Brearley, 1999). Like soils and paleosols on Earth and Mars, carbonaceous chondrites demonstrate the great antiquity of hydrolytic weathering in dilute acidic solutions, presumably of carbonic acid derived from water vapor and CO2. These remain the principal gases released from volcanoes, and soils remain important buffers for this environmental acid. 

Chondrules comprise the major portion of most chondrites, the most abundant type of meteorites. If the achondrites and terrestrial planets formed from chondrite-like precursors, then much, perhaps most of the solid matter in the inner solar system once existed as chondrules. Even if chondrules were restricted to the chondrites, the process that formed them was important in that region. The origin of chondrules is an important unsolved problem in cosmochemistry. Chondrules formed in the Sun s accretion disk through some sort of transient flash-heating event(s). Some CAIs apparently also were melted in the disk. What was the process (or processes) that melted the chondrules and CAIs Whatever it was, it dominated the disk for at least a few million years. 

A further important and yet incompletely resolved matter is the nature of the heating event in which chondrules formed. The experimental study of Cohen et al. (2004) shows that chondrules probably formed in a matter of minutes to hours. This implies a flash heating process (Boss, 1998), such as might be caused by shock-heating or lightning strikes in the solar nebula. If this is the case then chondrule melting temperatures represent transient temperatures rather than the ambient temperature of the solar nebula. 

The plant conditions considered in the safety analyses for LWRs range from relatively mild events to DBAs and natural events that have potential for serious consequences. The transient or event in an NPP can result due to (1) component and system failures or malfunctions, (2) operator errors, and (3) common-cause internally and externally initiated events. Most of the transient leading away from normal operation are results of imbalance between core heat generation and heat removal from the core. Reactor transients generally are classified based on fhe results of probabilistic studies and engineering judgment, into the four classes of evenfs as shown in Table 22.18. 

The results in Fig. 2 indicate that nucleation began after some time had passed, most likely due to a transient heat-up time and some time required for dissolution of the amorphous gel to achieve some threshold concentration. However, it is most noteworthy that the nucleation event in zeolite crystallization systems always has been determined to have ended when only about 10-15% of the aliunino-sificate material had been consumed. That is, it is remarkable that with 85 - 90% of the aliunino-silicate reagents left in the system, the nucleation process was somehow caused to cease, while crystal growA proceeded for the duration of the synthesis. This must be noted in the context of the amorphous gel dissolving sufficiently fast that the solution phase concentration was essentially constant up to almost 80% conversion in some cases [33-35],... 

As discussed in the introduction, disruptions cause the most severe thermomechanical loading experienced in a tokamak. In each of the 500 or so disruptions expected in ITER, approximately 10-20 MJ/m will be deposited onto the first wall in 0.01 to 3 seconds. Such a disruption will cause very high thermal stresses and significant material erosion (Section 4). As these events are transient in nature, the ability of the PFC to withstand the disruption depends on the material s ability to both conduct and to absorb the deposited heat, before reaching a temperature or stress limit. For comparative purposes, a disruption figure of merit takes this into account ... 

Function event trees include primarily the engineered safety features of the plant, but other systems provide necessary support functions. For example, electric power system failure amid reduce the effectiveness of the RCS heat-removal function after a transient or small UJ( A. Therefore, EP should be included among the systems that perform this safety function. Siipfiort systems such as component-cooling water and electric power do not perform safety functions directly. However, they significantly contribute to the unavailability of a system or group of systems that perform safety functions. It is necessary, therefore, to identify support systems for each frontline ssstcm and include them in the system analysis. 

The high flame front velocities prior to attainment of the steady state probably result from the transient conditions between the combustion front and shock front. Sufficient data were lacking to show whether the shock-heated gas ignited spontaneously, immediately behind the shock front, or whether the flame front overtook the shock front. In any event, the combustion wave finally moves along with the shock wave, thus forming a detonation wave... 

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