Quenching front

Quenching (front the solid state). Metastable alloys have been very familiar to metallurgists for a long time now. Several alloys employed in everyday applications contain metastable phases. Typical examples are quenched steels and precipitation hardened aluminium alloys. Until the 1960s, metastable alloys were always obtained by quenching (rapid cooling) from the solid state. 

Fig. 60. Schematic drawings of quenching methods and cooling rates in (a) liquid argon (b) a wedge-shaped cut in a copper block (c) between two copper plates. 1, ignitor, 2, quenched front 3, thermocouples 4, quenching medium (Adapted from Mukasyan and Borovinskaya, 1992).

Conduction limited. Here, the rate of removal of stored energy from the hot wall in front of the quench front is governed by conduction from the hot zone to the colder zone upstream of the front. 

The modeling of the quenching phenomenon has usually been focused on the conduction limited case, which is arguably the most important. Models are usually constructed in terms of a coordinate framework that moves at the quench front velocity (ur). The models take two main forms ... 

FIGURE 15.148 Movement of quench front calculated for an increasing flow transient (from Hewitt and Govan [298], with permission from ASME). 

The water rods are neglected which is conservative from the viewpoint of their role as a heat sink. The effect of the water rods on the quench front velocity should be assessed in a future study. The SCRELA reflood module is used in the calculation for which the flow chart is shown in Fig. 6.19. This module, also developed at the early stage of SCWR studies in the University of Tokyo like the SCRELA blowdown module, was based on the REFLA of IAEA [20, 21]. The flow chart of the calculation is shown in Fig. 6.19. The SCRELA reflood module includes the system momentum calculation, the thermal equilibrium relative velocity correlation, and the quench front velocity correlation . 

The core water level is calculated from a detailed core thermal-hydraulic calculation. The quench front is calculated by a theoretical correlation proposed by Yamanouchi, et al. [24]. The heat transfer coefficient sharply changes by about 2 orders of magnitude in the vicinity of the quench front. In order to prevent the numerical instability caused by the abrupt change in the heat transfer coefficient, the neighboring nodes of the quench front are more finely divided into a size 1/100 of the thickness of the normal node as shown in Fig. 6.21. The flow regimes assumed in the reflood analysis are described in Fig. 6.22. Various heat transfer correlations are prepared according to the flow conditions. Table 6.9 [21] summarizes them. 

Fig. 6.21 Noding near quench front in SCRELA reflood module...

Fig. 6.23 Comparison of quench front propagations. (Taken from ref [6] and used with permission from Atomic Energy Society of Japan)...

The analysis results of the reflooding phase following the end stage of Fig. 7.107 [37] are shown in Fig. 7.111 [37]. The changes of the quench front and hottest cladding temperature in the upward flow seed channel are quite similarly to those of the Super LWR. The peak temperature is much lower than the criterion of 1,260°C. 

On the other hand, the quench front propagates much faster, and hence, the increase in the hottest cladding temperature is almost zero in the downward flow seed channel. This is because the pressure drop from the quench front in the downward flow seed channel to the steam discharge point (break) is much smaller than that from the quench front in the upward flow seed channel to another steam discharge point (the quencher in the suppression pool). 

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