Thermal Analysis of The Start-Up SystemHeat flux margin during subcritical pressure

The main thermal issue during subcritical pressure is dryout. Dryout will happen if the

generated heat flux exceeds the critical heat flux (CHF). Dryout will increase the cladding surface temperature and endanger the integrity of fuel rods. In this study, CHF is calculated based on the 1995 look-up table for critical heat flux in a tube (Groeneveld, et al., 1996). The applicable ranges of the table and the parameter of the Super FR are shown in Table 3-3.

Fig. 3-9 Opening of main steam stop and turbine control valves

Fig. 3-10 Shutdown of the circulation system

In the past study the pressurization was performed independently from the power-raising. Pressure was raised by pressurizer pump while the power was kept constant.

Value of the constant power was selected to satisfy the criteria of cladding temperature and turbine inlet steam enthalpy which the turbine was started at subcritical pressure. The power should be small enough to satisfy the criterion of the cladding temperature at critical pressure

but the power should be high enough to satisfy the criterion of the turbine inlet steam enthalpy.

The power was selected at 20% of rated power and the flow rate was kept 35% to keep the MCST low around the critical pressure (Nakatsuka et al., 2001; Yi et al., 2004). This setting gives value of power to flow ratio of 0.57 during pressurization.

Table 3-3 Applicable ranges of the Groeneveld 1995 look-up table

* 4 E4 @ 8G8E CC?<64 5?8E4 A: 8F ,HC8E +

* E8FFHE8' * 4 k k

' 4 FF9?HK>: @^',(b^'+ k ?4 A>8G ,887 ,887 <4 @ 8G8E@ @ k ?4 A>8G

,887 ,887

& 8A: G;7<4 @ 8G8EE4 G<B ?4 A>8G ,887 ,887 ELBHGDH4 ?<GL GB

Fig. 3-11 Rise of pressure as function of the power (Sutanto and Oka, 2013a)

Mechanisms of the current start-up system are different from that in the past study. The pressure is increased by raising the power while the flow rate is kept constant at 25% of rated flow. The pressure will increase simultaneously with the power-raising. There is no criterion of minimum required turbine inlet steam enthalpy because the turbines are started by using supercritical steam. The pressure generation as function of the power is shown in Fig. 3-11. At

the beginning a small constant power is used to raise the inlet temperature to the operating inlet temperature. Pressure generation is started when the coolant temperature achieves the saturated temperature, 100 ^oC. The supercritical pressure is achieved at about 10% of rated power. The maximum power to flow ratio during pressurization is 0.4 which is smaller than the previous start-up system. The heat flux margin during subcritical pressure is shown in Fig.

3-12. The maximum actual heat flux in the hottest channel of blanket, seed 1 and seed 2 are much lower than the CHF. The CHF decreases with the increasing pressure while the maximum actual heat flux increases due to the increase of the power. The actual heat flux in seed 2 is the highest due to higher coolant temperature in the second pass. The margin becomes smaller near the critical pressure but the CHF is still higher than the maximum actual heat flux. There is no deterioration heat transfer near the critical pressure.

Fig. 3-12 Heat flux margin during subcritical pressure (Sutanto and Oka, 2013a)

The safety margin is also shown in departure from nucleate boiling ratio (DNBR). Fig.

3-13 shows the DNBR of each fuel channel as function of the pressure. The safety margin (DNBR) decreases with the increasing of the core pressure. Fig. 3-14 shows the DNBR of each fuel channel at 20 MPa. The value of minimum DNBR (MDNBR) of each channel is high. It is more than 8 for each channel. The safety margin is still large even around the critical pressure. However, the safety margin might be enlarged by using grid spacers in the fuel channels (Pioro et al., 2002; Zhu et al., 2014). Geometry of the grid spacers will influence the enhancement.

Fig. 3-13 DNBR of fuel channels during pressurization

<: 4 ( + B95?4 A>8G4 FF8@ 5?L4 G ' *4

<: 5 ( + B9F887 4 FF8@ 5?L4 G ' * 4

<: 6 ( + B9F887 4 FF8@ 5?L4 G ' * 4

Fig. 3-15 MCST during subcritical pressure (Sutanto and Oka, 2013a)

During subcritical pressure the MCST is low because of no dry out. The calculation results are shown in Fig. 3-15. The flow to power ratio is high. The MCST is only a little higher than the saturated water temperature. The highest MCST is happened at the second seed (seed 2) due to high coolant temperature. Based on the core design, the thermal power density of blanket is the smallest of other parts. The coolant flow rate flowing through the blanket is high enough to make the MCST low. Otherwise the thermal power density of seed 1 is similar to this of seed 2. However the coolant temperature of seed 2 is higher than this of

seed 1. It causes the heat transfer in seed 2 worse than the heat transfer in seed 1 so that the MCST of seed 2 is the highest of other parts.

Table 3-4 Sensitivity analysis during pressurization (Sutanto and Oka, 2013a)

Parameters MCST (ºC)

Inlet temperature (ºC) 1. 200

2. 280 3. 300

389 387 385 Gap volume (m3)

1. 4.14 2. 7.14 3. 13.14

387 387 387 Flow rate (%)

1. 20 2. 25 3. 35

386 387 387 3.4.2 Startup sensitivity

Variations of the inlet temperature, the gap volume and the flow rate are examined to view the influence of these parameters to the maximum cladding surface temperature (MCST).

The rates of power-raising are the same for all conditions. The calculation results are shown in Figs. 3-16 to 3-18.

Fig. 3-16 Influence of inlet temperature during pressurization (Sutanto and Oka, 2013a)

Fig. 3-17 Influence of gap volume during pressurization (Sutanto and Oka, 2013a)

In all the figures, the variation of inlet temperature, gap volume and flow rate changes the rate of pressurization but it does not influence the MCST as shown in Table 3-3. The higher the inlet temperature, the faster the rate of pressurization. Higher inlet temperature will increase the rate of vaporization which in turn increases the rate of pressurization. The MCST is not influenced much by the change of the inlet temperature due to high flow to power ratio.

The heat transfer is kept below the critical heat flux.

The larger the gap volume, the slower the rate of pressurization. Larger gap volume means the larger the whole volume of the core. It will reduce the rate of the pressurization.

The change of pressurization rate will be followed by the change of increment rate of saturated water temperature. The higher the rate of pressurization, the higher the increment rate of saturated water temperature. These different rates of pressurization do not influence significantly the MCST due to high flow to power ratio. The MCST is not sensitive to the gap volume. It is because the highest MCST is happened in the second seed. The change of the gap volume does not change the flow rate through the second seed.

Change of circulation flow rate has the same effect as that of the gap volume. Higher flow rate will decrease the rate of vaporization so that the rate of pressurization is decreased.

However, the MCST is also not sensitive to the change of the circulation flow rate.

Fig. 3-18 Influence of flow rate during pressurization (Sutanto and Oka, 2013a)

3.4.3 Thermal analysis during power raising

Fig. 3-19 MCST of the first seed assembly during power-raising phase (Sutanto and Oka, 2013a)

The calculation results of thermal analysis during power-raising are shown in Figs. 3-19 to 3-21. The MCST of the first seed assembly, blanket assembly, and the second seed is calculated for various ratios of power to main flow. According to core design (Liu et al., 2012), the MCST at rated value is 640 ^oC and it is used as the limit. Fig. 3-19 shows the allowable region in the first seed assembly during power- rising. There is a small region on the lower part where the MCST is higher than the limit. In this region the ratio between the total thermal

power of seed assemblies in the first path and the flow rate flowing through this part is high.

Fig. 3-20 MCST of the second seed assembly during power-raising phase (Sutanto and Oka, 2013a)

Fig. 3-20 shows the allowable region during power-raising phase for part of seed assemblies in the second path. Based on this figure the width of allowable region is smaller than this in the first path of seed. It is because the coolant temperature flowing through the second path is higher than this flowing through the first path. Even the coolant flowing through the second path is higher but the power to flow ratio is more limited.

Fig. 3-21 MCST of blanket assembly during power-raising phase (Sutanto and Oka, 2013a)

Fig. 3-21 shows the allowable region in the blanket. The whole region is allowable because the flow rate flowing through the blanket is high enough compared to its total thermal power of the blanket.