Study on thermal hydraulic characteristics under startup of SCWR

Progress in Nuclear Energy 122 (2020) 103266

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Study on thermal hydraulic characteristics under startup of SCWR Laishun Wang, Ph.d a, Yuan Yuan, Ph.d a, *, Jianqiang Shan, Ph.d b, Xiaoying Zhang a a b

Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-sen University, Zhuhai, Guangdong, China State Key Laboratory of Multiphase Flow in Power Engineering, School of Nuclear Science and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Supercritical water reactor SCTRAN Recirculation pressure startup

To investigate the startup characteristics of the supercritical pressure water-cooled reactor (SCWR) system, a complete startup system model of the SCWR was established with the analysis code SCTRAN, based on the highperformance light water reactor (HPLWR) steam cycle and SCWR circulation startup loop. The correctness of the model was verified in comparison with the steady-state parameters of the steam cycle of the HPLWR. A sliding pressure startup procedure with a circulation loop that employs a control system was analyzed, and the transient performances of the core, steam drum, turbine, reheaters, steam extraction and heaters at each stage were ob­ tained. The calculation results showed that the startup sequence and startup thermal parameters could fully meet the expectations: the system starts up stably and the core remains in the single phase; the inlet steam of the turbine stays superheated; the core inlet temperature can reach 280 � C after the adoption of the high-pressure and low-pressure heaters; and the inlet pressure of the high-pressure turbine can be kept constant. During the startup process, the maximum cladding surface temperature (MCST) remains below the limit of 650 � C, sug­ gesting that the entire startup procedure is safe and reliable.

1. Introduction Since the startup involves the process ranging from the subcritical region to the supercritical region, it has become an important research objective to analyze the thermo-hydraulic characteristics of SCWR startup (Oka and Koshizuka, 2001). The University of Tokyo was the first to analyze the startup characteristics of SCWR. There are three typical startup modes for SCWR, namely the constant pressure startup system, steam-water separator sliding pressure startup system and recirculation sliding pressure startup system. The latter two are classi­ fied as sliding pressure startup. The feasibility of the three systems has been studied from the thermal aspects by Nakatsuka et al. (2001). Both the more simplified procedure and better economy show that the sliding pressure startup system is generally better than the constant pressure startup system. Furthermore, the recirculation pressure startup scheme is startup using the recirculation system, where the core is in a single-phase state during the whole process, with no DNB phenomenon or flow instability. The previous investigation of startup characteristics was carried out by several researchers. The recirculation sliding pressure startup system of high-temperature supercritical-pressure light water-cooled thermal reactor (Super LWR) was introduced in detail by Oka et al. (2010) and

Tin et al. (2004). The sliding pressure startup characteristics of super­ critical light-water cooled fast reactor (Super FR) and sensitivity of the main parameters of the startup system were analyzed by Oka (2013). To simulate the thermal-hydraulic dynamic behavior of the mixed SCWR core during the startup process, Fu et al., 2011 modified the ATHLET-SC code to realize the trans-critical calculation under the two-fluid model. Moreover, SCWR sliding startup characteristics were analyzed by Chen et al. (2012). However, the above studies mainly focused on the ther­ modynamic characteristics of the core modeling during the startup process, neglecting the thermodynamic analysis of the recirculation loop and the once-through direct cycle. The objective of the paper is to analyze the startup process and the recirculation sliding pressure startup characteristics within the entire system. A variety of SCWR thermal cycle concepts have been proposed by Europe, Japan and other countries. The Super LWR steam cycle includes one steam separator, one two-stage reheater, one high pressure turbine, one intermediate pressure turbine and two low pressure turbines (Oka et al., 2010). Huang et al. (2013) proposed the design schemes of steam turbine and steam cycle based on the design of Super LWR. Brandauer et al. (2009) put forward the HPLWR steam cycle by improving the Super LWR steam cycle (Oka et al. (2010). A complete startup model was established with reference to the European HPLWR steam cycle

* Corresponding author. E-mail addresses: [email protected] (L. Wang), [email protected] (Y. Yuan), [email protected] (X. Zhang). https://doi.org/10.1016/j.pnucene.2020.103266 Received 9 May 2019; Received in revised form 22 January 2020; Accepted 25 January 2020 Available online 6 February 2020 0149-1970/© 2020 Elsevier Ltd. All rights reserved.

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Fig. 1. The heat transfer logic diagram of the new wall heat transfer model.

Fig. 2. SCWR sliding pressure startup with the circulation loop.

(Brandauer et al., 2009) and the Japanese SCWR recirculation startup loop (Oka et al., 2010). Then, the thermal hydraulic characteristics of the entire SCWR recirculation sliding startup process were analyzed based on the SCTRAN code (Wu et al., 2013).

are as follows. The mass conservation equation is:

2. SCTRAN theoretical models

where：

SCTRAN, developed by Xi’an Jiaotong University, is a onedimensional thermal hydraulic system analysis code for SCWRs (Wu et al., 2013). A homogenous equilibrium mixture model is applied in SCTRAN and the code has been verified using other codes, such as RELAP5 (Gou et al., 2012). Typical accidents, such as loss of coolant accident (LOCA) and loss of coolant flow accident (LOFA), of Canada deuterium–uranium SCWR (Canadian SCWR), CGNPC-SCWR and CSR1000 are calculated (Gou et al., 2012). Based on the SCTRAN code, the complete SCWR startup system model is established (Fig. 3 and Fig. 4). The basic equations of the homogenous equilibrium mixture model

ρ ¼ αg ρg þ αl ρl

∂ ∂ ρA þ W ¼ 0 ∂t ∂z

(1)

(2)

W ¼ Wg þ Wl ¼ αg ρg νg þ αl ρl νl

�

(3)

The momentum conservation equation is:

∂ ∂ W þ Wν ¼ ∂t ∂z

A

∂p ∂z

2Aρνjνj ftp þ ρAgz Dh

and the energy conservation equation is:

2

(4)

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Fig. 3. SCTRAN model for the CSR1000 core.

Fig. 4. SCTRAN model for the entire SCWR startup system.

� � X� � 1 � 1 L W2 Wg hg þ Wl hl þ Wg νg νg þ Wl νl νl 2A ρ 2 j � � þ Wg z zj þ Q

3.1. Control system design

d U¼ dt

The control of the SCWR circulation startup system should be able to meet the control requirements needed for the pressure, thermal power, temperature, steam drum water level and coolant flow rate. Therefore, we designed six different control systems for the SCWR startup pro­ cedure (Table 3). The basic control methods and equations can be introduced by reference (Ishiwatari et al., 2003, Dong et al., 2016).

(5)

Staggered grid discretization is applied in space and the basic equations are discretized using the control volume balance method. The wall heat transfer model used in the developed SCTRAN model is shown in Table 1. The heat transfer logic of the wall heat transfer model is shown in Fig. 1. Pressure is the criterion for judging the trans-critical and subcritical region heat exchange mode.

3.2. SCWR startup model A complete startup system is proposed (Fig. 2) based on the HPLWR thermal cycle (Brandauer et al., 2009), SCWR circulation startup loop, and CSR1000 core design parameters. The circulation loop (Fig. 2 left side) includes a steam drum, a heat exchanger and a pump for startup. The steam drum water level can ensure that the system pressure rises steadily, the heat exchanger helps maintain the inlet temperature, and the pump for startup provides the driving force of the coolant flow. The once-through direct steam cycle (Fig. 2 right side) consists of a high pressure turbine, an intermediate pressure turbine and a low pressure turbine. Since the outlet coolant of the high pressure turbine is

3. SCWR complete startup system model CSR1000 concept was proposed by Nuclear Power Institute of China (NPIC). Since CSR1000 is still in the conceptual design stage, only its core parameters are used in the system modeling, while the loop design scheme and operating parameters are determined by referring to HPLWR. Table 2 shows the main design parameters of CSR1000 core (Xia et al., 2013).

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Table 1 Heat transfer correlations used in the SCTRAN. Application Natural convection

Correlations Laminar Turbulence

Forced convection

Turbulence

Laminar Default Vertical rod bundle correction

Nucleate boiling

Subcooling correction Saturation

Transition boiling Film boiling

McAdams correlation McAdams (1954) Warner-Arpaci correlation Warner and Arpaci (1968) Sellars correlation (Sellers et al., 1956) Dittus-Boelter correlation Dittus and Boelter (1985) Inayatov correlation (Inayatov and AY, 1975) (parallel flow) Kutateladze correlation (Inayatov and AY, 1975) (Parallel flow and cross flow) Collier correlation (Collier and Thome, 1994) Chen correlation Chen (1966) Forster-Zuber correlation (Forster and Zuber, 1955) Butterworth correlation (Bjornard and Griffith, 1977)

Chen-Sundaram-Ozkaynak correlation Chen et al. (1977) Subcooling correction Sudo-Murao correlation Sudo and Murao (1976) Saturation Groeneveld-Leung PDO lookup table Groeneveld et al. (2003) Bromely correlation (Bromley, 1949) Sun radiation correlation Sun et al. (1976)

Critical heat flux Minimum film boiling temperature

Groeneveld CHF look-up table developed in 2006 Groeneveld et al. (2007) Berenson correlation (Berenson, 1961) (default) Groneveld-Stewart correlation (Groeneveld and Stewart, 1982)

Vapor generation rate

Laminar

Vertical tube

Nusselt correlation Nusselt (1916)

Laminar

Horizontal tube

Chato correlation Chato (1960)

Turbulence

Shah correlation Shah (1979)

Determine steam generation

Saha-Zuber correlation Ha (2004)

Vapor generation rate

Lahey correlation Lahey (1978)

h h h h

h¼

� � qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Gcross Dht k Pr0:4 ⋅ h2parallel þ h2cross hcross ¼ 0:21 Dht μ �

F ¼

Fsat

0:2ðTspt

q ¼ hmac ðTw 0:00122

1

Tf ÞðFsat

Tspt ÞF þ hmic ðTw

0:49 k0:79 C0:45 pf ρf f

!

0:24 σ0:5 μ0:29 h0:24 fg ρg f

qtb ¼ qCHF Af þ hg ðTw

1Þ Tsat > Tf > Tspt 5 Tf < Tspt 5 k Tspt ÞShmac ¼ 0:023⋅Re0:8 ⋅Pr0:4 ⋅ hmic ¼ Dht

0:75 ΔT0:24 w ΔP

Tg Þð1 Af Þ hg ¼ 0:0185Re0:83 Pr1=3 Af ¼ e

hf ¼ hBr f1 þ 0:025max½ðTspt

λðTw Tspt Þ0:5

Tf Þ; 0:0�g

Gas：hwfspt ¼ hf þ qwf =ðTw Tspt Þ Liquid: hfb ðTw Tspt Þ hwfspt ðTw Tspt Þ Tw Tspt #0:25 " gρg k2g ðρf ρg Þh’fg Cpg hBr ¼ 0:62 h’fg ¼ hfg þ 0:5Cpg ðTw LðTw Tspt ÞPrg 1 � qwf ¼ Fwf σðT4w T4spt ÞFwf ¼ � Rw Rw Rf 1 þ þ Rg Rf qCHF ¼ fðP; G; XeÞ hwgg ¼

Tspt Þ

� � � �1=2 � �1=3 ρg hfg gðρf ρg Þ 2=3 μg σ TMFB ¼ Tspt þ 0:127 k ρ þ ρ gð ρ ρ Þ gð ρ ρ Þ g f g f g f g 8 557:85 þ 4:41 � 10 5 P 3:72 � 10 12 P2 > > > > � � > 4 > 10 xe > < min 0; P � 9MPa 2:82 þ 1:22 � 10 6 P TMFB ¼ > > > > > Pcrit P > > : Tspt þ ðTMFB;9MPa Tspt Þ P > 9MPa Pcrit 9 � 106 TMFB ¼ 363:6 þ 38:37 ln P þ 0:02844P 3:86 � 10 6 P2 þ aðTw Tspt Þ 8 17:1 > < Tw > Tspt a 3:3 þ 0:0013P > : 0 Tw � Tspt " " #1 #1 3 3 kf 3μ2f Ref μ2f Ref ¼ 0:9086 hNusselt ¼ 1:2 � δ ¼ δ 4gρf ðρf ρg Þ gρf ðρf ρg Þ #1 " gρf ðρf ρg Þhfgb k3f 4 hchato ¼ 0:296 Dht μf ðTspp Tw Þ � � � � 3:8 kf 0:4 Re0:8 hShah ¼ hsf 1 þ 0:95 hsf ¼ h1 ð1 xs Þ0:8 h1 ¼ 0:023 f Prf Z ¼ Z Dht � �0:4 �0:8 � 1 P 1 xs 8 Pcrit St⋅Pe0:124 Cpf > ’ > Pe > 52000 > < hf;sat 0:0287 h’cr ¼ > > St⋅Pe1:08 Cpf > : h’ Pe � 52000 f;sat 918:525 } minðh’f ; h’f;sat Þ h’cr ρf ½h’f;sat minðh’f ; h’f;sat Þ� qf Aw Γw ¼ MulMul ¼ ’ εp ¼ ρg hfg Vðh’g;sat h’f Þ ðhf;sat h’cr Þð1 þ εp Þ

Chen correlation Chen (2011)

Condensation

k ðGr⋅PrÞ1=4 , 104 < ðGr ⋅PrÞ < 109 Dht k ¼ 0:10 ðGr⋅PrÞ0:3333 , 109 < ðGr ⋅PrÞ < 1013 Dht k ¼ 4:36 Dht k ¼ 0:023⋅Re0:8 ⋅Pr0:4 ⋅ Dht Pbundle ¼ hDB ⋅ Dht

h ¼ 0:59

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downcomer (315), first process water rod (340–349), first process core channel (350–359), second process water rod (360–369), second process core average channel (370–379), second process core heat channel (380–389) and lower plenum (320). The left side of Fig. 4 shows the recirculation startup loop, including the steam drum (502–503), heat exchanger (440–449) and pump for startup (130). The right side of Fig. 4 shows the once-through cycle, including the new steam starting from the reactor core decompression process until it enters the turbine (510 and 5), high pressure turbine (100–101), intermediate pressure turbine (102–104), low pressure turbine (105-105), the condenser (14), the condensate pump (818), the water tank (826), the feed water pump (828), the 9th steam turbine extractions and 7th heaters. Then the complete SCWR startup system model is established (Fig. 4), where the system temperature, pressure, power, liquid level and flow rate can be controlled by adjusting the valve opening, control rod insertion speed, discharge flow under steam drum and fluid flow rate.

Table 2 Main parameters of CSR1000. Parameters

Value

Parameters

Value 1

Thermal/Electric power/MW Thermal efficiency/% Neutron spectrum Average power density/kW⋅cm 3 Active core height/m Coolant flow scheme Core pressure/MPa

~2300/ 1000 ~43.5 Thermal ~60.0

Main coolant flowrate/kg⋅s Fuel type Quantity of fuel assemblies Quantity of fuel rods

UO2 177 39,648

4.2 Double 25.0

5.5 310S ＜39.0

Inlet/outlet temperatures/� C Hot channel factor

280/500 1.26

MCST/� C

650

Fuel rod diameter/mm Cladding material Maximum linear heat generation rate/kW⋅m 1 Fuel doppler feedback coefficient/$⋅K 1 Moderator density feedback coefficient/$⋅kg 1⋅m 3

1200

3.54 � 10 3 2.046 � 10 2

4. Comparison of steady state calculation results of SCWR

Table 3 SCWR Control system. Control system

Control method

Heat exchanger outlet temperature control

The temperature is kept constant by regulating the secondary side flow of the heat exchanger or the condenser. The change of thermal power is sensitive to insertion reactivity.

Power control

Pressure control

Steam drum water level control Once-through directcycle loop inlettemperature control Coolant flow rate control

The pressure is kept constant by regulating the opening of the control valves The water level is kept constant by regulating the flow rate discharge from the steam drum. The extraction steam flow rate is sensitive to new steam entering the heater. The coolant flow rate is kept constant by regulating the opening of the control valves.

To verify the correctness of the system model, the steady state value was compared with the reference (Brandauer et al., 2009) (Table 5), and the results were satisfactory. The maximum relative error is 6.5% occurring in the turbine efficiency and electric power. This is because the pressure loss caused by pipe wall friction is small, and that the turbine enthalpy loss is relatively small. The other relative error of the remaining parameters is less than 6%, suggesting that the model can be used to analyze and calculate the startup process.

Equation �� � uðsÞ KI ¼ KP þ � ΔTðsÞ s � 1 þ T1 s ⋅KT ð1 þ T2 sÞTset � vρmax e=b ðjej < bÞ vρ ¼ , vρmax ðjej � bÞ Z t ρ ¼ vρ dt

5. Analysis of recirculation sliding pressure startup system

0

ΔVðsÞ 1 þ T1 s ¼ K1 � þ K2 � ΔPðsÞ 1 þ T2 s 1 s � ΔmðsÞ 1 þ T1 s ¼ K1 � þ ΔHðsÞ 1 þ T2 s � p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 K2 � ⋅ ⋅kp b2 4ac s ρmix � � ΔVðsÞ KI ¼ KP þ � ΔTðsÞ s 1 þ T1 s ð1 þ T2 sÞTset ΔVðsÞ 1 þ T1 s ¼ K1 � þ K2 � ΔmðsÞ 1 þ T2 s 1 s

According to the SCWR startup sequence (Table 6), the startup procedure consists of four phases: the raising of feed water temperature, pressurization, switching from the recirculation line to the once-through line and power-raising. The once-through direct cycle is used for the phase IV startup and rated operation. The entire startup process takes about 61.59 h. The performances of the parameters are shown in Figs. 5 and 6. During the startup process, based on the control system, such as thermal power control system, flow control system, temperature control system and steam drum water level control system, the core power and the flow rate operate in accordance with the requirements, the core-inlet coolant temperature can be maintained at 280 � C, the outlet coolant temperature gradually in­ creases with time until the rated operating temperature reaches 500 � C, the same as system pressure increases with time until the rated operating pressure reaches 28 MPa, the maximum cladding surface temperature (MCST) does not exceed the limit of 650 � C throughout the startup process (Fig. 6). The thermo-hydraulic characteristics of the once-through cycle during the phase IV of the startup procedure can be seen from Fig. 7 to Fig. 11. The pressure, temperature, and flow changes of the first stage of the high-pressure turbine, first stage of the intermediate-pressure and fourth stage of the low-pressure turbine are shown from Figs. 7–9. As the core outlet coolant flow rate increases from 25% rated flow to 100% rated flow, in order to maintain the core-inlet coolant temperature of 280 � C, the once-through cycle core inlet temperature control system changes the new steam flow rate of reheater first by adjusting the new extraction steam bypass valve opening. The coolant flow in the steam turbine and condenser will curve up under the influence of new steam extraction and steam extractions (see Fig. 8). The turbine inlet void fraction values can be seen in Fig. 10 (b). Since they are all 1.0 in the startup procedure, the steam separator is not required and only the first stage reheater is needed to ensure that the turbine inlet condition is overheated. Since the extraction rate of the new steam is higher than that of the flow rate of the coolant at the turbine inlet, the outlet temperatures of the primary side and secondary side of the reheater will slowly rise with time (Fig. 10 (c)). In addition,

Table 4 Main parameters of the turbine on SCWR. Parameters

Value

Turbine power/MW Rotate speed/r⋅min 1 Life steam/t⋅h 1 Main steam pressure/MPa Main steam temperature/� C Low pressure turbine exhaust steam pressure/kPa Design cooling water temperature/� C Number of stages Feed water temperature/� C

1006 3000 4284 24.65 493 5.0 20.0 HP 2 þIP 3 þ LP 4 280.0

supercritical steam, no steam separator is required and the number of reheater is reduced to 1 (Fig. 2). The water tank between the high and low pressure feed water heaters is not only used to stabilize flow fluc­ tuations, but also serves as a deaerator and an open heater. The main parameters of the turbine are shown in Table 4. The CSR1000 core coolant flow adopts the two-pass design scheme, and its main parameters can be seen in the reference (Xia et al., 2013). Fig. 3 shows the core model based on the SCTRAN code. The model simulates core structures, including the upper plenum (305), 5

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Table 5 Comparisons between SCTRAN calculation results and literature results (Brandauer et al., 2009) under the rated condition. Location

Parameter/Unit

SCTRAN result

Literature result

Relative error/%

Core outlet

Temperature/ C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Temperature/� C Enthalpy/kJ⋅kg 1 Pressure/MPa Thermal power/KW Tubine net power/KW Efficiency/%

500.00 3181.40 24.00 488.24 3161.32 22.60 315.49 2808.93 4.25 300.64 1324.76 22.49 426.63 3275.76 4.04 353.29 3135.59 2.36 290.43 3014.59 1.41 225.28 2890.72 0.78 150.58 2754.20 0.37 109.53 2600.95 0.14 76.67 2435.03 0.04 32.83 2187.36 0.01 29.98 125.60 1.35 70.76 297.30 1.15 102.02 428.30 0.85 134.22 564.29 0.75 156.19 658.41 0.55 0.55 158.85 687.73 26.70 187.85 811.88 26.50 215.70 932.89 26.30 243.10 1054.90 26.10 280.04 1230.11 26.00 280.04 1230.10 25.00 2300.00 1065.33 46.32 2 þ 3þ4 ¼ 9 2 þ 3þ3 ¼ 8 0.90

500.00 3181.40 24.00 494.01 3181.40 22.60 322.69 2824.00 4.25 288.19 1271.70 22.40 441.01 3309.70 4.04 365.92 3166.00 2.36 300.58 3042.00 1.41 233.27 2915.00 0.78 158.85 2775.00 0.37 109.48 2621.00 0.14 76.75 2449.00 0.04 32.88 2196.00 0.01 29.97 125.60 1.35 70.97 298.00 1.15 102.69 431.00 0.85 135.24 569.00 0.75 155.57 656.26 0.55 0.55 159.68 689.85 26.70 188.86 814.00 26.50 216.29 935.00 26.30 243.38 1057.00 26.10 280.03 1230.20 26.00 280.00 1230.20 25.00 2300.00 1000.15 43.49 2 þ 3þ4 ¼ 9 2 þ 3þ3 ¼ 8 0.85

0.00 0.00 0.00 1.20 0.60 0.00 2.20 0.50 0.00 4.30 4.20 0.40 3.30 1.00 0.00 3.50 1.00 0.00 3.40 0.90 0.00 3.40 0.80 0.00 5.20 0.80 0.00 0.00 0.80 0.00 0.10 0.60 0.00 0.10 0.40 0.20 0.00 0.00 0.00 0.30 0.20 0.00 0.70 0.60 0.00 0.80 0.80 0.00 0.40 0.30 0.00 0.00 0.50 0.30 0.00 0.40 0.30 0.00 0.30 0.20 0.00 0.10 0.20 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 6.50 6.50

�

High pressure turbine first stage High pressure turbine second stage Reheater second side outlet Reheater outlet Intermediate pressure turbine first stage Intermediate pressure turbine second stage Intermediate pressure turbine third stage Low pressure turbine first stage Low pressure turbine second stage Low pressure turbine third stage Low pressure turbine fourth stage Condenser outlet Condensation pump outlet pressure HP7 outlet HP6 outlet HP5 outlet Water tank outlet Feed pump inlet pressure Feed pump outlet HP4 outlet HP3 outlet HP2 outlet HP1outlet Core inlet System Turbine stage Turbine extraction stage Power generation per kilogram of fluid/kw⋅kg

1

6

5.50

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Table 6 Sliding pressure startup procedure. Pressure/MPa

Pre-start state

I. The raising of feed water temperature

II. Pressurization

III. Switching from the recirculation line to the oncethrough line

IV. Powerraising

Pressure/MPa Power/% Outlet temperature/ � C inlet temperature/� C Flow rate/%

0.1 0.1 80

0.1 → 6.5 0.1 → 0.1 80 → 280

6.5 → 25.0 0.1 → 9.0 280 → 374.5

25.0 9.0 → 25.1 375 → 500

25.0 25.1 → 100 500

80 25

80 → 280 25

280 25

280 25

280 25 → 100

the increase of flow rate will lead to the increase of pressure drop loss in the loop. Through regulating the turbine inlet pressure control system and once-through cycle core inlet coolant temperature control system, the high pressure turbine inlet pressure and the core inlet temperature can be maintained at 24 MPa and 280 � C respectively (Fig. 10 (d) and Fig. 10 (a)). Moreover, the pressure in the loop will rise under the interaction of the two control systems (Fig. 10 (e)). Therefore, the tubine net power will first rise and then flatten (Fig. 10 (f)). The once-through cycle mainly includes two types of pumps: the condensation pump and the feed pump. As the lift pressure drop of pump decreases with the increase of flow, the outlet pressures of the condensation pump and the feed pump will decrease with the increase of the flow rate (Fig. 10 (g) and Fig. 10 (h)). The change of the outlet temperature at each heater during the fourth stage of the startup procedure can be seen from Fig. 11. The temperature of ph1-ph7 is basically unchanged during the startup. When both the flow rate and thermal power are increased to 100% rated value, the phase IV startup procedure is completed, with which the entire system startup procedure is completed. Then the system is in normal operation. Fig. 6. Transient MCST performance of the startup procedure.

6. Conclusion

the steam drum, reactor core, turbines, reheater, condenser, water tank and heaters are obtained during the startup based on the SCTRAN code for SCWR. The following conclusions can be drawn: 1) the system pressure, temperature, power, flow and other parameters will change according to the expected startup sequence through the control system adjustment, with a MCST below 650 � C; and (2) the system starts stably which

The entire recirculation sliding startup system model can be used to analyze the response of the steam turbine, extractions, reheater, and regenerators during the startup process which is different from the startup model only for the core modeling. The system-wide response curve can be obtained, which is more consistent with the actual situa­ tion. In this study, the startup characteristics of the entire recirculation sliding startup system were analyzed and the variation characteristics of

Fig. 5. Transient performance of the startup procedure. 7

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Fig. 7. Transient performance of the first stage of the high-pressure turbine.

Fig. 8. Transient parameters of the first stage of the intermediate-pressure turbine.

Fig. 9. Transient parameters of the fourth stage of the low-pressure turbine.

indicates that the entire startup process is safe and reliable. There is still a lot of work needed to be done on SCWRs thermal hydraulic research. The heat transfer coefficients obtained from the lookup table above the quasi-critical point lack the correction factor for the bundle structure even though the heat transfer correlation is very important. Also, the density distribution of the coolant in the reactor

varies greatly and leads to a large change in the core power distribution, but the existing model cannot calculate the axial uneven heat conduc­ tion. In the future, the physical model of SCTRAN should be modified through the collection of experimental data and the investigation of theoretical models to further improve the calculation accuracy of the code. 8

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Fig. 10. Transient performances of pressure, temperature and flow rate of the once-through cycle.

Fig. 11. Transient performances of the outlet temperatures at each heater.

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Progress in Nuclear Energy 122 (2020) 103266

Author contributions section

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