A.K. Nayak, PhD Reactor Engineering Division Bhabha Atomic Research Centre Trombay, Mumbai 400085

transcript

IAEA Meeting on INPRO Collaborative Project “Performance Assessment of Passive Gaseous Provisions (PGAP)”13-15 December, 2011, Vienna

A.K. Nayak, PhDReactor Engineering DivisionBhabha Atomic Research CentreTrombay, Mumbai 400085

GFR DHR Analysis for Transient 1

Computer code used : RELAP5/MOD3.2

Power = 2400 MWth No. of DHR Loops = 1 Full reactor is simulated in the RELAP5/MOD3.2

to study the passive decay heat removal behaviour of the reactor.

Thermal inertia of all the components in the main circuit have been considered.

Heat exchange between DHR hot and cold ducts through the insulation has been considered.

Steady state calculations are continued until 500 sec.

Inputs for Analysis of Main LoopPhysical parameters

Main CKT:

Power = 0- 2400 MW increased linearly in 100 secondsPressure = 6.98 MPa at t=0 secMass Flow Rate = 0 kg/s at t=0 secTemperature = 673K at t=0 sec

Main Secondary CKT:

Mass flow Rate = 2685 kg/s at t = 0 to t = 500 secInlet Temperature = 839 K at t = 0 to t = 500 secInlet Pressure = 6.5 MPa at t = 0 to t = 500 sec

Steady State Analysis

Transient Calculations continued for 500 sec to achieve the steady state

CODE achieved Steady state after 125 sec

Inputs for Analysis of DHR Loop – Initial ConditionsDHR secondary mass flow rate = 0 DHR secondary pressure = 1.0 MPa DHR secondary Temperature = 323 K

POOL INITIAL CONDITIONS:Pool pressure= 0.1 MPaPool Temperature= 323 K

Assumptions Local resistances in the fuel element is considered such

that the pressure difference in the core part is matched with the steady state conditions given.

Since the geometry of the core is complex, the lumped model is used for the simulation of the core.

The core is divided into 7 channels (6 heat generating and one bypass). Each channel is divided into 25 volumes.

The flow area and the heat transfer area are same as in the actual reactor core.

Heat transfer coefficient in the heat structure parts viz: in the core, in main IHX, in DHR IHX and in the pool IHX, is decided by the RELAP5 inbuilt models.

Dimensions Considered

BLOWERMain features are:

– Flow Area= 3.14m2– Length =3.0m– Rated velocity= 470.24 rad/s– Initial blower velocity/rated velocity=1– Rated flow =340.0m3/s – Rated head= 30000m– Rated torque= 15019N.m– Moment of inertia=0.0676 Kg/m2– Rated density of fluid= 5.58 Kg/m3– Pump closing takes place in 50seconds as per the velocity given.

Important dimensionsComponents Area(m2) Length (m) Volume (m3)

MainHeat

TransportSystem

Reactor Pressure vessel

Lower plenum 3.2 68.63Core0 0.27

0.25058160.27

Core1 0.540.50116320.54

Core2 0.810.75174480.81

Core3 0.540.50116320.54

Core4 0.630.58469040.63

Core5 0.90.8352720.9

Bypass 1.9068 5.8Upper plenum1 32.3713 3.3Upper plenum2 3.2 68.63Downcomer 5.5 7.0

Primary Circuit

Primary hot leg 1.89 2.0Inlet main IHX 2.0126 7.95IHX primary side 10.73 0.821Outlet main IHX 2.0126 7.95Buffer volume 6.0 100.0Blower 3.0 9.42V Circol 20.0 2.5Primary cold leg 1.53 2.0

Important dimensionsComponents Area(m2) Length (m) Volume (m3)

DHRSystem

Primary Circuit

Lower plenum 3.2 68.63Core0 0.27

0.25058160.27

Core1 0.540.50116320.54

Core2 0.810.75174480.81

Core3 0.540.50116320.54

Core4 0.630.58469040.63

Core5 0.90.8352720.9

Bypass 1.9068 5.8Upper plenum1 32.3713 3.3Upper plenum2 3.2 68.63Downcomer 5.5 7.0

Secondary Circuit

Primary hot leg 1.89 2.0Inlet main IHX 2.0126 7.95IHX primary side 10.73 0.821Outlet main IHX 2.0126 7.95Buffer volume 6.0 100.0Blower 3.0 9.42V Circol 20.0 2.5Primary cold leg 1.53 2.0

RELAP 5 Nodalization of Main circuit of GFR

GFR Nodalization

Mass Flow rate (Various Channels)

Mass Flow rate (total core)

Pressure in the lower and upper plenum

Helium temperature in lower and upper plenum

Variation of clad surface temperature along the height

Fuel Centre line Temperature (Steady State)

Individual Channel power at Steady State

-500 -400 -300 -200 -100 0-5.00E+007

0.00E+000

5.00E+007

1.00E+008

1.50E+008

2.00E+008

2.50E+008

3.00E+008

3.50E+008

4.00E+008

4.50E+008

5.00E+008

5.50E+008

6.00E+008

Time(S)

Power Core0 Power Core1 Power Core2 Power Core3 Power Core4 Power Core5

Total Core Power at Steady State

-500 -400 -300 -200 -100 0

-2.00E+008

0.00E+000

2.00E+008

4.00E+008

6.00E+008

8.00E+008

1.00E+009

1.20E+009

1.40E+009

1.60E+009

1.80E+009

2.00E+009

2.20E+009

2.40E+009

2.60E+009

Time(S)

Total Power To Core

Model Qualification – summary of Steady-state results

3. Error defined as:(Design value)-(RELAP5 value)

*100Design value

Reference RELAP5 Error(%)

Main Vessel Inlet/Outlet Gas Tempreratures (°C) 400/850 400/851

Core Outlet Gas Temperature (°C) 900 902 0.22

Main Vessel Inlet/Outlet Gas Pressure (MPa) 7.12/6.98 7.13/6.98

∆P Vessel (Uppper Plenum/Lower Plenum) (MPa) 0.14 0.12 14.28

Main Loop Mass Flow Rate (kg/s) 340.8x3 1019 0.09

Core Inlet Mass Flow 1020 1019 0.09

Main Loop IHX Exchanged Power (Mw) 803.3x3 2400

Model Qualification – summary of Steady-state results

Qchannel (kg/s) (reference) RELAP5 Error (%)

Downcomer 1020.2 1019 0.09

Core0 74.5 76.0 -2.01342

Core1 145.7 144.0 1.166781

Core2 208.4 206.0 1.151631

Core3 130.1 132.0 -1.46042

Core4 166.8 165.0 1.079137

Core5 193.3 194.0 -0.36213

Bypass 101.4 102.0 -0.5917

SBO Transient

DHR Analysis for SBO

0 t_Q3% t_Q3%+ 3 s

t_Q3%+ 5 s

t_Q3%+ 11 s

Core inletmass flow

Main loops isolating valves opening fraction

DHR loops isolating valves opening fraction

2 s 6 s

NCDHR phase

“DHR sequence”5 s

t_Q3%+ 16 s

Time lag for DHR blowersstart-up

0 t_Q3% t_Q3%+ 3 s

t_Q3%+ 5 s

t_Q3%+ 11 s

Core inletmass flow

Main loops isolating valves opening fraction

DHR loops isolating valves opening fraction

2 s 6 s

NCDHR phase

“DHR sequence”5 s

t_Q3%+ 16 s

Time lag for DHR blowersstart-up

After 500 sec transient calculation were continued for the DHR

Reactor Was Tripped at 500 sec

Blower Stops in 50 sec after 500 sec– valves in main

loops start closing at 47 sec and gets completely closed at 49 sec after 500 sec.

DHR Circuit Was Valved In After 55 sec Seconds And Valve Fully Opened In 60 Sec after 500 sec.

Main vessel pressure

DHR secondary side pressure

DHR secondary side Temperature

DHR water side flow rate

Gas Temperature at Main Vessel Inlet/ Outlet

Channel Flow rate

Total core flow rate

Power to the various channels

Power to the Core

Power to and from DHR secondary loop

Sensitivity analysis – Parameters considered and their variations

• Core ∆P variation ±15%• Core Power variation ±2%• Residual Power variation ±10%• Heat Transfer area variation ±25%• DHR Heat Transfer area variation ±25%• DHR inlet Loss coefficient variation ±200%• DHR outlet Loss coefficient variation ±200%• Thermal Inertia variation ±15%• Main Circuit Pressure variation ±2bar• Primary Blower Inertia ±25%

Failure Criteria

Criterion

SBO Transient

(DHR loop structural integrity)Maximum temperature of DHR structural material

850 °C

Maximum clad temperature

1600 °C

(Core upper structures integrity)Maximum temperature of gas at hot channel outlet

1050 °C

Effect of Blower Inertia ±25%

Effect of Core Power ±2%

Effect of Residual Power variation ±10%

Effect of Core ∆P variation ±15%

Effect of Heat Transfer area variation ±25%

Effect of DHR Heat Transfer area variation ±25%

Effect of Thermal Inertia variation ±15%

Effect of Main Circuit Pressure variation ±2bar

Effect of DHR inlet/outlet Loss coefficient variation ±200%

Effect of variation of all parameters (conservatively)

Effect of variation of all parameters (conservatively)The maximum clad surface

temperature is 1190 deg CThe maximum temperature of the

gas at channel outlet is 1167 deg C DHR structural temperature is 425

deg C.

Summary of results of sensitivity analysis Parameters Clad Surface

Temperature (0C)Nominal (1013 0C)

Gas Temperature at Core Outlet (0C) Nominal (1008 0C)

DHR Structural Temperature (0C)Nominal (386 0C)

Blower Inertia 1056.56 1053.00 388.50

Power 1043.78 1039.00 394.18

Residual Power 1033.00 1030.00 416.67

Pressure 1032.00 1028.00 385.90

Primary heat Transfer Area

1042.00 1041.00 382.0

Core Pressure Drop 1030.00 1025.00 390.50

Thermal Inertia 1013.33 1008.83 396.70

DHR Heat Transfer Area 1013.30 1008.79 390.60

Inlet Loss Coefficient 1013.44 1008.83 402.02

Outlet Loss Coefficient 1013.54 1008.93 402.35

Failure limits; Clad T > 1600 deg C

Gas T > 1050 deg C

DHR Structural T > 850 deg C

Statistical treatment on the effects of most critical parameters

Statistical analysis on the effects of most critical parameters

Clad Surface Temp (0C) Gas Temp at Channel Outlet(0C)

DHR Structural Temp(0C)

Average(µ) 1029.59 1011.75 389.8463

Standard Deviation( )σ 42.6107 54.62799 10.0725

Variation Coefficient( / µ) in %σ

4.138609 5.4 2.58

Minimum 962.61 901.18 374.00

Maximum 1134.22 1134.00 409.00

X90%1084.131 1081.674 402.7391

X95%1099.897 1101.886 406.4659

X99%1128.871 1139.033 413.3152

X99.99%1191.508 1219.336 428.1218

Reliability Assessment of Passive Decay Heat Removal System of GFR using APSRA Methodology

Identification of natural circulation failure

CriterionFailure limit

(DHR loop structural integrity)Maximum temperature of DHR structural material

850 °C

Maximum clad temperature1600 °C

(Core upper structures integrity)Maximum temperature of gas at hot channel outlet 1050 °C

For SBO conditions, natural circulation failure in GFR is considered to occur according to the conditions given in Table

Important parameters affecting the performance of the system

1. Core power2. Residual Power 3. Main Circuit Pressure4. Fuel Heat Transfer coefficient 5. Heat Transfer coefficient in DHR secondary side 6. DHR primary side inlet Loss coefficient 7. DHR primary side outlet Loss coefficient 8. Pressure drop in fuel channels9. Thermal Inertia of primary system components 10. Primary Blower Inertia Out of these parameters listed above parameters 1-3 are the

operating process parameters of DHR primary circuit & 4-10 are the model parameters.

Effect of process parameters in combination on failure without consideration of modeling uncertainty

Fig shows an example of the effects of increase of residual power and initial operating power from their nominal values on system behavior while the system operates at nominal pressure of 6.98 MPa. It can be observed that the gas temperature exceeds the failure criteria limits even though the clad surface temperature and DHR structural temperatures have large margins to failure.

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000

Time(s)

Clad Surface Temperature(0C)

DHR Structural Temperature(0C)

Gas Temperature at Channel outlet(0C)

Fig: Nominal Pressure Residual power and initial power varied (Without model uncertainty)

Effect of process parameters on failure without consideration of modeling uncertainty

Fig shows an example of the effect of decrease of main circuit pressure together with increase of nominal operating power on system behavior. In this case also the gas temperature at hot channel outlet exceeds the failure limit. Such failure cases are summarized in table.

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000

250300350

400450500

550600650

700750

800850900

95010001050

Time(s)

DHR Structural Temp. Clad Surface Temp. Gas Temp. at Channel out

Fig: Pressure decreased nominal Residual power and initial power increased

Parameters/Cases Normalized

Pressure

Normalized Residual

Normalized Power

1 0.98 1.1 1.02 0.98 1.0 1.013 1.01433 1.1 1.014 0.99 1.1 1.0125 1.0 1.1 1.0156 0.98 0.9 1.027 1.02 1.1 1.028 1.01433 1.05 1.029 0.99 0.99 1.02

Effect of model uncertainty on failure

Fig-40 shows an example of the effect of the variation of operating power which is decreased by 2% and residual power which is decreased by 10% keeping the system pressure at nominal value. The system in this case is found to be safe. However when model uncertainty is applied to this case the system is found to fail as shown in Fig-41. The model uncertainty is treated by considering the worst combination of all model parameters in this case.

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000

Time(s)

Clad Surface Temp. DHR structural Temp. Gas Temperature at channel outlet

Fig-40 Nominal Pressure, Residual power and initial power decreased

(Without model uncertainty)

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000100

Time(s)

Clad Surface Temperature(0C)

DHR Structure Temperature(0C)

Gas Temperature at Channel outlet (0C)

Fig-41 Nominal Pressure, Residual power and initial power decreased

(With model uncertainty)

Failure cases with Process and Model parameters

Parameters/Cases Normalized

Pressure

Normalized Residual

PowerNormalized Power

1 1.0 1.0 1.0

2 1.0 0.9 0.98

3 1.02 0.9 1.0

4 1.01433 0.9 0.987

5 1.02 1.0 0.98

6 1.01433 0.99 0.98

7 1.01433 0.985 0.985

8 1.019 0.9 0.995

Failure surface generation

Pressure

Residual Power

Root Diagnosis The root causes for the variation of the process

parameters are not known for GFR. Hence, the causes for failure are assumed in this exercise as an example of demonstration of application of APSRA methodology and not to accurately predict its reliability.

The failure probability of the PDHRS, depends on the variation of the three process parameters of the main heat transport system as discussed before.

Typical Fault Tree considering deviation of process parameters

Typical Fault Tree considering deviation of process parameters along with model uncertainty

Failure Frequency without model uncertainty

Failure Frequency with consideration of model uncertainty

The failure frequency of PDHR system in the GFR has been calculated and found to be 7.052 × 10−6/h, considering variation of process parameters only. With considerations of model uncertainty (all model parameters varied to their worst combination) the system is found to fail at nominal operating conditions.The failure frequency of the PDHRS system is found to be 7.3× 10−6/h by considering the model uncertainty. The result shows that contribution of model uncertainty is negligible (around 4%).

Conclusions For the benchmark-1 exercise during SBO

transient it is found that only one DHR natural circulation loop is sufficient for removing all the decay heat of the reactor to keep the reactor safe.

Even though the operating parameters of the reactors are varied to a possible range then no failure is found.

Clad surface temperature and DHR structural temperature are far below their failure limit. There is least margin in gas temperature at channel outlet which is also sufficient.

A.K. Nayak, PhD Reactor Engineering Division Bhabha Atomic Research Centre Trombay, Mumbai 400085

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