IAEA Meeting on INPRO Collaborative Project “Performance Assessment of Passive Gaseous Provisions (PGAP)” December, 2011, Vienna A.K. Nayak, PhD Reactor Engineering Division Bhabha Atomic Research Centre Trombay, Mumbai

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 Loop Physical parameters Main CKT: Power = MW increased linearly in 100 seconds Pressure = 6.98 MPa at t=0 sec Mass Flow Rate = 0 kg/s at t=0 sec Temperature = 673K at t=0 sec Main Secondary CKT: Mass flow Rate = 2685 kg/s at t = 0 to t = 500 sec Inlet Temperature = 839 K at t = 0 to t = 500 sec Inlet 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 Conditions DHR secondary mass flow rate = 0 DHR secondary pressure = 1.0 MPa DHR secondary Temperature = 323 K POOL INITIAL CONDITIONS: Pool pressure= 0.1 MPa Pool 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 BLOWER Main features are: –Flow Area= 3.14m2 –Length =3.0m –Rated velocity= rad/s –Initial blower velocity/rated velocity=1 –Rated flow =340.0m3/s –Rated head= 30000m –Rated torque= 15019N.m –Moment of inertia= Kg/m2 –Rated density of fluid= 5.58 Kg/m3 –Pump closing takes place in 50seconds as per the velocity given.

Important dimensions ComponentsArea(m 2 )Length (m)Volume (m 3 ) Main Heat Transport System Reactor Pressure vessel Lower plenum Core Core Core Core Core Core Bypass Upper plenum Upper plenum Downcomer Primary Circuit Primary hot leg Inlet main IHX IHX primary side Outlet main IHX Buffer volume Blower V Circol Primary cold leg

Important dimensions ComponentsArea(m 2 )Length (m)Volume (m 3 ) DHR System Primary Circuit Lower plenum Core Core Core Core Core Core Bypass Upper plenum Upper plenum Downcomer Secondary Circuit Primary hot leg Inlet main IHX IHX primary side Outlet main IHX Buffer volume Blower V Circol Primary cold leg

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

Total Core Power at Steady State

20 Model Qualification – summary of Steady-state results 3.Error defined as: ReferenceRELAP5Error(%) Main Vessel Inlet/Outlet Gas Tempreratures (°C)400/850400/851 Core Outlet Gas Temperature (°C) Main Vessel Inlet/Outlet Gas Pressure (MPa)7.12/ /6.98 ∆P Vessel (Uppper Plenum/Lower Plenum) (MPa) Main Loop Mass Flow Rate (kg/s)340.8x Core Inlet Mass Flow Main Loop IHX Exchanged Power (Mw)803.3x32400

Model Qualification – summary of Steady-state results Qchannel (kg/s) (reference)RELAP5Error (%) Downcomer Core Core Core Core Core Core Bypass

SBO Transient

DHR Analysis for SBO 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)

The maximum clad surface temperature is 1190 deg C The 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 ( 0 C) Nominal ( C) Gas Temperature at Core Outlet ( 0 C) Nominal ( C) DHR Structural Temperature ( 0 C) Nominal (386 0 C) Blower Inertia Power Residual Power Pressure Primary heat Transfer Area Core Pressure Drop Thermal Inertia DHR Heat Transfer Area Inlet Loss Coefficient Outlet Loss Coefficient 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 ( 0 C)Gas Temp at Channel Outlet( 0 C) DHR Structural Temp( 0 C) Average(µ) Standard Deviation(σ) Variation Coefficient (σ/ µ) in % Minimum Maximum X90% X95% X99% X99.99%

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

Identification of natural circulation failure Criterion Failure limit (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 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 power 2.Residual Power 3.Main Circuit Pressure 4.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 channels 9.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. 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. Fig: Pressure decreased nominal Residual power and initial power increased Parameter s/Cases Normalized Pressure Normalized Residual Power Normalized Power

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. Fig-40 Nominal Pressure, Residual power and initial power decreased (Without model uncertainty) Fig-41 Nominal Pressure, Residual power and initial power decreased (With model uncertainty)

Failure cases with Process and Model parameters Parameters/C ases Normalized Pressure Normalized Residual Power Normalized Power

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 × 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.