RELAP5-3D two-phase behavior and predictions at low pressures Documents... · operate also as...

Copyright © 2013 SCK•CEN

RELAP5-3D two-phase behavior and predictions at low pressures

Diego Castelliti, Tewfik Hamidouche SCK•CEN

[email protected]

IRUG 2016 Meeting INL, Idaho Falls, 6-7 October 2016

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Contents

l MYRRHA plant general description l MYRRHA depressurization transient l  Experimental validation at low pressures l  Conclusions

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MYRRHA plant: purposes and general design

l MYRRHA: Multi-purpose hYbrid Research Reactor for High-tech Applications

l  Pool-type Accelerator Driven System (ADS) with ability to operate also as critical reactor

l  Liquid Lead-Bismuth Eutectic (LBE) as primary coolant l Main purposes:

l Flexible fast-spectrum irradiation facility l Minor Actinides (MAs) transmutation demonstrator l ADS demonstrator l GEN-IV European Technology Pilot Plant (ETPP) in the roadmap for

Lead Fast Reactor (LFR) l MYRRHA project recognized as high priority infrastructure for

nuclear research in Europe

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MYRRHA plant: Primary Cooling System

1.  Reactor vessel 2.  Reactor cover 3.  Diaphragm 4.  4 Primary Heat

Exchangers 5.  2 Primary Pumps 6.  In-Vessel Fuel

Handling Machine 7.  Core barrel 8.  Above Core Structure 9.  Core plug 10.  Spallation window

l MYRRHA primary system design current status:

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l Completely enclosed in primary vessel (pool-type)

l Design power: 110 MW l Primary LBE:

l Lower plenum (270 °C) l Upper plenum (~325 °C)

l Cold plenum separated from hot plenum by Diaphragm supporting core barrel and components’ penetrations

l Above LBE free surface: Nitrogen layer


MYRRHA plant: Fuel Assembly

l MOX fuel, 30% wt. Pu l  Fuel pin with wire

spacer in 15-15Ti l  127 pins per Fuel

Assembly l  External wrapper

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MYRRHA plant: Sub-Critical Core Layout

l  Sub-critical core: l 72 FAs l Maximum Power: 75 MW l Keff = 0.95 à improved safety

characteristics l 6 Control Rods l 6 In-Pile Section positions l No safety rods required l High and hard flux in core center à

MA transmutation

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1Spallation Target72FA6IPS6Abs.Devices84Dummy(LBE)42Reflector(Be)


MYRRHA plant: Critical Core Layout

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l  Critical core: l 108 FAs l Maximum Power: 100 MW l 6 Control Rods l 3 Safety Rods l 4 In-Pile Section positions


MYRRHA plant: Secondary and Tertiary Cooling System

l MYRRHA secondary system (one loop out of four) conceptual diagram:

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MYRRHA plant: Secondary and Tertiary Cooling System

l  Secondary system: l Four independent secondary loops (linked through PHXs) l Operated with forced flow two-phase water mixture (16 bar, 200 °C) l Secondary water flow path:

l PHX inlet (~saturated conditions) l PHX outlet (x ~ 0.3, α ~ 0.9) l Moisture separated in steam drum

l In normal operation, secondary water temperature kept constant by control system (primary LBE temperature changing as a function of core loading)

l  Tertiary system: dissipating heat to external environment through air condensers (forced circulation air fans)

l  Condensed steam recirculated into steam drum

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MYRRHA plant: Decay Heat Removal l  Accidental conditions

à DHR in full natural circulation (primary, secondary and tertiary able to operate in passive mode)

l  Two systems to remove decay heat power: l DHR-1: secondary

and tertiary systems operating in passive mode

l DHR-2: Reactor Vessel Auxiliary Cooling System

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LBE

~15m

Liquid water level

~50

m

Silicon doping pool

Refilling line

Pressure relief line towards reactor hall


MYRRHA safety analysis: FP projects participation l  Participation to several European FP projects in last decade:

l FP6 IP-EUROTRANS, leading to finalization of MYRRHA/XT-ADS version of MYRRHA in June 2008

l FP7 Central Design Team (CDT), defining MYRRHA/FASTEF version in March 2012

l FP7 MAXSIMA (started in November 2012, ongoing), more focused on the MYRRHA safety analyses and component qualification

l  European FP projects outcome partly used to define the latest version of MYRRHA design (currently in verification phase)

l  Current version not definitive l MYRRHA design still evolving taking into account results from

parallel R&D program

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MYRRHA safety analysis: RELAP5-3D model

l  RELAP5-3D MYRRHA plant model: l 2518 volumes, 2590 junctions l All cooling systems (primary, secondary, tertiary) simulated l Main control systems (Control Rods, secondary pressure) included

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Core Pumps + Primary Heat Exchangers

Primary System

Secondary + Tertiary Cooling System (x4)


MYRRHA safety analysis: Steady State

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l  First step: steady state analysis l Main steady state results reported:

l Very good agreement between code-calculated values and design values l Limited differences due to different LBE physical properties (mainly Cp)

l Maximum clad temperature ~470 °C l Maximum fuel temperature ~1600 °C (low value due to low linear

power ~110 W/cm)

Parameter Unit RELAP5-3Dvalue Designvalue Lowerplenumtemperature °C 270.1 270 Upperplenumtemperature °C 322.9 325

Maximumcoreoutlettemperature °C 424.6 430.7 Primaryflowrate kg/s 13829 13800 Coreflowrate kg/s 7716 7711

Secondarywaterpressure bar 16 16 SecondarywaterPHXinlettemperature °C 198.2 200 SecondarywaterPHXoutletquality - 0.30 0.3


MYRRHA safety analysis: SCS depressurization transient l  Unbalance between power delivered by PHXs and removed by

aero-condensers à SCS depressurization (16 bar à 1 bar and below)

l Overcooling transient: l 4 (out of 4) tertiary fans accidentally restarting at full speed with DH

power level in the core l SCS depressurization l SCS feedwater pumps active

l  Transient evaluation performed with RELAP5-3D and RELAP5/Mod3.3

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MYRRHA safety analysis: SCS depressurization transient l  Different transient evolution evaluation by the two codes:

l Above 5 bar: ~same evaluation l Below 5 bar:

l Nearly no differences in pressure evolution l Diverging liquid and vapor temperatures

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MYRRHA safety analysis: SCS depressurization transient l Minor edit "sattemp" and "tsatt" plotted (supposed to coincide if

no non-condensable species present) for both codes:

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l  Different trends for "sattemp" and "tsatt"

l  "tsatt" provides same profile as vapor phase

l  "sattemp“ lower than the liquid phase

l  "sattemp" and "tsatt" profiles match l  "sattemp" and "tsatt" profiles ~equal to

vapor phase l  Liquid phase lower than saturation

temperature


MYRRHA safety analysis: SCS depressurization transient l  SCS temperature evolution important towards primary PbBi

coolant freezing risk:

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l  RELAP5-3D and RELAP5/Mod3.3 predicts LBE freezing time of ~1600 s and ~800 s respectively. This difference can have a certain impact for the safety case.


Experimental validation at low pressures: Zeitoun and Choukri

l  Preliminary review of available benchmarks: Zeitoun and Choukri experiment: l Void generation in heated channel l Sensitivity parameters:

l Pressure (1.07 bar - 1.56 bar) l Heat flux and mass flow rate (q/m: 0.867 kJ/kg – 2.64 kJ/kg) l Inlet subcooling temperature (11.4 °C – 23.5 °C)

l  Experiments included in RELAP5/Mod3.3 Validation matrix (volume 3) but not in RELAP5-3D developmental assessment documents (volume 3)

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Experimental validation at low pressures: Zeitoun and Choukri

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Experimental validation at low pressures: Zeitoun and Choukri l  RELAP5-3D predictions underestimated, especially for high q/m l  RELAP5-3D void fraction estimation quite less sensitive to

experiments condition: void fraction prediction very limitedly affected by experimental boundary conditions

l  Approaching the experimental channel top, RELAP5-3D predictions often present a maximum in the void fraction function (void fraction decreasing while channel is still heated)

l Other parameters such as wall temperature and water outlet temperature are in acceptable agreement

l  Possible reason: l RELAP5-3D two-phase SNB model based on former RELAP5/

Mod3.2 model (mainly based on high pressure subcooled boiling data)

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Experimental validation at low pressures: THTL l  Different experiment: Thermal-Hydraulic Test Loop

l Void generation in heated channel l Experimental conditions:

l Coolant: light water, upward flow l Inlet coolant temperature: 45°C l Outlet pressure: 1.7 MPa l Local heat flux range: 0.7–18 MW/m2 l Corresponding exit velocity range: 2.8–28.4 m/s

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Experimental validation at low pressures: THTL

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Experimental validation at low pressures: THTL l  Difference with respect to experimental data increases with

applied heat flux l Above 8 MW/m²: code simulation crashes with Onset of Fluw

Instability l This behavior is common for both RELAP5-3D and RELAP5/Mod3.3

codes l  At higher pressure (similar to MYRRHA SCS normal operating

conditions) there is no observed divergence between the two codes

l  Void fraction plotted in function of mass flow rate shows also a good agreement between the two codes l Some disturbances only arising when operating at higher heat fluxes

l  Two codes found in good agreement at a pressure of ~17 bar (~MYRRHA SCS operation)

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Conclusions l  Several transients studied for MYRRHA reactor pre-licensing

involve the depressurization of the Secondary Cooling System (SCS), cooled by low pressure two-phase water mixture

l  RELAP5-3D overestimates, with respect to RELAP5/Mod3.3, the freezing time (~1600 s vs. ~800 s), with potential consequences on the LBE freezing conservative predictions. This is caused by the SCS temperature prediction between the two codes

l  The code benchmarking for low pressure showed some shortcomings and limitations of RELAP5-3D subcooled model at low pressures

l  Predictions of the two codes are found to be in good agreement with experiments at pressures above ~10 bar. In particular, experimental evidence proves the correct behavior at 17 bar.

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