MYRRHAPrimary Heat Exchanger design status

Diego CastellitiSCK•CEN

diego.castelliti@sckcen.be

IAEA, Vienna, 21-22 December 2011Copyright © 2011 SCK•CEN

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Introduction

� MYRRHA project:� Targets� Characteristics� Design status

� MYRRHA Primary Heat Exchanger design� Current layout� T/H parameters� Design choices� Control strategy

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MYRRHA project [1/5]

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

� MYRRHA facility main targets� Flexible irradiation facility� Minor Actinides (MAs) transmutation aiming at "closed cycle“

(Generation IV requirement)� Accelerator-driven system (ADS) demonstrator� Lead Fast Reactor demonstrator� (Pre-) Gen IV plant

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MYRRHA project [2/5]

� MYRRHA facility main characteristics� ADS reactor operating in sub-critical mode with ability to operate in

critical mode as well� Fast neutron flux� No electrical power generation� Material testing and isotope production facility � flux performance

maximized in the In-Pile Sections (IPS)� Primary coolant: Lead-Bismuth Eutectic (LBE)� Pool-type reactor� 4 Primary Heat Exchangers (PHXs)� 2 Primary Pumps (PPs)� Secondary coolant: 2-phase water� Tertiary coolant (heat sink): air

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MYRRHA project [3/5]

� MYRRHA main plant parameters

General design parameters Unit MYRRHA ValueMaximum core power MW 100

Reactor power MW 110Primary System Temperatures

Cold shutdown state °C 200Maximum core inlet temperature °C 270

Average core ∆T °C 140Average core outlet temperature °C 410

Maximum hot plenum temperature °C 350

Secondary System Temperatures2-phase water temperature °C 200

2-phase water pressure bar 16Fuel Pin

Fuel type MOX, max. 35wt.% PuO2 from spent fuel, the balance is natural UO2

Pellet type Solid pellet

Fuel element length mm 1400

Fuel active height mm 600

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MYRRHA project [4/5]

� MYRRHA main plant parameters (continued)

Fuel assembly

Assembly type Hexagonal fuel bundle with wrapper

Number of pins - 127

Spacer type Wire spacer

Assembly length mm 2000

Maximum LBE bulk velocity m/s 2

Core

Number of positions 151

Core diameter mm 1450

Maximum core pressure drop bar 2.5

Reactor vessel

Internal diameter mm ~7600

Length mm ~11000

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MYRRHA project [5/5]

� Overview of the MYRRHA reactor primary system

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MYRRHA current PHX layout [1/2]

Shell and tubes counter-current PHX (water flowing in tubes)

Lower HX zone (Water tubes, tube plates and lower plenum)

HX shroud and feed-water tube double-walled against SG rupture accident

Upper HX zone (Feed-water inlet and 2φoutlet)

MYRRHA current PHX layout [2/2]

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MYRRHA current PHX functional requirements

� Normal operation: reactor core power (and all other heat sources) removal � PHX operating in forced circulation regimes on both sides

� Decay Heat Removal (DHR) condition: accidental situation �reactor must be able to operate in passive conditions (natural circulation) to guarantee the DHR function (conservatively estimated at ~7% of the total core power)

� Safe Shutdown condition: during shutdown periods (decay heat power low enough to be compensated by thermal heat losses) � PHX to provide power to the primary LBE in order to prevent freezing (Safe Shutdown Temperature: 200 °C) operat ing in"reverse" mode

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MYRRHA current PHX design choices

� Possible advantages:� No change in flow area � no phase separation, TH conditions preserved for the

whole tube length� Pressure drop � reduced need to orifice the water tubes for tube bundle flow

stabilization� Aspect ratio comparable to past LMFBRs � counter-current flow development

through the bundle� Possibility to adopt a wide inlet for LBE � full flow area for full power and partial

flow area (hot free surface at LBE inlet level) for DHR conditions� No need to adopt a double-walled structure for the upper part of the PHX shroud� Only one tube plate (� set of weldings) under LBE (upper tube plate above hot

free surface)� Relatively easy inspection and repair

� Possible disadvantages:� Tube length � stresses (weight, thermal-induced) in the tube plates� Upper fraction of tube bundle not in contact with LBE � free surface zone with

possible problems due to differential thermal expansion and level fluctuation (�thermal fatigue)

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MYRRHA PHX calculation and verification

� Heat Treansfer Coefficient (HTC) and material properties correlations used for design:� For LBE properties and HTC correlation: LBE handbook (2007)� For steel properties: RCC-MRx (2010)

� PHX T/H performances verified with RELAP5 and TRACE system codes

� High level of agreement has been found between design and system code verification

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MYRRHA current PHX control strategy [1/3]

� MYRRHA � research, experimental and material testing reactor

� Need to operate at different power levels when possible (in compatibility with performances required)

� Control issues arising when departing from T/H full power conditions:� Core parameters: Tin, Tout, ∆T, mass flow rate� Secondary side parameters: pw (and Tw), mass flow rate, exit quality

� Basing on the strategies comparison and taking into account the purpose and the needs of MYRRHA/FASTEF, it appears that the control solution involving a constant secondary loop conditions is preferable

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MYRRHA current PHX control strategy [2/3]

� Advantages in keeping primary conditions constant:� Unchanged stresses for primary structural materials� Control rods operation not required to compensate temperature

feedbacks

� Advantages in keeping secondary conditions constant:� No pressure variations in secondary side � no need to design the

loop for pressure > 16 bar� SGTR accident condition mitigation � more margins for vessel

design, safety valves, rupture disks, etc…

� IPS thermally decoupled from primary system � No real need (differently than power reactors!) to keep constant primary system conditions

� Power level varied only between irradiation cycles (in function of IPS requirements) � no drawbacks!

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MYRRHA current PHX control strategy [3/3]

� Control strategy proposed for PHX operation:� Constant secondary system condition� Primary system following load changes� Constant mass flow rate in both primary and secondary systems

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0 10 20 30 40 50 60 70 80 90 100

Te

mp

era

ture

(°C

)

Power (MW)

PHX EoL operating temperatures at different power levels

EoL lower plenum temperature

EoL upper plenum temeprature

Core average exit temeprature

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MYRRHA current PHXinspection and tube plugging

� Need to identify, inspect and plug failed PHX tubes

� Current available solution (Phénix experience):� PHX extraction� Lower plenum cut in the reactor hall (by remote

handling)� Tube plug� Re-weld� PHX insertion

� Inspection and plugging in-situ are considered

� Mechanical assessment of plugged pipe could show higher stresses than normal conditions

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MYRRHA current PHXHTC and tubes thermal stresses

� Very preliminary mechanical assessment performed on the tangential stress in water tubes

� According to the HTC values evaluated, top wall temperatures are the following:� LBE side: 281 °C (292 °C at EoL)� Water side: 221 °C

� Thermal stresses intensity (~174 MPa in EoL conditions) is well below the 3Sm for AISI 316L (342 MPa)

� Use of different steel (2Cr 1Mo) with lower thermal deformation and higher Sm could represent an improvement

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Flow instability problem

� Two kinds of instabilities can be found in a boiling (2φ) tube bundle of an HX:� Static instability� Dynamic instability

� Static instability (boiling channel problem): different pressure drop between liquid phase and vapor phase (assuming constant mass flow rate) � instability range in certain 2φ flow conditions (function of power, pressure, mass flow rate, quality, void fraction…) may appear in the heated channel

� Dynamic instability (tubes connected to plena problem): differences in mass flow rate distribution may appear among tubes � different flow regimes, obscillations, possible thermal fatigue, flow inversion, etc…

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MYRRHA PHX static instability [1/3]

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MYRRHA PHX static instability [2/3]

� Static instability:� Does not require time-dependent

approaches � can be evaluated with “steady-state" models without solving PDE systems

� Different approaches available, with different complexity

� Approach used: full non-equilibrium model

� Heated tube generally divided in 4 regions:� Single phase liquid� Subcooled nucleate boiling� Bulk boiling� Single phase vapor

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MYRRHA PHX static instability [3/3]

� In nominal operation conditions, 99% of the heated length is in the Bulk boiling region

� At 7% of power (start DHR), about 85% of the heated length is in the Bulk boiling region

� Results to be confirmed with system code (RELAP5) model

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MYRRHA PHXStatic instability – Results [1/3]

� In nominal conditions (full power, full mass flow rate), the working point of the system falls in a perfectly stable range

∆p = 1.15 bar

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MYRRHA PHXStatic instability – Results [2/3]

� Also in DHR conditions (7% power, full mass flow rate), the working point falls in a stable range: less power �higher stability (saturated steam production very unlikely)

� For lower mass flow rates (to be expected in NC), the working point is also stable

∆p = 0.44 bar

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MYRRHA PHXStatic instability – Results [3/3]

� Interesting behavior at very low power (with nominal mass flow rate): at very low void fractions, gravity pressure drops > friction pressure drops � overall pressure drop increase

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2.0E+04

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1.4E+05

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Pre

ssur

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rops

(P

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Power (MW)

PHX pressure drops vs. reactor power

Pressure losses

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Dynamic instability

� Dynamic instability evaluation � time-dependent problem� A perturbation of any kind (power peak, mass flow rate

maldistribution, etc…) can change conditions in one pipe �consequences will be felt in other pipes

� Need to consider thermal inertiae of pipe walls for accurate thermal perturbation propagation

� Solution requiring time-space PDE coupled system � use of system codes (RELAP5) could be adviced

� Instability analysis methods could provide results about bundle stability without fully PDE system solution

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Conclusions

� Further developements:� PHX inspection and tube plugging� PHX inlet – outlet thermal-hydraulic behaviour analysis and

design� Tube bundle spacer grids and their influence on the LBE flow� Complete mechanical assessment: water tubes, feed-water

inlet tube, upper and lower tube plates� Dynamic instability assessment� Cover and secondary system interfaces design

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