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Presentation Entitled, Evaluation Methodology for Stability Analysis of the NuScale Power Module.
	ML19192A166
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Site:	NuScale
Issue date:	07/10/2019
From:	; NuScale
To:	; Office of New Reactors
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NuScale Nonproprietary ACRS Full Committee Presentation

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Presenters Dr. Yousef Farawila (*)

System Thermal Hydraulics Matthew Presson Licensing Project Manager

(*) On the phone 2

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Agenda

Introduction and Main Message

Stability Solution Type

Stability Investigation Description

- Theoretical

- Numerical Using New Code PIM

- Experimental Benchmark

Procedure and Methodology

Summary

Questions and Discussions 3

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The Main Message

The NuScale power module design was found to be stable in the entire range of normal operation

Outside of normal operation, the reactor is destabilized when the riser flow is voided, however

- Unstable flow oscillation amplitude is limited by nonlinear effects and the critical heat flux ratio actually improves

The stability threshold is protected by scram upon loss of riser inlet subcooling

- Conceptually equivalent to a "region exclusion" not a "detect and suppress" solution type

- No action required to implement a stability solution hardware

These conclusions are based on extensive first principles, experimental, and computational studies. Details next.

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Stability Evaluation

Natural circulation instabilities were reported in literature

- See for example D.S. Pilkhwal et al. , "Analysis of the unstable behaviour of a single-phase natural circulation loop with one-dimensional and computational fluid-dynamic models," Annals of Nuclear Energy 34 (2007) 339-355.

a) HHHC: horizontal heater and horizontal cooler (the only unstable configuration) ;

Pressurizer b) HHVC: horizontal heater and vertical cooler; c) VHHC : vertical heater and horizontal cooler; Ill'!::==:> Superheated Steam d) VHVC : vertical heater and vertical cooler (qualitatively like NuScale module) 11 11

.I Feedwater (a) (b) (c) (d)

HHHC HHVC .. VHHC VHVC Core t t

Investigation of the NuScale module stability commenced to demonstrate stability, identify threshold conditions, and license stability protection methodology 5

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Stability Investigation Elements Module Design and Operational Domain First Principles Theory -- Experience -- Pl RT Construct Models and Main Code Independent Models (PIM) Experimental Data and Reduced Order Model Benchmarking (RADYA) (NIST-1)

Comprehensive Analysis & Results

Steady State Perturbations

Stability during Transients

Conclusions:

Stable within Operating Domain

Threshold is Riser Voiding 6

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Theoretical Investigation

Kick off with an expert committee to generate a first PIRT

Scoping review of thermalhydraulic instability modes and contrasting with the NPM design features

Identification of the possible instability mechanism

Analysis from first principles

- Riser-only mode (separate from cold leg)

- Stability trend with power using a simple SG model

- Shows decoupling of possible oscillations in SG tubes from primary flow and core

- Informs design of stability experiments

All medium ranked phenomena treated as highly ranked 7

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Theoretical and First Principles

A system with feedback processes may undergo oscillatory instability if the feedback is:

- Negative (positive feedback is unconditionally unstable)

- Delayed

- Sufficiently strong

NuScale natural circulation mode is examined

- Feedback is negative. A perturbation increasing core flow decreases exit temperature thus decreases riser density head

- Feedback is delayed. Transport delay for core exit condition to fill the riser and reach maximum density head effect.

- Feedback strength is related to liquid thermal expansion and possibility of phase change, riser length, SG characteristics, reactivity feedback ... Requires detailed modeling 8

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Main Stability Analysis Tool: PIM

Transient 1-D 2-phase non-equilibrium primary loop flow

- CONTROL ROD DR IVE MECHAN ISM Pressurizer PRESSURIZER Superheated Steam MA IN STEAM RIS ER Cooling (PR IMARY FLOW)

Heat Exchanger

- - STEAM GENERATOR (SECON DARY FLOW)

Feedwater

- coNTAINMENT VESSEL FEEDWATER DOWNCOMER Core (PRIMARY FLOW)

- - - REACTOR PRESSURE VESSEL 9

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Model Equations of the PIM code

Thermalhydraulic conservation equations t time Liquid and vapor mass balance M, liquid mass dMl,n - . . r Mg vapor mass

- - - - mz n-1 - m, n - n liquid mass flow rate dt ' ' mt mg vapor mass flow rate dMg,n . . r r rate of evaporation

- - - = mg,n-1 - mg,n + n dt I integrated momentum

/J.Pgrav gravitational press. drop Mixture momentum conservation with drift flux friction pressure drop Mfi-iction (integrated momentum) !J..~ocal local pressure drop residual pressure drop dl /J.P,*esid

-dt = 11Pgrav - /1P.fi*iction - /1P,local + 11Pres id hi liquid enthalpy hfg latent heat Q power Energy conservation (assume saturated vapor) n control volume index

!!_(M h )=m h -m h -r h +Q n-l upstream index df l,n l,n l,n-1 l,n-1 l,n l,n n Jg n -------------

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Model Equations of the PIM code

Point Nuclear Kinetics C Concentration of the delayed neutron precursors d<D ,,l Decay constant of the delayed neutron precursors A-== /J(p-l)<D +JC ct> Neutron flux amplitude dt fJ Delayed neutron fraction dC == /3 <D _ JC A Prompt neutron lifetime dt p Reactivity

- Thermalhydraulic model provides reactivity input

Moderator density reactivity feedback model (equivalent to moderator temperature reactivity under single-phase flow)

Doppler fuel temperature reactivity feedback

- Heat source from neutron kinetics feeds back to thermal hydraulics

Energy deposited in fuel pellets (proportional to neutron flux)

Fraction of fission energy deposited directly in coolant

Decay heat: input by the user as fraction of initial power 11 PM-0719-66233 Revision: 0 Copyright 2019 by NuScale Power, LLC.

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Model Equations of the PIM code

Heat conduction in fuel rods

- Pellet conductivity is function of temperature and burnup

- Driven by energy deposited in fuel pellets

- Heat flux at outer rod surface as power source to coolant

- Pellet temperature needed for Doppler reactivity

Heat transfer models for heat sink (steam generator)

- Secondary side flow is driven by user-provided inlet forcing function

- Secondary flow is subcooled, 2-phase equilibrium, and superheated

- Primary flow parameters calculated from transient conservation equations

- Heat transfer between primary and secondary flow

Heat transfer correlations

Transient heat conduction in tube walls

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Model Equations of the PIM code

Closing Relations and Correlations

- Frictional pressure drop (single- and two-phase friction and local losses)

- Drift flux parameters

- Non-equilibrium evaporation and condensation model

- Thermodynamic properties for water

- Physical material properties (pellets, cladding, SG tubes)

- Pellet-clad gap conductance

- Reactivity coefficients as functions of exposure and moderator density

What is not modeled

- Pressurizer; pressure is input provided constant or forcing function

- Heat transfer through riser wall, adiabatic riser is default option

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Flow Stability in Steam Generator Tubes

Secondary side flow changes from single-phase liquid, to two-phase mixture, ultimatelyto superheated steam

Flow in the tubes is subject to density wave instability

Experiments demonstrate flow oscillations under certain conditions (generally low flow)

- Oscillations in different tubes are not phase-locked

- Effect on primary flow cancels out, confirming first-principle finding

No impact on the primary flow and core stability

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PIM Results of Perturbing Steady State

Purpose is to calculate stability parameters of decay ratio and period at different conditions of power and exposure

- Following a user-applied small perturbation flow will oscillate

- Oscillations will grow with time if system is unstable

- Oscillations will decay eventually returning to the pre-perturbation state if the system is stable

Stability parameters, decay ratio and period, are extracted from the transient output. Observations:

- Unconditional stability in the entire operational range

- DR decreases with power and exposure

- Period also decreases with power

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PIM Application Methodology

For perturbations of steady state to get DR

- Vary power within 5-100°/o of rated

- BOC and EOC, and any point in between if warranted

- Conservative assumptions for MTC and decay heat fraction

For a depressurization transient (scram not credited)

- Verify that unstable oscillations limit cycle without CHFR decrease

Stability conclusion is generic, but confirmation is needed

- For plant upgrades such as power uprates

- Plant operation changes such as operating temperatures and.

maximum boron concentration

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Long Term Stability Solution

Region Exclusion for NuScale

- Unstable region defined by a single parameter (core exit subcooling)

- Monitor and protect margin to riser exit subcooling (with temperature margin below saturation point at pressurizer pressure)

- Operator alarm when subcooling margin is approached

- Riser exit subcooling will be controlled by the reactor protection system as part of normal operating limits- not only for preventing instabilities

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Summary and Conclusions

Stability of the NuScale module was evaluated using a dedicated code (PIM) and supported by first principles analysis and experimental data benchmarking

The module was found unconditionally stable within normal operation domain using conservative criterion

Stability boundary identified as associated with riser voiding (loss of riser inlet subcooling)

Stability protection methodology protects riser inlet subcooling with a margin to define the exclusion region enforced by the module protection system with scram 18 PM-0719-66233 Revision: 0 Copyright 2019 by NuScale Power, LLC.

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Portland Office Richland Office 6650 SW RedWJod Lane, 1933 JadVlin Ave., Suite 130 Suite 210 Richland, WA 99354 Portland, OR 97224 541. 360. 0500 971.371.1592 Arlington Office Corvallis Office 2300 Clarendon Blvd., Suite 1110 1100 NE Circle Blvd., Suite 200 Arlington, VA 22201 Corvallis, OR 97330 541. 360. 0500 London Office 1st Floor Portland House Rockville Office Bressenden Place 11333 Woodglen Ave., Suite 205 London SW1E 5BH Rockville, MD 20852 United Kingdom 301. 770.0472 +44 (0) 2079 321700 Charlotte Office 2815 Coliseum Centre Drive, Suite 230 Charlotte, NC 28217 980. 349. 4804 http://vwvw.nuscalepower.com

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ML19192A166

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