Modelling and Simulation of LOCA

7/28/2019 Modelling and Simulation of LOCA

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With great pleasure and deep sense of gratitude, we express our indebtedness to our

respected Sir Mr. Sandip Ghosh for his invaluable guidance and constant

encouragement at each and every step of our project work. He gave all the

requirements that we had to implement in the project in very structured manner, not

only that he helped us through proper analysis and discussions but always showed

great interest in providing timely support and suitable suggestions.

Last but not least we would like to thank Mr. Malay Kr Banerjee HOD, Mechanical

Dept for his encouragement, support and help us in completing this program

successfully.

Thanking you,

Subhra Roy

Shangarab Bera

Partha Sarathi Ghosh

Souvik Das

Nilkanta Mahato

Amit Roy 3


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1. AIM OF THE PROJECT.

2. PROJECT PLAN.

3. BASICS OF LOCA.

4. FLASHING FLOW.

5. BASICS OF FINITE

ELEMENT ANALYSIS.

6. WHAT IS ANSYS?7. WORKS ACHIEVED.

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1) A compressible two phase domain modeling for a typical pipe crack will be approachedby using ANSYS design modeler and FLUENT two phase flow setups using cavitation

subroutines.

2) The Void fraction or the volume fraction of the flashing phase at outlet as well as the flow

Mac No. for several upstream stagnation conditions are to be calculated and compared with

standard experimental results or analytical predictions on flashing flow or those specific toLOCA accidents.

3) Pressure distribution through the narrow slit channel ( looks like a short duct of minute

width) are to be found to know the flashing inception point for judging the flashing flow

domain properly.

4) Critical leakage flow rates through modeled slit will be an important findings related to

leak detection and plant safety shutdown factors. Effect of friction in channels can also be

separately investigated.

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A loss-of-coolant accident (LOCA) is a

mode of failure for a nuclear reactor;

if not managed effectively, the results

of a LOCA could result in reactor core

damage.

Nuclear reactors generate heat

internally; to remove this heat and

convert it into useful electrical power,

a coolant system is used. If this coolantflow is reduced, or lost altogether, the

nuclear reactor's emergency shutdown

system is designed to stop the fission

chain reaction.

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A PWR primary system is shownin Figure. The design basis

accident in the PWR is a double-

ended guillotine break in a cold

leg between the reactor coolant

pump and the reactor vessel.

The average peak cladding

temperature (PCT) during the

blow down phase of a large-

break LOCA is approximately 1

500F (815C) and the PCT at95% confidence level is about 1

750F (954C), assuming a loss-

of-offsite power and the worst

single failure assumption for the

emergency core cooling system.8


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The refill period occurs

between 30 and 40 s following

the start of the LOCA.

The reflood period occursbetween 40 and 200 s

Zirconium-water reactions

can occur for high

temperature regions of the

core

The average reflood PCTduring this period is

approximately 1 680F

(915C) and the PCT at 95%

confidence is about 1 975F

(1 080C)

The maximum amount of

cladding oxidized at a given

location during this phase of

the LOCA is about 10% for

beginning-of-life (BOL) UO2

fuel and the total oxidation is

less than 1%.

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Various LOCA tests were conductedat LSTF, the world's largest plant

simulator with the same height and

1/48 volume of a 3423 MWt PWR, to

examine the effectiveness of accident

management (AM) measures if there is

severe loss of coolant. In case of HPI total failure, it is

important for operators to depressurize

the primary coolant system by opening

SG relief valves (RVs) to activate the AIS

and LPI system.

The tests verified the effectiveness ofthis AM measure even in a LOCA caused

by vessel bottom break which is the

worst break location.

A code analysis also confirmed the

effectiveness.

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The design basis accident for a

BWR-6 is a double-ended breakin the suction-side of the

recirculation line. Shortly after the

break, the reactor scrams,

typically on drive flow pressure.

Because of the large flow

reductions immediately followingthe LOCA caused by the

depressurization, there is a rapid

increase in the core average void

fraction. The negative void

reactivity rapidly shuts down the

core. The flow reverses in thebroken loop jet pump. With the

flow reversal all the drive flow to

that jet pump is lost and one-half

the drive flow that is supporting

the core flow is lost.13


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Valves are closed to isolate

the system, typically within

four seconds after the LOCA.

As the LOCA and

depressurization continue , the

level inside the core region

decreases, as well as forming a

level in the lower plenum region.

Within 35-40 s following the

LOCA, the high pressure core

spray system begins to delivercoolant to the top of the core, the

time being determined by the

time to start the diesel generator

that drives the high pressure core

spray system.

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In a CANDU reactor, the fuel is loaded

into horizontal pressure tubes, and is cooled

by the flow of pressurized heavy water,

In broad terms, a LOCA in a CANDU

follows a similar sequence to that described

for a PWR. A break in the heat transport

system initiates reactor shut down.

Despite these similarities with the PWR

LOCA sequence, the horizontal pressure tube

design and heavy water moderator mean that

the details of the accident progression are

quite different.

In the CANDU-6 design, the coolant void

reactivity is positive but the coolant void

reactivity is negative in the advanced CANDU

reactor (ACR) design.15


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The term flashing flow is reserved

for the flow with dramatic

evaporation of liquid due to a drop

of pressure P. The process of

production of the vapour phase is

usually accompanied by massivethermodynamic and mechanical

non-equilibrium by virtue of a

difference in temperature and

velocity of both phases.

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This linear relationship

does not always hold true.

As the pressure drop is

increased, the flow reaches a

point where it no longer

increases. Once thishappens, additional

increases in pressure drop

across the valve do not

result in additional flow, and

flow is said to be choked.

Here we will call thislimiting or choking pressure

drop the Terminal Pressure

Drop, pT.

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Conservation of energy dictates that

since kinetic energy at the venacontracta has increased to a maximum,

potential energy, in the form of static

pressure, must decrease to a minimum.

If the vena contracta pressure

drops below the vapor pressure,vapor bubbles form at the vena

contracta. Because vapor takes up a

much larger volume that the liquid,

the vapor bubbles fill the vena

contracta and any additional lowering

of the downstream pressure simply

results in the bubbles getting bigger,

but the flow does not increase. It is

the formation of these bubbles in the

vena contracta that causes the flow to

become choked.19


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As the bubbles move down stream, the

cross sectional flow area opens up, the

velocity goes down and the pressure goes

up. Now we have bubbles with an internal

pressure equal to the vapor pressure

surrounded by a higher pressure. Thebubbles collapse in on themselves.

This damage can happen very quickly,

sometimes in as little as a few weeks or

months. Because cavitation damage

happens so quickly, we try to avoidcavitation at all costs. Very hard materials

give some improvement, but usually the

improved performance is not enough to

justify the cost.

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If we continue to decrease the downstreampressure, we reach a point where the pressure

downstream of the valve is less than the vapor

pressure of the liquid.

The damage mechanism is a sand blastingeffect. Downstream of the vena contracta the

flow consists of a large volume of vapor with

many tiny drops of liquid. Because the volume

increases greatly when liquid vaporizes,

The noise caused by flashing is usually

below 85 dBA and to the author's knowledge

there is no method for calculating flashing

noise.

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In reality, at pressure drops

approaching, but below the calculated

value of pT, there is usually some

formation of vapor bubbles and some

degree of cavitation. Figure 5 shows

what really happens as flow

transitions from non-choked to fully

choked flow.

It is interesting to note that current

control valve sizing methods do notinclude a method of calculating where

the transition from non-choked to fully

choked flow begins and ends.

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The value ofpT is a

function of both the

process condition andthe valve's internal

geometry represented

by the experimentally

determined Liquid

Pressure Recovery

Factor, FL.

Higher values of FL are

associated with valves

that have a lower

potential for cavitation,and smaller values of FL

are associated with

valves that have a greater

potential for cavitation.

A more reliable method of preventing cavitation damage

in control valves, according to one major control valve

manufacturer, is to avoid valve applications where the

calculated noise exceeds limits based on a broad range of

application experience. 23


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Computational Fluid

DynamicsThe physical aspects of any fluid flow are governed by the

following three fundamental principles:

1. Mass is conserved;

2. Energy is conserved.

3. It obeys the 2nd law of Newton.

Computational fluid dynamics is, in part, the art of replacing

the governing partial differential equations of fluid flow

with numbers, and advancing these numbers in space

and/or time to obtain a final numerical description of the

complete flow field of interest.

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Contd.

Now in the previous slide the given expression isnot all inclusive about ANSYS.

there are some applications which involve

integral equations rather than partial differentialequations.

all such problems involve the manipulation of,and the solution for, numbers.

The end product of CFD is indeed a collection ofnumbers, in contrast to a closed-form analyticalsolution.

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ANSYS is based on FEA ( Finite Element

Analysis ). Before proceeding to obtain

various solutions in it we should try to

understand its working process very briefly.

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WORKING PLATFORM OF ANSYS

The purpose of Finite Element Analysis (all varieties) is to mathematicallymodel a physical problem that cannot be solved satisfactorily by other means.Typical reasons for difficulties in finding solutions are:-1. that manual means of mathematical modelling cannot represent the

problem sufficiently accurately.

2. that physical (real) models are deficient.3. that full size prototypes are far too expensive.

In all Finite Element Analysis, the mathematical modelling of the problem is done by dividing the probleminto small pieces whose performance can be modelled simply; Finite (size)Elements.The relationships between each neighbouring element are controlled so that,taken as a whole, the Mesh of Finite Elements approximates to the originalproblem.

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Contd..

For small deflection, elastic, static structural

analysis, the system is modelled as:-

[K] {x} = {F} where

[K] is the stiffness matrix

{x} is the displacements (of the nodes)

{F} is the forces (at the nodes)

Solving this set of simultaneous equations yields

the basis of the desired solution.

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Contd..

Many FEA systems are now often capable ofmodelling time varying quantities with nonlinearproperties and large changes in geometry.However the core of FEA analysis, and the mostreliable, is still the static structural analysislimited to:-

elastic, homogeneous, isotropic materials

linear material properties

small deflections: geometry changes can beignored

all material well below yield: no plasticdeformation

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What is ANSYS?

ANSYS is a general purposefinite element modelingpackage for numericallysolving a wide variety ofmechanical problems, used

widely in industry to simulate

the response of a physicalsystem to structural loading,and thermal andelectromagnetic effects.

ANSYS uses the finite-element method to solve the

underlying governingequations and the associatedproblem-specific boundary

conditions.

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ANSYS MODULES

ANSYS can work integrated with other usedengineering software on desktop by adding CADand FEA connection modules.

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Advancement in 13.0 version

ANSYS 13.0 includes a great number ofnew and advanced features that make iteasier, faster and cheaper for customers to

bring new products to market, with a highdegree of confidence in the ultimate resultsthey will achieve. The product suite deliversnew benefits in three major areas:

* Greater accuracy and fidelity: As

engineering requirements and designcomplexity increase, simulation softwaremust produce more accurate results thatreflect changing operating conditions over

time.* Higher productivity: ANSYS 13.0 includesdozens of features that minimize the timeand effort product development teams investin simulation.

* More computational power: For someengineering simulations, ANSYS 13.0 can

provide speedup ratios that are five to 10times greater than previous softwarereleases. Even complex multiphysicssimulations can be accomplished morequickly and efficiently.

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OUR WORK

Started ANSYSworkbench 13.0 andcreated a new fluidflow analysis system

in fluid flow(FLUENT)toolbar.This creates anew ANSYS FLUENT

based analysis systemin the projectschematic.

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DESIGN MODELER

Started ANSYS DesignModeler and set the unitin mm.

Created the geometry ofthe pipe and the crackformation within it bygiving certain dimensionand selecting suitable

planes.

Generated the geometry.

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MESHING

Opened the ANSYSMeshing application.

Created name

selections for geometryboundaries.

Set some basicparameters for the

ANSYS meshingapplication.

Generated the mesh.

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FLUENT

Proceeded to setting up aCFD analysis Using

ANSYS FLUENT.

Enabled the properoptions,

Set some general settingsfor simulation and alsothe models,material

settings, cell zoneconditions, boundaryconditions.

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Continued.

Set the solutions for the CFD simulation.

Changed the convergence criteria for the continuity

equation residual. Calculated a solution.

Started a calculation by requesting 250 iterations.

As the calculation processes ,the residuals plotted inthe graphics window.

Solution is converged after 180 iterations.

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Some instant shots..38


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Some more.39


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Results and termination

Displayed results inANSYS FLUENT.

Displayed filled contours

of pressure in all planes. After displaying the result

Closed the ANSYSFLUENT and checked the

all generated file write ornot in the workbench

window.

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Problem Faced and Recovery

Installation problem

Meshing problem

Due to some unknown General settings

Lack of data of boundary condition initially In convergency

Key of Recovery

Our group discussion ANSYS tutorials

useful suggestions of our guide.

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REFERENCE

Nuclear Fuel Behavior in Loss-of-coolant Accident (LOCA) Conditions.

NUCLEAR ENERGY AGENCY ORGANISATION FOR ECONOMIC CO-OPERATION ANDDEVELOPMENT

Schemes to compute unsteady flashing flows,M. Barret, E. Faucher, J.M. Herard

Basics of CavitationE.I.P. Drosos, S.V. Paras and A.J. Karabelas.

Fluid Piping System.Indian Renewable Energy Development Agency,Core 4A, East Court,

1st Floor, India Habitat Centre,Lodhi Road, New Delhi 110003.

Basics of Computational Fluid Dynamics.J. Anderson (Jr.)

Basic aspects of Discritization.

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Documents

Modelling and Simulation of LOCA