Multi-physics couplingApplication on TRIGA reactor

Student Romain HenrySupervisors: Prof. Dr. IZTOK TISELJ

Dr. LUKA SNOJ

PhD Topic presentation

27/03/2012

FMF LJUBLJANA

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A nuclear reactor is a “boiler” in which heat is produced the fission of some nuclei of atoms having high atomic mass

Reactor principle (1/3)

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fission products radioactive delayed neutrons are emitted

2 to 3 prompt neutrons a chain reaction is possible

High energy photons

The reaction is exo-energetic (~ 200 MeV)

1 fission produce 10^8 times more energy that burning one atom of carbon

Reactor principle (2/3)

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Thermal reactor: PWR,BWR Fast reactor: SFR,LFR,GFR

Reactor principle (3/3)

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Pool reactor

thermal spectrum

Water cooled

Pmax=250 kW

TRIGA

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The multiplication factor k describes the evolution of the neutron density between 2 generations

k < 1 : The neutron density decreasesThe power decreasesThe reactor is sub-critical

k = 1 : The neutron density is constantThe reactor is critical

k > 1 : The neutron density increasesThe power increasesThe reactor is super-critical

Neutron physics (1/5)

i

i

n

nk 1

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Interaction neutron matter :Notion of cross section (expressed in barns)

Absorption (fission, capture), scattering

Total cross section:

interaction probability :

macroscopic cross sectionfor a given material (atoms density N):

Neutron physics (2/5)

sat

t

iiP

N

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Natural U: 99.3% of U238 +0.7% of U235

Fuel Enriched in U235

Neutron physics (3/5)

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Moderator:

◦ Very low atomic mass, optimal for the slowing down process

◦ Very low cross section for capture in the thermal range of energy

◦ high concentration of nuclei to favor the probability of neutron scattering

Water

Neutron physics (4/5)

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Transport equation :

Core modeling geometry (2D, 3D), isotopic composition (fuel, moderator, …)

Neutron physics (5/5)

),,(),,(),(),,(),(),,(.),,( ''''3 tvrstvrnvvrvvdtvrnvrvtvrnv

t

tvrns

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Flow phenomena for the coolant (turbulence ,heat transfer )

Phenomena of importance in the evaluation of fuel integrity.

CFD is a branch of fluid mechanics that uses numerical methods and algorithms to solve Navier-Stokes system

Thermal-hydraulics (1/3)

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Navier-Stokes system for incompressible flow with constant Newtonian properties:

◦ Continuity equation

◦ Momentum equation

◦ Energy equation

Fluid velocity Thermal diffusivity

Thermal-hydraulics (2/3)

TuTt

T

Puuut

u

u

2

2

).(

1).(

0.

)( coolantfuelfissionfuel

p TThPdt

dTmc

pc

u

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Example of CFD result

Thermal-hydraulics (3/3)

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The main goal : describe some behaviors that pure neutron transport equation or pure thermal-hydraulic models are unable to do

Research in neutron physics and nuclear thermal-hydraulics require long computational time on large parallel computer

Coupled models cannot rely on the most accurate and advanced models from both disciplines(simpler models that allow performing simulations in a reasonable time)

Coupling (1/4)

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Coupling (2/4)

NeutronicsThermal-

Hydraulics

Tfuel,Tmod

…

Power distribution

…

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Build a neutroniccore model accurate

Full 3D description

3D single phase flowdescription phenomena

2 codes working as 1

Coupling (3/4)

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Validation of the model through measurement with TRIGA reactor:

Detectors devices allowing to measure neutron flux for different configurations of the TRIGA core to deduce the power distribution

The temperature of the moderator is also easily accessible, with thermocouple, from the reactor pool

Coupling (4/4)

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Basic example (1/4)

Temperature

reactivityNumber of

neutron

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Reactivity ρ= (k-1)/k

Temperature increases

Absorption increases

Reactivity decreases

Basic example (2/4)

20 30 40 50 60 70 80 90 100 110

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

measurement

T(°C)

Δρ/ΔT(pcm/°C)

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Point kinetic (Boltzmann with no space dependence)

C precursorλ decay constant

l Neutron lifetimein critical reactor

β proportion ofdelayed neutron

Thermodynamic law

Basic example (3/4)

iiii

ii

i

Cnldt

dC

Cnldt

dn

)()( coolantfuelfissionfuel

p TTnPdt

dTmc

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Basic example (4/4)

Temperature

reactivityNumber of

neutron

Δρ/ΔT

Point kinetic

Pfission

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Build a full 3D model of the TRIGA reactor Simpler geometry Data easily available

See which application we can have for a power reactor

Conclusion

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Thank you for your attention