Multi-physics coupling Application on TRIGA reactor Student Romain Henry Supervisors: Prof. Dr. IZTOK TISELJ Dr. LUKA SNOJ PhD Topic presentation 27/03/2012 FMF LJUBLJANA 1

 A nuclear reactor is a “boiler” in which heat is produced the fission of some nuclei of atoms having high atomic mass 2

 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 3

 Thermal reactor: PWR,BWR Fast reactor: SFR,LFR,GFR 4

 Pool reactor  thermal spectrum  Water cooled  Pmax=250 kW 5

 The multiplication factor k describes the evolution of the neutron density between 2 generations  k < 1 : The neutron density decreases The power decreases The reactor is sub-critical  k = 1 : The neutron density is constant The reactor is critical  k > 1 : The neutron density increases The power increases The reactor is super-critical 6

 Interaction neutron matter : Notion of cross section (expressed in barns)  Absorption (fission, capture), scattering  Total cross section:  interaction probability :  macroscopic cross section for a given material (atoms density N): 7

 Natural U: 99.3% of U % of U235  Fuel Enriched in U235 8

 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 9

 Transport equation :  Core modeling geometry (2D, 3D), isotopic composition (fuel, moderator, …) 10

 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 11

 Navier-Stokes system for incompressible flow with constant Newtonian properties: ◦ Continuity equation ◦ Momentum equation ◦ Energy equation  Fluid velocity  Thermal diffusivity 12

 Example of CFD result 13

 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) 14

Neutronics Thermal- Hydraulics Tfuel, Tmod … 15 Power distribution …

 Build a neutronic core model accurate  Full 3D description  3D single phase flow description phenomena  2 codes working as 1 16

 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 17

Temperature reactivity Number of neutron 18

 Reactivity ρ= (k-1)/k  Temperature increases  Absorption increases  Reactivity decreases 19

 Point kinetic (Boltzmann with no space dependence)  C precursor λ decay constant  l Neutron lifetime in critical reactor  β proportion of delayed neutron  Thermodynamic law 20

Temperature reactivity Number of neutron Δρ/ΔT Point kinetic Pfission 21

 Build a full 3D model of the TRIGA reactor  Simpler geometry  Data easily available  See which application we can have for a power reactor 22

Thank you for your attention 23