INDEX Abstract………………………………………………………………..…Pg. 1 Abbreviations……………………………………………………………Pg. 3 Introduction……………………………………………………………..Pg. 5 Chapter 1 -

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Index_______________________________________________________________________

Improvement of the pebble mixing neutronics model

in Pebble-Bed HTR analyses

INDEX

Abstract………..…Pg. 1 Abbreviations………Pg. 3 Introduction………..Pg. 5 Chapter 1 - The High Temperature He-cooled pebble-bed plant…….Pg. 8

1.1 Power Plant Description………...Pg. 8 1.2 The PBMR-400 vessel and internals………..Pg. 12 1.3 Recirculation system………..Pg. 17 1.4 Conclusive considerations………...Pg. 18

Chapter 2 - Fuels for High Temperature pebble-bed Reactor……..…Pg. 20

2.1 Introduction………...Pg. 20 2.2 Fuel general description………..Pg. 22 2.3 PBMR-400 uranium pebble……….Pg. 24

2.4 PUMA project plutonium and plutonium-minor actinide fuels………..……….Pg. 26 2.5 Thorium pebble configuration………Pg. 27 2.6 Transmutation physics………..…Pg. 29

2.6.1 Theory………

Pg. 29

2.6.2 Innovative gas cooled thermal reactors………..

Pg. 30

2.6.3 Challenges………

Pg. 32 2.7 The Level Of Mine (LOM) concept as reference parameter definition

and discussion…….……….Pg. 32

2.7.1 Radiotoxicity………

Pg. 32

2.7.2 Level of Mine Balancing Time…….………..

Pg. 32 2.8 The HTR as plutonium burner…….………..Pg. 34 2.9 Symbiotic fuel cycles………..Pg. 36

2.9.1 A symbiotic LWR-HTR-GCFR fuel cycle……….…

Pg. 36

2.9.2 LOMBT evaluations for LWR-HTR-GCFR fuel cycles

Pg. 37 2.10 Conclusions………Pg. 39

Chapter 3 - Preliminary analysis of the energetic meshing for the neutronics

calculations………...Pg. 41 3.1 Introduction………..Pg. 41 3.2 SHEM (Santamarina Hfaiedh Energy Mesh)……….…Pg. 41

3.2.1 Actinide Resonances ……….….

Pg. 45

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3.2.2 Fission Products ……….

Pg. 45

3.2.3 Matrix and Bond Materials……….

Pg. 46

3.3 Grouping Methodology……….Pg. 47 3.4 Conclusions……….Pg. 51

Chapter 4 - Description and comparison of PANTHERMIX and MCNP5

codes………..………..Pg. 52 4.1 Introduction ………..Pg. 52 4.2 The PANTHERMIX code……….Pg. 52 4.3 Pebble-bed HTR without fuel discharge and reload…………..Pg. 53

4.3.1 Neutronics modelling ………...

Pg. 53

4.3.2 Thermal hydraulics modelling………...

Pg. 53

4.3.3 Combination of neutronics and thermal hydraulics

modelling……….….

Pg. 53

4.4 Fuel discharge and reloaded in Pebble-bed HTR……….Pg. 54

4.4.1 Physical quantities per volume element……….

Pg. 54

4.4.2 Axial shift principles………

Pg. 55

4.4.3 Pebble circulation actions………

Pg. 56

4.5 The MonteCarlo method and the MCNP5© _{………Pg. 58}

4.6 MonteCarlo method vs. Deterministic method………..Pg. 59 4.7 The MonteCarlo method………Pg. 60 4.8 Conclusions………..Pg. 61

Chapter 5 - Analysis of fuel pebbles at different burn-up levels: the mixing

model………Pg. 62 5.1 Introduction………..Pg. 62 5.2 Objective of this study……….Pg. 63 5.3 The Recirculation Problem……….Pg. 63 5.4 The PANTHERMIX recirculation model………..….Pg. 63 5.5 The limit of PANTHERMIX: Nonlinear trend of k-infinite vs.

burn-up………..Pg. 64 5.6 Characteristics of the method………...Pg. 67 5.7 Preliminary analyses………Pg. 67 5.8Operative method………..Pg. 68 5.9 Potential Mixing Models……….Pg. 69

5.9.1 Flux depression Cross-Section Mixing model……….

Pg. 70

5.9.2 Polynomial Average of K-infinite………..

Pg. 72

5.9.3 Linear Absorption Cross-Section Model………

Pg. 72

5.9.4 Linear Absorption Cross Sections with Flux Depression

Correction Mixing Model (LACFD)………..

Pg. 77

5.9.5 Slab Solution with Flux Spectrum Correction Mixing

Model……….

Pg.85 5.10 Conclusions………..Pg. 95

Conclusions ………..Pg. 97

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Appendix A - Additional data on PBMR-400 and PUMA simplified

models………

Pg. 100 A.1 Introduction………..Pg. 100 A.2 Equilibrium specifications………..Pg. 100 A.3 Boundary conditions……….Pg. 100 A.4 Material specifications……….Pg. 101 A.5 The Cross-Duct………Pg. 102 A.6 Definition and Analyses of LOFC………Pg. 103

A.6.1 Introduction……….

Pg. 103

A.6.2 P-LOFC………

Pg. 103

A.6.3 D-LOFC………..

Pg. 104

A.6.4 D-LOFC with air ingress………

Pg. 104

A.6.5 P-LOFC with ATWS……….

Pg. 105

A.6.6 D-LOFC with ATWS……….

Pg. 106 A.7 Conclusions………..Pg. 106

Appendix B - The “SHEM” energy grouping……….

Pg. 107

Appendix C - Extension of the Mixing Model to particular situations

Pg. 110 C.1 Introduction……….Pg. 110 C.2 Uranium pebble behaviour………Pg. 110 C.3 Uranium and plutonium pebble behaviour………Pg. 111 C.4 Power Fraction Mixing Model results………Pg. 112 C.5 Conclusions………Pg. 114