Characterisation of highly active nuclear waste simulants

300 Pages·2015·12.59 MB·English

by  Ananta Paul

Characterisation of highly active nuclear
waste simulants

Neepa Paul

Submitted in accordance with the requirements for the degree of
Doctor of Philosophy

The University of Leeds
Institute of Particle Science and Engineering
School of Chemical and Process Engineering

September 2014
ii

The candidate confirms that the work submitted is his/her own, except where
work which has formed part of jointly authored publications has been
included. The contribution of the candidate and the other authors to this work
has been explicitly indicated below. The candidate confirms that appropriate
credit has been given within the thesis where reference has been made to
the work of others. The title of the publications:

Characterising highly active nuclear waste simulants. Paul, N., Biggs, S.,
Edmondson, M., Hunter, T. N. & Hammond, R. B.
Synthesis of nuclear waste simulants by reaction precipitation: Formation of
caesium phosphomolybdate, zirconium molybdate and morphology
modification with citratomolybdate complex. Paul, N., Biggs, S., Edmondson,
M., Hunter, T. N. & Hammond, R. B., Maxwel, L.

The candidate confirms contribution to all publications included carrying out
relevant experimental studies, obtaining data, analysis of data and writing of
the paper. Contribution of co-authors included discussion of results, proof
reading and making relevant amendments.

This copy has been supplied on the understanding that it is copyright
material and that no quotation from the thesis may be published without
proper acknowledgement.

© 2014 The University of Leeds and Neepa Paul
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Dedicated to my parents
Ananta Sindu and Nomita Paul
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Acknowledgements
I would like to express gratitude to my supervisors, Simon R. Biggs, Robert
B. Hammond, Timothy N. Hunter and Michael J. Edmondson (NNL). for
giving me the opportunity to carry out this PhD research in the Institute of
Particle Science and Engineering (IPSE) at the University of Leeds and in
the National Nuclear Laboratory (NNL) at Sellafield, and their constant
support and assistance throughout this project. I would also like to thank the
nuclear and crystallisation group in IPSE.
I am very grateful to the EPSRC, NNL and Sellafield Ltd. for the support of
an Industrial CASE award scholarship and the opportunity to work on an
industrially motivating project and widening my knowledge on nuclear waste
processing. In particular I would like to thank the HALES team at
NNL/Sellafield Ltd for their support.
I would like to especially thank my wonderful parents, sisters, Anamika and
Jessica, my brother in law, Nikhil, and my amazing nephew, Hari for their
endless support throughout my Ph.D. Last but not least I would like to thank
my lovely Fiancé, Avi for the continuous encouragement and for being my
rock!
Thank you to all my family and friends!
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Abstract
Nuclear power is a non-carbon emitting energy resource generating 18% of
electricity to the UK. As with any type of industrial process the waste
management strategy is an important step to define considering the
environmental, economic and political factors. However, the nuclear industry
faces ongoing challenges to underpin a well-defined waste treatment
strategy due to the high heat load and the radioactive nature of the products
produced. Reprocessing of spent nuclear fuel produces a highly active liquor
(HAL) waste stream. HAL is currently stored in a number of highly active
storage tanks (HASTs). Within the HASTs, solid materials are known to have
precipitated from the HAL over time. Particle simulants provide a route for
understanding the physical behaviour, it is the synthesis of the particle
simulants and the characterisation of these solid-liquid systems that are the
interest of this study.
An understanding of the HAL waste properties is required for its safe
transport, storage and eventual disposal of the HASTs are to be safely
emptied and decommissioned. Collaboration with the National Nuclear
Laboratory (NNL), at Sellafield UK, provided the opportunity to manufacture
the HAL simulants, caesium phosphomolybdate (CPM) and zirconium
molybdate (ZM), on larger scale. Manipulation of the aspect ratio of ZM
particles is also investigated. The initial step of the synthesis produces
spherical CPM which leads to the production of cubic ZM, the final step
requires the addition of an organic additive, citric acid, where cuboidal
zirconium citratomolybdate (ZMCA) is formed. Molecular modelling analysis
revealed growth inhibition of the {2 0 0}, {-2 0 0}, {0 2 0} and {0 -2 0} faces,
due to greater number of Zr sites for citratomolybdate complex affiliation.
Distinct particle properties are established for CPM, ZM and ZMCA and
compared to a common oxide particle material titanium dioxide (TiO ).
2
The results of this study highlight the influence of key aspects of the HAL
particulates, such as size and shape, on relevant solid-liquid properties such
as sedimentation and rheology. The influence of bulk liquid properties such
as electrolyte concentration and pH were also investigated. Sedimentation
behaviour was characterised by fitting the experimental data to the
Richardson-Zaki model, yielding a fitting parameter n (cognate to particle
size and shape) and thus demonstrated a settling relationship with particle
shape, sphere > cubic > cuboidal.
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The rheological behaviour explored was categorised into four groups: (i) flow
behaviour data was fitted to a simplified Cross model yielding two
parameters K (related to viscosity) and n (extent of shear-thinning); (ii)
dependency of viscosity on particle volume fraction was characterised using
the Krieger-Dougherty model yielding fitting parameter [µ] (particle’s
contribution to suspension viscosity) and maximum packing fraction  , this
m
demonstrated the relationship, cuboidal > sphere > cube; (iii) yield stress
was characterised using an empirical model derived by Heymann et al
(2002) yielding a fitting parameter 𝜎∗ (cognate to particle shape and size)
and demonstrating a relationship, sphere > cuboidal > cubic; (iv)
characterisation of compressive yield stress demonstrated the relationship,
cuboidal > cubic > sphere.
The results indicate various possible behaviours within the tanks which may
impact the storage, remobilisation and pipeline transport of this class of
nuclear waste. Ultimately, it is of importance to establish the effect of solid-
liquid properties on the behaviour of HAL for current processing, post
operational clean out (POCO) and life-time assessment.
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Table of Contents
Acknowledgements .................................................................................... iv
Abstract ........................................................................................................ v
Table of Contents ...................................................................................... vii
List of Figures .......................................................................................... xiii
List of Tables ........................................................................................... xxii
List of Acronyms .................................................................................... xxiv
1 Introduction .............................................................................................. 1
1.1 Sustaining nuclear power ................................................................ 2
1.2 Organisation of the UK nuclear industry ......................................... 4
1.3 Research aims and objectives ........................................................ 4
1.4 Research application ...................................................................... 6
1.5 Thesis delivery ................................................................................ 9
2 Fundamentals of nuclear power and The Nuclear Fuel Cycle ............ 11
2.1 Nuclear Physics ............................................................................ 12
2.2 Nuclear power generation ............................................................. 15
2.2.1 Mining and milling of uranium ore ............................................ 15
2.2.2 Purification of uranium oxide ................................................... 16
2.2.3 Enrichment of uranium (U235) ................................................... 16
2.2.4 Fuel manufacturing .................................................................. 17
2.2.5 Nuclear fuel reactors ............................................................... 17
2.2.5.1 Gas reactors ..................................................................... 18
2.2.5.2 Pressurised water reactors (PWR) .................................... 18
2.3 Reprocessing of spent fuel............................................................ 19
2.3.1 Evaporator operation and design ............................................. 20
2.3.2 Storage of highly active liquor .................................................. 23
2.3.3 Vitrification of high level waste ................................................. 26
2.3.4 Waste Disposal ........................................................................ 27
2.4 Conclusions .................................................................................. 27
3 Synthesis and particle characterisation of nuclear waste simulants 28
3.1 Introduction ................................................................................... 29
3.2 Literature review ........................................................................... 29
3.2.1 Particle synthesis of nuclear waste simulants .......................... 29
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3.2.1.1 Influence of citric acid on molybdenum ions ...................... 32
3.2.2 Solid chemistry of HAL ............................................................ 33
3.2.2.1 Barium/strontium nitrate .................................................... 33
3.2.2.2 Magnesium lanthanide ...................................................... 34
3.2.2.3 Zirconium phosphates ....................................................... 34
3.2.2.4 Caesium phosphomolybdate ............................................. 34
3.2.2.5 Zirconium molybdate ......................................................... 35
3.2.3 Fundamentals of particle science ............................................ 35
3.2.3.1 Brownian motion ............................................................... 36
3.2.3.2 Contributing Forces of Interaction ..................................... 36
3.2.3.2.1 Van der Waals forces .................................................. 36
3.2.3.2.2 Electric double layer forces .......................................... 38
3.2.4 DVLO Theory ........................................................................... 40
3.2.5 Fundamentals of particle characterisation techniques ............. 42
3.2.5.1 Scanning electron microscopy (SEM) ............................... 42
3.2.5.2 Energy dispersive X-ray (EDX) ......................................... 43
3.2.5.3 Laser diffraction techniques for particle size distribution ... 44
3.2.5.4 X-ray diffraction technique (XRD)...................................... 47
3.2.5.5 Zeta potential measurements ............................................ 47
3.2.6 Particle Characterisation Techniques of Nuclear Waste .......... 49
3.2.7 Literature review conclusion .................................................... 51
3.3 Materials and Methods .................................................................. 51
3.3.1 Pre synthesised materials ........................................................ 51
3.3.2 Particle synthesis ..................................................................... 52
3.3.2.1 Synthesis of caesium phosphomolybdate (CPM) .............. 53
3.3.2.2 Synthesis of zirconium molybdate (ZM) ............................ 54
3.3.2.3 Synthesis of zirconium citratomolybdate (ZMCA) – Method
1 55
3.3.2.4 Synthesis of zirconium citratomolybdate (ZMCA) – Method
2 56
3.3.3 Particle characterisation .......................................................... 56
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3.3.3.1 Sample Preparation .......................................................... 56
3.3.3.2 Particle shape ................................................................... 57
3.3.3.3 Particle Size ...................................................................... 57
3.3.3.4 Particle density .................................................................. 57
3.3.3.5 Particle stability ................................................................. 58
3.3.3.6 Crystalline structure .......................................................... 58
3.3.3.7 Elemental analysis ............................................................ 58
3.4 Results and discussion ................................................................. 59
3.4.1 Particle synthesis ..................................................................... 59
3.4.1.1 Synthesis of Caesium phosphomolybdate ........................ 59
3.4.1.2 Synthesis of zirconium molybdate ..................................... 60
3.4.1.3 Staged synthesis of zirconium citratomolybdate with Method
1 62
3.4.1.4 Synthesis of ZMCA – Method 2 ........................................ 66
3.4.2 Particle characterisation .......................................................... 71
3.4.2.1 Particle shape ................................................................... 71
3.4.2.2 Particle density .................................................................. 71
3.4.2.3 Particle size ....................................................................... 72
3.4.2.3.1 Particle size distribution of non-spherical particles ...... 74
3.4.2.4 Particle stability ................................................................. 77
3.4.2.5 Crystalline structure of nuclear waste simulants ............... 78
3.5 Conclusions .................................................................................. 81
4 Morphological prediction of zirconium molybdate ............................. 83
4.1 Introduction ................................................................................... 84
4.2 Underlying science of crystallography ........................................... 84
4.2.1 Crystal lattice in two dimensions .............................................. 84
4.2.2 Crystal lattice in three dimensions ........................................... 85
4.2.3 Symmetry operators ................................................................ 87
4.2.3.1 Point Groups and Space Groups ...................................... 87
4.2.3.2 Space group Representation ............................................ 88
4.2.3.3 Point group Representation .............................................. 88
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4.3 Molecular modelling techniques and underlying science .............. 88
4.3.1 Atomistic force field ................................................................. 89
4.3.2 Lattice energy calculation ........................................................ 90
4.3.3 Energy minimisation ................................................................ 91
4.3.4 Morphological prediction .......................................................... 91
4.3.4.1 Bravis-Friedel-Donnay-Harker (BFDH) model ................... 91
4.3.4.2 Surface energy model ....................................................... 92
4.3.4.3 Attachment energy model ................................................. 92
4.4 Literature review ........................................................................... 92
4.5 Materials and methods .................................................................. 97
4.5.1 Crystal structures ..................................................................... 98
4.5.2 Molecular visualisation............................................................. 98
4.5.3 BFDH calculation ..................................................................... 98
4.5.4 Lattice energy minimisation ..................................................... 99
4.5.5 Surface and attachment energy calculation ........................... 101
4.6 Results and Discussion ............................................................... 102
4.6.1 Crystal and surface chemistry ............................................... 102
4.6.1.1 Surface chemistry of α-ZrMo2O8 .................................... 104
4.6.1.2 Surface chemistry of β-ZrMo2O8 ..................................... 106
4.6.2 BFDH prediction using Materials Studio ................................ 108
4.6.3 Lattice energy minimisation ................................................... 110
4.6.4 Surface and attachment energy morphological prediction ..... 111
4.6.5 Crystal chemistry of ZMH ...................................................... 121
4.6.6 Effect of citric acid on ZMH morphology ................................ 124
4.7 Conclusions ................................................................................ 128
5 Influence of particle properties on sedimentation behaviour .......... 130
5.1 Introduction ................................................................................. 131
5.2 Literature review ......................................................................... 131
5.2.1 Sedimentation fundamentals ................................................. 131
5.2.1.1 Particles falling under gravity through a fluid ................... 132
5.2.2 Settling regimes ..................................................................... 134