Medium-active waste form characterization

European Commission Directorate-General For The Information Society And Media Directorate-General For Research And Innovation

Description

The performance of cement-based systemsTask 3 Characterization of radioactive waste forms: A series of final reports (1985-89) No 1
Nuclear energy and safety

Informations

• Publié par: EUROPEAN-COMMISSION-DIRECTORATE-GENERAL-FOR-THE-INFORMATION-SOCIETY-AND-MEDIA-DI
• Nombre de lectures: 13
• Langue: English
• Poids de l'ouvrage: 4 Mo

Extrait

+ *
Commission of the European Communities
Medium-active waste form characterization:
the performance of cement-based systems
Tasks
Characterization of radioactive waste forms
A series of final reports (1985-89)
No1
Report
EUR 13542 EN y
Commission of the European Communities
nuclear science
and technology
Medium­active waste form characterization:
the performance of cement­based systems
Task 3
Characterization of radioactive waste forms
A series of final reports (1985­89)
No1
M. Atkins,1 N. Beckley,1 S. Carson,1 J. Cowie,1
F. P. Glasser,1 A. Kindness,1 D. Macphee,1 C. Pointer,1
A. Rahman,1 J. G. Jappy,1 P. A. Evans,2 G. McHugh,2
N. J. Natingley,2 C. Wilding2
1 University of Aberdeen
Meston Walk
Old Aberdeen AB9 2UE
\ United Kingdom
i
¡ ? UKAEA Harwell
[Harwell, Didcot
Oxfordshire 0X11 ORA
United Kingdom
Contract No FI1W/0025 UK
Final report
[ This report was prepared for the European Atomic Energy Community's
!cost­sharing research programme on radioactive waste management and disposal
!(Task 3­Section 1)
^­ ■,......­
\
\Directorate­General
PARL EURGP Bibliotk [Science, Research and Development
j mitrimi '*»'■
EUR 13542 EN I1991Published by the
COMMISSION OF THE EUROPEAN COMMUNITIES
Directorate-General
Telecommunications, Information Industries and Innovation
L-2920 Luxembourg
LEGAL NOTICE
Neither the Commission of the European Communities nor any person acting
on behalf of then is responsible for the use which might be made of
the following information
Cataloguing data can be found at the end of this publication
Luxembourg: Office for Official Publications of the European Communities, 1991
ISBN 92-826-2908-2 Catalogue number: CD-NA-13542-EN-C
© ECSC-EEC-EAEC, Brussels • Luxembourg, 1991
Printed in Belgium TABLE OF CONTENTS
Page
1 1. CHEMICAL MODELLING OF BLENDED CEMENTS.
2. PROPERTIES OF MATURED SLAG-CEMENT BLENDS 25
3. INTERACTION WITH WASTE AND CONSTRUCTIONAL
MATERIALS 4
4. IRRADIATION EFFECTS IN BLENDED CEMENTS 103
5. TEMPERATURE EFFECTS IN BLENDED CEMENTS 139
6. Ca(OH)2 SOLUBILITY: PRESSURE AND TEMPERATURE
DEPENDENCE 147
7. CEMENT NEAR-FIELD INTERACTIONS 151
III EXECUTIVE SUMMARY
The objective of this programme was to improve and document the data
base relevant to long-term performance of cement and concrete matrices.
At the outset, it was agreed that modelling would be required; simple
extension of short-term leach tests was unlikely to enable the long-term
performance of cements to be adequately established. Moreover, much
apparently relevant data recorded in the literature were not in a form
which were amenable for treatment within the context of modelling.
The total modelling of cement systems, including the impact of waste
components, of radiation stresses, of interactions with ground waters,
containers and backfills, etc. is too large a project to be assimilated at
once. Therefore, it was crucial to break the total system into
significant subsystems, each of which could be studied in its own right,
but in a way which would permit data on the separate subsystems to be
interfaced. This approach, together with a prioritization of the task
list, dictated the plan of the work programme.
With respect to these tasks, the relationship between aqueous solution
compositions and the gel calcium silicate hydrate (C-S-H) binding phase of
cement has been established at 20*C. The pH of solutions in contact with
C-S-H decreases with Ca/Si ratio of the gel; from ca 12.5 to 11-5.
Ageing up to several years does not significantly affect the gel structure
or solubility; examination of historic cements discloses that, very
probably, their chemical properties are unaffected for at least 102 years.
The properties of blended cements have been elucidated in more detail.
Slags contain more Si and less Ca than cement; moreover, they contain
significant Mg, an element which is essentially absent in Portland cement.
The Mg is shown to form low solubility phases which do not interfere with
the buffering action of C-S-H gel. Although free Ca(OH>2 is predicted to
disappear from slag-rich blends upon prolonged cure, the high system pH is
maintained and buffered by the C-S-H hydrogel. The sulphur chemistry of
slags may, however, lower the internal redox potential of the cement
system. Sulphide S2- and polynuclear species (Sn2-, Ss), and possibly
thiosulphates are present in pore waters and in solids. The redox
potential is not a linear function of slag content, but drops abruptly from
a high plateau at +100 to +200mV to as low as -400 mV at or above 70-80X
slag.
Fly ash is an intrinsically more complex additive on account of its
inhomogeneous and highly variable nature. The need for fly ash
characterization is stressed. Presently available UK fly ashes are shown
to have a significant effect on reducing free Ca(OH)2 contents and
decreasing the Ca content of the C-S-H gel. However, the consequences to
the pH are shown to be chemically similar to slag additions. Fly ash also
conditions formation of characteristic minor phases: hydrogarnet,
gehlenite hydrate, etc. Data for these phases discloses that they are
incongruently soluble, but that their total solubility is low. Thus they
are not effective in conditioning and buffering high system pH, leaving
C-S-H as the main effective control.
V The minor components of cement also play an important role in
influencing the short-term properties of the system. Sodium and
potassium, present in cement, slag and fly ash at the 0.5-3-OX level,
concentrate preferentially in the aqueous phase, where they repress the
solubility of calcium. The modelling studies confirm this and provide a
data base whereby the long term alkali and alkaline earth leaching can be
predicted. Work on sulphates has continued; ettringite is found to be
congruentIy soluble, and its K«P value has been determined. Monosulphate,
on the other hand, is incongruently soluble.
The potential for immobilizing radwaste ions is shown to arise from
three mechanisms. Firstly, the high pH precipitates many species as
hydroxides. Secondly, the high specific surface area and surface charge
of many of the cement phases, notably of amorphous C-S-H, condition
sorption characteristics. Finally, the more crystalline phases exhibit
lattice incorporation. Sometimes several mechanisms occur jointly, but in
general, each radwaste species behaves differently. Iodine, for example,
undergoes mainly lattice incorporation in the aluminate phase (AFM) as
iodate; IO3-, but is also sorbed onto C-S-H, etc. In the iodide
speciation, I-, similar mechanisms apply, but uptake values are considerbly
less relative to iodate. Under reducing conditions, I- is likely to be
the most significant I species.. Uranium (VI), on the other hand, is
rapidly precipitated, probably as a hydrous oxide. However, it is slowly
converted with time to more crystalline, less soluble phases. Several of
these are characterized, e.g. uranophane, and solubility data are given.
In general, the solubility of U decreases by one or two orders of magnitude
over the interval between 1-2 days and 1-2 years. Sorption is likely to
be the most rapid process and compound formation involving the solid cement
phases, the slowest.
The effect of elevated temperatures is to markedly enhance reaction
rates. Pozzolanic additives, such as slag and fly ash, react more rapidly
and completely with cement. As a result, Ca(OH)2 contents diminish more
rapidly relative to 20*C. The conversion of amorphous precipitates to
crystalline phases is accelerated.
An analysis of the factors leading to cement dissolution has shown that
the effect of dissolved CO2 is very important. Various engineering
approaches have been developed: these are mostly shown to be inadequate.
It has been necessary to approach the problem from first principles and
this has led to construction of a new type of plot, termed an aggressivity
map. Starting from a ground water analysis (pH, pC02 or some related
function) the aggressivity of the ground water, relative to distilled
water, can be determined. The general approach used enables interactive
actions to be calculated, e.g. dissolved CO2 in the presence of chloride.
The durability of steel in concrete is re-examined. The kinetics of
reaction are controlled by the state of the steel cement interface. It is
shown that comparatively low levels of chloride, 0*XX, interfere with the
passivation of the passivating layer.
The interaction of two specific wastestream analogues; a 225g/l
solution of Na2S0* and a 300g/l solution of NaNOa ; with OPC was
investigated, The presence of the former, at 20*C, was found to enhance
the amount of AFt formed in the solid phase, at the expense of Ca(OH)2.
- VI Consequently, leading to a decrease in the SO42- concentration of the
aqueous phase, and an increase in the pH. These effects were all enhanced
in samples aged at 55*C. The presence of NaN03 appeared to have little
effect on the relative abundance of the '