ESC Technical Memo
LLWR/ESC/Mem(12)176
Topic area
Monitoring
Subject
Response to Issue Resolution Form
ESC-RO-SUE-007 (Use of Future Monitoring to
Reduce Uncertainties)
Author
A J Baker and R Cummings
Circulation
Environment Agency, ESC Team, file
Related records
None
Security
Open
1
Date
19th October
2012
Introduction
The Environment Agency recently issued a regulatory observation, on the use of monitoring
to reduce uncertainties in the Environmental Safety Case (ESC) [1]. Specifically, it requires
us to:
" provide evidence of how the forward monitoring programme will be developed throughout
the PoA and linked to the ESC in order to reduce key uncertainties in the ESC. We wish to
understand how the ESC will be used to help drive the scope and content of the forward
programme."
and
" clarify the systems in place to ensure that linkages between the ESC team and the
monitoring team will be maintained going forward and that, where appropriate, site-derived
information is fed into the ESC on a regular basis, interpreted and utilised to review and
refine the monitoring programmes. We wish to understand how a feedback loop will be
established between the monitoring programme(s) and review and upkeep of the ESC."
In Section 2, we summarise our approach to the development of a forward monitoring
programme and in Section 3 provide some specific examples. The second action
concerning linkages between the ESC and monitoring programmes is addressed in
Section 4 and conclusions are drawn together in Section 5.
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2
Approach to Monitoring and Uncertainty Reduction
2.1
Definition
According to the GRA [2], monitoring is:
"Taking measurements so as to be aware of the state of the disposal system and any
changes to that state. This may include measuring levels of radioactivity in samples taken
from the environment and also measuring geological, physical and chemical parameters that
are relevant to environmental safety and that might change as a result of the construction of
the disposal facility, waste emplacement and closure."
In the 2011 ESC, we do not use the term 'monitoring' to describe the results of specific site
investigations or field experiments, unless those site investigations or field experiments
involve repeated measurements.
Based on this usage, much of the work that we envisage to reduce uncertainties would not
be characterised as monitoring, but as research or site characterisation.
2.2
Monitoring Forward Programme
In the Monitoring Level 2 report [3], we commited to a forward programme of work and a
further study has been undertaken to explore potential approaches to long-term
monitoring [4], but that report is an exploration of the issues rather than a commitment to a
specific and detailed programme. The monitoring programme is developed by regular
reviews, including a review that was completed to determine whether any changes to the
monitoring programme are required in the light of the results of the 2011 ESC [5]. As
described in Section 4 of this memo, the ESC makes use of the data provided by the
monitoring programme and provides input to define monitoring requirements. There is
regular exchange of information and data to ensure that the monitoring programme meets
the requirements of the ESC.
As noted in reference [4], monitoring at the LLWR is undertaken for a variety of purposes:
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to confirm that the repository system is not giving rise to unacceptable environmental
hazards by direct measurement of the impacts and to ensure compliance with the
relevant environmental standards;
to develop and build confidence in the models of the repository system by collecting
data that may be used to refine conceptual models or in model parameterisation,
calibration or validation;
to provide reassurance to stakeholders that the system is safe and is evolving in a
manner consistent with the models and assumptions in the ESC;
to define baseline conditions before specific engineering developments or activities,
such as the construction of a new repository component (for example, a vault).
We make wide use of monitoring data in the ESC and have summarised the uses to which
data re put in Subsection 4.3 of the Monitoring Level 2 Report [4].
2.3
Strategies for Uncertainty Reduction and Model Validation
We have identified the main uncertainties that bear on the 2011 ESC and will continue to
keep these uncertainties under review and to manage them. We also recognise the need to
validate models where appropriate and to continue to build confidence in numerical models.
In Section 5 of reference [6], we committed to a forward programme of technical work to
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reduce uncertainties and resolve issues. Some of the details of that work programme
remain to be specified and a number of hold points were envisaged at which decisions
would be made.
As the details of much of the programme remains to be defined, we cannot provide a
comprehensive account of what work will be undertaken to address a particular uncertainty.
However, we are committed to the process:
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identify uncertainties through a register of uncertainties;
decide on the best way of addressing these uncertainties (use of monitoring data,
experiment, site characterisation etc.);
implementing that programme of work and taking account of the findings in future
models.
It is noted that the LLWR follows the IAEA definition of model validation [7]:
"model validation. The process of determining whether a model is an adequate
representation of the real system being modelled, by comparing the predictions of the
model with observations of the real system."
In comparing model estimates with observation, account must also be taken of uncertainty
and spatial variability. Differences between observation and model estimates may well be a
consequence of uncertainty and spatial variability.
When considering particular uncertainties or outstanding issues, there is always a question
as to the most appropriate sort of work to undertake to reduce uncertainties. For example, a
given uncertainty might be reduced by:


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undertaking laboratory experiments;
a specific programme of site characterisation;
an in-situ or field experiment;
a programme of modelling;
desk study review;
use of monitoring data.
Only some uncertainties are best resolved through the use of monitoring data. For example,
information on the chemical conditions in the vaults after resaturation of the vault system
cannot be addressed by monitoring before the occurrence of such resaturation. Monitoring
data are helpful in calibrating or validating some models (for example the 3-D groundwater
flow model used in the 2011 ESC where the model is intended to provide a realistic model of
the physical processes), but less helpful in other cases (e.g. in relation to the estimation of
fluxes or concentrations of contaminants in the near field, where some of the models
cautiously omit any representation of the kinetic processes that sometimes control the
releases of contaminants from the waste).
3
Specific Examples
In this section, we provide some specific examples to illustrate our proposed approach.
Table 1 identifies some of the key uses of monitoring data in the ESC to constrain or build
confidence in conceptual and numerical models. Further examples are provided in
Subsection 4.3 of reference [4].
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Table 1
Use of Monitoring Data in the ESC
Monitoring Data
Groundwater levels
Measurements of
contaminant
concentrations in
leachate
Measurements of
contaminant
concentrations in
groundwater
Leachate chemistry
measurements
Monitoring of coastal
morphology walkover surveys,
LiDAR etc.
Use in ESC/Comments
Groundwater level data have been used to calibrate the
3-D groundwater flow model. Transient groundwater level
data have also been used to confirm that the 3-D
groundwater flow model has a reasonable representation of
the transient response of the system. It is likely that in
future iterations of the ESC attempts will be made to
improve the calibration and better represent the distribution
of heads in the system (e.g. around the groundwater
'mound').
Groundwater level data continue to be collected and will be
compared with the data used in the ESC to assess whether
there are any future changes in the groundwater system.
GRM has been used to estimate the concentrations of
C-14, Tc-99 and uranium in trench porewaters. These
estimates have been compared with observation [8].
Estimates of the concentrations of non-radiological
contaminants estimated with GoldSim have been
compared with observations. However, it should be noted
that the assessment model is not a detailed chemical
model and the relevant test would be that the estimates of
concentration from GoldSim are equal to or greater than
those observed. Detailed validation of the GoldSim model
is not considered appropriate.
Further measurements of radionuclides that are key
contributors to radiological impact are currently being
considered and if obtained would be compared with the
results of GoldSim calculations. However, as noted above,
detailed validation of GoldSim, as an assessment model, is
not considered appropriate.
Estimates of contaminant concentrations in groundwater
have been compared with the results of GoldSim
calculations (e.g. reference [9]). However, the objective of
such comparisons should be to check that the assessment
models are not unduly optimistic rather than to validate the
simple models used in GoldSim. It is noted for example
that a cautious representation is used in GoldSim for the
tritium source term and it would not be expected therefore
that the spatial and temporal distributions of tritium would
be replicated fully by the model.
Measurements are made of pH, Eh and other bulk
chemical characteristics of trench leachate. These are
compared with the results of GRM calculations.
Coastal erosion is being monitored both by regular walk
over surveys and LiDAR and other surveys. These data
are used as input to conceptual model development for
coastal erosion and hence form a basis for numerical
modelling. Continuation of this monitoring programme will
produce a better picture of rates of change, both of local
erosion and local accretion.
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LLWR/ESC/Mem(12)176
Table 1 (continued)
Monitoring Data
Gas compositions
Surface water
monitoring of
contaminant
concentrations
Surface water flows
Air monitoring
Cap subsidence
Long-term Trench
and Vault
Experiments
Monitoring of ISO
containers
Use in ESC/Comments
Gas compositions can be used to check that GRM is giving
reasonable estimates of gas composition, e.g. in relation to
the relative proportions of methane and carbon dioxide and
that generation rates are generally low. However,
measurements of flux are difficult and it is not considered
feasible to build confidence in detailed quantitative
estimates of bulk gas generation by direct measurements
in the trenches (as opposed to using instrumented
experiments).
Such monitoring is relevant to the radiological impacts that
arose as the result of past discharges from authorised
disposals. The monitoring will not be a good test of current
ESC models. The data do confirm that the system is
performing as expected.
Used as an input to water balance for assessing the
performance of the cap. The flow data have been used to
assess the water balance of the whole site and the
influence of the different sub-catchments.
Concentrations or fluxes of contaminated dust and radon
can be measured at the site boundary and used as an input
to estimates of radiation dose (e.g. reference [9]).
Observations will provide data on the behaviour of the
trenches during pre loading and as a result of waste
degradation. Effects can either be managed or compared
with the assumptions used in cap subsidence models.
A range of outputs have been used in the ESC, including
estimates of cellulose degradation rates in the trenches,
which are partly based on results from the Long-term
Trench Experiments.
Monitoring of the condition of waste container will
determine whether current assumptions are reasonable or
not.
Table 2 provides our current review of the approaches that offer the best potential to reduce
some of the key uncertainties in the ESC. It is not a commitment to undertake particular
pieces of work as in a number of cases we envisage further studies and decision points
before commissioning detailed research. The table addresses the uncertainties identified by
the Environment Agency in the IRF.
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Table 2
Potential approaches to address key uncertainties
Uncertainty
Coastal recession
rates
C-14 gas generation
and release
Bulk gas generation
rates
Dilution, effective
concentrations of
contaminants in
groundwater
Potential approach
Regular monitoring both by
inspection and by aerial survey will
provide further information on
coastal recession rates.
Laboratory experiments on wellcharacterised wastes would be
appropriate where further data are
required on gas generation rates.
Use might be made of experimental
results from other programmes. We
are intending to review our
requirements based on the results
from current re-assessment work.
Laboratory experiments of corrosion
rates or cellulose degradation rates
would be the likely approach, if
further data were required. We
have already obtained data on
cellulose degradation rates from
such experiments (including the
Long-term Experiments) and
experimental data on corrosion
rates are available from other
programmes.
The most effective way of reducing
uncertainties is to build confidence
in the groundwater flow model and/
or to better calibrate it.
There is some scope for obtaining
direct information on the dilution
factor by careful measurements of
contaminant concentrations at
different depths and we are giving
this further consideration. Tracer
tests might also be considered.
6
Comments
Monitoring data are a key
requirement to address this
uncertainty.
Monitoring data are not likely
to provide helpful data on
C-14 generation rates given
heterogeneity and the
difficulty of measuring fluxes
as opposed to concentrations.
Data on generation rates from
specific wastes cannot easily
be obtained from monitoring.
Monitoring data are not likely
to contribute towards
reductions in uncertainty, as
fluxes are difficult to measure.
Direct attempts at
measurement of a dilution
factor are likely to be difficult
because of heterogeneity.
We have identified the issue
during our recent monitoring
review [5] and propose to give
further consideration to this
issue in 2012/13.
LLWR/ESC/Mem(12)176
Table 2 (continued)
Uncertainty
Release of
contaminants from
the near field
Unsaturated zone
contaminant
transport
Evolution of the
Engineered Barriers
Potential approach
A number of processes are relevant
to understanding the release of
contaminants from the near field
including, processes controlling
release from the waste (e.g.
diffusion or metal corrosion),
sorption, chemical reaction,
groundwater flow, distribution of
water in the unsaturated zone etc.
Many of those processes are best
addressed using designed
experiments. However, monitoring
data are also important as they
provide a means of comparing
observed and calculated
contaminant concentrations and
monitoring data are also important
in parameterising and testing
groundwater flow models and
understanding the water balance.
Specific laboratory experiments
might be appropriate to study the
distribution of saturation in different
scale pores, column transport
experiments with sorption and
desorption to provide data for
comparison with ESC model.
During the Period of Authorisation,
monitoring is possible either of
engineered barriers or the effects of
those barriers on flow and
contaminant transport. We
recognise the need to continue to
address this question.
For processes that operate over this
period, useful data can be obtained
and there is some scope for
extrapolation.
7
Comments
An integrated approach is
needed drawing on
monitoring data, the results
from validated models and
experimental data.
Monitoring data are not likely
to be a key source of model
validation. It would be difficult
to acquire sufficiently detailed
information for a wellcharacterised region in order
to allow validation of any
model.
It is harder to use monitoring
data (collected during the
Period of Authorisation) to
understand barrier
performance after the end of
the Period of Authorisation.
Experimental approaches
might be possible to address
some phenomena.
We are planning to review
how to assess the
performance of the
engineered barriers in
2012/13
LLWR/ESC/Mem(12)176
Table 2 (continued)
Uncertainty
Elevated
groundwater levels
(mound)
Potential approach
We have recently analysed
available data to gain a better
understanding of the origin of the
groundwater mound [10].
Data on the effects of Groundwater level data are
construction of
available from before, during and
Vault 9
after construction of Vault 9. The
data can be used to assess the
performance of the hydrogeological
model in responding to changes in
recharge.
Settlement of
We will continue to monitor
grouted waste
subsidence of the interim cap and
containers and
will monitor the final cap when it is
capping system
installed. The contribution to
subsidence from the degradation
and settlement within the ISO
containers is not amenable to
monitoring since the containers will
remain intact for a long period.
4
Comments
The analysis indicates that
the ‘mound’ is defined by
groundwater levels in a small
number of boreholes drilled
during the same period.
Further work is required to
investigate the condition of
the boreholes to establish
whether the levels are correct.
The use of the Vault 9 data to
develop the hydrogeological
model will be considered as
part of the work being
planned to maintain
hydrogeological
understanding.
Further work may be defined
as an outcome of current
studies of voidage in Vault 8.
We recognise the need to
continue to monitor the
condition of the containers
themselves.
Linking the ESC and the Monitoring Programme
The existing arrangements for collaboration between the ESC Project and Monitoring teams
were explained in the Monitoring Level 2 report [3]. These arrangements have ensured that
the LLWR’s monitoring programme reflects the needs of the ESC and that the results of the
programme were properly taken into account in the development of the ESC. These
arrangements will continue, including the formal monthly minuted meetings between the two
teams at which ESC requirements and results from the monitoring programme are raised
and discussed.
Repository Site Procedure 02.20: Environmental Monitoring Data Review requires the
Monitoring team to consult with stakeholders, including the ESC Project team, about the
significance of unusual monitoring results if appropriate.
The implementation of the ESC, planning for which is underway, will include formal
arrangements for the implications of new information relevant to the ESC to be assessed.
Potential new information will include new results from the monitoring programme if these
have the potential to affect understanding of the characterisation of the site and facility. If
appropriate, the ESC will be revised or extended to take account of the new information
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under a formal change control process, similar to that used for the site’s nuclear safety
cases.
An Issues Register will be developed and maintained, as part of the implementation of the
ESC. Either this new register, or the existing Environmental Issues Register, will be used to
record monitoring issues relevant to the ESC whose resolution requires further work.
In our response to Issue Resolution Form ESC-RI-SCM-001, Maintenance of ESC Expertise
and Knowledge, we described our consideration of the creation of a new, combined
technical team at the LLWR, in which the Monitoring and ESC Project teams would be
brought together. This organisational change is now underway and will be completed in the
next few months (subject to Management of Change considerations). This organisational
change can only enhance the already close working of the two teams.
These organisational changes and new systems will complement and enhance current
arrangements, ensuring that the LLWR’s monitoring programme continues to reflect the
needs of the ESC and that the results of the programme are properly taken into account in
the development of the ESC.
5
Summary
The LLWR recognises the requirement to identify and manage uncertainties. Where this is
practicable through the collection and use of monitoring data, we will pursue this. However,
it is not always possible to use monitoring data to validate models in the sense of confirming
that the physical and chemical representations of the system are correct in detail.
Experimental work may often be the preferred approach and we consider such work to be
outside the conventional monitoring programme. We have provided some examples, based
on the uncertainties identified by the Environment Agency, to illustrate our approach.
The LLWR recognises the need to ensure very close links between the ESC and Monitoring
programmes. We have and will put in place arrangements and systems to ensure that this
occurs.
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References
Environment Agency, The Use of Future Monitoring to Reduce Uncertainties in the
ESC, Issue Resolution Form, ESC-RO-SUE-007, 2012.
Environment Agency, Northern Ireland Environment Agency and Scottish
Environment Protection Agency, Near-surface Disposal Facilities on Land for Solid
Radioactive Wastes: Guidance on Requirements for Authorisation, February 2009.
LLWR, The 2011 ESC: Monitoring, LLWR/ESC/R(11)10024, May 2011.
Hayes P, Keep M, Fretwell B and Smith G, Long Term Environmental Monitoring
Strategy, Entec report 27280 Issue 4, April 2011.
LLWR, ESC Review of the Monitoring Programme: Post 2011 ESC,
LLWR/ESC/R(12)10048, in preparation.
LLWR, The 2011 Environmental Safety Case, LLWR/ESC/R(11)10016, May 2011.
IAEA, IAEA Safety Glossary, Terminology Used in Nuclear Safety and Radiation
Protection, IAEA Vienna, 2007.
Small JS, Lennon C and Abrahamsen L, GRM Near-field Modelling for the LLWR
2011 ESC, NNL Report (10)11233 Issue 2, April 2011.
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LLWR/ESC/Mem(12)176
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LLWR, The 2011 ESC: Environmental Safety during the Period of Authorisation,
LLWR/ESC/R(11)10027, May 2011.
Jackson CP and Woollard H, Integration of Geology and Hydrogeology at the LLWR,
Amec Report 005864_002 Issue, 2 July 2012
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