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Regarding computational fluid dynamics (CFD),
work is progressing on the consolidation of best practice
guidelines for CFD application to nuclear reactor
safety (NRS) problems, CFD code assessment for NRS
problems, CFD extension to two-phase flow problems,
benchmarking activities and the organisation
of workshops. A new CFD benchmark exercise was
undertaken on turbulent flow in a customised rod
bundle with spacer grids and completed in September
2012. The results of this benchmark were presented
during the workshop on Experimental Validation
and Application of CFD and Computational Multifluid
Dynamics (CMFD) Codes to Nuclear Reactor
Technology (CFD4NRS-4), organised in the Republic
of Korea on 10-12 September 2012.
Regarding knowledge transfer, the NEA held
the 3 rd THICKET seminar on 25-29 June 2012. This
seminar series, organised with the support of the
European Commission (EC) and the IAEA, is dedicated
to transferring competence, knowledge and
experience gained through CSNI activities in the
field of thermal-hydraulics over the past three decades.
This time, new topics such as best-estimate
methods and uncertainty evaluation, computational
fluid dynamics, safety margin assessment and application
and OECD/NEA project outcomes were added
to the programme.
Work also continued on in-vessel behaviour of
degraded cores. A new activity is progressing on
code benchmarking to assess the ability of computer
codes to simulate in-vessel core melt progression
and degraded core coolability on the basis of
three severe accident sequences involving safety
systems failure or delayed operation. In 2012, there
were two meetings related to this benchmarking
activity, which is expected to be completed in
2014. In parallel, the work on the Containment Code
Validation Matrix approached completion at the end
of 2012 with a draft report distributed for independent
review. The activity on a state-of-the-art report
addressing molten corium concrete interaction and
ex-vessel molten core coolability was launched in
April 2012, and progressed with a technical meeting
in November 2012; the state-of-the-art report is
expected to be completed at the end of 2013.
Finally, the WGAMA submitted several proposals
to the CSNI regarding Fukushima-related issues.
All the proposals were approved and concern: i) a
status paper on filtered containment venting; ii) an
international benchmarking project on fast running
software tools to model fission product releases during
accidents at nuclear power plants; iii) a status
paper on hydrogen generation, transport and risk
assessment; and iv) a status report on spent fuel
pools under loss of cooling accident conditions.
Ageing and structural integrity of
reactor components
The main topics addressed by the Working Group on
Integrity of Components and Structures (WGIAGE)
concern the integrity and ageing of metal components
and concrete structures, and the seismic behaviour
of structures and components.
In 2011, the WGIAGE finalised the report that
documents the main findings and conclusions of the
2010 activity on improving the robustness of assessment
methodologies of structures impacted by missiles
(IRIS-2010). In 2012, a post-IRIS-2010 round robin
was initiated to give the opportunity to participants
to update and improve blind simulations.
In view of the increasing interest in safe, longterm
operation of existing nuclear power plants,
an activity is being conducted to identify technical
areas of common interest concerning age-related
degradation of materials in safety-related systems,
structures and components (SSCs) during NPP longterm
operation (60 years) and to capture operating
experience associated with degradation in buried
tanks and piping.
In 2012, a Fukushima-related activity on metallic
component margins under high seismic loads was
initiated to quantify the existing margins in seismic
analysis of safety-class components for high seismic
loads. The group continues to review current practices
in member countries regarding the definition of
seismic input from far- and near-field sources and its
control point, with a view to providing recommendations
on their harmonisation in the future.
Risk assessment
The main mission of the Working Group on Risk
Assessment (WGRISK) is to advance the understanding
and utilisation of probabilistic safety assessment
(PSA) as a tool to support nuclear safety decision
making in member countries. The activity on PSA
knowledge transfer is progressing as scheduled,
with the objective of developing an understanding
of the current needs and ongoing activities in member
countries on PSA knowledge transfer, including
related international activities, and supporting the
dissemination of lessons learnt and best practices.
A comprehensive questionnaire was circulated to
WGRISK members, the IAEA and the EC, and the
resulting report was approved by the CSNI.
The activities on PSA for the design and commissioning
of new NPPs and on PSA for advanced reactors
seek to identify and characterise current practices
in this area and to identify key technical issues,
current approaches to addressing them, associated
lessons learnt and issues requiring further work. In
addition, the proceedings of a joint workshop including
PSA for new and advanced reactors were issued.
A common report based on the outcome of this joint
workshop and on specific surveys on PSA for new
and advanced reactors was approved.
Work has continued on the development of best
practice guidelines on failure mode taxonomy for
reliability assessment of digital instrumentation
and control (I&C) systems for PSA with the aim of
developing technically sound and feasible failure
mode taxonomy for such reliability assessments,
and providing best practice guidelines on the use of
taxonomy in modelling, data collection and qualifi-
22 Technical Programmes