atw 2018-1

atw Vol. 63 (2018) | Issue 1 ı January
ENERGY POLICY, ECONOMY AND LAW 16
the terrorist attacks in the USA on September 11, 2001 and
the Fukushima Daiichi accident in March 2011 [24],
although the SFPs and the fuel stored in the pools remained
safe during the accident. Considering all possible initiating
events from safety as well as security perspectives, and the
assumption that the accident cannot be prevented or
mitigated, some SFP scenarios could possibly lead to large
radiological consequences on-site and off-site.
The main knowledge gaps are identified thanks to a
recently completed OECD/NEA/CSNI activity, led by IRSN
with the participation (among the international panel
of experts) of ETSON members Bel V, GRS, PSI and NRA,
on applying a Phenomena Identification and Ranking
Technique (PIRT) on SFPs under loss-of-cooling and
loss-of-coolant accidents conditions [20]. The resulting
phenomena of primary interest for further research can be
summarized as follows:
• Cladding chemical reactions with mixed steam-air
environments for all type of fuel claddings present in
SFPs and also the low temperature range,
• Thermal-hydraulic and heat transfer phenomena for
the coolability of partly or completely uncovered fuel
assemblies,
• Thermal-hydraulic behaviour and large-scale natural
circulation flow pattern that evolves in the SFP with
fuel assemblies covered with water,
• Spray cooling of uncovered spent fuel assemblies in
typical storage rack designs.
Quite a few experiments, specifically targeted to SFP
accidents, are underway or planned. Improvements of
models and simulation codes are still necessary, and their
validation will continue against the produced data.
Regarding applicability of codes, sensitivity and uncertainty
analyses should be considered an integral part of
their applications for SFPs accidents conditions.
National projects focusing on SFP issues are addressed
by several ETSON members, e.g. in cooperation with
universities and research institutes in case of Bel V [25], by
launching experimental programs by IRSN [26], related
to analysis of processes in SFP for LEI, sensitivity analysis
of various modelling options on SFP accidents in SSTC
NRS etc.
3.6 Corium thermophysical and thermodynamic
properties
During a severe accident sequence in LWRs, thermodynamic
models are required to predict the behaviour of
the melts (so-called corium) formed from the degradation
of the core materials, the fission product (FP) releases and
the residual power within the corium different phases.
Data such as the composition of the phases present in the
corium and its physical-chemical properties (solidus and
liquidus temperatures, heat capacities, enthalpies …) are
key parameters for modelling, among other things, the
­corium flow properties, the FP distribution between the
gas and the condensed phases and then for modelling of
the progression of the accident.
Since 1990’s, in the framework of projects in the frame
of the EC (COLOSS, SARNET…), the International Science
and Technology Center (CORPHAD and PRECOS) and the
OECD (MASCA [27]), SA experts have been interested in
the assessment of thermodynamic data for a number of
compounds of reactor materials and fission products and
more complex phases. The most common thermodynamic
data assessment approach for the chemical species of
interest is the CALPHAD method [28]. All properties are
derived from the Gibbs energy expression for each phase.
Based on physical models of the different phases, such
expression depends on various parameters, the values
of which are optimised in order to best fit available
experimental data.
Databases thus obtained are more than mere compilations
of thermodynamic data from various sources.
Their constitution and maintenance needs considerable
work for self-consistency analysis, to ensure that all
the available experimental information is satisfactorily
reproduced. Updating and improving the database
becomes then a regular task, tightly linked to the needs of
end-users.
IRSN is developing, with the SIMAP French Laboratory
scientific support, two consistent thermodynamic
data bases for use for the interpretation of SA experiments
and modelling. NUCLEA [29] is mainly used in research
related to the core degradation (in- and ex-vessel) while
MEPHISTA addresses the fuel and FP behaviour in normal
and off-normal conditions. Both databases are currently
used by a large number of institutes, industrial partners,
and universities, including a few ETSON partners (VTT,
soon PSI), EDF, CEA, Areva, KAERI (South Korea), JAEA
(Japan) and others. The OECD-NEA Thermodynamics of
Advanced Fuels – International Database (TAF-ID) project
[30] (2013-2016) made available a comprehensive,
internationally recognized and quality-assured database
of phase diagrams and thermodynamic properties of
advanced nuclear fuels. Its main goal consists in providing
a computational tool to perform thermodynamic calculations
on both fuel and structural materials for SA in
LWRs and for the design of advanced fuel materials (MOX,
metallic, carbide, nitride fuels) for Generation IV reactors.
The recently launched OECD/NEA Thermodynamic
Characterisation of Fuel Debris and Fission Products
(TCOFF) project (2017-2019), involving 16 partners, aims
at improving the existing thermodynamic databases
(e.g. NUCLEA and TAF-ID) for scenario analyses of SA
progression, looking particularly at the Fukushima-Daiichi
accident.
To date, the main gaps of knowledge in databases are
the following ones:
• The interactions between molten U-Zr-O and iron (and
steel) within the vessel since they impact the heat flux
to the vessel in order to determine the conditions (in
particular time and location) of an eventual rupture, in
particular for a molten metal layer located on top of the
oxide one. Some work has been done in the framework
of the MASCA and MASCA2 projects but it would be
necessary to extend it to MOX fuel.
• The impact of the stainless steel oxide components on
the thermochemistry of the corium-concrete mixtures
which should be experimentally investigated.
• The activity coefficients of the Ag-In-Cd control rod
elements in the melts are a very important item to
derive reliable expressions for vapor pressures of
absorber elements. Vaporization of these elements
during a SA is of prime interest for reactors with
Ag-In-Cd control rods. They actually constitute the
main contributors in terms of mass of the aerosol
release into the reactor coolant system and overall,
they greatly impact the aerosol deposition and
the source term behaviours. In fact, silver and cadmium
are very reactive with iodine which is known to
be a major contributor to the gaseous source term
to environment.
Energy Policy, Economy and Law
ETSON Strategic Orientations on Research Activities. ETSON Research Group Activity
J.P. Van Dorsselaere, M. Barrachin, D. Millington, M. Adorni, M. Hrehor, F. Mascari, A. Schaffrath, I. Tiselj, E. Uspuras, Y. Yamamoto, D. Gumenyuk, N. Fedotova, O. Cronvall and P. Liska