UK Research in LOCA-Related Activities and Co-operation with the Karlsruhe Research Centre - A Historical Perspective

From the 1970s onwards, very extensive research on Loss-of-Coolant Accidents (LOCA) was
carried out in the UK, well supported by the industry and regulator, much of it in support of the
safety case for the Sizewell B PWR, a Westinghouse 4-loop design. This work involved separate effect
and bundle tests on Zircaloy oxidation and creep, with investigation of methods for defining and
measuring bundle blockage. The dynamic bundle ballooning tests were supplemented by bundle
tests with a fixed blockage to investigate in detail coolability and coolant bypass effects, and by pellet
stack vibration tests investigating potential fuel slumping under LOCA conditions. There was
extensive international collaboration with experimental teams particularly in the US, Germany and
France, and the OECD Halden Reactor programme in Norway. Cooperation with the then
Kernforschungszentrum Karlsruhe (KfK, now Karlsruhe Institute of Technology, KIT) was particularly
important, concerning small-scale creep/ballooning and oxidation tests on Zircaloy cladding, along
with bundle/reflood tests in the REBEKA facility. The knowledge gained was encapsulated in the
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UKAEA modelling codes CANSWEL-2 for clad ballooning (2D r-theta) treatment taking account of
pellet/clad eccentricity and associated azimuthal temperature gradients) and MABEL-2 (with
CANSWEL-2 as the deformation module) for calculating ballooning and blockage in bundle
geometries and for reactor safety assessment, including mutual feedback with thermal hydraulic
effects and channel blockage.
Following this major effort, research continued including small-scale material tests
particularly concerning the properties of advanced cladding materials, such as binary alloys of the Zr-
Nb type, so that the LOCA response of cores containing these materials could be assessed. More
recently a new build programme in the UK, based on PWR and BWR technology, has been
established, and research capability is being established relevant to their safety needs. The MABEL-2
code has been further developed by Imperial College into the MATARE code, with additional thermal
hydraulic capability, and this remains the reference UK tool for independent assessment for clad
deformation and reflood response during a LOCA.
This presentation summarises the progress made over these years, indicating the ongoing
development of the MATARE code, and emphasising the cooperation with KIT. New work identified
here is being taken into account in an updated CSNI SOAR on fuel behaviour during loss of coolant
accidents, currently in progress.