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@BOOK{Moormann:1304,
      author       = {Moormann, Rainer},
      title        = {{A} safety re-evaluation of the {AVR} pebble bed reactor
                      operation and its consequences for future {HTR} concepts},
      volume       = {4275},
      issn         = {0944-2952},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {PreJuSER-1304, Juel-4275},
      series       = {Berichte des Forschungszentrums Jülich},
      pages        = {VIII, 37 p.},
      year         = {2008},
      note         = {Record converted from VDB: 12.11.2012},
      abstract     = {The AVR pebble bed reactor (46 MWth) was operated 1967-88
                      at coolant outlet temperatures up to 990°C. A principle
                      difference of pebble bed HTRs as AVR to conventional
                      reactors is the continuous movement of fuel element pebbles
                      through the core which complicates thermohydraulic, nuclear
                      and safety estimations. Also because of a lack of other
                      experience AVR operation is still a relevant basis for
                      future pebble bed HTRs and thus requires careful
                      examination. This paper deals mainly with some
                      insufficiently published unresolved safety problems of AVR
                      operation and of pebble bed HTRs but skips the widely known
                      advantageous features of pebble bed HTRs. The AVR primary
                      circuit is heavily contaminated with metallic fission
                      products (Sr-90, Cs-137) which create problems in current
                      dismantling. The amount of this contamination is not exactly
                      known, but the evaluation of fission product deposition
                      experiments indicates that the end of life contamination
                      reached several percent of a single core inventory, which is
                      some orders of magnitude more than precalculated and far
                      more than in large LWRs. A major fraction of this
                      contamination is bound on graphitic dust and thus partly
                      mobile in depressurization accidents, which has to be
                      considered in safety analyses of future reactors. A
                      re-evaluation of the AVR contamination is performed here in
                      order to quantify consequences for future HTRs (400
                      MW$_{th]$). It leads to the conclusion that the AVR
                      contamination was mainly caused by inadmissible high core
                      temperatures, increasing fission product release rates, and
                      not - as presumed in the past - by inadequate fuel quality
                      only. The high AVR core temperatures were detected not
                      earlier than one year before final AVR shut-down, because a
                      pebble bed core cannot yet be equipped with instruments. The
                      maximum core temperatures are still unknown but were more
                      than 200 K higher than calculated. Further, azimuthal
                      temperature differences at the active core margin of up to
                      200 K were observed, probably due to a power asymmetry.
                      Unpredictable hot gas currents with temperatures > 1100°C,
                      which may have harmed the steam generator, were measured in
                      the top reflector range. After detection of the inadmissible
                      core temperatures, the AVR hot gas temperatures were
                      strongly reduced for safety reasons. Thus a safe and
                      reliable AVR operation at high coolant temperatures, which
                      is taken as a foundation of the pebble bed VHTR development
                      in Generation IV, was not conform with reality. Despite of
                      remarkable effort spent in this problem the high core
                      temperatures, the power asymmetry and the hot gas currents
                      are not yet understood. It remains uncertain whether
                      convincing explanations can be found on basis of the poor
                      AVR data and whether pebble bed specific effects are acting.
                      Respective examinations are however ongoing. Reliable
                      predictions of pebble bed temperatures are at present not
                      yet possible. [...]},
      cin          = {IEF-6},
      cid          = {I:(DE-Juel1)VDB814},
      pnm          = {ohne FE},
      pid          = {G:(DE-Juel1)FUEK307},
      typ          = {PUB:(DE-HGF)3},
      url          = {https://juser.fz-juelich.de/record/1304},
}