atw 2018-07

atw Vol. 63 (2018) | Issue 6/7 ı June/July
RESEARCH AND INNOVATION 382
external TSO. Due to a double-barrier
concept of the experiments, the TSO
verifies that even in case of D 2 or H 2
ignition, neither the beam tube
integrity is affected nor any damage to
the core is expected.
In the assessment of the MUEEF,
the FR MZ fulfils a protection level 3
and the RSK sees no further need for
robustness analysis on this subject.
In case of a SBO the FR MZ is
equipped with an emergency-power
supply which consists of a combination
of battery buffer, which drives all
necessary reactor control units and
radiation surveillance systems for at
least one hour, and an emergency
diesel generator which starts within a
few minutes after the power blackout.
With a permanent diesel reservoir of
about 600 L, the diesel generator
supplies electric power to the FR MZ
infrastructure for up to 40 hours.
4 Evaluation of the
measures
As a follow-up, the points mentioned
above have been re-evaluated by the
RSK in 2017 [2]. The conclusions can
be summarized as follows:
4.1 Evaluation of the measures
taken by FRM II until 2017
4.1.1 Emergency drills
The FRM II has significantly revised
its emergency concept and mostly
implemented the RSK recommendations.
Some recommendations have
not been addressed in full detail yet:
The RSK recommends that the FRM II
should enlarge its concept of emergency
drills. The internal emergency
organisation as a whole should train at
least once yearly, the relevant external
authorities should be included in these
exercises at least every five years.
At the time of writing, however,
the internal emergency exercise concept
is fully functional and even an
external exercise has been done.
These measures, though, have not
been evaluated by the RSK yet.
4.1.2 Emergency measures
to supply water to the
reactor pool
The RSK recommends having a system
in place to supply water to the reactor
pool in case of a failure of the relevant
barriers against loss of pool water.
While this recommendation has not
been addressed explicitly by the
FRM II yet, at FRM II already now with
existing measures or minor changes it
would be possible to supply water to
the pool in case of emergency without
access to the reactor hall. Since no
explicit evidence has been provided by
FRM II yet there is also no evaluation
of the RSK.
4.1.3 Robustness of the
emergency data
acquisition systems
The RSK recommends an analysis on
the availability of the relevant DAQ
systems in case of beyond design base
accidents, since emergency measures
require reliable information e. g. on
the pool water level, temperature,
neutron flux and radiation levels.
While such information – especially
pool level and temperature – can be
acquired easily by rather primitive
means the recommended prove has
not yet been provided by the FRM II.
4.1.4 Emergency communication
The FRM II is equipped with several
independent and diverse communication
channels (e. g. landlines and
GSM mobile phones). On top of that,
the RSK recommends the FRM II
emergency communication should
have priority over other’s communication
needs. This recommendation has
not yet been implemented. However,
the relevant communication channels
(e. g. land line telephone service)
have large reserves and therefore the
safety gain through priority might be
negligible.
4.1.5 Seismic robustness/
implementation of an
additional system to
maintain long term
undercriticality
Additional very detailed and thorough
analysis confirmed that the earlier
only assumed robustness of the
reactor building and the reactor pool
even towards magnitude VIII ½
(MSK) earth quakes. This has been
confirmed by the TSO.
Such a beyond design base event
might impede the proper functioning
of the primary (control rod) and
secondary (four out of five shut down
rods) shut down system. Therefore the
implementation of an additional
system to maintain long term undercriticality
is recommended by the RSK.
The FRM II is exploring several
options to implement such a system.
Ideas include diluting the D 2 O with
H 2 O in the moderator or adding Boron
to the primary cooling loop or the D 2 O
moderator. Calculations show that
even small amounts of such impurities
would already lead to the required
long term undercriticality. No final
design has been drawn up yet.
4.2 Measures taken by the FR
MZ resulting from the RSK
analysis
4.2.1 Emergency communication
Although the FR MZ infrastructure
contains several communication systems,
the RSK suggests, similar to
section 4.1.4 for the FRM II, the
prioritization of the mobile phones in
the public network. The request to the
telephone network provider is under
progress.
4.2.2 Emergency drills
The RSK recognizes that the emergency
management of the FR MZ is
upgraded by creating two new safetydedicated
reactor staff positions. It
furthermore appreciates the idea of
triannual exercises with external
forces and under the involvement of
the MUEEF. In addition to that, the
RSK request to implement annual
internal drills, including the complete
reactor crisis management, into the
FR MZ emergency drill concept.
Preparations for the establishment of
the triannual exercises are currently
ongoing.
4.2.3 Earthquake
Based on the all-embracing event of
an airplane crash, the RSK confirms
the MUEEF’s evaluation to robustness
level 2. Additionally the RSK suggests
describing measures how to shut
down the reactor manually following
an earthquake with a subsequent
malfunction of the control rods. This
description should be integrated in
the reactor operation regulations.
5 Conclusion
After the events in the Fukushima-I
NPP the RSK has analysed the robustness
of the German nuclear reactors in
general and also the FRM II and the
FR MZ with respect to beyond design
base accidents. Already the analysis in
2012 [3] had given a positive result
and only few recommendations to
even further improve the overall
safety of the research reactors in
Germany were presented.
In its 2017 re-analysis [1] the RSK
confirmed that most recommendations
were met by the FRM II. The
FRM II is working to answer the last
open points. For the FR MZ the RSK
confirmed the Mainz MUEEF’s assessment
of the TRIGA research reactor.
No open questions remained from the
2017 assessment of the FR MZ. Both
facilities are working on reaching full
compliance with all the RSK recommendations
in the near future.
Research and Innovation
Safety Assessment of the Research Reactors FRM II and FR MZ After the Fukushima Event
ı Axel Pichlmaier, Heiko Gerstenberg, Anton Kastenmüller, Christian Krokowski, Ulrich Lichnovsky, Roland Schätzlein, Michael Schmidt, Christopher Geppert, Klaus Eberhardt and Sergei Karpuk