Page 44 - July-August 2012

By Mr. Thomas Fink, General Manager
of the Nuclear Safety Division at
SCHOTT Electronic Packaging
The severe accidents, which exceed-
ed the in-place safety levels at the Fu-
kushima Daiichi nuclear power plant in
March 2011, have created a worldwide
re-evaluation into severe accident safety.
Significant information is now in on the
events that lead to the hydrogen explo-
sions.
The Tokyo Electric Power Company
(TEPCO), the operator of the facility in
Fukushima, reconstructed the accident
and found that the temperature inside
the containment structure rose to four
times the normal operating temperature,
while the design pressure was exceeded
by more than twofold. The extreme tem-
perature and pressure levels overstrained
the epoxy seals of some Electrical Pen-
etration Assemblies (EPAs) at Fukushima
and likely led to leakage of explosive
hydrogen outside of the containment
vessel. Once outside of the containment
vessel, the hydrogen built up to an ex-
plosive concentration, which ignited
with devastating effects.
Japan’s Ministry of Economy, Trade
and Industry (METI) provided corrobo-
rating evidence. In a presentation at
the International Atomic Energy Agen-
cy (IAEA) summit in Vienna, Austria, in
March 2012, METI reported the likeli-
hood of containment damage due to
over-pressurization or over-heating, or
both, stating that “it is highly possible
that the leakages were caused by dete-
rioration of the organic [epoxy] sealing
as a result of high temperatures by ther-
mal radiation directly from the pressure
vessel…[the] possible location of leak-
age was top flange, penetration of the
containment vessel, and/or equipment
hatches.”
Containment leakage through epoxy
EPAs under severe accident conditions
was not unanticipated. Nearly 30 years
ago, the U.S. Nuclear Regulatory Com-
mission raised concerns about use of
polymer (such as epoxy) seals on EPAs
because of its vulnerability to heat and
radiation. A 1982 severe accident analy-
sis by Oak Ridge National Laboratory
based on the Browns Ferry Unit 1 reactor
(MK1 containment design)1 concluded
that “CEPA (Containment Electric Pene-
tration Assembly) over-temperature con-
stitutes an important containment failure
mode during degraded core accidents.
For those accident sequences in which
containment failure would be caused by
over-pressurization, failure would most
likely also occur in the CEPA seals.
Since the design of CEPAs is similar for
all nuclear power plants, results of this
study are applicable to other boiling wa-
ter reactors and pressurized water reac-
tor containments.”
EPAs are the vital conduit for power,
control and instrumentation circuits
within nuclear power plants. Their
performance is critical to running vital
functions within the reactor, and their
seals must be strong enough to main-
tain the pressure boundary integrity of
the containment vessel. While organic
polymers, such as epoxy, meet the cur-
rent design basis specifications for EPAs
in first- and second-generation NPPs,
nuclear experts have voiced concern
that the effects of severe accidents
which exceed the design basis condi-
tions could compromise the integrity of
the seal and therefore result in leakage.
Fortunately there are better seal tech-
nologies available for EPAs. One of the
better seal technologies is the Glass-
to-Metal Seal (GTMS) technology. Al-
though just one part of the design mix,
GTMS offers a number of safety advan-
tages over organic sealants to ensure
the integrity of EPA seals and contain-
ment vessels. GTMS technology is an
inorganic, non-aging glass seal with
significant heat- and radiation-resistant
properties. GTMS sealed EPAs have been
maintenance-free for 60 years of use.
The performance range of GTMS EPAs
is staggering having achieved over 400
bar (5,800 psi) and 400 °C (752 °F).
First Learning from Fukushima:
An Area of Focus – Improvement in
Severe Accident Containment Integrity of
Nuclear Power Plants
Glass-to-Metal Sealed
Compression
Compression glass-to-metal sealed
feedthroughs comprise a metal housing,
a glass sealant and metal conductors.
The preassembled component is heated to
a temperature where the glass melts to the
metal. During the cooling process, the metal
housing contracts at a rate much higher than
that of the glass. This compression creates
a highly pressure-resistant and hermetically
sealed unit that offers the highest safety.