Assessment of Risk Studies and Severe Nuclear Accidents - The Risks of Nuclear Energy Technology - page 209

10.3.2.4 Hydrogen Release in BWRs
The inner containments of modern BWRs (Chap. 3 ) are inertized by nitrogen in
order to avoid hydrogen ignition. Hydrogen recombiners decrease the released
hydrogen concentration and spray systems or steam condensers can decrease
steam pressure buildup [ 102 ]. The inner containment of BWRs must also be
equipped at key locations with instrumentation for the measurement of the hydro-
gen concentration and gamma radiation.
10.3.3 Break of a Pipe of the Residual Heat Removal System
in the Annulus of the Containment by Steam
The reference case for loss-of-coolant accidents with leaks in primary pipes in the
annulus outside the containment as referred to in the German Risk Studies Phase A
[ 10 ] and Phase B [ 64 ] was the assumed break of a pipe or the failure of valves of the
residual heat removal system. This can ultimately cause core meltdown at low
primary pressure. The radioactivity from the core melt in this case would bypass the
leaktightness function of the outer containment and escape directly into the annulus
and, through filters, on into the environment.
This weak spot in the design of existing early pressurized water reactors must be
avoided in future PWRs by appropriate design measures, according to the KHE
Safety Concept. To avoid such possible bypasses, the function of the multiple
barrier system (containment) of retaining the radioactivity must be maintained for
all pipes connected to the primary cooling system (double containment function) [ 8 ,
64 , 65 ]. This is technically feasible.
10.3.4 Core Meltdown After an Uncontrolled Large Scale
Steam Generator Tube Break
In the highly unlikely case of a large scale steam generator tube break, primary
coolant can flow to the secondary side. The loss of primary coolant causes the
primary pressure to drop and the high-pressure safety feed system to be automat-
ically initiated. Should the necessary shutdown of the high pressure safety feed
systems fail, this would ultimately cause overfeeding of the steam generators, and
the main steam relief valves would open. When these main steam relief valves do
not close again, primary coolant will flow straight into the environment. In the
further course of the accident, there could be core meltdown, and the radioactivity
released would escape directly into the environment.
Also this very rare core meltdown accident, which could become possible as a
result of the present design of steam generators and main steam pipes, must be