Due to the Three Mile Island accident in 1979 the Swedish government took the
decision in 1986 to impose a pressure relief system for Swedish BWR’s which prevents
containment overpressure in case of LOCA. This pressure relief system consists of a
rupture disks in two different systems, non-filtered system 361 and filtered system 362.
During a steam line break it is not clear if an unjustified activation of rupture disk 361 or
362 could possibly occur. If significant amount of nitrogen will leak out from the
containment then, there is a risk of low pressure in the containment (e.g. due to
activation of containment spray) with leaking rupture disks, which might cause air inflow
to the containment and burning of hydrogen, so conditions of activation of rupture disk
must be studied.
The main objective of this master thesis is the investigation of conditions of activation
of rupture disk in BWR containment filtering system. In order to find out these
conditions specific software called GOTHIC has been used.
The methodology of this master thesis has been modeling different containments with
GOTHIC software; this thesis work will go from a simple GOTHIC model, that consist in
nine lumped control volumes connected by flow paths, until a more complex GOTHIC
model that consist in a combination of lumped and 3D control volumes, connected
among them by flow paths and 3D connectors.
A large LOCA in the upper part of the reactor vessel will be considerate, due to this
severe accident; conditions for the activation of the rupture disk will be complying. It
has to be mentioned that pressure in the lumped modeling will be lower than pressure
in the 3D volumes. Activation time for the lumped modeling will be 8,5 seconds after
the steam break for system 362 and activation time for 3D modeling will be 2,8
seconds for system 362 as well. In neither case 361 system will be activated.
Considering this is a nuclear safety study and accuracy must be a key point, for further
investigations it might be more than advisable using 3D control volumes instead of
lumped control volumes.
It has to be mentioned also that due to there is no experimental data, uncertainty
regarding to the results exist, and if a further safety analysis want to be done, sensitive
study of the parameters implemented on GOTHIC software should be performed in the
future. |