Nuclear Production of Hydrogen, Fourth Information Exchange ...

CONCEPTUAL DESIGN OF THE HTTR-IS NUCLEAR HYDROGEN PRODUCTION SYSTEM
On the other hand, the IS process in the HTTR-IS system should be built outside and set at an
appropriate distance from the reactor building in order to prevent flammable and toxic gas inflow to
the building from the reactor safety point of view (Sakaba, 2008). Hence, the secondary cooling system
will be modified to penetrate the CV and reactor building in order to provide heat to the IS process.
The IHXTR in the HTTR-IS system would result in radionuclide transportation to the outside of the CV.
In order to reduce radionuclide transportation from the primary to the secondary cooling system,
containment isolation valves should be installed. These valves will be automatically closed by the
signal detecting the heat transfer tube rupture of IHX. Meanwhile, the current engineered safety
features actuating system is not designed to detect the rupture since the scenario did not impact on
the reactor safety of the original HTTR. Therefore, a detection method of heat transfer tube rupture of
IHX should be established.
Detection method of IHX tube rupture
The strategy of detection method establishment is to apply the simple and reliable method since it
would comprise the engineered safety featured actuation system.
During the IHXTR, helium flows through the rupture point from the secondary to the primary
cooling system until the pressure of both balances. In the meanwhile, pressure control systems try to
maintain the set pressures of cooling systems. The following parameters would vary sensitively:
• differential pressure between primary and secondary cooling system;
• primary helium gas recovery flow rate due to primary helium pressure control system;
• secondary helium gas supply flow rate due to primary-secondary differential pressure control
system.
Monitoring differential pressure between the primary and secondary cooling systems would be
one of the candidates for detecting the heat transfer tube rupture. Meanwhile, the pressure varies
even at normal operation and therefore there is a concern of malfunction of CV isolation valves. In case
of primary helium gas recovery flow rate, primary helium pressure control system only covers the
rated operation since the system activates when the primary coolant pressure reaches about 3.95 MPa.
On the other hand, the primary-secondary differential pressure control system covers start-up,
shutdown and rated operations. The control system keeps supplying helium gas to the secondary
cooling system during the scenario. Thus, monitoring the entire secondary helium gas supply would
be an effective way to detect the tube rupture.
Numerical evaluation of the detection methods
The availability of the proposed detection method was evaluated. Furthermore, system behaviour is
assessed during the scenario. A system analysis code developed for VHTR systems is used for these
calculations.
Outline of the system analysis code
JAEA conducted an improvement of the RELAP5 MOD3 code (US NRC, 1995), the system analysis code
originally developed for LWR systems, to extend its applicability to VHTR systems (Takamatsu, 2004).
Also, a chemistry model for the IS process was incorporated into the code to evaluate the dynamic
characteristics of process heat exchangers in the IS process (Sato, 2007). The code covers reactor power
behaviour, thermal-hydraulics of helium gases, thermal-hydraulics of the two-phase steam-water
mixture, chemical reactions in the process heat exchangers and control system characteristics. Field
equations consist of mass continuity, momentum conservation and energy conservation with a two-fluid
model and reactor power is calculated by point reactor kinetics equations. The code was validated by
the experimental data obtained by the HTTR operations and mock-up test facility (Takamatsu, 2004;
Ohashi, 2006).
Figure 3 shows the nodalisation of the HTTR-IS system model. The reactor consists of the internal
flow path (P2), permanent reflector blocks (HS25), upper plenum (B4), reactor pressure vessel (RPV)
(HS30), vessel cooling system, reactor core bypass flow (P10), lower plenum (B12) and reactor core. The
390 NUCLEAR PRODUCTION OF HYDROGEN – © OECD/NEA 2010