Reactor System Reactor
Type Power
(MW•th) Passive Safety Systems
Long operating cycle Simplified Boiling Water
Reactor (LSBWR) Toshiba Corporation, Japan
BWR 900 CORE:
• Gravity Driven Core Cooling System CONTAINMENT:
• Passive Containment Cooling System
• Suppression Pool
VII-1. Introduction
The long operating cycle simplified boiling water reactor (LSBWR) is a modular boiling water reactor (BWR) plant that is designed by Toshiba Corporation. The reactor concept described in this section has a small power output, a capability of long operating cycle, and a simplified BWR configuration with comprehensive safety features. To be economically competitive, simplification of systems and structures, modularization for short construction period, and improvement in availability are included into the LSBWR design. For comprehensive safety features, the aim is to need no evacuation by utilizing highly reliable equipment and systems such as large RPV inventory, bottom located core layout, in-vessel retention (IVR) capability and passive emergency core cooling system (ECCS) and primary containment vessel (PCV) cooling. Figure VII-1 shows conceptual drawing of the LSBWR.
FIG. VII-1. Conceptual drawing of the LSBWR.
Turbine
PCV Reactor
Seismic mechanism PCCS
69 VII-2. Description of passive core cooling system
Natural circulation core cooling is applied for eliminating recirculation pumps. This results in high reliability in operation. For attaining natural circulation core cooling, the fuel length is shortened to 2.2m from the conventional 3.7m to decrease the pressure drop. Figure VII-2 shows the core and fuel bundle for the LSBWR. Figure VII-3 shows the LSBWR reactor internals and configuration.
Part Length Rod Control Rod
1.2C Lattice Bundle Core 1.2C lattice bundle Water rod
Internal water gap
Fuel Rod
Part Length Rod Control Rod
1.2C Lattice Bundle Core 1.2C lattice bundle Water rod
Internal water gap
Fuel Rod
FIG. VII-2. Core and fuel bundle for the LSBWR.
Ｂ ＢＢ E L 1 2 0 0 ＴＢＢ E L 32 0 0 C R D S u pp o r t P la t e
S h r o ud H e a d F e ed Wa t e r Ｎ Ｎ Ｎ E L 10 0 0 0 M ain S t e a m Ｒ ＲＲ T o p E L 14 0 0 0
S h ro ud In ne r Dia . ４４ ４４φ V e s s e l Inn e r Dia . ５ ４４ ４φ
LONG CYCLE SI MPLI FI ED BWR 300MWｅ CLASS REACTOR VESSEL
S t e a m D r y e r
U p p er E nt r y C R D
Ｇ Ｇ ＧＧ C R
S t e a m S e p a r at o r
F u e l
FIG.VII- 3. LSBWR reactor concept.
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VII-3. Description of passive containment cooling system (PCCS)
The cylindrical type drywell with small diameter can be designed by routing the safety relief valve piping through the spacing between the RPV and the drywell wall and the main vent pipe from the RPV top to the suppression pool. The drywell air space is minimized and contains only SRV and depressurization valve (DPV) components, the gravity driven core cooling system (GDCS) and drywell loading piping. Since MS and FW piping is routed through suppression pool air space, which are protected by the guard pipe, GDCS piping is contained in the access tunnel placed in the lower part of suppression pool, and isolation valves are installed outside PCV etc.
Since the reactor core is placed at the bottom of the RPV, the emergency coolant injection system consisted of DPV and GDCS can achieve high reliability of the water coverage of the reactor core following an accident.
The containment wall with ship hull structure is filled with cooling water that is boiled off to the atmosphere to cool the PCV passively during an accident. This containment wall cooling system is also used for the drywell cooling during the normal operation and therefore the drywell arrangement is simplified without drywell cooling components used in the current BWR containment.
When cooling water in the PCCS pool above PCV is exhausted, external pool or seawater is supplied by gravitation so that the highly reliable and long term PCV cooling is achieved.
The double cylindrical raised suppression pool with the ship hull structure is installed around the cylindrical drywell and above the core elevation. This makes the structure stronger and simpler, and the suppression pool water can be easily used for GDCS and drywell lower part flooding. LSBWR safety system concept including PCCS is shown in Figure VII-4.
The performance of the safety system has been analyzed for a feedwater line break accident. The analysis has been performed using TRAC code incorporated with the heat transfer models for the natural convection cooling and the steam condensation cooling with a noncondensable, which have been used to estimate the heat transfer coefficients in the containment space and the containment wall coolant channels. The analysis results for the containment pressure and the heat removal rate by the passive containment cooling system are shown in Figure VII-5 and VII-6. After taking its peak value during the blowdown phase, the containment pressure keeps decreasing while the GDCS coolant flow is sufficient to suppress the steam production in the reactor core. The containment pressure begins to increase around 3 hours since the GDCS flow decreases and the steam is produced by the decay heat.
The pressure increase is, however, suppressed by the containment wall cooling and is maintained well below the design pressure for 24 hours. The heat removal rate of the containment wall cooling becomes almost comparable with the decay heat after 12 hours. The condensate produced by the containment wall cooling flows from the drywell to the RPV through the GDCS injection line and the reactor core is kept covered.
VII-4. Conclusions
The LSBWR design is still at the conceptual design stage and licensing reviews have not yet been started. Recently, Compact Containment BWR development is going on based on various experience obtained at LSBWR development.
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FIG. VII-4. LSBWR safety system concept.
0
FIG. VII-5. PCV pressure response. FIG. VII-6. Long term containment pressure transient.
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