The ACR-1000 incorporates multiple features utilizing natural circulation to prevent or mitigate accidents. The intrinsic passivity and inherent reliability of these features greatly strengthens the safety case for the ACR-1000.

— Thermosyphoning of the HTS primary coolant can remove decay heat from the fuel upon shutdown for normal maintenance outages and/or when forced circulation is not available.

— Natural circulation of the moderator in the calandria prevents formation of ‘hot spots’ when forced moderator circulation is unavailable. Natural circulation in the calandria can also limit damage to the fuel channels, thereby preventing severe accidents.

— Natural circulation airflows in containment prevent formation of regions of locally high temperature and dilution of hydrogen concentration following an accident.

The inclusion of features using passive natural circulation, in addition to the ‘traditional’ active safety systems used for active mitigation, is intended to improve the overall safety of the ACR design, by virtue of the simplicity and reliability inherent to passive designs.

Enhanced reliability through use of passive natural circulation features helps prevent events from progressing to the level of ‘severe’, or ‘beyond design basis’ accidents, and reduces severe core damage frequency for the ACR.

By utilizing natural circulation to remove heat from the HTS and the moderator, and to cool and circulate the containment atmosphere, the ACR-1000 design objectives are to improve overall reliability for the key safety functions, and to have a greatly enhanced safety case for postulated severe accidents.

Condensation occurs when the temperature of vapor is reduced below its saturation temperature. Presence of even a small amount of Non-condensable gas (e. g. air, N2, H2, He, etc.) in the condensing vapor leads to a significant reduction in heat transfer during condensation. The buildup of non­condensable gases near the condensate film inhibits the diffusion of vapor from the bulk mixture to the liquid film. The net effect is to reduce the effective driving force for heat and mass transfer. This phenomenon is the concern of industrial applications and nuclear reactor systems.

In nuclear plants, the condensation of steam in the presence of non-condensable gas becomes an important phenomenon during LOCA (loss of coolant accident) when steam released from the coolant system mixes with the containment air. Besides this, nitrogen gas in accumulators is a source of non­condensable gas, which can affect the condensation heat transfer inside the steam generator tubes of nuclear power plants, and may effect the core make-up tank performance. The effect of non­condensable gases on condensation heat transfer is also relevant to certain decay heat removal systems in advanced reactor designs, such as passive containment cooling systems. [4]

The effect of non-condensable gases on the condensation of steam has been extensively studied for both natural and forced convection flows. In each of them, geometries of interest (e. g. tubes, plates, annulus, etc.) and the flow orientation (horizontal, vertical) can be different for various applications. The condensation heat transfer is affected by parameters such as mass fraction of non-condensable gas, system pressure, gas/vapor mixture Reynolds number, orientations of surface, interfacial shear, Prandtl number of condensate, etc. Multi-component non-condensable gases can be present.

ANNEX I. ABWR-II

TEPCO, Ge, Hitachi Ltd, Toshiba Corporation

I — 1. Introduction

The ABWR-II, an evolutionary reactor based on the advanced BWR (the ABWR), is now being jointly developed by six Japanese BWR utilities led by Tokyo Electric Power Company (TEPCO), General Electric Company, Hitachi Ltd, and Toshiba Corporation.

By adopting a large electric output (1700 MW(e)), a large fuel bundle, a modified ECCS, and passive heat removal systems, among other design features, a design concept capable of increasing both economic competitiveness and safety performance has been achieved.

The key objectives of ABWR-II are further improvement in economics against alternative forms of electric generation and enhancement of safety & reliability. ABWR-II plant system features are summarized in Figure I-1.

Under low pressure conditions, elevated tanks filled with cold borated water can be used to flood the core by the force of gravity. In some designs, the volume of water in the tank is sufficiently large to flood the entire reactor cavity. As shown in Figure 3, operation of the system requires that the isolation valve be open and that the driving head of the fluid exceed the system pressure plus a small amount to overcome the cracking pressure of the check valves. The performance of the gravity drain tank may be limited under core uncovery conditions due to steam production in the core region. This is a Category D passive safety system.

Some advanced PWR designs incorporate a system to remove decay heat passively through the steam generators. This is done by condensing steam from the steam generator inside a heat exchanger submerged in a tank of water or an air cooled system as shown in Figures 4 and 5, respectively. The system shown in Figure 4 has some similar characteristics to isolation condenser. These are Category D passive safety systems.

This phenomenon involves heat and mass transfer from the containment atmosphere towards the surrounding structures. This phenomenon would occur in existing reactors in case of a coolant release into the containment. It also occurs in advanced designs where containment surfaces are cooled externally, usually by natural mechanisms. Good examples are the designs of the AP series by Westinghouse, where the steel containment is cooled externally by water flowing on its exterior surface from a reservoir above the containment, and by ascending air driven by bouyancy.

Steam condensation is largely affected by conditions which can be split into two groups depending on the relevance of the physical dimensions of the system. The ‘scale-independent factors’ are variables like the fraction of non-condensables, the pressure, the gas composition and so on, the effect of which could be well investigated through separate effect tests. The ‘scale-dependent factors’ are those phenomena that require to be investigated in actual or scaled geometries (i. e. Integral Effect Tests) since physical dimensions largely influence their quantitative effect. Examples of this kind are the natural convection process at both sides of the metallic structures and the potential gas stratification.

One of the new features of the ABWR-II safety design is the adoption of passive safety systems. The passive heat removal system (PHRS) consists of two dedicated systems, namely the passive reactor cooling system (PRCS: the same as Isolation condenser) and the passive containment cooling system (PCCS), that use a common heat sink pool above the containment allowing a one-day grace period (Figure I-2). These passive systems not only cover beyond DBA condition, but also provide in-depth heat removal backup for the RHR, and practically eliminate the necessity of containment venting before and after core damage as a means of overpressure protection.

With regard to ABWR-II development, the horizontal type PCCS has been designed with a focus on anti-seismic structure and decreasing PCCS pool depth instead of conventional vertical type PCCS. Figure I-3 shows PCCS functional schematic and an example of containment pressure transient following typical low pressure core melt scenario. Containment venting for overpressure protection under severe accident condition is practically excluded by adopting PCCS.

Passive residual heat removal (PRHR) heat exchangers are incorporated into several advanced PWR designs. Their primary function is to provide extended periods of core decay heat removal by transferring heat using a single-phase liquid natural circulation loop as shown in Figure 6. The PRHR heat exchanger loop is normally pressurized and ready for service. Single-phase liquid flow is actuated by opening the isolation valve at the bottom of the PRHR heat exchanger. The PRHR system design is optimized for single-phase (contrary to isolation condenser which is optimized for boiling and condensation) liquid heat transfer. It is particularly useful in mitigating the station blackout scenario. In general it eliminates the need for ‘bleed and feed’ operations for plant cool-down. This is a Category D passive safety system.

This CRP does not deal with natural draft air flow cooling of tubes. Figure 5 is added for completeness.

REACTOR

VESSEL

FIG. 6. Core decay heat removal using a water-cooled passive residual heat removal heat exchanger loop.

Nuclear power reactor containments are equipped with safety systems which protect the containment integrity under various accident conditions. The focus of this phenomenon is the natural circulation cooling and heat transfer in various containment passive cooling systems under accident conditions to remove the energy out of the containment by natural circulation and condensation heat transfer. Typical systems are the tube condensers such as the passive containment cooling system (PCCS) and external air cooling system or external liquid film cooling and internal condensation of steam in the containment by natural circulation. The major purpose of these containment systems is to protect the containment under both design basis accidents and severe accidents involving serious core damages and to prevent the significant release of radioactive materials to the atmosphere. These systems are required to remove the load on the containment from the LOCAs and other accidents by removing the heat but containing the mass within the structure. Most of load comes from the released steam from the primary coolant system due to the LOCA or venting of the pressure relief valves. The major part of the non-condensable gases consists of the original containment atmosphere such as air or nitrogen, however with the core damage, hydrogen or fission gases can be also released into the containment atmosphere. The thermal-hydraulic phenomena of importance are tube surface condensation with non­condensable gases, natural circulation of steam and non-condensable gases, degradation of condensation by the accumulation of non-condensable gases and purging of non-condensable gases from condenser systems. The passive containment cooling system can be vertical or horizontal tube condensers in external water pool, exposed condenser tube system in the containment cooled by natural circulation water through the tubes from the external pool or by external air circulation and others.

In the present, various testing programs are being performed or planned to consolidate their feasibilities and to find further room for improvements. ABWR-II results are planed to reflect the new next generation light water reactor development program sponsored by the government.

FIG. I-3. Example of containment pressure transient following typical low pressure core melt scenario.

ANNEX II. ACR-1000
Atomic Energy of Canada Ltd, Canada

I — 1. Introduction

The Advanced CANDU Reactor ™ (ACR™) is a Generation III+ pressure tube reactor designed by Atomic Energy of Canada (AECL). The ACR-1000 is an evolution of the proven CANDU reactor design. It is a light-water-cooled reactor that incorporates features of both pressurized heavy water reactors (PHWRs) and advanced light-water PWRs. It incorporates multiple and diverse passive systems, wherever necessary, for mitigation of any postulated accident scenarios, including severe accidents. The ACR-1000 uses passive design elements to complement active features, thus enhancing reliability and improving safety margins.

The ACR retains the core features of previous CANDU designs, such as horizontal fuel channels surrounded by a heavy water moderator. The major innovation in the ACR is the use of low enriched uranium fuel and light water as the coolant.

The overall layout of an ACR-1000 reactor and its primary components are shown in Fig. II-1 and Fig. II-2. The ACR-1000 reactor is designed by Atomic Energy of Canada Ltd to produce a nominal gross output of 1165 MW(e). The reactor employs active as well as passive safety features, the latter relying on gravity, compressed gas or natural circulation (thermosyphoning). As with all CANDU reactors, the high pressure, (11.1 MPa) heat transport system (HTS) and the low pressure and low temperature moderator are separate systems.

The low pressure, low-temperature moderator is contained in a tank called the Calandria. The primary coolant system consists of fuel channels, stainless steel feeders, four inlet headers, four outlet headers, four steam generators, four electrically driven heat transport pumps, and the interconnecting piping. There are 520 fuel channels and each fuel channel contains 12 nuclear fuel bundles (approximately 50 cm long x 10 cm in diameter). The primary coolant system is arranged in a two ‘figure of eight’ loop configuration. The pumps circulate the water in the two loops in opposite directions.

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In normal operation heat is generated in the reactor fuel bundles and exchanged with the fast-flowing coolant. The heated water is transported through the feeder pipes to the headers and into the U tubes inside the steam generators (SGs). The heat is then transferred to the water on the secondary side to produce steam, driving the turbine to generate electricity. The cooled water leaving the steam generator is pumped by four HTS pumps through two separate HTS loops connected to a common pressurizer, which maintains the HTS at a constant pressure.

Passively cooled core isolation condensers are designed to provide cooling to a boiling water reactor (BWR) core subsequent to its isolation from the primary heat sink, the turbine/condenser set. As shown in Figure 7, during power operations, the reactor is normally isolated from the isolation condenser (IC) heat exchanger by closed valves. In the event that the core must be isolated from its primary heat sink, the valves located in the IC lines are opened and main steam is diverted to the IC heat exchanger where it is condensed in the vertical tube section. Heat is transferred to the atmosphere through the heat exchanger and the ICS/PCCS pool. The condensate returns to the core by gravity draining inside the tubes. This is a Category D passive safety system.

FIG. 7. Isolation condenser cooling system.