Variations of a passive safety containment for a BWR with active and passive safety systems

Nuclear Engineering and Design 237 (2007) 74–86

Variations of a passive safety containment for a BWR with active and passive safety systems Takashi Sato ∗ , Yoshihiro Kojima Toshiba Corporation, IEC, Gen-SS, 8, Shinsugita-Cho, Isogo-Ku, Yokohama, Japan Received 1 March 2006; received in revised form 22 August 2006; accepted 24 August 2006

Abstract The paper presents variations of a certain passive safety containment for a near future BWR. It is tentatively named Mark S containment in the paper. It uses the operating dome as the upper secondary containment vessel (USCV) to where the pressure of the primary containment vessel (PCV) can be released through the upper vent pipes. One of the merits of the Mark S containment is very low peak pressure at severe accidents without venting the containment atmosphere to the environment. Another merit is the capability to submerge the PCV and the reactor pressure vessel (RPV) above the core level by flooding water from the gravity-driven cooling system (GDCS) pool and the upper pool. The third merit is robustness against external events such as a large commercial airplane crash owing to the reinforced concrete USCV. The Mark S containment is applicable to a large reactor that generates 1830 MW electric power. The paper presents several examples of BWRs that use the Mark S containment. In those examples active safety systems and passive safety systems function independently and constitute in-depth hybrid safety (IDHS). The concept of the IDHS is also presented in the paper. © 2006 Elsevier B.V. All rights reserved.

Abbreviations: ABWR, advanced boiling water reactor; ABWR-II, advanced boiling water reactor-II; AC, alternating current; ACR, advanced CANDU reactor; AFC, active fuel cladding; ALWR, advanced light water reactor; AOT, allowable operable time; ASD, adjustable speed drive; BWR, boiling water reactor; CCFP, conditional containment failure probability; CDF, core damage frequency; CFR, code of federal regulations; CR, control rod; CRD, control rod drive; CV, containment vessel; D/G, diesel generator; DBA, design basis accident; DW, dry well; ECCS, emergency core cooling system; EDF, Electricit´e de France; EPRI, Electric Power Research Institute Inc.; EUR, European utility requirements for LWR nuclear power plants; FMCRD, fine motion control rod drive; FP, fission product; FW, feed water; GDCS, gravity-driven cooling system; GIRAFFE, gravity driven integral full-height test for passive heat removal; HCU, hydraulic control unit; HPCF, high-pressure core flooder; HPFL, highpressure flooder; HVAC, heating; ventilation and air conditioning; IC, isolation condenser; ICSS, isolation and connection switching system; IDHS, in-depth hybrid safety; IORV, inadvertent openings of safety relief valves; LOCA, loss of coolant accident; LPFL, low-pressure flooder; LWR, light water reactor; NRC, Nuclear Regulatory Commission; PCCS, passive containment cooling system; PCCV, pre-stressed concrete containment vessel; PCT, peak cladding temperature; PCV, primary containment vessel; PSA, probabilistic safety assessment; R/B, reactor building; RCCV, reinforced concrete containment vessel; RCIC, reactor core isolation cooling system; RCW, reactor coolant water system; RHR, residual heat removal system; RIP, reactor internal pump; RPV, reactor pressure vessel; RSW, reactor sea water system; RWCU, reactor water clean up system; SBWR, simplified boiling water reactor; SGTS, standby gas treatment system; SP, suppression pool; SSE, safe shutdown earthquake; TAF, top of active fuel; TMI 2, Three Mile Island 2; TSBWR, total safety boiling water reactor; USCV, upper secondary containment vessel; WW, wet well ∗ Corresponding author. Tel.: +81 45 770 2066; fax: +81 45 770 2179. E-mail address: [email protected] (T. Sato). 0029-5493/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2006.08.009

1. Introduction Recently passive safety reactors are prevailing in the licensing review by the U.S. NRC (General Electric Company, 2003, 2005a,b; Cummins et al., 2005; Brettschuh and Meseth, 2005). Among them there is the ESBWR as a passive safety BWR. It is based on the technology developed for the SBWR (Rao et al., 1991). Toshiba participated in the international joint study for the SBWR development. We conducted GIRAFFE tests for the passive containment cooling system (PCCS) development (Nagasaka et al., 1991). The basic design is almost the same between the SBWR and the ESBWR. However, the plant output is quite different. While the SBWR was only about 600 MWe, the ESBWR is about 1535 MWe (General Electric Company, 2005a). The larger plant output is, the bigger impact on containment design becomes. Sato proposed the use of an improved Mark III type containment to reduce the containment pressure at severe accidents (Sato et al., 2005). The Mark III containment is the containment for BWR/6 (General Electric Company, 1980). It has the biggest free volume of the pressure suppression type containments for BWRs. This can provide the potential to reduce the pressure due to hydrogen compression at severe accidents. Firstly a very safe containment concept named Mark X containment was proposed (Sato et al., 2005). The Mark X containment has a steel secondary containment and can be cooled

T. Sato, Y. Kojima / Nuclear Engineering and Design 237 (2007) 74–86

by natural circulation of outside air. A steel containment is also used for the standard design of the Mark III containment. In some actual plant design of the Mark III containment, however, a steel containment is not used and reinforced concrete containment is used as in the case of Grand Gulf nuclear power station and Clinton nuclear power station (U.S. NRC, 1985). Therefore, in order to simplify the Mark X containment, Sato also tried to replace a steel containment with a reinforced concrete containment and thought of another containment concept. It is named Mark S containment in the paper tentatively. The Mark S containment consists of the primary containment vessel (PCV) and the upper secondary containment vessel (USCV). The Mark S containment has a large free volume and enables a very low peak pressure at severe accidents. It has walk-away safety during its grace period. The grace period is no less than 3 days. The paper presents a basic example of a passive safety BWR that uses the Mark S containment as a benchmark. It is tentatively named Total Safety BWR+ (TSBWR+ ) in the paper. 2. Design objectives of the Mark S containment The design objectives of the Mark S containment are as follows: (1) to provide a containment for a large reactor generating no less than 1830 MW electric power; (2) to provide a containment that can accommodate severe accidents within the design pressure without venting hydrogen outside of the containment; (3) to provide a containment that can be cooled passively and also has a configuration to submerge the reactor pressure vessel (RPV) above the core level passively; (4) to provide a containment that has no potential risk of hydrogen detonation without relying on uncertain measures such as igniters; (5) to provide a containment that is robust against external events including a crash of a large commercial airplane; (6) to provide a containment that can cool and stabilize core debris with only passive means even if an ex-vessel core melt occurs; (7) to provide a containment that has a compact and simple configuration for both construction and economy. The recent remarkable progress of core design has enabled a larger plant output such as 1700 MWe with slightly larger core and RPV (Aoyama et al., 1997). The advanced BWR-II (ABWRII) was pursuing this larger plant output with slightly larger core and RPV (Mochida et al., 2003). Based on these remarkable progresses of core design, even 1800 MWe plant output is expected to be feasible in the very near future. Therefore, the Mark S containment needs to be designed for a large power reactor that can generate no less than 1830 MW electric power. Recently, all the advanced light water reactor (ALWR) requirements require us to assume 75–100% metal–water reaction (EDF et al., 1995; Taylor, 1990; EPRI, 1997). The U.S. NRC general design criterion 50 also prescribes containment design basis that requires consideration of metal water reaction as required by 10 CFR 50.44 (U.S. NRC, 2006). It requires that future water cooled reactors must assume 100% metal–water reaction of the active fuel cladding (AFC) for the analysis of containment structural integrity (U.S. NRC, 2003). Therefore, we thought that the design pressure of the Mark S containment must be decided considering 100% metal–water reaction of the AFC.

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In the conventional active safety BWRs the RPV is not submerged in the primary containment vessel (PCV) at a loss of coolant accident (LOCA). The active emergency core cooling system (ECCS) must continue to pump up the suppression pool (SP) water and inject it into the core. This active ECCS operation must continue for a very long time period such as more than 3 days after the initiation of a LOCA. If the active ECCS stops the core will be damaged. This constitutes a very low but a certain risk of all loss of the active ECCS. The potential risk comes from mainly all loss of AC power source due to a very large earthquake, a very large hurricane and so on (Sato et al., 1995). On the contrary in the case of passive safety BWRs, there is no active ECCS and the core and the RPV can be submerged in the PCV after the injection phase of the passive ECCS is finished. Therefore, passive safety BWRs can eliminate the above-mentioned risk of all loss of active ECCS in the long term and are much safer in this regard. The conventional level 1 probabilistic safety assessment (PSA) cannot tell the difference because it only looks at the short-term risk after a LOCA (Sato et al., 1995). Owing to this safety function of submergence they also have potential to survive excessive events beyond the DBA LOCA. Examples are multiple pipe breaks and a large RPV bottom break caused by a very big earthquake that is far beyond the design basis earthquake. Even for these excessive events the core can be submerged in passive safety BWRs. This is the very important safety performance of passive safety BWRs but it also requires a large amount of water in the gravity-driven cooling system (GDCS) pool. In some cases, actually, the GDCS pool is not enough to submerge the PCV and causes a serious water management problem. In the design of the Mark S containment, some other passive water source should be considered in addition to the GDCS pool to solve the water management problem. A large commercial airplane crash on a nuclear power plant became a safety concern after the terror in the USA on September the 11th in 2001. The EUR originally requested protection against a small airplane crash. It requires 1.3 m thickness of reinforced concrete wall (EDF et al., 1995). Therefore, all the European future plants meet this requirement. For example, the reactor building of the EPR is made of 1.8 m thick reinforced concrete to protect its pre-stressed concrete inner containment against a large commercial airplane crash (Stoll and Waas, 2005). Therefore, it is determined that the Mark S containment also must have at least 1.8 m thick reinforced concrete wall to withstand a large commercial airplane crash. After the Chernobyl accident in 1986 the importance of a containment for severer accidents became highlighted. From the standpoint of containment integrity at severer accidents, conditional containment failure probability (CCFP) is the sole important safety measure. A very low core damage frequency (CDF) value is meaningful to protect the core but meaningless to protect the containment because it never reduces the CCFP value. Assuming severe core damage accidents and providing safety features to protect the containment against them only can reduce the CCFP. In order to improve the CCFP of the Mark S containment, it is determined to provide a core catcher that can withstand ex-vessel molten core debris with only passive cooling

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Table 1 Comparison of reactor system parameters Parameters

ABWR

ESBWR

TSBWR+

TSBWR II, III

TSBWR IV

Thermal power (MWt) Electric power (MWe) Vessel height (m) Vessel diameter (m) Number of fuel bundles Active fuel height (m) Power density (kW/l) Number of CRD

3926 1356 21.1 7.1 872 3.7 51 205

4500 1535 27.6 7.1 1132 3.0 54 269

4500 1535 27.6 7.1 1132 3.0 54 269

5300 1830 23.1 7.4 1180 3.7 51 281

5300 1830 25.1 7.4 1180 3.7 51 281

systems. This only passive means approach is very important to eliminate the need of active cooling systems and electric power sources at a severe accident and improve reliability and CCFP value. 3. Reactor system and safety systems 3.1. Reactor system of the TSBWR+ In order to make a benchmark concept, the total safety BWR (TSBWR+ ) adopted the reactor system of the ESBWR. It has exactly the same core, reactor pressure vessel (RPV), and control rod drive (CRD) as the ESBWR. It is also a natural circulation reactor. Table 1 shows the comparison of design parameters of the reactor systems among the ABWR, the ESBWR, and the TSBWR+ . Some forced recirculation reactor concepts are also developed for comparison and explained in the later sections. They are named TSBWR II, TSBWR III and TSBWR IV tentatively in the paper. Design parameters of these forced circulation reactors are also shown in the table. The TSBWR+ uses the Mark S containment that can accommodate a large reactor generating 1830 MWe. However, we do not confirm the feasibility of 1830 MWe for a natural circulation reactor yet. Therefore, as the core design of the TSBWR+ , 1535 MWe plant output is put down in the table. As for the other forced recirculation plant concepts, a larger plant output is feasible as core design and 1830 MWe is put down in the table. The vessel diameters are also slightly larger for these forced recirculation plant concepts because of the larger power.

the top of active fuel (TAF). This configuration insures that the flooded water at a LOCA submerges the RPV deeply above the core level. There are also the equalizing lines connecting the SP and the RPV. If direct steam condensation occurs on the surfaces of components and structures in the DW, the condensate accumulates on the flooded water and also flows into the SP through the spillover holes. This constitutes the bypass of the above-mentioned passive recycling core cooling capability of the GDCS and the PCCS. This bypass phenomenon causes very slow water level decrease in the RPV in the long term. The lost water drains into the SP and causes SP water level increase. However, even if level increase occurs, the SP can return water into the RPV through the equalizing lines. There is the upper pool over the PCV head. The upper pool works as a shielding for the radiation from the core during normal operation. However, in case of accidents, it can deluge the lower DW in addition to the GDCS pool water. There is the core catcher on the bottom of the lower DW. Even if an ex-vessel core melt accident occurs, the GDCS deluge line and the core catcher can cool the core debris passively.

3.2. Safety systems of the TSBWR+ Fig. 1 shows the safety systems of the TSBWR+ . It has the isolation condenser (IC), the passive containment cooling system (PCCS) and the gravity-driven cooling system (GDCS). The PCCS cools the steam from the primary containment vessel (PCV) directly and returns the condensate to the GDCS pool. Therefore, the GDCS can continue to inject water into the reactor pressure vessel (RPV) up to depletion of the PCCS pool. The combination of the PCCS and the GDCS provides a passive recycling core cooling capability in the TSBWR+ . There are several spillover holes on the loss of coolant accident (LOCA) vent pipes. The flooded water in the dry well (DW) at a LOCA overflows to the suppression pool (SP) through the spillover holes. The level of the spillover holes is about 4 m higher than

Fig. 1. Safety systems of the TSBWR+ .

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Fig. 2. Containment of the TSBWR+ (Mark S containment).

4. Containment of the TSBWR+ 4.1. Main features Fig. 2 shows the Mark S containment of the TSBWR+ . The naming of S comes from submergence. It has the upper secondary containment vessel (USCV) above the primary containment vessel (PCV). Both the PCV and the USCV are made of reinforced concrete and constitute the monolithic containment vessel (CV). The wall of the CV is made of 2 m thick reinforced concrete including the dome head of the USCV. This 2 m thick reinforced concrete wall follows the standard design of the current ABWR reinforced concrete containment vessel (RCCV) and provides a very good protection against a large commercial airplane crash. As mentioned before, the reactor building of the EPR is made of 1.8 m thick reinforced concrete to sustain a large commercial airplane crash (Stoll and Waas, 2005). Therefore, the Mark S containment also has potential to reduce its wall thickness to 1.8 m in the final design. There is the optional external event shield surrounding the CV. The wall of the external event shield is also made of 0.6 m thick reinforced concrete. It can protect the CV against minor external events such as a large hurricane or an explosion of chemical materials near the plant. However, the 2 m thick CV wall itself is enough to sustain such a

minor external events. Therefore, the external event shield is just optional for property damage protection and can be eliminated in the actual design. This is one of the biggest advantages of the RCCV against the pre-stressed concrete containment vessel (PCCV). Because the PCCV is very week against external forces due to its pre-stress, it requires additional protection made of reinforced concrete wall. Therefore, in the case of the PCCV, it looks like as if it were a double containment. In the case of the RCCV, however, the RCCV itself is strong enough against external forces and an expensive double containment can be avoided. The USCV is connected with the PCV through the upper vent pipes. Although only one of the upper vent pipes is shown in the figure, there are actually several upper vent pipes. The PCV consists of two compartments, the dry well (DW) and the wet well (WW). The DW installs the reactor pressure vessel (RPV) and the WW installs the suppression pool (SP). The DW is connected with the SP through the LOCA vent pipes. There are about 15 LOCA vent pipes in order to moderate the overshoot pressure at a DBA LOCA. This PCV configuration is almost the same as the ESBWR containment. However, the pressure transients of the Mark S containment at accidents are much more moderate. This is because the WW air space is connected to the USCV through the isolation and connection switching system (ICSS)

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and the upper vent pipes. The ICSS can be rupture disks, vacuum breakers, or automatic valves. The ICSS is closed during normal operation and opened automatically if the pressure difference through it reaches to the preset pressure difference. For transients and small LOCA significant pressure increase in the WW air space does not occur. The ICSS does not open for these events. FP release is limited in the PCV and contamination of the USCV does not happen at these minor events. On the contrary, if a large LOCA or a severe accident occurs, pressure increase in the WW air space activates the ICSS automatically. Noncondensable gases such as nitrogen and hydrogen compressed in the WW air space can be released into the USCV. Therefore, the pressure increase of the PCV is limited very low. The USCV has about 20,000 m3 free volume. This plenty of volume can accommodate a large amount of hydrogen generated at severe accidents. This is the biggest advantage of the USCV of the Mark S containment. There is the upper pool on the PCV head. The upper pool has the deluge line to dump its water into the PCV. The upper pool exists outside of the PCV but inside of the USCV. Normally it is difficult to dump water into the PCV only by gravity from outside because the PCV pressure increases at an accident. In the case of the Mark S containment, however, if the PCV pressure increases at an accident, the ICSS opens automatically and the PCV pressure propagates into the USCV through the upper vent pipes. Therefore, the PCV pressure rather works as backpressure to dump the upper pool water into the PCV. To be safe, the deluge line has the U seal to prevent a direct pass from the DW to the USCV. Normally the injection point of the deluge line is covered with the flooded water and the U seal is not necessary. This upper pool injection can solve the water management problem inside the PCV more easily. In order to install a larger diameter RPV for 1830 MWe plant, it is necessary to increase the inner diameters of the core catcher, the lower DW and the RPV annulus. All these things require more water to flood the increased volume. The upper pool injection can flood the increased volume without increasing the GDCS pool. Hence it becomes possible to install a larger diameter RPV for 1830 MWe plant in the same compact PCV as 1535 MWe ESBWR. Table 2 shows the comparison of containments among the ABWR, the ESBWR and the TSBWR+ . The paper also presents some variations of the Mark

S containment for forced recirculation reactor concepts. They are explained precisely in the later sections. The containment design parameters of one of those forced recirculation cases are also shown in the table for comparison. Both the PCV and the USCV are inerted with nitrogen in order to eliminate the risk of hydrogen detonation completely. The volume of the USCV is the same order of the PCV free volume. Based on our experience, it will take about 5 h to inert or deinert the USCV respectively. The USCV is normally isolated from the PCV by the ICSS. Therefore, it is possible to inert and deinert the USCV independently of the PCV. Deinerting of the USCV can be started earlier for refueling. 4.2. Safety performance of the Mark S containment A large amount of hydrogen generation is one of the biggest challenges for a very compact primary containment vessel (PCV) of a passive safety BWR. The amount of hydrogen generated by 100% metal–water reaction of the AFC is about 26,500 m3 at atmospheric pressure in the case of 1830 MWe plant. The amount of nitrogen in the DW is about 7500 m3 . The total of 34,000 m3 of non-condensable gases is released into the 20,000 m3 free volume of the USCV and about 5200 m3 of the WW air space. Because the PCCS is available, the containment pressure is determined mainly by the compression of the noncondensable gases. The estimated USCV peak pressure by the compression is only about 137 kPa (1.40 kg/cm2 g). This is well below the USCV design pressure 207 kPa (2.11 kg/cm2 g). The PCV peak pressure is at most 49 kPa (0.5 kg/cm2 ) higher than that of the USCV due to the pressure difference across the vent pipes. It is at most 186 kPa (1.90 kg/cm2 g) that is well below the PCV design pressure 310 kPa (3.16 kg/cm2 g). When a DBA LOCA occurs, the situation is much easier because it is not necessary to assume a large hydrogen generation owing to the single failure proof ECCS. As for the non-condensable gas compression, we only have to consider preexisted nitrogen in the PCV before the accident. The pressure increase of the USCV due to the nitrogen compression is very small and estimated only about 29 kPa (0.3 kg/cm2 g). However, we also have to consider the overshoot pressure in the DW before the LOCA vent pipe clearing occurs. The Mark S con-

Table 2 Comparison of containments Characteristics

ABWR

ESBWR

TSBWR+

TSBWR II

Containment type PCV design pressure (kPa) (psig) USCV design pressure (kPa) (psig) PCV inner diameter (m) PCV inner height (m) Total free gas volume (m3 ) DW free gas volume (m3 ) WW free gas volume (m3 ) USCV free gas volume (m3 ) SP volume at low water level (m3 ) SP depth at normal water level (m) LOCA DW pressure (kPa) (psig) SA DW pressure (kPa) (psig)

RCCV 310 (45) – 29.0 29.5 13,310 7350 5960 – 3580 7.0 248 (35.9) 690 (100)

RCCV 310 (45) – 36.0 35.0 12,638 7206 5432 – 4383 5.45 241 (34.9) 597 (87)

Mark S 310 (45) 207 (30) 36.0 35.0 33,719 7558 5296 20,865 4314 5.45 197 (28.5) 186 (27)

Mark S 310 (45) 207 (30) 36.0 35.15 33,454 7293 5296 20,865 6370 8.0 209 (30.3) 186 (27)

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that much water in the GDCS pool. The drainable water inventory in the GDCS pool is at most 1700 m3 . Additional 300-m3 water is necessary to flood the lower DW up to the spillover holes. In the case of the Mark S containment the upper pool can supply the additional 300-m3 water. Therefore, the Mark S containment can keep its compact configuration although it can contain a larger diameter RPV for a 1830 MWe plant. 5. Ex-vessel molten core coolability of the TSBWR+

Fig. 3. Pressure transient of the Mark S containment at a DBA LOCA.

In order to cope with an ex-vessel molten core accident, the Mark S containment has the core catcher on the floor of the lower dry well (DW). Fig. 4 shows the basic configuration of the core catcher. The main frame of the core catcher is composed of the 0.2 m thick steel plate and the integrated 0.2 m depth cooling

tainment has about 15 LOCA vent pipes. Fig. 3 shows the result of the PCV pressure transient at the DBA LOCA that is a guillotine pipe break of the feed water (FW) line. During the first 10 s the PCV pressure increases and reaches the peak pressure of 200 kPa (28.5 psig). After the water level in the RPV becomes lower than the break at about 10 s the break flow decreases and the PCV pressure also decreases rapidly. The pressure transient of the ABWR is also shown in the figure for comparison. The peak pressure of the ABWR is higher and the pressure decrease delays considerably due to the compression of the nitrogen in the limited volume of the WW air space. In the case of the Mark S containment, however, the nitrogen is purged into the large volume of the USCV and the pressure transient is much moderated. Owing to the same reason the pressure transient of the WW of the Mark S containment is much more moderated. There is actually no acute pressure increase in the WW. The DW pressure also drops quickly and comes close to the WW pressure after about 100 s. After the rapid drop of the containment pressure the Mark S containment settles at very low pressure in the long term. This is very effective to limit the containment leak rate and FP release into the environment. On the contrary, in a conventional passive BWR, PCV pressure remains high due to nitrogen compression and lack of active containment spray. This is the worst combination for FP release and completely solved in the design of the Mark S containment. 4.3. Flooding capability of the Mark S containment The Mark S containment has the configuration that can submerge the reactor pressure vessel (RPV) above the core level. If a loss of coolant accident (LOCA) occurs, the gravity-driven cooling system (GDCS) injects water into the RPV and the coolant flowing out the break floods the lower dry well (DW). There are several spillover holes on the LOCA vent pipes at the level of 1.6 m higher than the suppression pool (SP) normal water level. If the flooded water in the lower DW reaches to the spillover holes, it drains into the LOCA vent pipes and returns to the SP. The amount of water necessary to flood the lower DW up to the spillover holes is about 2000 m3 that is more than that of 1535 MWe ESBWR in order to potentially install a larger diameter RPV for a 1830 MWe plant. However, the very compact design of the Mark S containment does not allow maintaining

Fig. 4. Core catcher of the Mark S containment.

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channel section on the bottom surface. The cooling channels are running centrifugally form the center to the periphery. The number of the cooling channels is increased stepwise across the header region to keep the cooling efficiency. The main frame also consists of the elemental pieces that have trapezoid shapes to fit the overall shape of the main frame. Each elemental piece has the radial cooling channels and the header region. The header region is the mixing volume of the cooling water and connects with the increasing cooling channels in the subsequent elemental pieces. The height of the elemental piece is 0.4 m. The height of the sidewall, however, is 3.5 m. The reason why it is so tall is to embed the drain sumps inside the core catcher. The inside of the core catcher is stuffed with the 1.5 m depth solid melt resistant material made of magnesium oxide. Other melt resistant materials such as zirconium oxide can be also used. This melt resistant material works as the structure to embed the floor drain sumps. It is further covered with the 0.1 m thick sacrificial concrete to prevent its dispersion. The depth of the core catcher above the sacrificial concrete is then 1.5 m. The cooling water is supplied from the GDCS deluge lines and injected into the distributor on the bottom center of the core catcher. Then the cooling water flows centrifugally in the cooling channels, through the header region, to the sidewall channels and overflows onto the core catcher. Every cooling channel in a stage has uniform configuration and uniform flow friction. This uniform channel configuration assures uniform flow distribution. A fishbone type channel configuration is not used because uniform flow distribution is difficult and bypass of the central region channels might happen. The cooling channels are also declined to accelerate exhausting stagnant voids in the channels. In order to decline the cooling channels it is necessary to increase the height of the core catcher. The taller the core catcher, the higher the cooling water level. The high level location of the GDCS pool enables the tall core catcher design very easily. If a very low-level water storage tank is used for the cooling water, it is necessary to make the core catcher as shallow as possible to get the spillover flow onto the molten core as soon as possible. If a core catcher is very shallow, it is difficult to decline the cooling channels. Moreover, if a core catcher is very shallow and wide, the length of the cooling channels must be also very long. This also makes the flow frictions of the central region channels bigger. The high-level cooling water pool, the declined short cooling channels and the centrifugal uniform channel distribution make the Mark S containment core catcher more reliable. The internal diameter of the lower DW at the core catcher region is extended 1.0 m in order to enlarge the debris dispersion area of the core catcher. The enlarged debris dispersion area is sized to accept the molten core for a 1830 MWe large plant. The upper portion of the lower DW is, however, not enlarged to minimize the lower DW volume. The lower DW is flooded completely up to the spillover holes opened at about 4 m higher than the top of active fuel (TAF) passively. The residual core debris in the RPV is also completely submerged. Therefore, the local DW temperature never reaches very high temperature that can threat the PCV integrity even in the long term. There is no need to rely on active containment spray systems and dedicated diesel generators even for this long-

term cooling. The core debris heats up the submerging water and generates steam. Then the PCCS cools the steam and returns the cooling condensate to the GDCS deluge lines passively. The GDCS deluge lines continue to supply cooling water into the cooling channels of the core catcher. The PCCS condensate is the coldest in the DW and firstly cools the core catcher. This cooling mechanism of the core catcher is recycled passively using the decay heat as the driving force without any active pumps. The combination of the passive cooling core catcher, the submerged lower DW and the PCCS passive cooling mechanism provides an excellent solution for an ex-vessel molten core accident in the Mark S containment. 6. Other probable variations 6.1. Forced recirculation reactor in the Mark S containment Instead of a natural circulation reactor, we can also use a forced recirculation reactor. This concept is named TSBWR II in the paper tentatively. The TSBWR II has reactor internal pumps (RIP) as the current advanced boiling water reactor (ABWR) does (Yamada et al., 2003). The plant concept is shown in Fig. 5. It can control the power by changing the core flow. On the contrary, the TSBWR+ is a natural circulation reactor and cannot control the power by changing the core flow. It always uses the control rods (CR) to control the power. In the TSBWR II, however, we can control the power much more easily and frequently with the RIP. TSBWR II does not rely on the natural circulation in the reactor pressure vessel (RPV) and can reduce the RPV height. In the case of a natural circulation reactor, in order to keep enough core flow, it was necessary to have much water head and thus the 27.6 m tall RPV that is 6.5 m taller than that of the current ABWR. The RPV height of the TSBWR II is decided to 23.1 m that is 2 m taller than the current ABWR RPV and 4.5 m shorter than the TSBWR+ RPV. The TSBWR II also uses the Mark S containment. It has a lot of safety margin for the amount of hydrogen generation at severe accidents. Therefore, the amount of hydrogen is not a limiting factor of the plant output. Using the RIP we can increase the plant output easily. The fuel length is also increased to the conventional length of 3.7 m. With these things we can increase the plant output of the TSBWR II. It has a good potential for plant output of 1830 MWe. The TSBWR II has both active safety systems and passive safety systems based on the safety philosophy of in-depth hybrid safety (IDHS) (Sato et al., 2004, 2005). Fig. 6 shows the basic concept of the IDHS in the TSBWR II. The active ECCS copes with the design basis accident (DBA). If all of the active safety systems fail, the passive safety systems back up. The passive safety systems can work independently of the active safety systems. Thus the IDHS can truly provide in-depth safety. The IDHS is a new concept as a safety philosophy. However, as hardware, it is just a combination of the ABWR and the SBWR. The active safety systems came from the ABWR and the passive safety systems came from the SBWR. They have been well established and there is no more necessity for time consuming research and development.

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Fig. 5. Containment of the TSBWR II (forced recirculation reactor in the Mark S containment).

The HDIS consists of minimum but enough active safety systems. They have important safety functions besides coping with the DBA LOCA. The high-pressure flooder (HPFL) has a safety function of the IC back up to avoid unnecessary depressurization of the vessel and the GDCS initiation at some minor

Fig. 6. Basic concept of in-depth hybrid safety in the TSBWR II.

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accident scenarios. These minor accident scenarios include transients followed by loss of all the IC, small pipe break of less than 5 cm diameter, and inadvertent openings of safety relief valves (IORV). In these minor accident scenarios, plant property is not so damaged at first but eventually badly damaged if the GDCS initiates. Once the GDCS initiates the containment is submerged and electric systems of the FMCRD and the RIP might be damaged. The RPV is also submerged from the outside and encounters thermal stress. The HPFL can keep the core level and avoid the GDCS initiation at these minor events. The safety grade residual heat removal system (RHR) also has the cold shutdown function at the safe shutdown earthquake (SSE). Without the safety grade, seismically qualified RHR a plant would have to keep hot shutdown state after the SSE and might encounter the subsequent larger earthquake under a high pressure condition. The safety grade RHR can remove this risk completely. Of course, the active ECCS needs the safety grade diesel generators (D/G). However, a passive safety reactor like the TSBWR+ still must have non-safety grade D/Gs to operate the fuel pool cooling system, operate the DW cooler, operate the HVAC system in the control room, and monitor plant safety parameters at a loss of offsite power. The passive safety systems never take care of these operational safety matters because they only take care of major reactor safety. A passive safety reactor can use inexpensive non-safety grade D/Gs. However, there is no insurance that non-safety grade D/Gs can certainly work in the actual emergencies. They might not be tested more than 10 years or might not be qualified against seismic events because the passive safety systems have no interest in D/Gs. On the contrary, owing to the IDHS philosophy, the TSBWR II has two safetygrade D/Gs. It is possible to cope with the above-mentioned operational safety matters more reliably with the safety grade D/Gs. Passive safety is cheep, simple and reliable but only covers reactor safety. The IDHS covers plant total safety, including reactor safety, operational safety, plant property safety, and operator safety more reliably. Using the safety grade D/Gs it is also possible to install the standby gas treatment system (SGTS) for the DBA LOCA. This is optional but important to get licensing in a country where the alternate source term is not allowed yet like Japan. Fig. 7 shows the three division configuration of the hybrid safety systems of the TSBWR II in comparison with the current ABWR ECCS configuration. There is no reactor core isolation cooling system (RCIC) because the isolation condenser (IC) is more reliable and inexpensive. The high-pressure core flooder (HPCF) of the current ABWR is replaced with the HPFL. The HPCF injects water into the core shroud. The HPFL, however, injects water outside of the core shroud, namely, the downcomer. The HPCF has boiling suppression capability after a LOCA but the HPFL does not. However, the Mark S containment can provide complete submergence of the core after a LOCA and prevent boiling off of the core even if the active ECCS stops. Therefore, now the HPCF can be replaced with the HPFL. The HPFL is also preferable because the large pipe break connected inside the core shroud can be eliminated. Now the DBA LOCA is a pipe break accident of the HPFL injection line connected outside the core shroud. Assuming a single failure of another

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Fig. 7. Hybrid three-division configuration of the TSBWR II safety systems in comparison with the current ABWR.

HPFL, there is no high-pressure injection system. However, the 2 m longer vessel of the TSBWR II has enough coolant inventory and can establish no core uncovery at the DBA LOCA. Fig. 8 shows the result of the ECCS analysis for the HPFL line break. As for the GDCS performance, the severest break is the bottom drainpipe break and loss of all the active ECCS. This is because the bottom drainpipe is connected to the bottom of the inside core shroud region. Even for this case, the PCT is less than 1200 ◦ C. The largest break outside the core shroud region is the RHR suction pipe break. For this case the PCT is moderated to 900 ◦ C without all the active ECCS. All these analyses used a very conservative 5300 MW thermal power that is quite enough to generate 1830 MW electrical power. Elimination of the RCIC can bring about a certain cost saving. Two-division

reactor coolant water system (RCW) and reactor sea water system (RSW) also contribute much cost saving. The very simple two-division active ECCS is suitable for being installed under the main steam tunnel. It should be noticed that the passive safety systems also can cope with the DBA independently of the active safety systems. They work as a complete set of diversity of the active safety systems. Therefore, with the passive safety systems available, on-line maintenances of the active safety systems, including the D/G, the reactor coolant water system, and the reactor sea water system are possible. On the other hand, with the active safety systems available, on-line maintenances of the passive safety systems are also possible. Therefore, owing to the in-depth hybrid safety (IDHS), there is no limitation on allowable operable time (AOT) for on-line maintenances from the standpoint of safety regulation. During on-line maintenances, however, the IDHS is partially lost. Therefore, the AOT for the on-line maintenance of one safety division should be limited to about 30 days. As for the containment design, the water level of the suppression pool (SP) is increased to 8 m from 5.45 m of the TSBWR+ . This is because in the case of the TSBWR II the active ECCS uses the SP as the water source. The level of the spillover holes is also higher than that of the TSBWR+ . They are opened near the top of the vent wall. Fig. 9a shows the situation of the TSBWR II after a DBA LOCA. The flooded water from the SP submerges the PCV almost up to the diaphragm floor that is the floor of the upper dry well (DW). Once the PCV is submerged to this level, the recirc screen is opened and the flooding water in the RPV annulus flows into the RPV. Thus boiling off of the core can be avoided even if the active ECCS stops in the long term. The increased SP water is enough to submerge the PCV and the RPV. In case of all the active ECCS failure from the initiation of a LOCA, the GDCS pool submerges the lower DW. Fig. 9b shows this situation of the TSBWR II. The amount of the water necessary to submerge the PCV is about 2250 m3 due to the higher elevation of the spillover holes. The amount of drainable GDCS water is about 1700 m3 . The rest of 550 m3 water is supplied from the upper pool. It can deluge the lower DW by gravity after the isolation and connection switching system (ICSS) is activated. The space of the equipment room of the TSBWR II is smaller compared with that of the TSBWR+ . This is because the SP water is increased. The equipment room, however, has still enough room to install the adjustable speed drive (ASD) for the RIP, the hydraulic control units (HCU), electric controllers of the FMCRD, the reactor water clean up system (RWCU) and so on. The active ECCS pumps and the residual heat exchangers can be installed under the main steam tunnel like the Mark III plant. 6.2. ABWR in the Mark S containment

Fig. 8. ECCS analysis results of the TSBWR II.

The advanced boiling water reactor (ABWR) is the latest nuclear power plant that is constructed and operated successfully in Japan. There are two operating units of the ABWR in Kashiwazaki-Kariwa site (Kobayashi et al., 2003). Both have

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Fig. 9. (a) Submerged Mark S containment of the TSBWR II at a DBA LOCA. (b) Submerged Mark S containment of the TSBWR II at a SA.

1356 MWe plant output. The third unit of the ABWR, that has 1380 MWe, started commercial operation in Hamaoka site in 2004 (Yamazaki and Takahashi, 2003). The fourth unit is planed to start commercial operation in Shiga site in early 2006. Several units are also under construction or expected to be constructed in the near future both in Japan and Taiwan (O’Neil and Hucik, 2005). The U.S. ABWR has acquired standard design certification from the U.S. NRC in 1997 (U.S. NRC, 1997). Therefore, it is the most realistic and certain option when a nuclear renaissance emerges in the U.S. in the near future. It is a forced recirculation plant with reactor internal pumps (RIP) and also can use the Mark S containment. This concept is called TSBWR III in the paper tentatively. The plant concept is shown in Fig. 10. This concept is somewhat similar to the Mark III containment. Actually this containment configuration is the Mark III containment for a RIP plant, although it is better in that the wet well (WW) is completely separated from the upper

dome by the isolation and connection switching system (ICSS). The routing of the main steam lines is similar to the ABWR and much better than the Mark III containment because the dry well (DW) is not surrounded by the WW. Using the Mark S containment the TSBWR III also can reduce the peak pressure of the containment at severe accidents below the design pressure. It also can increase plant output up to 1830 MWe. It is protected against external events including a large commercial airplane crash. The large surrounding R/B is eliminated. Only the lower portion of the containment is covered with the R/B like the Mark III plant. The inner diameter of the Mark S containment of the TSBWR III is 37.8 m that is exactly the same as that of the Mark III containment. As for the ECCS, exactly the same ECCS configuration as the TSBWR II sown in Fig. 7 can be applied to the TSBWR III. The SP is located at the bottom of the PCV like the current ABWR. The equipment room is located over the SP. In the

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Fig. 11. Containment of the TSBWR IV (raised SP in the Mark S containment). Fig. 10. Containment of the TSBWR III (ABWR in the Mark S containment).

equipment room the adjustable speed drive (ASD) for the RIP and the electric controllers of the FMCRD are installed. This closer layout of electric systems to the RPV contributes to the reduction of electric cables. The hydraulic control units (HCU) are also installed in the equipment room. The reactor water clean up system (RWCU) are installed in the lower R/B. The active ECCS pumps are installed below the steam tunnel like the Mark III plant. The number of the active ECCS pumps, however, is only 4 that are fewer than 6 of the Mark III plant. The number of the diesel generator (D/G) is 2 that are fewer than 3 of the Mark III plant. Therefore, the building volume can be smaller than that of the Mark III plant. The plant output, however, is about 1.5 times larger than that of the Mark III plant. Therefore, the cost competitiveness of the TSBWR III is much improved. 6.3. Raised SP in the Mark S containment The raised suppression pool (SP) is a primary containment vessel (PCV) concept that has the SP at the highest portion of the PCV. This PCV also can be installed in the Mark S containment.

This plant concept is called TSBWR IV in the paper tentatively. The plant concept is shown in Fig. 11. The main steam lines go down to the lower DW to avoid the raised SP. This causes the larger lower DW volume. In order to make up the larger lower DW up to the higher SP the TSBWR IV needs a large amount of water inside the PCV. The SP water depth is increased to 9 m resulting in 7988 m3 SP water. It is also possible to make up 500 m3 for the SP from the upper pool in the USCV. The total amount of 8488 m3 water is quite enough to submerge the lower DW both at the DBA and severe accidents. As for the ECCS the TSBWR IV also uses the hybrid 3 division ECCS shown in Fig. 7. In the case of the TSBWR IV, however, there is no space to install the gravity-driven cooling system (GDCS) pool in the upper DW because the raised SP preoccupies there. From the standpoint of probabilistic safety assessment (PSA), the GDCS is not essential because the active ECCS, the IC and the PCCS are quite enough to establish a very good core damage frequency (CDF). The reason why the other plant concepts presented in the paper all have the GDCS is that they all need the GDCS water to submerge the lower DW in case of all the active ECCS failures. In the case of the TSBWR IV, however, the water of the raised SP can submerge

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the DW passively. Therefore, the GDCS pool is not necessary in the TSBWR IV. Nevertheless, if one still wants the GDCS function that is effective to reduce the CDF furthermore, it can be installed in the dome of the USCV. This is similar design to the reserve water tank supported from the reactor building wall and dome of the ACR (Hopwood et al., 2005). It is shown in Fig. 11 but still optional. The merit of this design is that this GDCS has higher injection head owing to its higher elevation. Another merit of the raised SP is that it can have the biggest equipment room under the SP among the plant concepts presented in the paper. Therefore, it can have the maximum layout efficiency and minimize the volume of the surrounding R/B. With the 40 m inner diameter of the equipment room it is possible to layout almost all the systems including all the active ECCS pumps. Exception is the two diesel generators (D/G). A separate D/G building is necessary like the Mark III plant but the total building volume can be much smaller. The volume of the D/G building is also smaller because it has only two D/Gs instead of the three D/Gs of the Mark III plant. Hence much savings in the plant capital cost can be expected. The TSBWR IV is also a 1830 MWe large plant. It is possible to get about 50% cost savings in MWe basis as compared with a Mark III standard plant.

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radial cooling channel core catcher can prevent the molten core concrete interaction in the basemat. Therefore, the conditional containment failure probability (CCFP) of the Mark S containment is expected very low. The TSBWR II, the TSBWR III and the TSBWR IV all have active safety systems and the passive safety systems based on the in-depth hybrid safety (IDHS). Owing to the IDHS on-line maintenances are possible both in active and passive safety systems. Combination of the IDHS and the Mark S containment can cover plant total safety, including reactor safety, operational safety, property safety and operator safety. Four variations of the Mark S containment were presented. They are all very compact and well protected against external events including a large commercial airplane crash. The total building volume can be less than that of the Mark III type plant. The plant output is, however, much more than that of the Mark III type plant. Therefore, the cost-effectiveness of the TSBWR series is about twice of the conventional plant. All the TSBWR series use only proven and well established technology. They are just combinations of the ABWR and the SBWR contained in the Mark S containment. Nothing is new in the hardware but everything is new in the performance. No more research and development are necessary. The TSBWR series are one of the most realistic solutions for the next generation light water reactors (LWRs) in hand.

7. Conclusions References Plant concept of the TSBWR+ that uses the Mark S containment was presented. The Mark S containment uses the operating dome as the upper secondary containment vessel (USCV). The primary containment vessel (PCV) can be vented to the USCV at accidents. The peak pressure at severe accidents can be limited below the design pressure considering 100% metal water reaction of the active fuel cladding. It should be noticed that severe accidents are accommodated within the envelope of the DBA in the Mark S containment. The Mark S containment is inerted including the USCV and eliminates the risk of hydrogen detonation completely. It is designed to potentially contain a large reactor generating 1830 MWe. The TSBWR+ is a natural circulation reactor as a benchmark and generates 1535 MWe. Forced circulation reactor concepts, TSBWR II, TSBWR III and TSBWR IV were also presented. They all use the reactor internal pumps (RIP) and the Mark S containment. For these forced circulation reactors the 1830 MWe electric power is assumed extending the current ABWR core design. The Mark S containment is designed to submerge the dry well (DW) at accidents using the gravity-driven cooling system (GDCS) and the upper pool. The level of the submergence is above the top of active fuel (TAF). This safety performance of the Mark S containment is very effective to cope with excessive accidents including a large bottom break of the RPV or multiple pipe breaks due to excessive events such as a large earthquake beyond the design basis. The passive containment cooling system (PCCS) can cool the containment and return condensate to keep the submerging water in the DW passively. Therefore, an ex-vessel molten core accident is expected quite impossible in the TSBWR+ . The Mark S containment is, however, still well protected against severe accidents. Even if an ex-vessel molten core accident occurs, the

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