Design Study of Sodium Cooled Small Fast Reactor

Transcription

1 GENES4/ANP2003, Sep , 2003, Kyoto, JAPAN Paper 1114 Design Study of Sodium Cooled Small Fast Reactor Nobuyuki UEDA 1, Izumi KINOSHITA 1, Akio MINATO 1, Shigeo KASAI 2 and Shigeki MARUYAMA 2 1 Central Research Institute of Electric Power Industry (CRIEPI) , Iwado Kita, Komae-shi, Tokyo , Japan Phone: , Facsimile: , nob@criepi.denken.or.jp 2 Toshiba Corporation Isogo Nuclear Engineering Center, 8, Shinsugita-cho, Isogo-ku, Yokohama , Japan Phone: , Facsimile: CRIEPI has been exploring to realize a small-sized nuclear reactor for the needs of dispersed energy source and multi-purpose reactor. A conceptual design of 4S (Super-Safe, Small and Simple) reactor was proposed to meet the following design requirements: (1) All temperature feedback reactivity coefficients including whole core sodium void coefficient are negative; (2) The core integrity is secured against all anticipated transient without reactor scram; (3) No emergency power nor active mitigating system is required; (4) The reactivity core life time is more than 10 years; (5) Its construction, maintenance and operation are expected to be very simple by eliminating active components from inside of a reactor vessel. The 4S reactor is a metallic fueled sodium cooled fast reactor. The primary system is a pool type (an integrated type). A target of an electrical output is MW. A remarkable feature of 4S is that its reactivity is not controlled by neutron absorber rods but by neutron reflectors to cope with a long core lifetime and a negative coolant void reactivity. In this paper, design consideration of the 4S is described, which are focused on various core designs to meet above requirements. As a core lifetime and reflectors reactivity worth dominate core height, a tall core active height is favorable. RVACS (reactor Vessel Auxiliary Cooling System) is introduced for a passive decay heat removal system. As for the steam generator design, a double tube type steam generator is proposed to prevent a sodium-water reaction accident. Safety analyses based on a reference design have been carried out to demonstrate the passive safety features. KEYWORDS: Small Reactor, Fast Reactor, Passive Safety, Metallic Fuel I. Introduction The 4S reactor is a small sodium-cooled fast reactor in which intensive efforts are concentrated with an aim at meeting the global power source market. To correspond to the global market, 4S reactor has been designed on the principle of simple operation, simplified maintenance including refueling, higher safety and improved economic features. More specific design policy for 4S reactor includes ten items as follows; 1. no refueling for 10 years, 2. simple core burn-up control without control rod and its rod driving mechanism, 3. removal of control and adjustment components from the reactor system, 4. quality assurance and short construction period based on shop fabrication, 5. load following without operation of reactor control system, 6. minimum maintenance and inspection of reactor components, 7. negative reactivity temperature coefficients including coolant void reactivity, * Corresponding author, Tel , Fax , nob@criepi.denken.or.jp 8. no core damage in any conceivable initial events without reactor scram, 9. safety system not dependent on the emergency power and active decay heat removal system, 10. complete containment of reactivity under any operational conditions and decommissioning. Items from 1 through 6 relate simplification of system and maintenance. Items from 7 through 10 relate safety. Based on above design requirements, it was the first time that the 4S reactor was embodied with higher safety and passive characteristics in ,2,3). The 4S reactor introduces neutron reflectors to control the core reactivity without neutron absorber rods. The reflectors are driven from outside of the reactor vessel and move very slowly. The moving speed is about 1or 2 mm/day. Electromagnetic pimps are applied to the primary pumps. From these design features, there is no moving or rotating part, which can decrease component failures and maintenance works. All temperature reactivity coefficients are designed to be negative, which strongly helps to realize the passive safety features 2). It also enables to simplify a power control system so that only feed water control can regulate a reactor power. However, there were several problems to be resolved to enhance the feasibility for an earlier realization of 4S reactor. The extracted problems are (1) the core height of 4m having difficulty to do irradiation tests in consideration with the 1

2 existing facilities, (2) the reliability of a reactor shutdown system including a reflector control system, (3) the chemical activity of the secondary sodium systems. The 4S reactor has been modified in 2001 to make its critical path the metallic fuel core development. A reactor concept is built up in core configuration centered manner in this type of reactor because the safety requirements are strongly dependent on core performance. In this paper, at first, the basic concept of the 4S is explained. In second a core design for a modification is described. Finally, a reactor configuration and the passive safety capability are presented. BOC MOC EOC Core Reflector Axial Power Profile Fig.1 Schema of reflector controlled core burning process II. Basic Concept of the 4S 1. Core configuration Requirements for the core design from the reactor kinetics and fuel management are the negative temperature feedback coefficient of reactivity and a long core lifetime. Especially, the positive coolant coefficient is a difficult problem in sodium cooled fast reactors. In general, the negative coolant reactivity coefficient requires the large neutron leakage. On the other hand, a long core lifetime requires good internal conversion, which means a good neutron economics. Namely, above two characteristics are opposed each other. The superior characteristics of metallic fuel (superior internal conversion, excellent thermal conductivity) are utilized to make the requirements into practice. Also a suitable core configuration is adopted, which is a combination of a long tall core and reflectors. An axial height can gain the lifetime. Partially covered core can enhance the neutron leakage. Fig. 1 shows the axial power profiles in several periods. At the BOC (beginning of core life), a bare core in sub-critical becomes critical by inserting reflectors to reduce the neutron leakage. The peak position of the reactor power is at lower part of the core. As the core burning, the reflector gradually goes up to cover fresher part at the MOC (middle of core life). At the EOC (end of core life), the reflector is almost at the top of the core. The sum of the power, where the core is covered by reflector, comes to the maximum at the EOC. The coolant reactivity coefficient and the coolant void reactivity is severest at the EOC. 2. Reflector Control The 4S reactor employs a reactivity control system with an annular reflector in place of the control rods and driving mechanism which traditionally require frequent maintenance service. Control rods must be replaced a number of times due to their lifetime. Reactivity is controlled only by the vertical movement of the annular reflector during plant startup, shutdown and power generation, thus eliminating the necessity for complicated control rod operations. The reflector is installed inside the reactor vessel and the heat generated in the reflector is cooled by coolant sodium. The reflector is gradually lifted up to control the reactivity according to core burn-up. Regular power operation is attained at a constant speed which is regulated in scheduled maintenance according to the reflector differential reactivity worth. Since no feedback system or control system are used due to the simplicity, reactor thermal output drifts in several percent during a maintenance interval. The reflector drive mechanism consists of hydraulic system that operates at startup and shutdown and ball screw system connecting to motors which are operating during normal operation. The mechanism has six driving systems corresponding to the azimuthally separated reflector. The six ball screw systems are fixed on the platform supported by the hydraulic system. For the reactor shutdown, the hydraulic pressure is released to move the reflectors downward by opening scram valves (Fig.2). The mechanical part of the reactor shutdown system has redundancy that the platform is divided and the scram valves are set in parallel. Reflector Up Motor driven Normal operation Platform Shutdown Drive Unit Down Fig.2 Schema of Reactor Shutdown Mechanism 3. Passive Decay Heat Removal System The 4S reactor introduces the passive decay heat removal system to realize the passive safety features. An RVACS (Reactor Vessel Auxiliary Cooling System) is adopted, in which the natural circulating air flow removes the decay heat through a guard vessel in radiation mechanism. Heat removal capability depends on an irradiation area. A relative area per thermal power of small reactors is larger than that 2

3 of medium or large reactors. It is expected that about 1% of nominal power can be removed by the RVACS. II. Core Design A core design modification has been done to solve the raised problem as the core height of the former 4S reactor of which the design parameters are shown in Table 1. Major restrictions on core design are a core lifetime of 10 years and a coolant void reactivity in the design modification. Core performance parameters are kept as much as possible. The former 4S core had the large height and the slender radius. The 4m height gained long core lifetime and the slender radius enhanced the neutron leakage which helped to make the coolant void reactivity negative. Table 1 Main Design Specification of 4S Core Thermal output [MW] Electrical output [MW] Pri. T. [ o C] (outlet/inlet) 510/355 Sec. T. [ o C] (outlet/inlet) 475/310 Steam cond. [ o C/MPa] 453/10.8 Core dia. [m] Core height [m] /1.5 No. of S/As No. of reflector units 6 6 Reflector thickness [m] Core lifetime [yr.] Plant lifetime [yr.] No. of fuel pins Fuel pin dia. [mm] Cladding thickness [mm] Smear density [%TD] Pitch/Dia Duct thickness [mm] 3 2 Duct gap [mm] 4 2 Bundle pitch [mm] Ave. burnup [GWd/ton] Pu enrichment [%] 18.5/ /20.0 Max. LHR [kw/m] Conversion ratio (MOC) Coolant void reactivity (EOC) [%ρ] -0.3 ~0 Burnup swing [%ρ] ~8 ~9 Core pressure drop [MPa] ~0.2 ~0.1 These design parameters are fixed that are the maximum linear heat rate (25 kw/m), core inlet and outlet temperatures (355/510 o C), the maximum Pu enrichment (20 wt%hm) and fuel smear density (75 %TD). Although the larger Pu enrichment helps to make a core size compact and the enrichment larger then 20 wt%hm would be expected 4), this upper value is set up in consideration of the irradiation results. The limitation of the fuel smear density is also fixed the same reason. The pressure drop at the fuel sub-assembly is aimed below 0.1 MPa from the point of view of the safe margin for ULOF (Unprotected Loss Of Flow) event which is the major initiator of HCDA (Hypothetical Core Disruptive Accident). Inner core 2.1 m 1.5 m 1.0 m 1.2 m Effective core dia. Fig.3 4S Core Configuration (2001) Ultimate shutdown rod Outer core Reflector unit The latest core design in 2001 is summarized in Table 1 after the survey calculation for the optimization compared with the former design 5). Thermal output is reevaluated to 135MW from 125 MW, because the previous design seemed to use optimistic heat efficiency. The inner core height is lower than that of outer core to decrease the coolant void reactivity. The upper 0.5 m of the inner core region, in which no fuel is arranged, can gain negative reactivity in case of the coolant density loss. 3

4 The target value of an averaged fuel burn-up is set to 100 GWd/ton, however, the final value results in 70 GWd/ton due to an enlargement of core size. In the former 4S design, the reflector is shorter than the core height. This feature has some advantage in abnormal reflector withdrawal accident in which the partial moving of an azimuth-separated reflector brings negative reactivity 6). The latest design, however, has the higher reflector than the core. Reflector control system ought to have more consideration for reactivity insertion accident. Fig. 3 shows the latest core configuration. There are 6 inner subassemblies and 12 outer subassemblies. And the ultimate shutdown rod is arranged at the centre of the core. This core has rather square configuration. It brings disadvantage for the coolant void reactivity, and advantage for the fuel burn-up and the core lifetime which can increase economic performance, because the neutron leakage decreases to improve internal conversion. Though an enlargement of fuel pin decrease the burn-up reactivity swing under the core pressure drop fixed, the number of fuel pins becomes to be the double to keep the linear heat rate (LHR) criterion. The coolant void reactivity is kept below zero during the 10 years core lifetime, and the value is almost zero at the EOC. The former core has margin in the coolant void reactivity of ~1% ρ. The latest core spends this margin to satisfy requirements with the shorter core. The height of the reflector is higher than the active core to enhance the compensation of the burn-up reactivity loss. Graphite reflectors are adapted. In the former design, the reflector was made of the steel. And there was cavity region above the reflector. As replacement of the reflector material form the cavity has larger positive reactivity effect than that of form the sodium, this above-reflector cavity enhances the reactivity worth of the reflector. The initial fissile inventory, however, increases, as the absorption of neutron becomes higher, especially at the BOC. The active core height of the inner core is shorter than that of the outer core (see Fig. 1). This 0.5 m sodium region above the inner core helps to decrease the coolant density reactivity coefficient over the entire core. The inner core less affects the reactivity worth of the reflector. Optimum configuration to gain good neutron economics may bring better core performance. Doppler Fuel Coolant Structure Table 2 Feedback Temperature Coefficients dk dt T -2.80x x 10-3 k / kk' C o x x10-6 k / kk' C o x10-6 ~0 k / kk' C o x x10-8 Table 3 Main Design Parameters of Latest 4S Reactor Items Specifications Reactor Configuration Pool type Diameter [m] 3.0 Height [m] 18.0 (*1) RV Thickness [mm] 25 GV Thickness [mm] 15 Inner Cylinder Inner Diameter [m] 1.84 Thickness [mm] 15 Reflector Material Graphite Height [m] 2.1 Thickness [mm] 300 Core barrel Inner Diameter [m] 1.33 Thickness [mm] 10 Primary EM pump Rated Flow [m 3 /min.] 50 Head [MPa] 0.08 x 2 (*1) from bottom to coolant free surface Table 2 shows the comparison of the feedback temperature coefficients integrated over the core region. The graphite reflector helps to increase the Doppler coefficient, which is slightly smaller than that of large MOX core. At the outer core, neutron energy spectrum is relatively soft comparing to that of the former core with the steel reflector. Passive safety characteristics of this core are investigated and presented the following chapter. In transient analyses, spatial feedback coefficient parameters (R-Z model) are estimated and modelled. The guard vessel covers the reactor vessel to secure the loss of primary coolant. The guard vessel also forms the containment boundary together with the top dome. The natural air cooling system between the guard vessel and cavity wall is designed as the passive decay heat system (RVACS). The primary pump system consists of two EM pumps in series arranged. The EM pump has an annular single stator coil and is sodium immersed self-cooled type. The total rated flow is 50 m 3 /min. and each pump has 0.08 MPa head. This series pump system has a good response in case of a single pump seizure to mitigate to decrease the core flow. Reverse flow may occur at a failed pump in a parallel. On the contrary, a working pump can support the flow due to its Q-H (flow-head) property. The secondary piping is set at the top of the reactor vessel and it connects to the SG (steam generator). Fig. 5 shows a schematic vertical cut of the reactor building. The reactor assembly is supported by a massive structural platform that is seismically isolated. The SG and the dump tank for the 4

5 secondary coolant are arranged on that plat form. As the single tube is utilized for the reference design of SG, a sodium-water reaction accident due to tube rapture must be taken in account to the design basis accident. There are two optional designs to avoid this accident. One is to introduce a double-wall tube type SG. The other is to replace the secondary sodium coolant to an inert material. The optional designs are under evaluation to put the sodium-water reaction accident as the beyond basis accident. flow (ULOF) and an unprotected transient overpower (UTOP) accidents are chosen to evaluate the passive shutdown capability. All accidents are simulated by the plant dynamics analysis code CERES including the major components of the 4S. CERES can solve multidimensional (2D and 3D) in-vessel thermal hydraulics that has been verified by the benchmark experiments. As the selected transients change symmetrically, whole plenum including the primary components is modeled into two dimensions. R-Z model has 16 radius cells and 58 vertical cells. R-Z plane size is 1.5 m by m. Inner cylinder Fig.5 4S Plant Layout (vertical view) (2001) Fig.4 4S Reactor Assembly (2001) IV. Passive Safety Capability 1. Analytical modeling The typical hypothetical accidents are analyzed to demonstrate the passive safety capability of the 4S. The accidents to be analyzed are chosen for the passive heat removal capability and the passive reactor shutdown to play a significant role or not 7). A term of passive reactor shutdown dose not mean a true shutdown but a function of reducing a reactor power to a level where heat removal can be possible with no core damage by passive decay heat system of RVACS. A protected loss of heat sink (PLOHS) accident is chosen to evaluate the RVACS capability. An unprotected loss of All components are modeled into one dimension, which are IHX, EM pumps, SG and secondary piping. Fig. 6 shows analytical network model of the 4S, which has a unique flow pass configuration to enhance the RVACS performance. Exhausted coolant from the primary EM pumps flows to dual directions in right side schema of Fig. 6. One is the main direction to the core inlet through shielding. The other is upward along the inside of the reactor vessel returning to the pump inlet as bypass flow. Without the pump head, coolant flow at this region turns to reverse direction after reactor shutdown in left side schema of Fig. 6. It makes the effective surface of the heat radiation increase. The bypass flow is 10% of rated flow. CERES covers the plant system from feed water pump to turbine inlet. Boundary conditions can be adequately fixed to represent the event sequence from several assumptions. CERES also has additional reactivity feedback models which are thermal core expansion including the support grid 5

6 plate and load pads, and the relative displacement between the reflector and the core due to the thermal expansion of the reactor vessel and the reflector drive line. The design safety criteria are no coolant boiling and no fuel melting for fuel element and 650 o C for the primary boundary structure. The temperatures are evaluated for the nominal hottest pin, which is assumed to have a nominal hot channel factor of 1.53 without the engineering safety factor. The outlet coolant temperature is 593 o C at normal operation. Steam respectively. Although the PRACS can remove the decay heat under natural convection mode, the PRACS is assumed to be out of work and the only RVACS can be expected in this analysis conservatively to evaluate the heat removal capability of the RVACS. The accident starts with primary pump trip followed the reactor shutdown after 1 second. Feed water stops at 6 seconds and the steam/water blow valve has been opened for 34 seconds. 2.5 Feedwater 2.0 Decay Heat RVACS RVACS Heat collector Air flow 2D(R-Z) R:16 Z:58 IHX EMP Piping Pump model IHX model SG model junction Heat [MW] Time [hr.] Fig.7 Predicted Heat Removal Capability of RVACS in PLOHS Core air After shutdown Normal operation Fig.6 Schematic Network Model of 4S 2. PLOHS results PLOHS event is simulated to predict the heat removal capability of the RVACS. PLOHS assumes to be initiated by loss of the external AC power, resulted in loss of total AC power, because the 4S does not have onsite emergency AC power. The steam/water system cannot remove the decay heat in this event. The primary coolant flow shifts to natural convection mode. Designed heat removal capability of the PRACS and the RVACS are 2.5 MW and 1 MW, Temperature [ o C] Core outlet HP Na (RV Top) Core Inlet Time [hr.] Fig.8 Predicted Temperatures in PLOHS 6

7 Fig. 7 shows the predicted heat removal capability of the RVACS. As the temperature distribution in the primary hot plenum in transition and the secondary flow is instable at around 1 hour after shutdown, the heat and flow move up and down. The RVACS removes about 0.8 MW after stable condition that overcomes the core decay heat. The difference from the design value of 1.0 MW comes from the difference of the temperature at the primary hot plenum. The design value is defined with 650 o C. The heat transfer area of RVACS is 130 m 2 at the outer surface of the reactor vessel with the effective height of 13.8 m. The maximum averaged heat flux is 6.2 kw/m 2. The primary coolant flows at 3.4%. The coolant at the bypass region flows 3.3 kg/s, which is about 5% of the core flow rate in stable condition. Fig. 8 shows the temperature variation during the event. The legend of HP Na (RV Top) denotes the coolant temperature at the top of down comer, which represents the primary boundary temperature. The maximum temperature of coolant is low enough for the criterion. The maximum primary boundary temperature is also lower than 650 o C. The thermal-hydraulic analytical result by CERES predicts that neither stagnant area nor local vortex flow is observed in flow pattern due to simple plow path configuration. It is also predicted relating the flow pattern that neither hot spot nor cold spot is observed in temperature distribution 7). 3. ULOF results In case of an off-normal event occurs, lowering the reflector performs the reactor shutdown and the decay heat is removed. Anticipated transient without scram (ATWS) seems the off-normal event in which the active reactor shutdown system does not work. ULOF and UTOP events are categorized in ATWS. ULOF event is initiated by loss of the external AC power of the primary pumps without reactor scram. As a result, core temperature rises due to power to flow mismatch. The core flow decreases faster than the core power reduced by the negative reactivity feedback. Because the typical fast reactor has the positive coolant void worth (coolant density feedback is also positive), the transient may result in catastrophic core damage after the coolant boiling onset. A non-positive void worth enhance the negative feedback and prevent from inserting the large positive reactivity. Flow halving time and core kinetic characteristics, especially, the Doppler and the sodium density feedback coefficients mainly govern the ULOF consequence 8.9). Core flow coastdown profile, define by the flow halving time, affects the severity of ULOF. An EM pump does not have mechanical inertia comparing to a mechanical pump. Some electrical equipment is needed to realize the coastdown. VVVF (variable voltage and variable frequency) controller supplies electricity to both an EM pump and a synchronous machine in normal operation of the 4S reactor. The synchronous machine acts as a motor in normal operation and as a generator in transients. A stator current, which is generated by connected generator in coaxial or supplied by DC battery, controls energy supply for the EM pump. The power supply control of the synchronous machine can intend to make an adequate coastdown profile of EM pump. In this paper, a coastdown shape similar to that of a mechanical pump is used for the comprehensive discussion. Reactor Power / Core Flow [-] Reactivity [$] Power Core Flow Time [sec.] Fig.9 Predicted Power and Flow in ULOF Structure Coolant Fuel Doppler Net Time [sec.] Fig.10 Predicted Reactivity Components in ULOF Fig. 9 shows the predicted power and flow in ULOF event with 10 seconds flow halving time. The low-pressure drop in the primary system can develop the sufficient natural convection. The flow rate is 20% of rated flow. 7

8 Fig. 10 shows the variation of the reactivity feedbacks. As the negative component of the Doppler and the fuel are greatly large enough to overcome the positive components, the net reactivity is kept deeply negative during the sequence. The passive reactivity feedback effects are neglected in this analysis, which are the core radial expansion and the relative displacement between the core and the reflector. Temperature [ o C] Fuel Collant Time [sec.] Fig.11 Predicted Temperatures in ULOF Fig. 11 shows the predicted temperature changes of the fuel and the coolant at the nominal hottest pin, which has the hottest temperature over the all pins. The both maximum temperatures are lower than the fuel melting point of 1180 o C and the boiling point of 960 o C. In addition, severer ULOF is analyzed, which is an instantaneous loss of circulation head of a single EM pump out of the two EM pumps in series. This ULOF is initiated by loss of one EM pump and the core flow rate is followed by the flow coastdown maintained by the living EM pump in 1 second after onset of the transient. The duration time from 100% to 50% of rated flow is shorter than that of the previous ULOF, and the coastdown shape maintained by only one pump is below. From these flow decreasing characteristics, an analytical result of the additional ULOF is prospected severer than that of the previous ULOF. More than three pumps are required in parallel arranged system, because there may be some reverse flow through fault pump. In the serial arranged two pumps system in 4S reacotr, the core flow rate can be sustained more than 50 % of rated flow, because there is no reverse flow and the living pump covers the flow due to its Q-H (flow-head) curve. The analytical result predicts that this severer ULOF pushes only ~10 o C which is fully accepted. From this result; there are some margins, which are a core pressure drop and reactivity feedback characteristics, to improve the core performance by making the fuel pitch closer. 4. UTOP results UTOP events are analyzed to estimate an allowable external reactivity insertion. The reflector is regulated to compensate the burn-up reactivity loss. This event assumes to initiate by unexpected reflector lifting without scram. Analytical assumptions for a reference case are summarized below. - no radial core expansion except the core support grid - constant coolant flow at the primary and the secondary - constant heat removal via SG (135MWth) - out of consideration of PRACS and RVACS The external reactivity is inserted by ramp rate of 0.1 /s, which is ten times larger than the ramp rate required to change the reactor power by 1 %/min. The peak coolant temperature at the nominal hottest fuel reaches 970 o C up to 1$ insertion, while the hottest fuel temperature is below the melting point. The inserted reactivity is almost canceled by the Doppler reactivity. The calculated reactor power rises to 1.31 of rated power. However, it was reported that the cladding of the irradiated fuel element breached and fuel liquefaction occurred at ex-reactor test 10). This furnace test was conducted to evaluate the behavior that could be expected during a LOF event with irradiated EBR-II Mk-V-type fuel element, which was the combination of U-19Pu-10Zr fuel and the HT-9 cladding. The fuel element was kept at about 820 o C for 112 minutes. The cladding breached due to the cladding thinning by the fuel/cladding metallurgical interaction. The fuel/cladding interaction also caused fuel foaming, because the iron atom diffused into the fuel matrix to form low melting alloy. The reactor power rises and becomes stable at 1.30 of rated power in another UTOP with 70 insertion. The tendency is the same as 1$ insertion case. The coolant temperature rises to 860 o C and comes down to 800 o C after the reactivity insertion. The peek fuel temperature is 940 o C. If the transient would last for hours, fuel elements might be damaged resulted in molten fuel dispersion due to the iron atom diffusion and the liquefaction. These results predict that the slightly below 1$ reactivity may be acceptable to avoid the fuel melting and the coolant boiling in short term behavior, and that the external reactivity is limited to around 70 in long term behavior. It is noted that the acceptable reactivity depends on the analytical assumptions, which are the heat removal condition and the passive reactivity mechanism, especially, the core radial expansion (bowing). The latest 4S reactor has about 24 $ reactivity loss during core lifetime of 10 years. This value is large enough to initiate severe core damage, in case of all reflector reactivity may be inserted. The reflector reactivity control system must have the stopper system to prevent unacceptable reactivity 8

9 insertion, which can be designed by various UTOP analyses. The raised problem of the reliability of the shutdown system is resolved to modify the detection system to the dual system. The reliability is evaluated to be the order of 1x10-6. V. Future Works years core The present 4S core has 10 years core lifetime. The refueling will be required due to the more than 30 years plant lifetime. A 30 years core lifetime may not need the refueling and is better to satisfy a non-proliferation requirement. A larger size and a good neutron economics are required to gain 30 years lifetime satisfying the negative coolant void reactivity. We are trying to look for this 30 years core, if necessity, the reactor power will be decreased. 2. Sodium-water reaction free SG The problem is remained in the sodium-water accident. In near term design, we are to introduce double-wall tube SG. And a plate type heat exchanger is proposed instead of the ordinary SG for an advanced design. This new type SG is similar to the double-wall-tube SG in respect of layer configuration. Both SG has three layers that are sodium, gas and steam/water. Sodium and steam/water are separated by double boundaries. Helium gas is used in general because of its good heat conductivity. A plate type or a plate fin type heat exchanger is used in many industrial fields. The prime merit of SG is the compactness comparing to tube-to-tube type SG which enables a pool type reactor configuration. A heat exchange unit is fabricated by HIP (hot isostatic press). Rectangular tubes are bent into a plate-like case formed by outer plates with inner plates which envelops the side of tubes. Hundreds of units of plate type heat exchangers may be azimuthally installed instead of IHX. The future works remained are how to assemble the parts for HIP and pre-service inspection. VI. Conclusion The design study has been carried out to embody the ten design policy, and improve the 4S reactor to realize the earlier practical use, of which a core is controlled by neutron reflectors. As an extracted major problem is the core height, the core active height is shortened to 1.5 m from 4.0 m to keep the design requirements of which the major items are the thermal output, the core lifetime, the fuel burnup and the feedback reactivity coefficients. Important requirements are the long core lifetime of 10 years and the negative reactivity coefficients. The several transient sequences are analyzed to evaluate the passive safety capability of the latest 4S by the suitable analytical code CERES modified to model the 4S configuration. The passive heat removal system of RVACS demonstrates its ability through the simulation of PLOHS event. The negative reactivity feedback set of 4S acts to the best of its ability to secure the fuel integrity in ATWSs, especially in ULOF event. These calculations predict that passive safety features help to simplify or to exclude the safety related reactor system. The simplification is expected to improve the economics of small reactor. An autonomous characteristic of 4S is skipped in this paper due to limitations of space. The reactor power regulation by the feed water control is investigated and presented another paper 11). Acknowledgements The authors wish to thank Ms. Yumi Yamada and Mr. Osamu Watanabe belonging to Advanced Reactor Technology Co., Ltd. (ARTECH), and Mr. K. Umemura belonging to Denryoku Computing Center, Ltd. (DCC) for their helpful support of conducting analyses by CERES code and useful comments on the results. References 1) N. Ueda, A. Minato, N. Handa and S. Hattori, Super-Safe, Small and Simple Reactors for the Global Energy Demand, Proc. Of Int. Conf. on Fast Reactor and Related Cycles (FR 91), Kyoto, Japan, Oct. (1991) 2) S. Hattori, A. 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10 10) L.L. Liu, et al., Behavior of EBR-II Mk-V-type Fuel Elements in Simulated Loss-of-Flow Tests, Journal of Nuclear Materials, 204, pp (1993) 11) N. Ueda, Y. Nishi, H. Matsumiya and T. Yokoyama, Passive Reactor Dynamics and Load Following Characteristics of Sodium Cooled Super-Safe Small and Simple Reactor, Proc. of the 10th ASME-JSME Int. Conf. on Nuclear Engineering (ICONE-11), 36539, Tokyo, April (2003) 10