Ability of New Concept Passive-Safety Reactor "KAMADO" - Safety, Economy and Hydrogen Production -

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1 GENES4/ANP2003, Sep , 2003, Kyoto, JAPAN Paper 1092 Ability of New Concept Passive-Safety Reactor "KAMADO" - Safety, Economy and Hydrogen Production - Tetsuo MATSUMURA*, Takanori KAMEYAMA, Yasushi NAUCHI and Izumi KINOSHITA Central Research Institute of Electric Power Industry (CRIEPI) , Iwado Kita, Komae-shi, Tokyo , JAPAN New concept of a passive-safety reactor "KAMADO has negligible possibility of core melting and flexibility of total reactor power. The present concept has simple plant system design without a reactor pressure vessel, ECCS, re-circulation systems (of BWR) and others. Therefore construction cost per electric power generation is expected to be sufficiently low comparing with conventional large scale LWRs. As KAMADO has a reactor water pool of atmosphere pressure and low temperature similar to swimming pool type research reactors, irradiation of neutron and γ-ray are available around the reactor core. With γ-ray heating around the reactor core, a few kilo m 3 / hour of hydrogen are expected to be produced with 1000 MWt reactor core. KEYWORDS: passive-safety, simple plant system, neutron and γ-ray irradiation, hydrogen Production I. Introduction In normal operation, generated heats of the fuel rods are removed by cooling systems. In case of loss of coolant/flow such as pipe break, turbine trip, etc., decay heat of fuel rods are cooled by natural heat transfer from surfaces of the fuel element to the reactor water pool of atmospheric pressure (1 atm.) and low temperature (< 60 0 C). We have proposed new concept of a passive-safety reactor "KAMADO" (#), which has negligible possibility of core melting and flexibility of total reactor power 1). The reactor core of KAMADO consists of fuel elements of graphite blocks, which have UO 2 fuel rods and cooling water pipes. These fuel elements are located in a reactor water pool of atmospheric pressure (1 atm.) and low temperature (< 60 0 C). In case of loss of coolant/flow events such as pipe break, turbine trip, etc., decay heat of fuel rods are removed by natural heat transfer from surfaces of fuel elements to the reactor water pool. The reactor water pool has enough heat capacity of decay heat for 3 days away safety of operators. KAMADO has the simple plant system design with a reactor water pool similar to swimming pool type research reactors. Therefore irradiation of neutron and γ-ray are available around the reactor core. Present paper will show concept of a passive-safety reactor KAMADO and outstanding ability of the present reactor concept such as hydrogen production, neutron and γ-ray irradiation, easy development and demonstration, small amount of radioactive waste, easy maintenance, flexibility of fuel and core design, and others. UO 2 Fuel Rod RReactor ater PPool ooll Fuel Element (graphite) Turbine Pump Fig. 1 Basic concept of a passive-safety reactor KAMADO II. Concept and passive-safety feature Fig. 1 shows basic concept of the present reactor design. Cooling water is fed from bottom of a fuel element. Heated steam drives steam turbine similar to conventional Light Reactors. Generated heat of a fuel rod is not cooled directly with cooling water but cooled via graphite blocks of a fuel element. Cooling water is heated with high temperature graphite of the fuel element. * Corresponding author, Tel (ext. 0991), Fax , matsu-t@criepi.denken.or.jp Fuel elements are arranged in the reactor water pool. Cross type control rods are inserted from the upper part between the fuel elements. A fuel element is combined with cooling water and steamy pipes through flanges. A reactor core consists of two or more fuel elements, control rods, etc. within the reactor water pool. y pipes are bundled and led to a steam turbine (Fig.2). In the present concept, normal LWR fuel rods are used to eliminate development cost of fuel rods. However carbon coated particle type fuel of high temperature gas cooled reactor also can be applicable.

2 Graphite Gap for controlling heat flux Flange Fuel Element About 4m tube (downwards ) Cooling water tube (upwards) Box of fuel element Reactor Pool Control Rod Flange Fig. 2 Arrangement of fuel elements in the reactor water pool Cooling water is fed from bottom of the fuel element. Heated steam is also discharged from bottom of the fuel element. In a cooling water tube, water is heated by fuel rods through graphite and changed to steam. Within a steam tube, steam flows below and is super heated (Fig. 3). Since heat of a fuel element escape from it to the reactor water pool in normal operation, the gap is prepared on the fuel element surface to control heat flux to the reactor water pool. Preliminary neutronic calculations show enough negative void reactivity coefficient and negative temperature reactivity coefficients of fuel and graphite. Therefore, in case of loss of coolant/flow, reactor will be stopped passively by these negative reactivity coefficients. Increase of fuel elements temperature will be suppressed by enough heat capacity of the graphite of the fuel elements during several seconds after LOCA. Decay heat will be transferred passively to the reactor pool. Therefore maximum temperature of fuel elements can be suppressed less than C after LOCA (Fig.4). The reactor water pool has enough heat capacity, which can keep decay heat for more than 3 days without operation. Therefore safety feature of 3 days away safety of operators can be achieved. Even if the reactor does not stop operation after LOCA, temperature of the fuel elements will be passively suppressed under melting temperature of fuel and graphite. Additionally reactivity induced accidents can be controlled by reactor design such as gravity driven safety rods. Therefore the present concept of reactor "KAMADO" has negligible possibility of core melting. Fig. 4 Pipes Fig. 3 Conceptual design of a fuel element turbine Fuel Element Heat Reactor pool Decay Heat (a) Normal operation Negative void coefficients Reactor stop Fuel Element Reactor pool (b) LOCA Passive-safety characteristic of present reactor concept III. Preliminary conceptual design of reactor core and cost estimation In the preliminary neutronic calculation, we use fuel rod design and volume ratio (fuel / water) of standard BWR 8x8 type 2). Neutron energy spectrum of the fuel element is a little softer than that of BWR 8x8 type of fuel, because of neutron moderation effect of graphite. Since decrease in reactivity of the fuel element with burn-up is similar to that of BWR 8x8 type, the fuel element has possibility of high

3 burn-up (more than 55 MWd/kgU) with 5% enriched UO 2 fuels. Void coefficient of reactivity is very important for passive safety of reactor core. Continuous energy Monte Carlo code MVP 3) calculations show enough negative void coefficient of 15 % δk/k (at 40% void, BOL) and temperature coefficient of -2.3E-4 % δk/k/ 0 C for graphite. Therefore, the present fuel elements have negative coefficients of reactivity enough for passive safety of reactor core. With the preliminary neutronic calculation, we set up tentative design parameters of the fuel element and reactor core as in Table 1. The fuel element size is defined by sub-criticality (<0.9) of one fuel element filled with cold water in cooling tubes. Thermal output of the core is defined by electricity output of 300 MWe, assuming generation efficiency of 33%. Thermal hydraulic analyses of the reactor water pool may be necessary to confirm this thermal output value for several accident events. Table 1 Tentative design parameters of the fuel element and reactor core of the present concept core size Items /cooling tube pitch Fuel element size Number of fuel rods in a fuel element Liner heat rate of a fuel rod Height of a fuel rod Thermal output of a fuel element Thermal output of core Number of fuel elements in a reactor core Values 13.1 mm (d=3 mm) 223 mm x 223 mm kw/m 3 m 3.6 MW 1000 MW (~300 MWe) m x 4.5 m "d": Distance between the outer surfaces of a fuel rod and a cooling tube. The tentative design of the fuel element has 60 fuel rods and 229 cooling water/steamy tubes, which are located on a grid (Fig. (a)). Temperature distribution in the fuel element is calculated with 2D thermal diffusion equation and design parameters of Table 1. Thermal conductivity values of 35.0, and 2.53 (W/m/K) are used for graphite, fuel and water respectively in present calculation. Heat transfer coefficients of and (W/m 2 /K) are assumed for fuel element surface and cooling water tube surface. Heat transfer coefficient of fuel element surface to the reactor pool is reduced remarkably considering gap at surface of the fuel element (see Fig. 3). This gap reduces thermal heat leakage to the water pool within a few % during normal operation. In normal operation, temperature of graphite is estimated to be C (Fig. 5). After LOCA (5% decay heat of normal power, that is, decay heat at 13 seconds after reactor stop), maximum temperature of graphite is estimated to be less than C without heat removal of cooling water tubes mm Distance from surface Cooling tube Graphite (a) s arrangement within a fuel element Temperature 0 C Fig. 5 Fuel Rod Normal operation After LOCA (5% decay heat) Distance from surface (cm) (b) Temperature distributions Temperature distribution of the fuel element in normal operation and after LOCA Present concept "KAMADO" has simple plant system design without a reactor pressure vessel, ECCS, re-circulation systems (of BWR) and others. Therefore construction cost per electric power generation of the present plant is expected to sufficiently low comparing with conventional large scale LWRs (Table 2). As the present concept has passive safety characteristics (reactor stop and decay heat removal), no special engineered safety system is necessary. Therefore cost reduction target of engineered safety system is 0 % in Table 2. A small-scale nuclear power plant has disadvantage of scale demerit in electric power generation cost per kwh. In case of 300 MWe, scale demerit factor is around Therefore present concept plant of 300 MWe has almost compatible electric power generation cost per kwh with conventional large scale LWR, assuming cost reduction target of ~ 60%. The module composition is possible with this reactor concept, which installed two or more reactor cores in one reactor water pool, because present concept does not need reactor pressure vessel and other large scale engineering structures and equipment. For this reason, even if the output

4 of one reactor core is 300 MWe, a large-sized total output is synthetically possible for it. Therefore electric power generation cost of present concept is expected to sufficiently low comparing with conventional large scale LWRs. Systems Primary Cooing System Engineered Safety Features Systems Other reactor systems Turbine system Electronical system Buildings Others Total Table 2 Target of cost reduction Cost reduction target and methods 1/4 by omitting reactor pressure vessel (RPV), recirculation sys (PLR) and others 0 % by omitting ECCS and all of other engineered safety systems 60% by reduction of radioactive waste treatment systems, etc. 1/2 by unifying R/B and T/B ~ 60 % of conventional LWR IV. Ability of present reactor concept The present concept has several abilities besides passive-safety or economical efficiency. As present concept KAMADO has the reactor water pool of atmosphere pressure and low temperature similar with swimming pool type research reactors, irradiation of neutron and γ-ray are available around the reactor core (Fig. 6). An irradiation pipe enables manufacture of various kinds of short half-lived radio actives, which can be used for medical treatments. A neutron irradiation room is utilizable for cancer medical treatment and others. γ-ray heating serves as a significant quantity at the surrounding high gamma ray field of a nuclear reactor. In JMTR (Japan Material Test Reactor), γ-ray heating ratio of 1 to 10 W/g is measured 4). With this reactor concept, γ-ray heating elements, such as 10 t stainless steel, can be installed in the circumference of the reactor core. 10 MW of γ-ray heating is expectable from 1 W/g of γ-ray heating ratio. This 10 MW of γ-ray heating is 1 % of the thermal output of the nuclear reactor (1000 MWt). It is considered appropriate quantity of heat. With the steam (about C) generated at the reactor is re-heated by this γ-ray heating of 10 MW, about 8 kg/sec of very high temperature steam (> C) can occur. With high efficient hydrogen production technology, such as the hybrid method 5, 6), the IS method 7), the UT-3 method 8) and others, a few kilo m 3 / hour of hydrogen are expected to be produced with 1000 MWt reactor core. In addition, this reactor concept of KAMADO has the following features. Small amount of radioactive waste produced in operation and in decommissioning, because of less radioactive leakage from fuel rods and no radiation shielding concrete and material. Simple and easy maintenance, because control rod drive mechanism and other important equipment are located outside of the reactor water pool or in non radiation area. Easy development and demonstration, because LWR technology (including fuel rods) can be used for development of the present concept. And irradiation test of fuel elements can be carried out in existing test reactors. Flexibility of changing fuel and core design such as carbon coated particle type fuel (for high temperature), metallic fuel (for longer operational period) and others. γ-ray heating pipes Neutron Irradiation Room FFuel EE lement /y pipes Irradiation pipes Fig. 6 Concept of neutron and γ-ray irradiation methods V. Summary Present reactor concept of KAMADO has the following features. 1. Passive-safety, which has negligible possibility of core melting. 2. Flexibility of total reactor power from small scale (order of MW) to large scale (order of GW) with module composition of a reactor core. 3. Sufficiently low construction cost comparing with conventional large scale LWRs by simple plant system design without a reactor pressure vessel, ECCS, re-circulation systems (of BWR) and others. 4. Easy development and demonstration, because LWR technology (including fuel rods) can be used for development of the present concept and irradiation test of fuel element can be carried out in existing test reactors. 5. Small amount of radioactive waste and simple and easy maintenance because of small radiation area. 6. Flexibility of changing fuel and core design, because the reactor consists of only fuel elements in the reactor water pool. 7. Irradiation of neutron and γ-ray are available around the reactor core because of the reactor water pool of atmosphere pressure and low temperature similar to swimming pool type research reactors. 8. Remarkable hydrogen production is available with γ-ray heating around the reactor core. A few kilo m 3 / hour of hydrogen are expected to be produced with 1000 MWt reactor core.

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