Reactor and Guard Vessels

The heat exchange surface of the steam generator is arranged above the core in the annular gap between the reactor pressure vessel and the in-vessel barrel. The steam generator is of the once-through, cassette-type. The maximum possible inter-circuit leak is equivalent to an orifice of 24 mm diameter for feedwater and of 40 mm diameter for steam. The SG cassettes are combined in twelve independent sections with individual supply of feed water and removal of steam out of the reactor and guard vessel. 

Sodium fire prevention and extinguishing measures are mainly provided by the passive means. First of all, these are reactor guard vessel and jackets covering pipeline sections attached to the reactor up to the shut off valves, as well as the main secondary sodium pipeline sections from the IHX to the reactor cell wall. The space between the main and guard vessels is filled with the inert gas preventing sodium from burning in case of leak. 

Boundary for primary sodium Double boundary reactor vessel (RV) and guard vessel (GV) ... 

The reactor vessel and guard vessel boundary for primary sodium ... 

Rail transportability imposes a size limitation upon the reactor vessel and guard vessel of 6.1 m in diameter and 18.9 m in height [XXIII-25]. The fission gas plenum height is based upon an assumed conservative gas release from nitride fuel of 2.5% per atom % of bum-up. The fuel volume fraction was held fixed in the thermal hydraulic design analyses at the value of 0.215 determined by the core design. The fuel rod outer diameter and pitch-to-diameter ratio were varied to determine an optimum combination. Figure XXIII-5 shows the relationship... 

Reactor vessel Double-wall monoblock-type cylindrical vessel with built-in primary systems. Dimensions of main and guard vessels are 08400x80 mm and 08000x80 nun, respectively. Vessel height is about 10 m. 

The sodium pools are mainly composed of a main vessel and guard vessel, with a temperature and pressure measurement instrument on the wall and a sodium leak detector in the gap of the vessels. The main vessel acting as the boundary of the primary circuit is a very important item of safety equipment. The internal strucmres involve the inner pool used to separate the hot and cold pools, the reactor core and its pressure header, and supports and shieldings. 

When there is no coolant flow in the circuit, temperature difference along the main route of sodium is insignificant. However under these conditions, the risk of stagnation zones appearance in the vicinity of the vessel bottom is high. It should be taken into account in the analysis of this phenomenon, that cold sodium from the cold trap enters lower section of the reactor vessel and both reactor pit and reactor guard vessel are cooled permanently by air. 

In the 4S, the decay heat is removed by two systems consisting of the decay heat removal coil installed in the reactor (PRACS) and the natural air ventilation from outside the guard vessel (RVACS). The analysis considers the destruction of PRACS and the RVACS cooling stack by a large falling aircraft. In addition to this extreme severe condition, 50% of the cross sectional area of the RVACS stack is assumed to be blocked. 

The Double Pool concept has been driven by an objective to reduce fast reactor construction costs to the same level as those of an LWR. A major contribution has been to reduce the overall size of the intermediate heat transport system by installing the steam generators in the sodium filled annular space between the primary vessel and the guard vessel. These two examples serve to show some of the ways in which the basic modular concept can be modified to meet different objectives. 

Integral pnmaiy circuit, reduced fliicncc, small dtamcto- of nozzles in the reactor vessel, guard vessel, low parameters reduce initiator frequency and limit consequences R... 

The reactor plant is located within the containment structure, which is arranged in the reactor building. Since the reactor and the primary circuit equipment are located inside the guard vessel a single ferroconcrete containment has been selected without prestressing, but with metallic liner. It is designed for an excess pressure of 0.1 MPa. 

The containment serves to protect the reactor plant against external impacts, as well as for confining radioactive products in the event of incidents during reactor refuelling and following accidents with postulated loss of the guard vessel integrity. 

The containment, annexes to the reactor building, reactor, guard vessel, primary circuit equipment and the safety systems are designed for a magnitude 8 earthquake (MSK-64 scale). 

Residual heat removal in all heat removal accidents is provided by passive reactor vessel cooling system by a leak-tight air circuit. Reactor heat is removed through the guard vessel to a leak-tight containment and then to the environment. 

The containment is composed of the guard vessel installed outside the reactor vessel and the top dome over the reactor vessel. The top dome has only few penetration to maintain high leaktightness. 

The MBRU-12 has maintained a conservative approach, providing for the shuffling of fuel under a closed guard vessel cover, which could help achieve early market availability. The 4S reactor, however, incorporates a small-diameter core of high neutron leakage rate with moving reflector control of bum-up reactivity loss, as a way to assure negative sodium void worth under all conditions. The reflector in the 4S is located outside the core and the power control is executed via the feedwater control from the steam-water power circuit. Some further related R D is required on these features (ANNEX XIV). 

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