Sodium-to-air Heat eXchanger

In case off-site power is available, the decay heat is removed through normal heat transport path of secondary sodium and water/steam circuits. Additionally, an independent safety grade passive direct reactor cooling system consisting of 4 independent circuits of 6 MWt nominal capacity each has been provided. Each of these circuits comprises of one sodium to sodium heat exchanger dipped in reactor hot pool, one sodium to air heat... 

Decay heat removal loops. The normal route for the removal of decay heat in a fast reactor is via the secondary sodium circuits and the steam plant. Should this route not be available, decay heat in PFR could be rejected by one or more of three thermal syphon loops, each filled with eutectic sodium/potassium alloy. Each loop extracted heat through an immersed coil, intercq>ting some of the primary sodium as it flowed from the core towards an intermediate heat exchanger, and delivered the heat by natural convection to the outside atmosphere through a sodium-potassium/air heat exchanger built into the wall of the secondary containment building. 

RAPSODIE was constituted by two secondary loops. Heat removal was made by sodium/air heat exchangers. At the decision of the final shutdown of the reactor, the two loops were drained. One loop was kept and transformed in order to develop a technological program to test a sodium pump (CARUSO program), it was then dismantled in the 90 s. The other loop was quickly dismantled. The two circuits were dismantled without preliminary in situ treatment. Pipes were cut and then treated with water in specific facility held at Cadarache. Global activity in tritium of the secondary sodium was estimated at the date of 1 January 1994, to 2.21 GBq/t. The activity of the residual sodium in the loop was estimated to 15 Bq/g of sodium. 

The PSACS uses the primary sodium cleanup lines and pump to reject heat via an Na-to-N2 heat exchanger coupled to the same N2 loop used for the primary sodium cold trap. The N2 loop rejects heat to an air dump heat exchanger. Electrical power is needed for this system to actuate valves, and run fans and pumps. The PSACS has a minimal impact on plant cost since it shares equipment with the sodium cleanup system. 

There are two Direct Reactor Cooling Systems (DRC 1 and 2) of three loops each. All loops extract heat from the hot pool of the primary sodium by immersed sodium/sodium heat exchangers (DHX) and reject the heat to the environment by sodium/air heat exchangers (AHX) arranged above the DHXs [5.31]. 

If all main transport circuits and power are available the decay heat is removed via the normal path. In case of loss of offsite power or station blackout decay heat is removed via the 4 decay heat removal circuits in the hot pool. Each circuit has a Na/Na decay heat exchanger (DHX) immersed in the hot pool and linked to an air heat exchanger (AHX), and is rated for 6 MW decay heat removal based on 18 MW capacity and a single failure criterion. The sodium flows between the DHX and the AHX by natural convection, as a consequence of the temperature and elevation differences between the two. There are dampers on the air side of the AHX which have to be opened. Apart from the damper, the rest of the system is passive. The system is designed to sustain a delay of up to 30 min. in opening the dampers. 

Carbonate Process. In this process the ores are leached with hot sodium carbonate for 24 hours, with sparging with air to provide oxidation. The leachate is cooled in counter-current heat exchangers, heating the carbonate solution for the next batch. The carbonate leachate is filtered on rotary drums, and the uranium is precipitated with sodium hydroxide and filtered. The filtrate is converted back to carbonate by sparging with carbon dioxide, usually from a boiler flue gas, and... 

The thermal and nuclear properties of sodium (it scatters neutrons without absorbing them) made it the heat exchange fluid of choice for fast-flux reactors in spite of its nasty chemical properties when exposed to air or water. The French Superphenix, a commercial-scale sodium cooled reactor, was beset with technical problems, but demonstrated that fast-flux reactors can produce electric power at the 1000 MW level. 

It is also possible to supplement the RVACS heat removal capacity using a direct reactor auxiliary cooling system (DRAGS) based on natural circulation of an intermediate coolant from bayonet heat exchangers in the reactor vessel to air-cooled heat exchangers. This type of DRAGS system was used in the Experimental Breeder Reactor II (EBR-II) with sodium-potasium as the intermediate coolant. There are a variety of potential intermediate coolants, several of which have been used extensively in industry for similar heat transfer applications. 

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