Decay Heat eXchanger

Fig. 2.2. Elevation view of the AHTR with metallic internals (AHTR-MI) for normal operation (left) and refueling (right) modes. IHX = intermediate heat exchanger DHX = decay heat exchanger PHX = pool heat exchanger Source Peterson and Zhao, 2006.)...

IHX = intermediate heat exchanger DHX = decay heat exchanger PHX = pool heat exchanger. 

The 500 MW(e) prototype fast breeder reactor (PFBR) is under construction in India. PFBR is a 500 MWe capacity, pool type sodium cooled fast reactor with 2 primary pumps, 4 intermediate heat exchangers and 2 secondary loops. There are 8 integrated steam generator (SG) units 4 per secondary loop where steam at 763 K and 17.2 MPa is produced. Four separate safety grade decay heat exchangers are provided to remove the decay heat directly from the hot pool. The hot and cold pool sodium temperatures are 820 and 670 K, respectively. 

Ring header Decay heat exchanger (DHX, Auxiliary cooling system)... 

Decay heat removal (DHR) via immersed decay heat exchangers (DHXs) in the hot or cold pool, intermediate sodium loop, sodium-air-cooler,... 

Birbraer, P.N., et al.. Comparison of Decay Heat Exchangers Placing in the Primary Circuit of Pool Type Fast Reactor, Proc. of the IAEA Specialists Meeting on Evaluation of Decay Heat Removal by Natural Convection, Oarai, Japan, 1993. 

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. 

At the outlet of the pump, water is accelerated by a venturi, passes into a diffuser and then through the decay heat exchanger tube bundles. 

The decay heat exchanger of the RRP system consists of bayonet tubes in which the outside surface is wetted by the primary fluid. The secondary water flows first in the internal tube and then upward through the annular space bound by the two tubes. The water box is located in a dead zone, behind the venturi. This type of heat exchanger was selected, since it does not require a water box at the exit of the heat exchanger. This reduces the primary pressure drop and allows free expansion of the tubes. Thermal loads are reduced leading to an increased mechanical resistance and an enhanced reliability. 

The decay heat exchanger-pump module can easily be extracted from the reactor vessel once the steam generator has been removed. The pump power supply and the heat exchanger secondary feed-lines are set in the vessel via a removable opening in the upper part of the reactor vessel. Main characteristics of the reactor coolant pump are as follows ... 

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