HKNIC - A Safe & Reliable Design

A Safe & Reliable Design
To minimise the chance of radiation being released, modern nuclear power stations incorporate the principle of duplication into their design.

Systems with the same safety objective are duplicated, so that if one system fails, the second back-up system can take over. As far as possible, these systems are designed in different ways, in order to reduce the possibility of a simultaneous failure brought about by the same factors. As a further safety feature, redundant items of equipment are installed in duplicate circuits, so that even if part of a system fails, the system itself can still operate.

To prevent radiation release, or reduce the consequences of any release, Daya Bay uses a "defense in depth" concept. This means that if one safety feature fails, those remaining should be capable of either preventing an accident or minimising its consequences.
The engineering systems controlling the rate of nuclear reaction are based on different physical principles. The pressure, temperature and water level in the reactor system are each monitored by several different systems. An anomaly in any one of these systems will lead to the reactor being shut down automatically within two seconds by the insertion of the control rods into the reactor core. These rods are held in place above the reactor by electro-magnets, so that should the electricity supply be interrupted, gravity will cause them to fall, shutting down the reactor. If for any reason the control rods cannot be inserted into the core, the reactor can still be shut down by injecting a solution of boric acid into the reactor core, which absorbs the neutrons produced during nuclear fission and stops the nuclear chain reaction.

Control rod drive mechanism  Control rod drive shaft 
Control rod in the up position  Control rod guide tube 
Neutron detector  Reactor core 
Reactor vessel  Control rod in the down position 


Several engineering systems are in place to continue cooling the nuclear fuel even after the reactor has shut down, so as to remove any residual heat. There is also a comprehensive system to deal with leakages at the primary circuits, by injecting water using pumps which have their own back-ups, or from tanks containing water under pressure, to maintain cooling.

Boron Injection Tank 
Water Tank 
Charging Pumps 
Low Head Injection Pumps 

Multiple barriers are in place to prevent the escape of the radioactive products of nuclear fission held in the fuel pellets:

the cladding of the fuel rods is made of a highy ductile zirconium alloy and will seal in the uranium fuel
the 20 centimetre thick reactor pressure vessel and its pipework will contain any radioactivity in the primary cooling water
the containment building, made of 90 centimetre thick pre-stressed reinforced concrete with a 6 millimetre internal steel lining will prevent the escape of radioactivity into the environment

Fisson Products 
Fuel Cladding 
Reactor Pressure Vessel 
Steel Lining 
Containment Building 

Daya Bay has been built to the highest standards using stringent quality control procedures. The design incorporates risk analysis and features that aim to ensure a very low level of risk from external factors such as fire, earthquake, high winds, flooding and aircraft crashes.