Nuclear power station thermal-hydraulic safety

the Institute of Nuclear Thermal-Hydraulic Safety and Standardization (NCEPU) validates feedwater system with Simcenter Flomaster

Safety and reliability are the most important considerations in the operation of nuclear power stations.

Many factors such as centrifugal pump start up, valve adjustment, bubble collapse can cause rapid changes of velocity and pressure, resulting in water hammer which can cause severe damage to the piping system including rupture. Conversely, low pressure can lead to tube collapse, and damage to valves and other components.

Auxiliary feedwater system is designed to provide water for steam generators in three scenarios:

  • Normal starting or shutdown of nuclear power station;
  • Work as the backup system when the main feedwater system fails; and
  • Perform its required functions during or after a natural disaster such as an earthquake

Its safety function is to prevent damage to the reactor core and get rid of the heat buildup in the reactor core until the residual heat removal system is put back into operation after a failure.

The inability of this system to operate properly when the primary systems fail can be catastrophic as was evident at the Fukushima Nuclear Power Plant in Japan when it was hit by a massive earthquake and Tsunami.

During the design of the auxiliary feedwater system for the Daya Bay second generation nuclear power station, the Institute of Nuclear Thermal Safety and Standardization at the North China Electric Power University turned to Simcenter Flomaster to evaluate different operational scenarios to minimize the risk of water hammer occurring in the feedwater system.

A feedwater system is designed with two operational flow paths. The primary path is a steam driven pump that is powered by bypass steam from the main steam system. The auxiliary flow path consists of two electrically driven pumps that are configured in parallel.

Each flow path has its own piping system to supply the water to the steam generators. Each piping system has independent control valves and orifices that are used to properly maintain the correct flow rate to each steam generator. Flow paths of auxiliary feedwater system are shown in figure 1.

Figure 1. Flow of auxiliary feedwater system.

The scenarios that they wanted to evaluate were when the main supply of feed water fails, and when the auxiliary system is operational, therefore the primary system is not included in the Simcenter Flomaster model as shown in figure 2.

The specific case that they wanted to investigate was if there was a line failure feeding one of the steam generators. In figure 2 the steam generator that fails is represented by the pressure source no.15.

What the designers wanted to know is: what is the best control strategy for closing valve 14 to prevent water hammer from occurring in the rest of the system?

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