PHWR Historical Problem Areas: Sources of Incidents
The pressure tubes of a CANDU® are in a hostile environment that includes a high neutron flux, hot, high temperature water, and a certain amount of hydrogen and oxygen released by the decomposition of water by radiation.Though the CANDU® has proven itself to be a reliable, cost effective and safe power generation system, there are some known weaknesses considered to be class problems requiring corrective action.
In some cases, the solution has been improved materials, in others it has been increased monitoring and planned replacement before failure. Often a combination of the two techniques has been used. None of the problems are particularly unusual in a complex industrial machine.
The pressure tubes of a CANDU are in a hostile environment that includes a high neutron flux, hot, high temperature water, and a certain amount of hydrogen and oxygen released by the decomposition of water by radiation.
It is not surprising that these tubes suffer degradation over time that has occasionally resulted in a tube breech. As a result of this known problem, the pressure tubes are normally scheduled for replacement at the mid-point of a CANDUs operating life. There has also been a large amount of research and development aimed at developing improved alloys. The Canadians have had to fully support the costs of this research, since the pressure tubes are unique to their reactor design.
Like all high pressure fluid systems, CANDUs depend on pressure relief valves to protect the piping from excessive pressure. Because relief valves must, by nature, balance the need for sensitive response to inputs with the fact that a false alarm could cause a dramatic and dangerous loss of pressure, their design is a challenge.
For example, a failed relief valve was the root cause of the loss of primary coolant pressure and subsequent damage caused during the Three-Mile Island accident.
Although the consequences were far less severe due to better operator and system response, several relief valve problems combined to cause a significant loss of coolant incident at Unit 2 of the Pickering station in 1994.
Improvements have been made to prevent the same failures from recurring, however, relief valves remain an item that requires operator and maintenance action. This is a problem whose solution is sought by a huge number of other interested industries both inside and outside of the power generation field.
The steam generators used for CANDU reactor plants are very similar to the U-tube design used in many pressurized water reactors. They require a good deal of care in construction and operation to allow them to withstand the difficult environment caused by boiling water, corrosion deposits and chemical attack.
So far, however, the operating record of the CANDU steam generators is better than that of many pressurized water generators. The need for replacement before the end of the design life has been rare.