In boiling water reactors (BWRs), oxygen and hydrogen peroxide in the reactor coolant promote stress-corrosion cracking (SCC) of reactor internals and other components in contact with the reactor coolant. These two gases, as well as hydrogen, are created through radiolytic decomposition of the reactor coolant. Experience shows that, over the operating lifetime of a BWR plant, there is a steady increase in SCC attack and in the rate of crack growth, even though this increase is very slow and easy to predict.

New Gas Generator Needs No Auxiliary Equipment
Continuous injection of hydrogen into the reactor feedwater is an effective method for combating such corrosion of the reactor vessel internals. There are basically two different ways of generating and injecting the quantity of hydrogen required in the reactor water as well as the oxygen subsequently needed for recombining the excess hydrogen in the plant's offgas system. In conventional systems these gases are either procured from an off-site source or generated on-site using standard electrolyzers, and then stored in tanks. The gases are distributed by a central gas supply system to the reactor feedwater system and the offgas system.
       In the simple and inexpensive system developed by Siemens, both hydrogen and oxygen are generated in the feedwater system itself, without any need for auxiliary equipment. Part of the feedwater is continuously bypassed through the gas generator where it is electrolytically decomposed on proton-exchange membranes (PEMs) to produce the amount of hydrogen presently required for injection. Downstream of the gas generator, the hydrogen and oxygen then are directly fed into the relevant plant systems.

OSART Identifies Gas Generator as "Good Practice"
In the spring of 2000, a Siemens hydrogen/oxygen gas generator of this design was installed in the feedwater system of Switzerland's 372-MWe Mühleberg BWR plant, which is operated by the Swiss electric utility Bernische Kraftwerke (BKW FMB Energie AG). The new generator successfully completed the first part of its trial run in July 2000 before the start of the plant's annual refueling outage. This advanced membrane-type gas generator, the first of its kind in the world, has continuously generated up to 10 Nm3/h of hydrogen and 5 Nm3/h of oxygen since October 2000.
       The customer is extremely satisfied with the new system's trouble-free and reliable performance as well as the high degree of user-friendliness afforded by the generator's electrical and control equipment.
       In addition, the Operational Safety Review Team (OSART) from the International Atomic Energy Agency (IAEA) that visited the plant for three weeks in November 2000, pronounced the new gas generator installed at Mühleberg, a "good practice." This system provided by Framatome ANP, offers similar benefits to all BWR types worldwide.

Benefits of Framatome ANP's Gas Generator

• Simple system configuration; i.e. no need for demineralized water supply, cooling water system or compressors
• Compact modular design permits the generator to be installed in the turbine building
• High degree of safety:
  - No need for hydrogen and oxygen shipments
    or gas storage tanks
  - Low operating pressure
  - Short pipe runs
  - Simple monitoring equipment
  - Easy-to-perform accident analyses
  - No need for additional physical security measures
• Low operating and maintenance costs.

The new 100-cell gas generator installed inside the turbine building at Mühleberg. The two compact units each contain 50 electrolytic cells, and each is assigned to one of the plant's two turbine generators.