Weld shape control system for TIG welding of small pressurization hole

A welding system for sealing a pressurization hole in an end plug of a nuclear fuel rod, comprising:

• a weld box for containing said end plug in an inert gas atmosphere having a first pressure, said weld box having a first predetermined volume;
• a welding electrode arranged inside said weld box for melting a weld material inside said pressurization hole;
• means for supplying current to said welding electrode for a brief duration;
• and
• means for reducing the pressure of said inert gas atmosphere outside said fuel rod while said weld material inside said pressurization hole is molten,
• wherein said pressure reducing means comprises a ballast tank having a second predetermined volume at a second pressure less than said first pressure, a valve having an open state and a closed state, said ballast tank being in fluid communication with said weld box when said valve means is in said open state, and a delay circuit for outputting an actuation signal at a predetermined time subsequent to the supply of current to said welding electrode, said valve changing from said closed state to said open state in response to said actuation signal.

A welding system for sealing a pressurization hole in an end plug of a nuclear fuel rod, comprising:

• a weld box for containing said end plug in an inert gas atmosphere having a first pressure, said weld box having a first predetermined volume;
• a welding electrode arranged inside said weld box for melting a weld material inside said pressurization hole;
• means for supplying current to said welding electrode for a brief duration;
• means for reducing the pressure of said inert gas atmosphere outside said fuel rod while said weld material inside said pressurization hole is molten;
• and
• means for outputting a timing signal to said pressure reducing means at the instant when said current is supplied to said welding electrode,
• wherein said pressure reducing means comprises a delay circuit for outputting an actuation signal upon expiration of a predetermined duration of time subsequent to receipt of said timing signal and means for expanding said inert gas in said first predetermined volume in response to said actuation signal using a second predetermined volume in fluid communication with said first predetermined volume.

A method for sealing a pressurization hole in an end plug of a nuclear fuel rod having a plenum space which is in fluid communication with the atmosphere external to said fuel rod when said pressurization hole is unsealed, comprising the steps of:

• filling said plenum space, said pressurization hole and a predetermined volume surrounding said end plug with a gas atmosphere having a first pressure;
• melting a weld material inside said pressurization hole, said weld material closing off said plenum space and said pressurization hole from said predetermined volume surrounding said end plug;
• and
• reducing the pressure of said gas atmosphere in said predetermined volume while said molten weld material inside said pressurization hole is cooling, whereby a seal weld having a concave surface is formed across said pressurization hole.