Do you have copy of Section IX?
And are you sure its Section VIII?
U-1 is quite specific in the type of 'steam generating' 'vessels' should and should not be built under Section VIII requirements.

Barbaros,

No enough information to answer your question concerning Welder Performance Qualification.

What country is boiler being manufactured and what country & state is boiler being installed?

As Js55 pointed out U-1 describes the Scope of pressure vessels specific to Sec VIII Div1 Code requirements. 

If you are in the USA, you must have applicable ASME certification stamp which means you have a QA/QC Manual and program in place that has been reviewed and approved by ASME.  AI involvement is required as he/she must approve not only the welding procedures & MTRs but the design & calculations of the boiler and/or vessel that you propose to fabricate.  Also inorder to manufacture said boiler or pressure vessel, your company must have current edition of applicable Code books.

QCRobert