Hi Sun riser,
There are multiple links to refer. Hope this will help.
How to handle scrap occured during processing
Movement type 601 is used to issue goods in sales order. In that case I assume you must have sales order for scrap.
Scrap is being maintained with 551 through MB1A. But if that is a by/co product, will be maintained in BOM with negative qty with 531 movement type. That also can be scrapped later on.
Regards,
Krishnendu.