It's not very difficult, the hardest thing is getting the discipline to make yourself read, read and re-read ALL of the applicable ASME Codes. 

My suggestion, if your involved in welding, read ASME IX FIRST and foremost.  If you can afford it, take one of Walt Sperko seminars.  www.sperkoengineering.com if not, just be prepared to spend a few years learning... well, maybe make that a few decades!  Always keep in the forefront of your thoughts that ASME IX is NOT a stand-alone Code, it is intended to work in conjunction with its constructing counter-codes; ASME III, VIII, B31 series, etc.  ASME IX Rules may be modified, eliminated or added to by the constructing codes so understanding it is just a beginning.

Also, it depends on what type of power plant your thinking about... nuclear?  Almost any Company that hires QC types will have in-house testing and certification.

Good luck, keep us posted!

Everything depends on which direction you wish to go.

A cwi should get you in the door for qc visual weld/certain walkdown crews etc. under an N45/NQA-1 program

Remember there are a variety of nuc certs for qc.. civil, mechanical, electrical, weld with many more under those 4.

I also suggest you read section XI and more importantly, the appendixes. You will find the  base requirements for the PDI (performance demonstrated initiative) UT
You should also have a good understanding of 10CFR http://www.nrc.gov/reading-rm/doc-collections/cfr/part050/part050-appb.html
and part 50.55a http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=6ed032d53fadcc2d9ee0fe34eae5ad75&rgn=div8&view=text&node=10:1.0.1.1.29.0.108.47&idno=10

If your talking about new construction You need to be reading ASME BPV section II, section III, section VIII div 1, section IX, B31.1 to start. Then start chasing the references.

Average age of the experienced and qualified nuclear QC hand is 57, average age of the same for NDE LII's is 40. It's not just the plants that are aging, it's the knowledge base.

Keep in mind when your reading section III that it, as well as XI, etc have mods via NRC mandates, and CFR's. It's a minefield if you go in blind. The best advice I can give you is read up on the NRC and CFR docs first, branch to the codes, and read it all in light of the Appendix B to Part 50--Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing Plants

Regards,
Gerald

To answer your question about the PDI more specifically:

10CFR50.55(a)
( 4 ) By September 1, 2009, ultrasonic examinations shall be performed using personnel, procedures and equipment that have been qualified by blind demonstration on representative mockups using a methodology that meets the conditions specified in (50.55a(g)(6)(ii)(D)( 4 )( i ) through (50.55a(g)(6)(ii)(D)( 4 )( iv ), instead of the qualification requirements of Paragraph -2500 of ASME Code Case N-729-1. References herein to Section XI, Appendix VIII shall be to the 2004 Edition with no Addenda of the ASME BPV Code.

( i ) The specimen set shall have an applicable thickness qualification range of +25 percent to -40 percent for nominal depth through-wall thickness. The specimen set shall include geometric and material conditions that normally require discrimination from primary water stress corrosion cracking (PWSCC) flaws.

( ii ) The specimen set shall have a minimum of ten (10) flaws which provide an acoustic response similar to PWSCC indications. All flaws shall be greater than 10 percent of the nominal pipe wall thickness. A minimum of 20 percent of the total flaws shall initiate from the inside surface and 20 percent from the outside surface. At least 20 percent of the flaws shall be in the depth ranges of 10-30 percent through wall thickness and at least 20 percent within a depth range of 31-50 percent through wall thickness. At least 20 percent and no more than 40 percent of the flaws shall be oriented axially.

( iii ) Procedures shall identify the equipment and essential variables and settings used for the qualification, and are consistent with Subarticle VIII-2100 of Section XI, Appendix VIII. The procedure shall be requalified when an essential variable is changed outside the demonstration range as defined by Subarticle VIII-3130 of Section XI, Appendix VIII and as allowed by Articles VIII-4100, VIII-4200 and VIII-4300 of Section XI, Appendix VIII. Procedure qualification shall include the equivalent of at least three personnel performance demonstration test sets. Procedure qualification requires at least one successful personnel performance demonstration.

( iv ) Personnel performance demonstration test acceptance criteria shall meet the personnel performance demonstration detection test acceptance criteria of Table VIII--S10-1 of Section XI, Appendix VIII, Supplement 10. Examination procedures, equipment, and personnel are qualified for depth sizing and length sizing when the RMS error, as defined by Subarticle VIII-3120 of Section XI, Appendix VIII, of the flaw depth measurements, as compared to the true flaw depths, do not exceed1/8inch (3 mm), and the root mean square (RMS) error of the flaw length measurements, as compared to the true flaw lengths, do not exceed3/8inch (10 mm), respectively.

( 5 ) If flaws attributed to PWSCC have been identified, whether acceptable or not for continued service under Paragraphs -3130 or -3140 of ASME Code Case N-729-1, the re-inspection interval must be each refueling outage instead of the re-inspection intervals required by Table 1, Note (8) of ASME Code Case N-729-1.

( 6 ) Appendix I of ASME Code Case N-729-1 shall not be implemented without prior NRC approval.

Thats where they are getting the PDI from. Sept 9 09 fwd inservice inspections utilizing PDI are mandated in the above noted manner.