RT Source Size?

[Mr168]

Does anyone know what aproximate dimensions would be for a 60 curie Cobalt 60 source? I was doing some unsharpness calcs using our Iridium size for our heavier wall, and we're teetering on the ASME limits with .068. I was under the impression that the source size for Co-60 was a bit larger, but can't get any actual values as the site hasn't picked a sub for their RT yet.

 

[hogan666]

A quick search found this link:

http://www.testima.eu/prilohy/143/infonormy.pdf

 

[unclesyd]

I believe that the sizes are contained in one of the following regulations. Size also depends on the model and or manufacturer of the projector. They are not too many sizes if I recall correctly.
The size numbers I remember using for anything 100 or under Curies was 0.060" x 0.060".

http://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-notices/1987/in87047.html

 

[Mr168]

Perfect, thank you both.

 

[JR97]

Mr168,

Your best bet is to check with your isotope supplier. We use QSA Global, nee. Amersham, nee. Tech-ops. The size of our Co-60 (purchased as 100 Ci) is:

.150 dia
.175 length
.200 maximum (diagonal)

JR97

 

[ndeguy]

Interesting thread and document from hogan666 - the gamma source effective focal spot size is one of the keys to the quality and flaw detectability of a radiograph. Its a pity Agiris dont/cant give actual dimensions for Co60 industrial isotopes - maybe its just impossible to foretell due to the vagaries of the production process. Such information is often difficult to obtain (Sentinel provide Se 75 source dimensions in their catalogue but steer shy of doing the same for either Ir192 or Co60) and if one does get some figures it is impossible to check the veracity of some of the dimensional claims.

Assuming a 60 Curie 2220 GBq Cobalt60 source is cylindrically-shaped (either a monolithic pellet or several wafers pushed together) then one possible way of minimising geometric unsharpness of these higher-activity, larger size isotopes is to ensure that the pig-tail guide tube is facing the weld end-on (rather than transverse) so that the maximum effective source dimension will be the pill diameter.

I am not sure how this will affect radiation output, but at a guess, as each pill is supposedly omni-radiating I would hazard a guess at 'not signifciantly'.

I would like to hear other peoples' knowledge or views.


Nigel Armstrong
Lloyds Register
Independent Verification Body Surveyor

 

[Mr168]

JR97: I actually tried to get in touch with the subcontractor to provide me with the actual dimensions available. Problem is, it turns out the site hasn't even selected an RT sub yet.

I too am curious as to just how much the source orientation would affect GUS. Unfortunately, we're very borderline right now, and the size of our main steam (4.094" wall, 22.5" dia) will be what dictates whether or not we need to perform UT/AUT on site. Which then means getting a P91 dropoff from the fab shop, machining a test block, etc...

That 3mm/.118" value has come up for me in a few different searches now, but who knows at this point. As little difference as .004 in the spec will bump us out.

 

[unclesyd]

JR97,

I believe that is the actual size of the pill/case and not the size of the pellet. The pill varies from camera to camera. A pellet of Co60 that size would be a whopper.
The only time pellet size didn't matter was when we used fish pole radiography.

Here is a very good site on whys and wherefore's of Gamma Radiography.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Physics/GeometricUnsharp.htm

 

[ndeguy]

No, unclesyd, I cannnot believe that is the size of the sealed source - it would be far too small and fiddly to handle when replacing.

According to its literature the Nordion G3 capsule (sealed source) can take Co60 up to 80 Curie (2.96 TBq) and its OD is 5.1mm (0,204 in), length 16.2mm.(0,64"). The Co60 discs have a diameter between 1mm (0,040) and 3mm (0,120) and a height of 1mm. Various diameters will have various activities, but unfortunately that info is not to hand. Stacking the discs will give greater activity. The sketch shows the capsule loaded with 6 discs and stoppered. If that is the maximum number of discs it can take (I dont know that) then fthat could imply each wafer has an output of about 16 curie and even for 60 Curie yourlength would still be 5mm. Now that is far too big for any reasonable unsharpness level if used transverse, giving an effective focal spot size of almost 6 mm (0,240").

After consideration I think that AUT with TOFD would be the way to go if you really wish to detect any defects!!! Good luck!

Nigel Armstrong
Lloyds Register
Independent Verification Body Surveyor

 

[unclesyd]

I just called to check on my notes and find that you are quite possibly correct. The size I quoted was when we had a 30 Curie ¹⁹²Ir camera. They are looking for the Budd manual to verify this. All the information and manuals for the 100 Curie ¹⁹²Ir and ⁶⁰Co60 cameras went with the equipment when we got out the business.

 

[JR97]

Unclesyd,
The dimensions I quoted (.150 x .200) are the pellet size. The dimensions are listed as a specific data entry on the source decay table provided with the source.

Mr168,

What ASME Code are you using? ASME Pressure Vessel Code Sections I and VIII allow larger than specified Ug provided required wire or hole is still visible.

I am running the Ug formula from Section V, greater than 4" thickness the allowable Ug is .07". You will meet that even using the .200 diagonal measurement and having the source at center of pipe.

 

[Mr168]

JR97: B31.1 is the governing code for fab and NDE.

 

[ndeguy]

What technique do you plan on using? Panoramic would seem to be excluded as you will not meet Code Ug requirements on such geometry (22,5" OD/4,094 wt = 14,3" ID?) as the SOD will be 0,5 ID = 7,156". And unless the penny can be placed source side then you will know nothing about the radiographic quality.

ASME Ug values are by definition Lowest Common Denominator - the very minimum acceptable radiographic quality, suitable for detection of workmanship slag and porosity defects perhaps, but under these conditions not lack of fusion or -hush! - cracks. Expected flaw type depends heavily on the welding process. If planar defects are a possibility then a Co60 exposure with an Ug of 0,068 will render any captured image of a planar flaw practically undetectable to the ordinary human eye due to the low contrast across the image.

My view is that for a reaonable POD of planar defects then properly controlled phased array/TOFD/AUT is the way to go.

Nigel Armstrong
Lloyds Register
Independent Verification Body Surveyor

 

[Mr168]

More than likely they will be going AUT or Phased Array on the main steam. As for what method they plan on using, that's for our Level III/QC Manager to call. I think its going to come down to whether they decide to shoot partials or not.

 

[JR97]

Mr168,
ASME B31.1 paragraph 136.4.5 specifically allows Ug of up to 0.07 inches. Even assuming 0.200 diagonal measurement for the Co60 source, you meet your specification with a panaramic exposure. IMHO, RT of the joint would meet all specification requirements (assuming source size of 0.200" or less).

I have to respecifully disagree with NDEGUY, the imposed specification is written to allow radiographs with Ug of up to .07. Therefore, by definition, adequate radiographs can be made with that unsharpness and there is no reason to consider alternative methods. If the specification imposed contained a more stringent requirement, then alternatives would be necessary. Bottom line, just because there may be a potentially better inspection out there is not justification to use it unless contractually imposed and compensated by the customer.

 

[ndeguy]

JR97

My Ug calc for a panoramic exposure was based on 22"OD, 4,094"wt, 14,312"ID and 0,2" effective source size giving Ug = 0,11"

Exchanging 22"OD for 22"ID (30"OD)gives panoramic Ug = 0,074".

If correct then both are outwith Code requirements and both giving poor defect image definition and detection.


I agreee with JR97 that the legal obligation is to meet the contract and specified Code requirements and nothing else. I believe that the customer is entitled to receive what they require and similarly they should pay for that. I also believe that in high pressure safety situations each professional should be enabled to make his/her engineering judgement input to the overall quality of the product. Of course I am aware that this is an ideal and the engineering industry does not run on ideals, but if a better inspection is available then the customer should at least be given the option to use it.

Rant over, good luck with the work Mr168, might even be completed before this thread ends!!!

Nigel Armstrong
Lloyds Register
Independent Verification Body Surveyor

 

[Mr168]

I agree that if it is below that .07 requirement, that it's good enough. We're not one to piss away a LOT of money on uneccessary NDE unless the client tells us otherwise. We'll leave the interpretation on the part of the NDE sub.

Given the difference in opinion of what the source size is going to be, looks like we we're just going to have to wait until they pick their sub and find out the actual size before making the call.