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Technical Discussions
Richard Saints
Other, Piping and Welding Inspector
Romania, Joined Oct 2015, 7

Richard Saints

Other, Piping and Welding Inspector
Romania,
Joined Oct 2015
7
10:30 Oct-15-2015
Barriers for Ir192 (HVL)

Hello,
I'm new in this forum and I would like to know if someone can help me understand how to calculate the HVL thickness.
I'm working as a Piping and Welding Iinspector in a project in Norway and the NDT company gave me a calculation for barriers to do x-ray in the plant and I just want to confirm that they are doing everything right... I will try to explain the situation:

They say that with a Ir192 source with 30Ci at a distance of 25meters of the barriers they need approximately 190mm of steel to secure the dose rate for general public.
I need to know how can I calculate the thickness of Steel I need to have to reduce the dose rate from a source of Ir192 with 30Ci for general public.
Best regards

Shankar Arumugam
Other, NDE Engineer
India, Joined Oct 2013, 39

Shankar Arumugam

Other, NDE Engineer
India,
Joined Oct 2013
39
11:26 Oct-17-2015
Re: Barriers for Ir192 (HVL)
In Reply to Richard Saints at 10:30 Oct-15-2015 (Opening).

The radiation intensity @ 25m, for 30Ci Ir192 source is 24mR/Hour.

To reduce the 24mR/Hour to General public dose

Required thickness =Tenth value thickness (TVL) x log (RF)

RF (Reduction Factor) = Radiation Level without Shield (24mR/H)/Radiation Level with Shield (General Public dose limit).

Tenth value thickness (TVL) for steel is 40mm.

Eric
Eric
04:41 Oct-21-2015
Re: Barriers for Ir192 (HVL)
In Reply to Richard Saints at 10:30 Oct-15-2015 (Opening).

What is the maximum General Public Dose per hour in your jurisdiction?
What is the maximum dose total applicable for the duration of the exposure?

Cheers!

Richard Saints
Other, Piping and Welding Inspector
Romania, Joined Oct 2015, 7

Richard Saints

Other, Piping and Welding Inspector
Romania,
Joined Oct 2015
7
09:53 Oct-22-2015
Re: Barriers for Ir192 (HVL)
In Reply to Eric at 04:41 Oct-21-2015 .

Hello,
First of all thank you for the help Shankar and Eric!
Eric,
In Norway the dose limit is the one used in Europe.
I check and I can you see the dose limit for year.
General Public :
1 mSv/year effective dose
50 mSV/year equivalent dose (Skin, arms, legs)
15 mSv/year equivalent dose (Eyes)
Does this tell you something? I'm sorry for not giving you the information you ask but I just cant find it...
Thank you again for all your help.
Best regards

jon wallis
NDT Inspector, -
Netherlands, Joined Feb 2010, 626

jon wallis

NDT Inspector, -
Netherlands,
Joined Feb 2010
626
10:52 Oct-22-2015
Re: Barriers for Ir192 (HVL)
In Reply to Eric at 04:41 Oct-21-2015 .

According to the Norwegian Radiation Protection Authority the barrier maximum is 7.5µSv/hr. This level is not standard throughout Europe unfortunately, different European countries have different maximums.
I believe that this is due to different ways of calculating down from the starting point of 1mSv/year.

Richard Saints
Other, Piping and Welding Inspector
Romania, Joined Oct 2015, 7

Richard Saints

Other, Piping and Welding Inspector
Romania,
Joined Oct 2015
7
11:36 Oct-22-2015
Re: Barriers for Ir192 (HVL)
In Reply to jon wallis at 10:52 Oct-22-2015 .

I'm sorry Jon but can you please tell me were did you saw that value?
I check with NRPA and the Regulations on Radiation Protection and Use of Radiation says this:
The dose limit for ionizing radiation regarding the general public and employees who are not occupationally exposed, is 1mSv/year.
The dose limit for occupationally exposed employees over the age of 18 is 20
mSv per calendar year.
Thank you very much for your help
Best regards

jon wallis
NDT Inspector, -
Netherlands, Joined Feb 2010, 626

jon wallis

NDT Inspector, -
Netherlands,
Joined Feb 2010
626
17:48 Oct-22-2015
Re: Barriers for Ir192 (HVL)
In Reply to Richard Saints at 11:36 Oct-22-2015 .

If you need confirmation of this, I suggest you make contact with an ndt company and ask their radiation protection supervisor (or whatever the Norwegian equivalent is) what the levels are.

Richard Saints
Other, Piping and Welding Inspector
Romania, Joined Oct 2015, 7

Richard Saints

Other, Piping and Welding Inspector
Romania,
Joined Oct 2015
7
18:18 Oct-22-2015
Re: Barriers for Ir192 (HVL)
In Reply to jon wallis at 17:48 Oct-22-2015 .

Thank you Jon!
I asked NRPA on this matter and they sent me 2 documents:
In this documents the dose limit is the one I said but I will contact them again to see what they say...
Thank you again for your help Jon.

Eric
Eric
23:45 Oct-22-2015
Re: Barriers for Ir192 (HVL)
In Reply to Richard Saints at 18:18 Oct-22-2015 .

For your reference, I can explain how the numbers may be calculated. Please note that the numbers may fluctuate a little because of the slightly different intensity values used by different agencies for the same isotope. The science does not change, but you may see small differences in safety distance, for instance.

I will stay in the SI system for the purpose of this discussion.
A 30 Ci source has an activity of 1110 GBq in SI units.
The specific activity of Ir192 is 0.16 mSv/hr at 1 m per GBq.
Therefore, your 1110 GBq source has an unshielded intensity of 177.6 mSv/hr at 1 m.
Adding steel shielding will reduce the intensity by ten for every 40mm of steel. (TVL of 40mm)
This new intensity will reduce by four for every doubling of the distance (inverse square function).
I hesitate to do the math for you, given the uncertainty about the exact limits in your jurisdiction but it would seem that placing 190mm of shielding 1m away from the unshielded source of radiation will reduce intensities to 0.005 mSv/hr or below at 25m. That would meet NRPA guidelines as I read them. Note that HVL and TVL numbers vary depending on the source of the information.

Norway has extremely tight limits for this, which accounts for this requirement. Have you considered carrying out the procedure while the building is empty or using lead shielding?

I must also point out that most radiography is carried out with the use of a collimator, which reduces the intensity of the fully projected source by a factor of approximately 30. Although many operators calculate safety distances based on the use of the collimator, there are a few situations when this will result in excessive exposure to the public, for instance in the event of a malfunction or in the event that the exposure requires the collimator aperture or source hole to point at the public. It is always safer to assume that there is no collimator being used, as in our previous calculation.

Hope this helps!

Richard Saints
Other, Piping and Welding Inspector
Romania, Joined Oct 2015, 7

Richard Saints

Other, Piping and Welding Inspector
Romania,
Joined Oct 2015
7
09:25 Oct-23-2015
Re: Barriers for Ir192 (HVL)
In Reply to Eric at 23:45 Oct-22-2015 .

Good morning Eric and thank you very much for your help!!!!

You are right, in Norway they have very rigid rules in all safety issues and I need to be sure that the NDT company is doing every thing right because if anything goes wrong they will call my name and I this is not really my area...

Just tell me one thing and I'm sorry for my ignorance but that thickness would be the same if the steel barrier was at 25 meters?

One thing that confuse me is that they establish an dose limit in mSv/year but the calculations are in mSv/hr...

Thank you again for the explanations and help all of you are giving me!

Eric
Eric
18:06 Oct-24-2015
Re: Barriers for Ir192 (HVL)
In Reply to Richard Saints at 09:25 Oct-23-2015 .

Good day Richard.
You get more attenuation for the same safety distance if the shielding is at 25 m from source because distance has already attenuated the radiation significantly by then. Everyone needs to be behind the shield though, so you may need a huge shield to protect everyone. It is usually easier to place a huge chunk of steel close to the source for that reason. An effective shield is something like a bulldozer or a crane, if you have the room. People have also used portable cement barricades, water barrels, pallets of steel castings, metal recycling dumpsters or simply loads of earth.
There are two limits at play:
The dose rate in mSv/hr sets a limit of the intensity of radiation at any given moment. That is what shielding will attenuate in the short term.
The cumulative dose sets a limit on the quantity that can be received over the longer term. This will depend on the above dose rate and on the length of the exposure.

New calculations:
Intensity (unshielded) at 25m: 0.284 mSv/hr
Intensity after 76mm of steel: 0.005 mSv/hr

As you can see, the public would be safe just behind the steel, but at significant risk if in direct line of sight at 25m. Your contractor may dispute any of my figures due to their own procedures and company regulations. Would the contractor not be liable for any breaches of safety standards, instead of you? Based on the information you provided, the contractor wishes to place the shielding close to the source, which would also be my first choice.

Shankar Arumugam
Other, NDE Engineer
India, Joined Oct 2013, 39

Shankar Arumugam

Other, NDE Engineer
India,
Joined Oct 2013
39
09:18 Oct-26-2015
Re: Barriers for Ir192 (HVL)
In Reply to Richard Saints at 09:25 Oct-23-2015 .

If the allowable does limit to general public is 0.005 mSv/Hr,
Radiation intensity @ 25 m, for 30Ci source is 0.24mSv/Hr.

RF (Reduction Factor) = Radiation Level without Shield /Radiation Level with Shield (General Public dose limit).
RF = 0.24/0.005 = 48

Required thickness =Tenth value thickness (TVL) x log (RF)
=40 x Log(48)
=40 x 1.68 = 67 mm (is enough)

If you have further confusion then calculate in simple with half value thickness (12.5 mm for steel for Ir-192).
Initial intensity - 0.24 mSv/Hr
1) 12.5 mm steel - 0.12 mSv/Hr
2) 12.5 mm steel - 0.06 mSv/Hr
3) 12.5 mm steel - 0.03 mSv/Hr
4) 12.5 mm steel - 0.015 mSv/Hr
5) 12.5 mm steel - 0.0075 mSv/Hr
6) 12.5 mm steel - 0.00375 mSv/Hr
So it found to reduce 0.24 mSv/Hr to 0.005 mSv/Hr we need ~75 mm of steel for Ir-192.

Richard Saints
Other, Piping and Welding Inspector
Romania, Joined Oct 2015, 7

Richard Saints

Other, Piping and Welding Inspector
Romania,
Joined Oct 2015
7
12:34 Oct-26-2015
Re: Barriers for Ir192 (HVL)
In Reply to Shankar Arumugam at 09:18 Oct-26-2015 .

Good morning
First of all thank you all again for the help.
Eric thanks a lot for the explanations!
They are liable to breaches but I just want to be sure of this because its a very small plant (25m2X30m2) and there are an other plant stuck to this one and I'm trying to see all of the hypotheses I have...
I understood all the calculations and explanations and I just want to ask one more thing...
Is there factor or a formula to calculate this values using a collimator?
Thank you again for the help!

jon wallis
NDT Inspector, -
Netherlands, Joined Feb 2010, 626

jon wallis

NDT Inspector, -
Netherlands,
Joined Feb 2010
626
13:05 Oct-26-2015
Re: Barriers for Ir192 (HVL)
In Reply to Richard Saints at 12:34 Oct-26-2015 .

It depends on the collimator shielding type and thickness and, more importantly, the direction of the exposure.
Knowing the half value layer thickness is only fully relevant if the material is completely surrounding the exposed source on all sides. If you place a flat piece of steel 190mm thick (how will you transport it?) in front of the exposed source you will still get radiation scattered from the surrounding structures and this cannot be calculated.
Why isn't the ndt company proposing to use lead as a shield? This can be placed as a 'blanket' over the weld being examined and would be more effective. Lead is used widely in radiography for this purpose.

Shane Feder
, Quality Co-ordinator (SubSea)
Thailand, Joined Dec 2014, 91

Shane Feder

, Quality Co-ordinator (SubSea)
Thailand,
Joined Dec 2014
91
14:07 Oct-26-2015
Re: Barriers for Ir192 (HVL)
In Reply to jon wallis at 13:05 Oct-26-2015 .

Richard,
Really think you are getting carried away with something that is well outside your scope of work.
Why do you need to confirm what they are doing ?
You are a Piping and Welding Inspector, not an RPO or Safety Officer.
You are quoting milli Sieverts per year but that is for long term exposure and has nothing to do with short term high levels of radiation.
Find out what the maximum dose allowable at the barriers is (Jon Wallis gave you some info), borrow a dosemeter from the NDT techs and patrol the barriers to ensure they have set them far enough away.
Ensure you take readings when the collimator is pointed directly at you, not downwards or in the opposite direction.
I speak as a Welding Inspector of 15 years and a Radiographer of 8 years.

And remember - if you get into a major contractual fight with the NDT company stating "I read about it on an internet forum" will not go down very well.

Cheers,
Shane

Richard Saints
Other, Piping and Welding Inspector
Romania, Joined Oct 2015, 7

Richard Saints

Other, Piping and Welding Inspector
Romania,
Joined Oct 2015
7
17:12 Oct-26-2015
Re: Barriers for Ir192 (HVL)
In Reply to Shane Feder at 14:07 Oct-26-2015 .

Shane,
First of all thank you for your help.
Everyone should know something about radiation safety if you are in the welding area and that's what I trying to get with all my answers... personal knowledge from experienced NDT technicians that write in this forum.
Of course I will not discuss or argue something that its not my area with the NDT company or even with the contractor but with this explanations I will have some bases to know what is being discussed in the meetings and to check some calculations. because the NDT company will send me all the calculations and I need to send them to the Contractor but I need to be sure before doing that.
Again thank you for your help.

Andreas
NDT Inspector, Specially in the oilfield
Tuboscope Vetco(Deutschland)Gmbh, Germany, Joined Oct 1999, 131

Andreas

NDT Inspector, Specially in the oilfield
Tuboscope Vetco(Deutschland)Gmbh,
Germany,
Joined Oct 1999
131
09:35 Oct-28-2015
Re: Barriers for Ir192 (HVL)
In Reply to Richard Saints at 17:12 Oct-26-2015 .

Gents ,
for radiation safety you must have attend a course and do a exam acc to european laws.
You need have the cert acc to the national law of the countryyou working in.Must are very similar but may some local laws are differrent.

In germany we dived the min aloud level of radiation for public and inspector.

You nee to keep the control area less than 40µSv/h
if higher you need to make a barrier.

With the 60Ci Source , we need to convert to GBq 1CI = 37 GBq.
37x30= 1110 GBq = 1,1TBq
Acc to DIN 54115-1 I see a distance off app 80m.

Regards

Andreas

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