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South_African

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What is considered a contamination area?
« on: Nov 20, 2006, 07:31 »
What are the dpm levels at which you would consider an area to be contaminated at a USA nuclear facility? I have heard >1000dpm/100cm2.

Thanks.

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Re: What is considered a contamination area?
« Reply #1 on: Nov 20, 2006, 07:51 »
That is the normal level used by most plants.  Some facilities use 100 or 400, but they have to count all their smears on a scaler or automatic counter.  You can't get levels that low by counting smears with a frisker.  This is why most facilities choose 1000dpm.
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Offline SloGlo

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Re: What is considered a contamination area?
« Reply #2 on: Nov 20, 2006, 10:21 »
That is the normal level used by most plants.  Some facilities use 100 or 400, but they have to count all their smears on a scaler or automatic counter.  You can't get levels that low by counting smears with a frisker.  This is why most facilities choose 1000dpm.

and that would be ~20cpm.
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Offline Rennhack

Re: What is considered a contamination area?
« Reply #3 on: Nov 20, 2006, 10:35 »
It depends on the nuclide.

Here is an example from Reg Guide 1.86

www.philrutherford.com/Dose_equivalents_of_RG_1-86.pdf

South_African

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Re: What is considered a contamination area?
« Reply #4 on: Nov 22, 2006, 06:47 »
The next question is how do you calculate the dpm/100cm2?

Do you assume a 10% efficiency and a 10% pick up factor for the smear?

i.e dpm/100cm2 = ccpm-bg/ efficiency % x pick up factor

Thanks

Offline Rennhack

Re: What is considered a contamination area?
« Reply #5 on: Nov 22, 2006, 07:48 »
The efficiency depends on the instrument, nuclide, and probe geometry.  No instrument detects a hard bata at the same efficiency as a soft beta.

Try our HP study guide area, in particular, download and read the entire DOE CORE manual.  It will answer all of your questions.

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Re: What is considered a contamination area?
« Reply #6 on: Nov 22, 2006, 09:11 »
and that would be ~20cpm.
That's true, but the manufacturer doesn't admit to any detectability below 100 net cpm.  Even though you can SEE 20 ncpm, it's hard to quantify it because the scale isn't all that easy to read and the needle jumps around a bit.  In order to read a level less than 100 ncpm, you have to hold the sample (smear, hand, shoe, etc.) at exactly the same distance from the detector for a LOOOOONG time.  Even doing so, you would have to measure your average background for a very long time with the same geometry. (i.e. you have to hold a clean smear under the detector for around 20 minutes without moving to get an accurate background reading to use for counting smears)  To accomplish this, you have to have those sample trays and planchettes installed, or have a very steady hand.

This kind of precision always comes at some cost - either more expensive equipment with longer and more complicated counting methods or very difficult methods of using the cheap, simple instrument.  The question then becomes:  How much is it worth?  IS the difference between 1000dpm/100cm2 and 400 really worth all the added expense and trouble?  In my opinion, with the newer, more sensitive protal monitors and body contamination monitors (PCM's, IPM's and the like) you are more likely to need to go down to around the 100dpm level in order to prevent lots of personnel contaminations in "clean" areas.  With a 1000 limit, you can pick up enough contamination in areas that are not posted as contaminated that you will alarm pretty frequently.

We Americans have the tendency to overpost areas.  If we find one smear out of 50 to have >1000 dpm/100cm2 we would almost always post the entire room, elevation, or building at the door rather than put a barrier around the actual contamination.  This is lazy, and tends to allow contamination to spread, but it gives a buffer zone that keeps the unposted areas from getting contaminated.  It also keeps people from having to change in and out of anti-c's many times to work in different areas in the same vicinity.  This is how we have gotten away with the 1000dpm limit up to the moment.

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Offline Marlin

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Re: What is considered a contamination area?
« Reply #7 on: Nov 22, 2006, 10:00 »
What a contamination area is depends on the regulatory agency (NRC, DOE, DOT, Military Branch, Army Corp. of Engineers) the most common for commercial power is 1000dpm/100cm2 for beta-gamma and 20 for alpha. The DOE has a broader isotope specific standard as Mike mentioned earlier. The way the areas are treated are different from licensee to licensee. I worked one facility that had a 1000 limit but required a decon of the area if multiple smears exceeded 100, effectivley mking the limit 100. I worked a facility that posted its area if it had persistent smears greater than 1000 (uranium processor) and only required a decon within 12 hours (no posting) if a clean area smear exceeded 1000. Another facility posted an Equipment Contamination Area, the floors had to be <1000 and gloves donned prior to touching equipment.

South_African

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Re: What is considered a contamination area?
« Reply #8 on: Nov 22, 2006, 10:34 »
Thanks guys for having the patience to help me. That's what makes this nukeworker site so great.

I understand that not all detectors will have the same eff. But I do know that some plants in the USA use a 10% eff - especially for a Ludlum-177.

So let me maybe rephrase the question:

If I have a Ludlum-177 with an eff of 10%
Assuming a 10% pick up factor
I measure 100ccpm
Then would this calculation be correct?

dpm/100cm2 = 100 / 0.1 x 0.1 = 10000 dpm/100cm2

I have downloaded all of the information I can, including the manuals mentioned. I have also been through many websites trying to confirm the above calcuation, but most only calculate dpm and do not calculate dpm/100cm2. 

I have done a calculation from Bq/cm2 to dpm/100cm2 and that works out fine. But when I calculate from ccpm to dpm/100cm2, I am not getting the same answer. And I think its because I left out the pick up factor in the calculation. I was calculating dpm and not dpm/100cm2.

I really hope this is all not sounding to stupid for you guys. But sometimes even the most simple things seem complicated until someone helps you along or points you in the right direction. HELP....





Offline Rennhack

Re: What is considered a contamination area?
« Reply #9 on: Nov 22, 2006, 11:32 »
We typically don't figure any pickup factor in the equation, so if you were assuming a 10% eff, it would be 1k here, not 10k.
« Last Edit: Nov 22, 2006, 11:32 by Rennhack »

South_African

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Re: What is considered a contamination area?
« Reply #10 on: Nov 22, 2006, 02:05 »
Thanks that's where I was going wrong. We use a pick up factor i.e 10% for smears and every time I calculated from Bq/cm2 to dpm/100cm2 I get a factor of 10 higher. So when we calculate 100ccpm (1.67ccps) into dpm/100cm2 instead of 1k such as you get we get 10k. Which makes it confusing when you are trying to compare.

Thanks for the help.

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Re: What is considered a contamination area?
« Reply #11 on: Nov 22, 2006, 08:16 »
Thanks that's where I was going wrong. We use a pick up factor i.e 10% for smears and every time I calculated from Bq/cm2 to dpm/100cm2 I get a factor of 10 higher. So when we calculate 100ccpm (1.67ccps) into dpm/100cm2 instead of 1k such as you get we get 10k. Which makes it confusing when you are trying to compare.

Thanks for the help.

i'm going out on a limb here, but if yer using a 10% pickup factor, then is your equation an attempt to measure total contamination?  usually, stateside and commercial plants, we measure loose contamination via smear and total via direct reading with a probe.  as mike pointed out, we don't factor the pickup or retention rate because if we are to look at the total contamination we use a direct approach too. 
what media do you utilize for assessing large areas?  typically we use what is called a masslin sheet.  it is a cloth fabric that has a very high retention rate and due to the material factors also shows attentuation of the contaminates.  however, for rapid measurement of large areas we will sweep it over an area.  at that point, please remember that we are talking rapid evaluation, it is customary to use a 1:1 factor for cpm to dpm with a gm tube, frisker style probe.  so if you were showing a 3000 cpm rate, then the measure could be recorded as 3000 dpm/100cm2.  please also remember that there may be other considerations involved in this type of measurement and it is not generally considered a final release of a contaminated to clean area survey method.
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South_African

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Re: What is considered a contamination area?
« Reply #12 on: Nov 23, 2006, 04:22 »
Thanks Slo Glo,

That info it really helps with what I am trying to do.

We use the same systems as you do in the USA. Most of our instruments are the same as many of you are familiar with.

For large area we also use a masslin sheet, that same as you guys do. But we don't quantify the masslin. We only use it to detect the presence of contamination and then if we locate anything by masslin, then we will go and quantify with a smear survey. We will not record a masslin on our surveys. Which from what I understand is different to what you would do. If you measure directly from a masslin then you record it in dpm?

The problem I have at the moment is when we get WANO and INPO come to do reviews then they mostly talk in dpm/100cm2. Now clearly if you look at the way we calculate with a pick up factor then we are not talking the same language when it comes to contamination. If you are saying 100ccpm is 1000dpm/100cm2 - then we are calling it 10000dpm/100cm2. And that doesn't make sense. So we need to be talking the same language and that is what I am trying to do - bring our method on line with yours.

So I appreciate the help.

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Re: What is considered a contamination area?
« Reply #13 on: Nov 23, 2006, 10:08 »
Most facilities use Masslinn the same as you do.  However, we will record a negative result as <100ncpm/LAS.  LAS=Large Area Smear.  It is very rare to quantify results from this type of survey, but some people have done the math on this one.
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Re: What is considered a contamination area?
« Reply #14 on: Nov 23, 2006, 11:52 »
[If you measure directly from a masslin then you record it in dpm?

The problem I have at the moment is when we get WANO and INPO come to do reviews then they mostly talk in dpm/100cm2. Now clearly if you look at the way we calculate with a pick up factor then we are not talking the same language when it comes to contamination. If you are saying 100ccpm is 1000dpm/100cm2 - then we are calling it 10000dpm/100cm2. And that doesn't make sense. So we need to be talking the same language and that is what I am trying to do - bring our method on line with yours.
[/quote]

recording masslin is a site specific method.  so it would depend on how that site dictates. i have recorded masslin results both ways, however a common denominator is that it is, as beer court said, per las. 

if i say that 100ccpm is 1000dpm, then it is 1000dpm/unit.  dpm is a function of counts/efficiency, and is source dependent.
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