HP Essay question.

[Rennhack]

Today is October 3rd, 2004.  You are an ALARA engineer in a pre-job briefing. You have been informed that a NDE tech has dropped his source through a grate in the floor in the Rx Building.  It has landed on a 3 inch valve with that sits one and a half feet behind a half inch wall of lead which has manipulator arms.  You are familiar with the valve.  It is shielded because inside it is 10 grams of Cobalt 60.  You also know that it is about a ten min. journey from the air lock.

The Rx is at 50% power. Remote detectors tell you there is a neutron field in that area reading 30 mRad/hr.  The source certificate the NDE tech hands you states that on September 11th, 2001 his Ir-192 source had an activity of 1.4 Curies.

An operator in the room tells you that he is familiar with the remote manipulators in that location, because he uses them on a regular basis to operate the valve.  He estimates that he can retrieve the source, and deposit it back in its shielded container in fifteen minutes.

What would the total man rem exposure for this job be?

[wave_theory]

Zero.  Job cancelled when I calculate Ir-192 source activity.  NDE tech fired.

[Rennhack]

Zero.  Job cancelled when you calculate Ir-192 source activity.  NDE tech fired.

Wrong.  667 mRem is hardly a job stopper.

MeV      &  Yield for Ir-192            
0.316      0.87            
0.486      0.52      
0.308      0.32      
0.295      0.3      
      

6CEN = 6 Rem/hr @ 1 foot
667 mRem/.25 hr @ 18 inches
 (Still not taking into account the shield.)

HVL for Pb
0.5 MeV = .38 cm
1.0 MeV = .86 cm
1.5 MeV = 1.2 cm
2.0 MeV = 1.3 cm

I=I_O(.5)ⁿ

Need more help?  I thought 'yous-guys' were smarty-pants.

[maxparity]

Mr. Rennhack,
           I agree with you that some of "uss guise" wear trousers of at least a fair degree of intellect("smarty pants").Being that as it is or is not, I will suggest to you sir that if the given data you put out is indeed the data to be used in your little brain teaser,then before you offer up your massive intellect to help us lesser gifted than yourself,you may consider decay-correcting your source prior to using it in the rest of your calculations.Although I had to make an assumption as to the deposition time of the Cobalt in the valve as it was not given,I have that answer directly in front of me in case you encounter any further difficulties.Have a  great day and Karma to all.  

[Rennhack]

When I said

"Need more help?  I thought 'yous-guys' were smarty-pants."

  I was only trying to stimulate interest in the question, to challenge others, I was not trying to be a smart ass, or offend you.  

As for decay correction...I only gave an example, not the answer... I was showing that the exposure was not great. (Hence, I neither decay corrected, nor took into account the shielding.)

It is a question to help those that wish to exercise their minds the ability to do so.  No one is required by law to try to answer the question.  Attitudes should be left at the door.


FYI:
Strong Gamma-rays from Decay of Ir-192
This table was derived from ENSDF as of September 1993.

   Gamma-ray
   energy(keV)  Intensity(%)  Decay mode / Half-life
   -------------------------------------------------------
    56.68            --           IT        1.45     M
   295.96            --           B-        1.45     M
   316.51            --           B-        1.45     M
   612.47            --           B-        1.45     M
   -------------------------------------------------------
   155.16            --           IT        241      Y
   -------------------------------------------------------
   205.8             3.3          EC        73.831   D
   308.46            30.          B-        73.831   D
   316.51            82.8         B-        73.831   D
   374.49            0.72         EC        73.831   D
   468.07            47.8         B-        73.831   D
   484.58            3.18         EC        73.831   D
   -------------------------------------------------------

[Rennhack]

I had to make an assumption as to the deposition time of the Cobalt in the valve as it was not given.

So, what is the deposition time you presume, so that we may all use the same variable?

[wave_theory]

Well Mike, the half life of Ir192 is 74 days.  The final activity I arrived at was 0.00004 Ci.  Using the 6CEN formula I come up with way less than 1 mR/hr at 1 ft.

My point was that the source activity is insignificant compared to the background dose rates.  Why retrieve it now?

[Rennhack]

Well Mike, the half life of Ir192 is 74 days.  The final activity I arrived at was 0.00004 Ci.  Using the 6CEN formula I come up with way less than 1 mR/hr at 1 ft.

My point was that the source activity is insignificant compared to the background dose rates.  Why retrieve it now?

OOOOhhhhhh.  My mistake.  I took it the other way.  Sorry.

[RAD-GHOST]

Co-60

1 Gram = 1,100 Curies

1 Curie = 1 Rem/Ci at 1 meter ( Point Source )

[Jr8black3]

Mike all I have to say is you came along way from being an old deconner at DC Cook..

 About the essay question, personally I'd just look at the HP and say it's your problem now, you deal with it, and I'd leave and go on break..

 Always practice ALARA,, Always let another run ahead!!!

 

[maxparity]

This actually turns out to be a somewhat interesting scenario.As far as a deposition time-10.54 years for the whole 10 grams would get the doserate at a 100' from the manipulator arms < an R. The BIG variable is how directional are the beams from the Cobalt? If the load is blasting the shield wall directly,then we are roughly looking at 159 R/min. at the arms.Kinda makin the Ir source and the 0n1's negligible. Great stuff-sorry for the previous attitude-an operator was giving me the business at the time.

[Phideaux]

Excellent thought-provoking question, Mike! We could use a few more of these now and then. I'd dig in and offer an answer, but I have to do laundry.  

We actually ran into something similar to this at Besse years ago. The source was hotter, but we didn't have to deal with power dose. We were able to "sneak" up on it with a cart full of lead blankets and tuck it away safely. I wasn't in on the ALARA engineering portion of the job, but we picked up a lot less dose than they calculated. Made for an exciting evening......unless you were an MQS Radiographer!

Do another one, Mike! One day when I have time, I'll do one about an 80Ci (?) Ir192 source that was vaporized inside of a tank at an oil refinery that was under forced ventillation with the exhaust stack discharging about 30' off the ground. One of the tricks will be to try to calculate the plume. Oil refineries must generate odd counter-currents.

Gotta go. Time to load the dryer!

[RAD-GHOST]

Mike, a great question to make people think, size doesn't matter.  Obviously the radiographer source is of little concerned.  The 10 grams of Co-60 seems like a very small number, until you apply the specific activity associated with it! Maybe some people can't appreciate the curie to gram aspect.  One pound is 454 grams.  One pound of C0-60 is equal to 500,000 curies, 7,500,000 REM @ 1 foot, point source!  I hope I did the math right.  Here is a great site to check out for quick calc's on specific activities and weight/volume distributions, eetcorp.com.

[SloGlo]

boy, am i glad this got settled over the weekend.  i saw it friday, 'n since i do my utmostest to knot think ona weekend, i only played with it a little bit.  the co gram thing bothered me as i knew i would hafta look that  up monday am.  also, since itz been awhile since i was in a plant mode, the gamma/neutron percentage o/s the biodhield was another tickler to me.  i had anudder thought of " how long would it take to stage the source container inside the shielded area for the operator to deposit the ir source into (10 min job) 'n retrieve it after depostition... that might be the most mitigating scenario to blow the job offa the planning board".  now ya know why i try not to think on the weekend, so thanx to all that i don't hafta do the mental acrobatics.

[DeathDose]

Screw the exposure.  Send in a green junior and just bury him later.

[nowhereman]

What exactly are they doing with only a 1.4 curie source in ctmt at power?  NDE on copper tubing? and I can't remember the last time I saw manipulator arms in a CTMT. and it falls behind shielding?  

[Rennhack]

What exactly are they doing with only a 1.4 curie source in ctmt at power?  NDE on copper tubing? and I can't remember the last time I saw manipulator arms in a CTMT. and it falls behind shielding?  

It's meant to make you think.... it wasn't a real story.  BTW, if you were paying atention, it was 1.4 Ci a LONG time ago, realitive to its half life, just one aspect people are meant to catch.

[SloGlo]

and I can't remember the last time I saw manipulator arms in a CTMT. and it falls behind shielding?  

nowhereman nails the answer!  look up the dose received on the job to install the shielding/manipulator arms, because obviously they were installed post-depostition of the 10 gms co60!  tack on the neutron 'n yer happening baby!  karma to the nowhereman! [smiley=beerchug.gif]

[HousePuke]

The total accumulated dose should be zero.  Just have them leave their dosimetry at the control point.

Badges, we don't need no stinking badges!

[Rennhack]

These questions are straight forward:

At what energy can a photon lose at most one-half of its energy in Compton scattering?

What fraction of the energy in a 10 keV X-ray beam is deposited in 5 mm of soft tissue?

[RP Instructor]

"At what energy can a photon lose at most one-half of its energy in Compton scattering?"

1.022 MeV?

[Rennhack]

"At what energy can a photon lose at most one-half of its energy in Compton scattering?"

1.022 MeV?

You may be thinking of Pair Production.

You need to take into account electron recoil, fractional energy loss, scatter angle...

[darkmatter]

Yes, Yes, as a theoratical question this is fine, most rad geeks with the Rad Health Handbook and a calculator will come up with a answer that is within reason depending on their assumptions.
However, I see two possible scenarios to a dropped source thru a grate:
1. The Really Poor Manager (RPM) catches wind of it and requires a Plant Problem Report, forms Alara Fast Action Response Team. Has the Rad Engineer recalculate everything plus Fax from the original manufactor. Directs a resurvey over and over then the committee decides to contract a vender in so as to be able to point fingers if anything goes wrong with the recovery. Everyone returns to "Cubicle World" once again their jobs secure.
2. The Field Technician fabricates a duct tape tip on a broom stick, applies the goop from a mouse glue trap, reaches thru the grate . Sticks the source on the tip and places it in the bucket he has prepared with a lead blanket wrap. Sprays the tip with WD-40 to release the glue and hands it to the Radiographer and says "Here you go Pal, don't let it happen again" The Radiographer buys the whole RP crew drinks at the bar that night telling the story over and over how the RPs saved his butt. The RPM catches wind of this and has a Plant Problem Report written for violating 3 Procedures, 2 Shopguides, Chemical Permits and (Gasp!) Management Expectations which results in disciplinary action up to and including Termination of the RPs.

[SloGlo]

darkmater.... yinze bin a bad boy one too many tymes.  karma 2 ya.

[Rain Man]

GK DarkMatter.  You understand the difference between the right answer and the true answer.