Plane Source

[sunpaw]

Anyone remember the thumbrule formula for calculating plane sources? I am not looking for the long formula, just the short version that turns into a point source at .7 times the diameter.

Thanks!!!

Roy

[johnnieslingshot]

I am not 100% sure on this since it has been over 13 years since I worked at Charleston Naval Shipyard, but the way we did it there was:  Contact to 0.1r (r = radius) the dose rate remained the same.  from 0.1r - 0.7r the dose rate was 1/3 the contact rate.  Beyond 0.7r it was treated as a point source.

slingshot

[sunpaw]

That sounds mighty close to the way I remembered it. I have all the numbers in my head but forgot how they were interrelated..

Thanks Johnnie!

Roy

[nuke_girl]

Planar or surface sources of radiation can be the floor or wall of a room, a large
cylindrical or rectangular tank or any other type of geometry where the width or diameter
is not small compared to the length. Accurate calculations for these types of sources
require the use of calculus; however, a relationship can be described for how exposure
rate varies with distance from the source.

When the distance to the plane source is small compared to the longest dimension, then
the exposure rate falls off a little slower than 1/d (i.e. not as quickly as a line source).
As the distance from the plane source increases, then the exposure rate drops off at a rate
approaching 1/d2.

1/3 the contact dose rate at a foot is probably your best estimate.

[Adam Grundleger]

I think the thumbrule was 3x from surface to 0.1r, constant 1x from 0.1r to 0.7r, and inverse square (point source) from 0.7 on out.  X is in this case defined as DR at 0.7r.  r is defined as the effective radius.

[Adam Grundleger]

That's probably why nobody really remembers it. 

[sunpaw]

Thats all true, why would a field tech need it, unless that tech was looking at something a rad engineer said it should read and calculated in the long form that it did not need shielding, and then the field tech used the thumbrule, in his/her head, and found the radeng calc was way off base.

you don't need to remember this type of thing when your work is in the same area day in and day out, but when you get into a project that poses something different, these things come in handy as second checks.

its not trivial, its our job.

Roy

[RDTroja]

What does all this mean to a field technician?  I'll tell you.  It means two things if he/she understands the concept:

(1) They'll get two-points towards passing a 50 question entrance exam.

(2) They'll be able to impress their non-nuke friends at the next keg party.

A field tech will not estimate a dose rate by pressing buttons on a calculator.  Never have, never will.  He/she will grab an RO2 or 2A, check the cal date and battery level, make sure it has a current source check and then go out and swing it!

1) If all this means to you is 2 points on a test, then that is probably all you will ever get from it.

2) Your non-nuke friends will not be impressed by this any more than the fact that you are a nuke in the first place. You could make a bunch of words and numbers up and they would never know the difference.

For the average tech, you are probably right -- you don't need to be able to do this calculation, just like you don't need the 6CEN thumb rule, shielding half-value layer calculations, or most of the nuclear science that you find on tests, either. If you want to be an average tech, you can ignore all of that extraneous stuff. If you want to be above average, it certainly helps to know your craft better than the next guy. I have been able to work a wide variety of RP tasks that were 'above average' because I demonstrated a knowledge level higher than the average tech. I happen to enjoy swinging a meter and have turned down ALARA jobs for job coverage slots. But, I have also held ALARA slots, Rad Engineer positions and am teaching again because I could demonstrate an 'above average' level of competence. I get paid more, arrive first, stay longer, and do more interesting work because I took the time to understand all that stuff I didn't need to know. Oh, yeah... I sweat less now, too.

Taking the NRRPT (and passing it) shows that you care about your craft and that you want to know more than just enough to get by. You will not get to practice all the esoteric points often, but when you do it is satisfying that you know how to do it. It also helps when you understand 'why' and not just 'how' you do what you do. Or maybe that is just me.

[Rennhack]

And now, lady's and Gentleman, the topic is "What technicians need on a test." -- Oh, Wait... No! The topic is "What is the thumb-rule formula for calculating plane sources?"

There are a half dozen topics that run on for hundreds of reply's about what technicians need on a test, and what they want on a test.  Your thought provoking question should have been it's own topic, as it stands, you just hijacked this topic.

HydroDave, post that Godwin's law video for us again.

[Rennhack]

Found it.  Please watch this Short educational video.  Pay attention to Step 9.

[M1Ark]

I think the thumbrule was 3x from surface to 0.1r, constant 1x from 0.1r to 0.7r, and inverse square (point source) from 0.7 on out.  X is in this case defined as DR at 0.7r.  r is defined as the effective radius.

I believe this is correct.

[Chimera]

Anyone remember the thumbrule formula for calculating plane sources? I am not looking for the long formula, just the short version that turns into a point source at .7 times the diameter.

Thanks!!!

Roy

"A quick and dirty plane source thumb rule for Co60 energy levels:
Reff = effective radius
from contact to .3 Reff dose rate = 3 x
from .3 Reff to .7 Reff dose rate = x
from .7 Reff out use the point source equation

It worked very well in practical use during spills and work on large source geometries such as tank walls and floors."

After searching through every book and old study guide I could find, I went on the web and found the above information on the RadSafe web site.  I haven't had to use this information since the late 70's when taking the Northeast Utilities tech entrance exam at Millstone Unit I.

As to the comment of never having to use 6CEN . . . The RPM at a PWR comes up to you on the night shift and says the radiographers will be in tomorrow with a 65 Ci Iridium source to do some x-raying on the turbine floor where people go by without dosimetry (the layout of the plant is that way).  Your job is to set up the rad boundaries before the radiographers show up.  That's a real event from 28 years ago . . . and is the reason I know the formula to this day.

Anyone can swing a meter.  I learned how by copying my elders and betters back in the days when I had trouble spelling "RO-2" (actually, it was an RO-1).  Then I started learning my craft - both the available instruments and the working environment.  The ability to anticipate events and, therefore, have the necessary equipment, supplies and manpower for a particular evolution is priceless whether you be a house tech or a road tech.  Not all events are repeat performances where someone will tell you what to have and what to do.

[moabmusher]

Just throwing my 2 cents in on the practical application of this for a field tech.

Thanks for this thread by the way, I had forgotten the exact thumbrule and needed it to direct the craft on the loading of intermodal containers as an RP tech. The intermodals were placed about 18" away from the wall of containment making it very difficult to get a 1 meter reading. The crane in containment is constantly used, and will only lift the containers when scheduled to send them out of containment. Picking it up just to get a dose reading isn't a very efficient way to use time and manpower, and with this handy thumbrule, I was able to predict whether to load some of the higher level material in a different intermodal to stay withing shipping guidelines before lifting the container and finding out the 1 meter readings were over limit and the container having to be set down and repacked.