NukeWorker Forum
Reference, Questions and Help => Nuke Q&A => Topic started by: RAD-GHOST on Oct 23, 2003, 03:20
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Mike posted a couple of these a few weeks back. I thought this would be a good one for the group.
Units associated with a Very High Radiation Area - Rad/Hr
This topic was generated from another thread and may make a few people scratch there head and wonder why. The answer is actually quite simple.
As per the definition of a Very High Radiation Area, the unit of measure is Rad/Hr, not Rem/Hr. Besides the definition, a special note also addresses the use of the term " Rad ".
Radiation Area's and High Radiation Area's, use the unit Rem/Hr, not Rad! Since all of these definitions address the concern for Exposure to personel, why does the unit change?
If you get that one, give your best guess why the definition includes the stipulation, " External to the Body "?
Give it your best shot!
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I've been told thatat the higher levels, the math goes out the window, that the conversionfactors are not applicable.... I don't but that. Ergs/gm are Ergs/gm.
However, a VHRA would bot be due toalpha, they can't travel tyhat far... and few Beta's can as well. But those gamma's can.... (with a 1:1 Ration q). This leaves us with those Neutrons with a 10-20 Q.....hmmmmm
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The problem is in converting exposure in air to absorbed dose. At levels associated with Very High Radiation Areas it would be not be necessary to determine the absorbed dose, only that the warning should be in place. If someone is exposed to those levels then medical treatment is going to be undertaken and the actual damage will be the concern, not the precise meter reading in such a high dose field.
The nitty-gritty explanation lies somewhere in Kerma, Bragg-Gray cavity theory, and electronic equilibrium.
Hopefully someone else will chime in with a comprehensive explanation.
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at a time like this, i can only ask, "what happened to moe?"
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Almost off topic....different plants seem to have different ways of deciding what is VHRA.
I recieved a briefing to enter VHRA at quad...I think it was in the circulation pump room?
Anyways, when we finally found the surveys, the highest dose in the room was 150mR/hr.
Smearable was pretty low, but we were crawling all over the top of the pumps (heat was the worst).
What are the "usual" limits for jumping from HR to VHR?
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different plants seem to have different ways of deciding what is VHRA.
Plants can establish any level for a VHRA as long as it at least is equal to or more restrictive than the 10CFR20 or 10CFR835 definition: >500 Rads (5 Grays) in 1 hour @ 1 meter from the source. Facilities are only obligated to at a minimum meet the federal limits. For numerous reasons (many legal) they have opted to be more conservative. Hence, different facilities-different definitions.
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i hate it when this stuff gets started on my friday! let's go with a layman's approach. roetgens are in air. rems are based on a conversion factor, rems/roetgen, based on photon energy in mev. so you go from r to rem.
rad is a measure of energy deposited in matter, a little more technically known as the quotient of the deposited energy by the mass. but a rad is not necessarily a rad, as some radiations cause more damage than others, neutrons vs xrays for example, due to the linear energy transfer. the rbe referenced earlier is used to describe this. thus begat the "q", quality factor which is used to determine the biological injury according to the present conditions. q is a function of the let and is based on the kev/um. this begat the hsubt....yadda, yadda, yadda. why do the regs go to rads at vhra? because they want the personnel involved to be aware of the damage that may be involved!
'n i won't know yer beating me up until i get back frum my weekend adventures! ;)
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Dose rates are in rad per hour for the VHRA because it is outside of the ranges normally considered in radiation protection that use quality factor for the types of radiation and give us rem (rad times QF = rem). So here the 500 rad is just energy absorbed per gram without the quality factor.
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Everyone has been dancing around the answer as it was explained to me back in '92.
If my failing memory serves... the VHRA regulatory posting was not originally considered a posting for the "Commercial Power" nukes, but for the research end of the Nuclear industry. Excelerators(sp?) are concerned more with absorbed dose in matter, rather than the biological effects, ergo RAD/hr instead of REM/hr.
Or so it was splained to me when I was but a wee lad, back in the old country....
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It appears that Slo Glo is pretty close.
It has something to do with the fact that the NRC did not feel it necessary to have to bother with Quality Factors at such high dose rates because, at these dose rates, the biological effects are non-stochastic (the dose can cause serious effects in a short time and the severity of the effects are directly dependent on the dose). I don't know how they arrived at the 500 Rad number, but I guess they HAD to have a number.
At lower (than 500 Rad) doses, the biological effects are supposedly more stochastic (there is a varying chance of a biological effect). The RBEs were used as the basis for the development of QFs because, as we know, different types of radiation, measured in Rads, cause different amounts of biological effect, measured in Rems.
That's the short answer. You'll have to talk to somebody smarter than me for a longer explanation.
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If you get that one, give your best guess why the definition includes the stipulation, " External to the Body "?
Give it your best shot!
'k, not knowng whether i got the 1st part yet, i'll take a flyer at the 2nd..... 'cause that phrase takes medical treatments out of the realm.
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Don't forget "Posting for Potential".
If an area has a chance of going VHRA due to evolutions, it can be posted. The legal posting is, as we know, 500 Rad at a meter.
A year ago I was working a job to remove a HEPA filter that had been let go much too long. 1200 Rad/hr at 1 meter through 3 layers of lead wool blankets.
Interesting job.
Care to guess how it got done for less than 100 man-millirem?
Steven