Re: Abnormal Event ALARA Optimization

["Janet Westbrook" <janet.westbrook@HOME.COM>]

Tom --



I seem to recall that in my files I have a couple of articles from

conferences where people talk about your question. I think ICRP has weighed

in one this. Below are some thoughts that I have.



When you say "abnormal", do you mean "off-normal" or "accident" or both?



My frequently reiterated opinion is that ALARA should not be applied to

accident situations. Once you have provided measures to reduce the accident

doses to below regulatory limits, perhaps plus a judicious selection of

defense-in-depth measures, you should stop. This is due to the fact that the

accident situation is so improbable that resources should not be spent on it

past the limits; the resources should be spent on "the dose you know you're

going to get", i.e., the normal and anticipated operational occurrence

doses.



So ALARA should be applied to normal activities and to anticipated

operational occurrences; also, good practice measures should be employed

analoguously to the defense-in-depth measures. The good practice measures,

such as unrequired redundancy of a monitor, should help minimize the effects

of an unanticipated but not accident-level occurrence. I believe that for

the normal and AOO doses, you should not multiply by the probability of

occurrence. This is a simplification, but again, if the probability of these

doses ranges from certain or reasonably likely (5-10%?) during the lifetime

of the facility, then there would not be a strong need to adjust for

probability. The probability is usually just a rough estimate anyway.



As we all realize, it is hard to decide how to optimize when the situation

is one where there is not a direct condition-to-effect link. That is, if we

have a person in a known gamma field, we can mak some assumptions about

exposure time, etc., and can easily calculate a conservative but realistic

dose for him. We can then do the optimization on the basis of that effect,

i.e., that dose. But a situation in which you have surface contamination

that does not readily translate into an airborne concentration, or in which

there is the potential for an unquantified release that thus results in an

unknown airborne concentration, is hard to optimize on. I suggest that you

use conservative but rationally realistic assumptions (I hope that makes

sense!) to calculate a dose based on the surface contamination or release

getting into the body. You can do a bounding calculation with, say, the

entire release and if the dose results aren't too alarming, you can use them

to optimize(with suitable caveats, of course). If they are alarming and

would thus require expensive measures to mitigate, you would need to sharpen

your pencil and make more realistic assumptions in order to see whether the

expense is truly justified.



I gave a course at the February Health Physics Society meeting. It was not

about accident stuff per se, but your question below is one of the reasons

why I titled the course "ALARA: Art and Science". The area between, say, the

"perhaps-once-or-twice-during-the-40-year-life-of-the-facility" occurrences

and the DBA is especially murky and requires careful judgment as to whether

and how to optimize.





Janet Westbrook



----- Original Message -----

From: "Goff, Tom" <gofft@WIPP.CARLSBAD.NM.US>

To: <radsafe@list.vanderbilt.edu>

Sent: Monday, July 30, 2001 8:56 AM

Subject: Abnormal Event ALARA Optimization





>

> Does anyone have any references/opinions as to ALARA optimization for

abnormal

> events?

>

> In general, optimization says that an the cost of an ALARA activity/dose

> saving component is justified if that cost is less than the dose averted

times

> a $/person-rem value.

>

> Is not the dose averted a function of the probability of the occurrence of

the

> problem that causes the dose?  If a particular abnormal event has a dose

> consequence of 10 rem but the probability of occurrence is only 1/20

years,

> what value do you use for the dose averted calculation?  10 rem or 10 rem

/20

> years or 0.5 rem?

>

> Any thoughts/references???

>

> Tom Goff

> WIPP ALARA Coordinator

> WIPP Radiological Engineering

> (505) 234-8861

> (fax)  234-6027

> e-mail  GoffT@WIPP.Carlsbad.NM.US

> page (505) 234-8850  (pager 479)

> P.O. Box 2078

> Carlsbad, NM 88221

> Radiological Engineering:  Anticipating Radiological Problems

>              Developing Techniques to Deal with Them

>

>

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