Answer to Question #129 Submitted to "Ask the Experts"

Q

If a TLD is exposed to a light bulb or a bright flash such as that from a camera, is that exposure equivalent to some gamma ray exposure which is above the regulatory limit? Also, how safe is the water in a spent fuel despository pool in a reactor--like the glowing water that is seen so often in reactor advertisements?

A

Question No. 1 Some TLD phosphors are light sensitive and those will have a light-tight covering. In some cases the binder that holds the phosphor may be light sensitive and it would require protection against light. For commercial TLD suppliers there should be no light contribution that would appear to be a gamma-ray exposure. Thus you should have no concerns in this regard. John Cameron, Ph.D. If you expose a bare TL element, such as LiBO or CaSO (Panasonic TLD) or LiF, CaF (Harshaw TLD), there will be some uniform response that will be assumed to be a high-energy photon response. This usually results in 20 to 50 mrem month of "assumed" dose. If the TL element is exposed longer, the response will increase. There is also a fade that occurs, so with the increasing response there is also fading occurring. Therefore is no finite equation to be used to eliminate what is real and what is not real dose. In conclusion, the response will NOT exceed regulatory limits, be they quarterly or annual limits (NRC or state) or for declared pregnant females (500 mrem gestation period). Sander C. Perle, ICN Question No. 2 The exposure increases as proximity to the fuel bundles occur. The dose is manageable and regulatory limits are generally not approached. While I was at Florida Power and Light, we routinely sent in divers to work in the spent fuel pit. They wore multiple TLDs (10 to 12) and the doses were generally less than 100 mrem. This is obviously dependent on the time in the area and the geometry. The area is safe with the proper precautions taken. The glow mentioned is usually seen during reactor refueling operations, and is called Cerenkov Effects. Sander C. Perle, ICN