Answer to Question #4503 Submitted to "Ask the Experts"

I worked on South East Island, Monte Bello Islands, Western Australia, during my employment as a radionavigation field engineer in 1983. At the time I had to sign a declaration form from the government of Australia stating that I would not remain longer than five days on the Island. I was on the Island for five weeks and four days.

Based on the year of exposure and the radiation type on the island, what was my likely exposure and how does this compare with safe limits? What long-term effects are there for this type of exposure?

What is the likely safe exposure limit in days in 2005?

You asked about the possible dose received and any related risks to having worked in 1983 on South East Island of the Monte Bello Islands. As you probably know, the Monte Bello Islands was the site of three nuclear tests. South East Island is about 3 km southeast from the first test site which was on a ship (HMS Plym) anchored 600 meters off the coast of Trimouille Island. Two other tests were conducted in the Monte Bello Islands at later dates, one farther north on Trimouille Island and the other on Alpha Island. The yields were about 25 kt, 15 kt, and 100 kt, respectively, and the detonations took place in 1952 (HMS Plym) and 1956 (Trimouille and Alpha Islands). A map of the islands showing the locations of the three tests as well as South East Island where you worked can be found at http://nuclearweaponarchive.org/Uk/MonteBello800.gif.

There have been radiological assessment reports issued by the Australian Radiation Protection and Nuclear Safety Agency in 1980, 1982, 1983, and 1990 and a report of a survey carried out in June 2001 ("Gamma radiation levels at the Monte Bello Islands June 2001," Manning, Toussaint and Upton, Radiation Protection in Australasia [2002] Vol.19, No.1). Though I don’t have access to the earlier reports, it should not prevent providing some reasonable answers to your questions.

In 2001, the maximum gamma dose rates measured at 1 m above the ground were about 0.6 μGy (microGray) per hr on Trimouille with levels at background of 0.1 μGy per hour in the southeast part of the island. The peak rate is attributable to fallout from the first detonation (HMS Plym). Quotes from earlier surveys indicate dose rates at the ground zero of the test on Trimouille Island (farther north than the zone of fallout from the 1952 HMS Plym test) were 35 μGy per hr in 1962, 18 μGy per hr in 1978, and 0.22 μGy per hr in 2001. These data don't seem to fit together particularly well; however, the 2001 plot of measured dose rates implies the fallout path from the HMS Plym test was in a northeast direction. Since readings moving in a southeast direction are down to background levels within about 1 km of the assumed center of the fallout trajectory, it seems unlikely that radiation levels on South East Island, 3 km from the HMS Plym ground zero location, would have been much above background in 1983. Hence, it is likely that you received little or no exposure from the tests (certainly nothing of consequence to you) during your stay. Even if the dose rate were as much as 20 times background, your accumulated dose over a 40-day stay would have been about 2 mGy, about the same as the natural radiation dose you receive each year from terrestrial and cosmic radiation, and well within any occupational dose limits. A stay even 100 times as long as yours would not likely have resulted in a dose nearing occupational dose limits. There are no long-term effects to be expected from this small of exposure.

The above discussion relates exclusively to external gamma doses. Generally, fallout deposited at close-in locations contains radioactivity that is fused into metallic or glass-like particles. Moreover, the contamination is usually removed from the soil surface over the years by wind and rain. Because of the size and insoluble nature of these particles, dust is usually of low hazard. So, it is unlikely that you received any internal dose of any significance.

We recommend you not worry about your time spent working there. We hope this information is useful.

Steven L. Simon, PhD (Bethesda, Maryland, USA)
Andrew McEwan, PhD (Christchurch, New Zealand)