Underwater Search for the Lost Radioisotope Device in the Hard-to-Reach Region of the Sea of Okhotsk Accidental drop of IEU-1 type RTG during its transportation to the lighthouse site of Maria Cape of the Sakhalin island as external load with Mi-8t helicopter belonging to Nickolayevsk flight took place not in the vicinity of the Cape from the height of 100 meters on the 8-th of August 1997 at 5 PM. Radioisotope power generators convert the heat energy generated during fission of the active part of the radionuclide source into electric energy due to the semiconductor thermoelectric converter, store it and supply it to the consumer – navigation equipment. This allows operating the equipment for a long period of time without additional servicing of it. Strontium-90 with the half-life of 28.4 years is the radionuclide source. Activity of radionuclide by the time of the generator decommissioning was 12.95x1014 Bq. The radionuclide itself is located inside the sealed cylindrical container of stainless steel. Gamma radiation presenting potential threat of external radiation does not exceed 200mR/h. Radiation shielding of the device consists of the lead, tungsten-nickel alloy, depleted uranium, and of alloys on its basis. Transportation of RTG is allowed in a special steel transportation container. The sizes are – 1.8x1.3x1.7 m, its weight is 2800 kg. Manufacturing plant guarantees integrity if IEU-1 RTG when it falls from one hundred meters. Search for the sunken RTG was started by the Hydrographic service of the Pacific Fleet, which is in charge of the navigation equipment, in August 1997 and was carried out practically every year. The search area was defined by the observations of witnesses and the photo-picture of the “dropping spot” of the RTG made from the coast after the accident. This location is at the distance of 1000- 2200 m from the lighthouse, according to the bearing its area is approximately 1.500.000 m2. The depth in the region is 20-30 m. The region was studied by magnetometric (using magnetic properties of the steel transportation container) and sonar (using the sizes of the device) equipment. Several tens of the supposed items were found. Later some of the most probable items were studied by the divers in the heavy three-bolt diving-suits. 1 Fig.1a. Lighthouse at Cape Maria, close to which IEU-1 RTG was Fig. 1b. IEU-1 RTG similar to the lost one. lost 2 Fig.2. Map of the region. It is difficult to conduct the search in this region from the standpoint of hydrometeorology. The time of carrying out the work is limited to 2-3 months a year (July – September); it is only possible to use divers for 3-4 hours a day because of the strong (up to 4 knots) tidal currents. They can work in low and high tides. Underwater visibility is 20-50 cm. With consideration of the errors in item positioning it was not always possible for the divers to contact the item and to identify it. Sea-bed geomorphology is characterized by the basement rock yield with the chines of underwater mountain ridges of 5-6 m high from the bottom, and by stone fragments (comparable in sizes with the looked for item) spread in the region. Krasnoarmeysk Scientific and Research Institute of Mechanization possessing side-scan sonar devise “Gidra” was engaged into the work in 2004 and 2005, four divers in the light diving-suits participated in identification of contacts. The Ministry of the Russian Federation for civil defense, emergencies, and mitigation of consequences of natural disasters (EMERCOM of RF) joined the work upon the request of the Pacific Fleet. In the course of the missions to find and inspect underwater potentially hazardous items in the Far Eastern seas the work 3 was performed jointly with the hydrographic and rescue ships of the fleet with the use of modern equipment for the underwater work. In the framework of the RF EMERCOM mission the work was carried out by the experts of ZAO “Altair- NTPZ). Research ship (SRS) “Professor Khromov” of DVNIGMI, “HS-404” (with “MGK-975” on board) PF was involved into the work in August 2005. The following devices and equipment were used by the expedition to help with the search of the lost radioisotope source: - side-scan sonar device – Klein System 3000 (SSSD S-3000); - underwater remote-controlled survey devise – Sea Lion; - underwater gamma-spectrometer; - special equipment for accommodation of the searching equipment on the floating vessel of small displacement. It was assumed that SSSD S-3000 and remote-controlled device would be used for the search of the lost generator, and gamma-spectrometer would help identifying (indicate that the item is the radioactive source) it. Reconnoitring with the use of the photo-picture of the “dropping spot” was carried out on the coast close to the lighthouse. Slide of the photo-picture of the falling process was put into the specially fabricated photolens (sort of “Obscure chamber”), and superpositioning of the coastal objects in the picture with the real ones in the locality was performed. When the direction from photographing location to the “spot” was identified, the leading beacons were installed on the coast. These leading beacons showed the direction for the ship with the GPS receivers. When the ship was passing the “dropping spot” the buoy was put, coordinates were measured, and the “spot” location was identified on the map. 4 Fig.3. Photo-picture of the witness. Dropping location (light spot on the water surface) is Fig.4 Direction-finding of dropping location encircled with red. with the help of projecting camera 5 Falling location was found to be at the distance of 950-1000 meters from the lighthouse. According to the assumed dropping location and with consideration of potential examination errors, the search region was defined as the square 300 meters on side. With the help of SSSD S-300 mounted onto MGK triple (in different directions) examination of the selected region was carried out. Figure 5 presents examples of the contacts, which can be interpreted as the looked for item from the standpoint of the size. Example of the ship mechanical trajectory during single scanning with the help of SSSD is presented in fig. 5a. Fig.5. Examples of echo contacts obtained with the help of SSSD. 6 Fig.5a. Example of the ship mechanical trajectory during single scanning with the help of SSSD Totally 13 contacts to be examined for being the RTG were found (Fig. 6). All of them are located in a certain region. It is possible that there are less than 13, as if some were found twice. Reasons for that can be: difficult towage that was mentioned above (impossibility of getting into the water at the same location twice), and navigation capabilities in this region. GPS fixation accuracy for Maria Cape is approximately 30 meters. It is not comparable with the sizes of the looked for item, therefore, it is necessary to improve the accuracy to meters, i.e. it is necessary to set up special navigation devices. The attempts were made to identify the found underwater items with the help of remote-controlled examination device (fig. 7) and submerged gamma- spectrometer (fig. 8). Unfortunately these attempts were unsuccessful. Even technically it was impossible to use the remote-controlled device at the depth exceeding 10 meters due to very bad underwater visibility and high current velocities. Points of placing gamma-spectrometer on the sea-bed happened to be aside from the found (due to complicated hydrology) contacts. The measured values were 51-55 mR/h, which is in compliance with the background values. In order to register radiation from RTG gamma-spectrometer is to be at the distance of no more than 1.5 m from the RTG. 7 Fig.6. Schematic of the search in 2005. 8 Fig.7. Underwater remote-controlled device - Sea Lion. Fig.8. Submerged gamma- spectrometer. 9 The results of the work indicated clearly that in order to be successful in finding the RTG next year it was necessary to achieve accurate positioning comparable with the results of the item. It is also necessary to involve highly qualified divers in the light diving-suits (the item is at the depth of only 18-20 m) with special equipment allowing orientation in the muddy troubled water. Measurements of Sr-90 and Cs-137 content in the probes of soil close to the Cape of Maria indicated concentrations of 1-3 Bq/kg (concentrations in the sea water are by an order of magnitude less). This testifies to the fact that there is no radioactive contamination of the marine environment in the region, because these values agree well with the average values at the same depth in all the regions of the great oceans. Participants of the 2006 expedition as well as the equipment of the RF Emercom were located on board the research ship “Pavel Gordiyenko” of DVNIGMI. Pacific fleet was represented by the hydrographic ship “HS-404” from Sovietskaya Harbor, and rescue towboat “SB-521” from Petropavlovsk- Kamchatsky. Along with the previously used equipment the expedition possessed: - underwater acoustic system - Sonardyne Scout (fig.9) to identify positioning of the divers and items under the water; - underwater hydro-acoustic device – ARMA (sound-imaging device) (fig. 10); - light diving-suits to work at the depth of up to 25 meters (2 sets). Fig.9. Sonardyne Scout - underwater acoustic system for positioning divers and items under the water 10 Fig.10. AKMA - underwater hydro-acoustic device (sound-imaging device). On the site of the lighthouse at Cape Maria specialists of the Head department of the navigation and oceanography (HDNO) of the RF MoD deployed the mobile station for differential corrections with three portable systems for global positioning (GPS) for accurate (up to 1-3 meters) determination of coordinates during the work.