The sinking of the Komsomolets submarine off Norway in 1989 resulted in several in-situ investigations of the submarine and surrounding area by Russian research institutions and scientists. In order to effectively carry out these studies (see Table 2.5), equipment, instrumentation, and sensors have been developed by Russia's Central Design Bureau for Marine Engineering (RUBIN) to enable researchers to assess the contamination effects from the nuclear reactor and nuclear missiles. For example, two small ROVs were developed by Intershelf for deployment from the Mir submersibles to allow closer inspection of the submarine. These neutrally buoyant ROVs have a depth capability of 6,000 m, two TV cameras, and a radiometer.
Komsomolets Submarine Investigations
Because of absorption of alpha and beta emissions by almost any intervening material (seawater, hull, etc.), radiation measurements from seabed sources or sunken vehicles is almost exclusively monitored by the use of gamma spectrometry. In response to the Komsomolets submarine's sinking on April 7, 1989, the Russian Research Center's Kurchatov Institute developed several versions of a gamma spectrometer deployed or used from the Mir submersibles (Nejdanov 1993). Basically, gamma rays emit between 0 and 3 Mev (mega electron volts) in the spectrum, and the point in the spectrum where a peak occurs directly identifies the isotope (in this case, Cs-137) causing the radiation.
Gamma radiation is measured using a sodium iodide scintillation detector that emits flashes of light when struck by gamma radiation. These are converted to electrical pulses by a photomultiplier tube coupled to the end of the detector. The pulses are amplified and recorded in the submersible. The brightness of the light flashes, and hence the size of the electrical pulses, is directly proportional to the energy of the gamma radiation striking the detector.
Several different gamma spectrometers were developed for use from the Mir submersibles, as well as a version that can be utilized from a ship. The minimum detectible activity of Cs-137 in seawater is claimed to be 0.5 pCi/l. The spectrometers are designed for minimal weight and size, and maximum sensitivity for undersea measurement use. Figure 2.1 depicts a model REM-2 gamma spectrometer that was deployed near the Komsomolets submarine with the Mir submersibles. Figure 2.2 shows some typical Cs-137 measurements, while the K-40 peak is the radiation signal detected in most seawater. An autonomous, self-contained version has been developed that allows deployment for longer time measurements. These instruments are not unique; similar designs of portable NaI crystal gamma spectrometers have been available commercially for approximately a decade, that is, at Exploranium G.S. Limited, Toronto, Canada.
Figure 2.1. Gamma Spectrometer REM-2
Figure 2.2. Gamma-Radiation Spectrum of Water in the Vicinity of Reactor Compartment