The reactor in question was one of two used in a land-based nuclear submarine training facility, and utilized by the Soviet navy. The two reactors were subsequently concreted over.

These facts of the matter were ascertained via transillumination, a technique of sampling illumination via transmission of light through the object under scrutiny.. 

This knowledge is important in regard to the future safe demolition of the reactor, AK reported.

The company tasked with the work, GScan, uses its own patented tech in detecting naturally-occurring muon radiation,.

Andres Nurme, project manager at GScan said: "These are Cobalt-60 radiation sources. With the ongoing current work and its results, we can state with certainty that there is no more nuclear fuel left inside the reactors."

Cobalt-60 is a synthetic radioactive isotope of cobalt with a half-life of 5.2714 years, produced artificially in nuclear reactors.

The muon tomography scan of the second reactor at the former Paldiski nuclear facility was completed this week, and imaged the object.

In this case, most of the heaviest radiation sources are located at or near the top of the reactor lid, the survey revealed.

Rooms, passageways and hatches had also been concreted, to prevent access to the interior.

Mapping the interior of the reactors is important for planning the future of the former nuclear facility.

AS A.L.A.R.A is tasked with the actual demolition of the Paldiski reactors.

Alari Kruusvall, AS A.L.A.R.A.'s environmental engineering specialist, told AK the complex will be demolished in its entirety, starting with the recently investigated reactor and layer by layer, from top to bottom.

The recent radiation source mapping will ensure that these sources are not disturbed in the destruction process.

The object must be destroyed because it is not known how durable its structural integrity, particularly at its base, is.

Paldiski became a Soviet navy nuclear submarine training center in 1962, with the construction of a facility known locally as the pentagon, decommissioned in 1994 and demolished in 2007.

At its peak, this pentagon employed around 16,000 personnel across the two reactors (rated at 70MW and 90 MW), making it the largest facility of its kind in the Soviet Union.

Due to what the occupying Soviet authorities would have seen as its highly sensitive nature, the whole of Paldiski was a closed town, meaning native Estonians could not easily enter.

The most commonly-used nuclear fuels are the radioactive metals uranium-235 and plutonium-239. As noted no traces of these or any other fuel types were detected in the course of the recent work.

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