Image copyright S.Gashchak/Chornobyl Centre Image caption These images are believed to be the first photographic evidence of a brown bear in the Chernobyl Exclusion Zone (CEZ)

Scientists have captured what is believed to be the first photographic evidence of brown bears within the Chernobyl Exclusion Zone (CEZ).

Camera traps, used by a project assessing radioactive exposure impacts on wildlife, recorded the images.

Brown bears had not been seen in the area for more than a century, although there had been signs of their presence.

The exclusion zone was set up after an explosion at the Chernobyl nuclear power plant in Ukraine in April 1986.

"Our Ukrainian colleague, Sergey Gashchak, had several of his camera traps running in one of our central areas over the past few months in order to start to get a feel for what (wildlife) was there," explained project leader Mike Wood from the University of Salford.

He told BBC News that data retrieved from one of the cameras in October contained images of a brown bear.

"There have been suggestions that they have existed there previously but, as far as we know, no-one has got photographic evidence of one being present on the Ukrainian side of the exclusion zone," Dr Wood said.

"We are basically working on the assumption that as you move people out of the equation and human pressure and disturbance is removed, then any animals that have a corridor into the exclusion zone find they are suddenly away from the pressures and dangers presented by people."

Following the April 1986 explosion - described as the world's worst nuclear power plant accident - more than 110,000 were moved from their homes as a 30km-radius exclusion zone was established around the damaged nuclear reactor.

In the subsequent years, the area has provided a valuable source of data for scientific research into the impact of radioactive contamination.

Dr Wood's team's project is part of a five-year research programme called Transfer, Exposure, Effects (Tree), which will aim to "reduce uncertainty in estimating the risk to humans and wildlife associated with exposure to radioactivity and to reduce unnecessary conservatism in risk calculations". Most of the fieldwork will be carried out within the Chernobyl Exclusion Zone.

"We have our models to predict radiation exposure but it does it in a very crude way - an average over a very large area - but we know that animals interact with the environment in different ways," observed Dr Wood.

Image copyright S.Gashchak/Chornobyl Center Image caption The study will assess the impact on large mammals within Chernobyl's exclusion zone

"They have habitat preferences, certain ones will want to be closer to a river while others will want to be in deep forest. When you have patchy contamination across an area, those habitat preferences and food preferences will actually change the way in which the animals are interacting with the contamination."

In order to get a comprehensive overview of what species are found in the CEZ, the team has identified three different areas: high, medium and low contamination.

Each area has a radius of 5km, containing 84 randomly generated locations where the cameras will be deployed. At any one time, 14 cameras will operating in each area.

The team will focus its attention on larger mammals, explained Dr Wood: "As you can imagine, a lot of the areas within the exclusion zone are very overgrown.

(Wild)life inside the exclusion zone The Chernobyl Nuclear Power Plant Zone of Alienation covers an area of 2,600 square-kilometres

Before the 1986 accident, the area was home to an estimated 120,000 people

There is an ongoing debate among the scientific community regarding the impact of radioactive contamination on the area's flora and fauna

"What we have to do when we are setting up these cameras is that we have got to clear the area a bit to make it possible to get photographs of passing animals.

"If you did not cut back the grass then you would never see the hares or foxes passing through the area, for example.

"It is also necessary to make sure there are no tree branches that could be blown into the camera's trigger point as it could set the camera off and store a lot of false images and fill memory cards.

Dr Wood and the team will continue to operate remote camera traps in the CEZ as part of the first stage of the project until late 2015.

"Once we have completed this particular stage of the study, looking at what animals are there and in what density, we are then going to be selecting one particular species to target for a trapping and collaring campaign," he said.

"We will be fitting collars with GPS to these animals, and also dose-measurement technology so that we are then able to track movement over the course of a year through the exclusion zone and get a real measurement of the exact radiation exposure that these animals get.

"This opens up the opportunity for us to not only test of models of how well we can predict radiation exposure but opens up the opportunity to do some very direct studies on the results between the field radiation exposure and radiation effects."