KYIV, May 12 (Reuters) - Scientists have recorded a rise innuclear activity in the destroyed nuclear reactor at theChernobyl power plant in Ukraine since it was covered over in2017, but the rise has levelled off and does not exceed safetystandards, staff said on Wednesday.

Staff at the plant said the rise in "neutron flux density",which if significant could indicate an uncontrolled nuclearreaction, did not pose a threat of such an event based on theirmathematical models.

High levels of radiation and damage mean it is not possibleto determine precisely the situation under the destroyed block.

"After the establishment of a new safe confinement which hasbeen in the designed position for more than four years, anincrease in the neutron flux density is actually observed,"scientists at Ukraine's Institute For Safety Problems Of NuclearPower Plants said in a statement.

"At present, the readings of the sensors in all rooms havestable values without an upward trend. The current levels do notpose a threat of a self-sustaining chain reaction," theChernobyl plant said in a separate statement.

The fourth rector at Chernobyl, 108 km (67 miles) north ofthe capital Kyiv, exploded in April 1986 during a botched safetytest, in the world's worst nuclear accident. Clouds of radiationwere sent out across much of Europe, and tens of thousands ofpeople were forced to evacuate.

Scientists say the increase in neutron flux was recorded ina unit that nuclear fuel from the destroyed reactor had gotinto, possibly during the installation of a new shelter over thereactor.

Scientists at Ukraine's Institute For Safety Problems OfNuclear Power Plants said that before work on installing the newshelter began in late 2016, fuel was cooled by rainwater, whichhad since disappeared.

"Based on predictive estimates, it is expected that in thefuture there will be an increase in the neutron flux density,which will be determined by the process of moisture loss," theysaid in a statement.

"Current experimental data has confirmed this scientifichypothesis," they said, adding that they were carefully studyingand monitoring the damaged unit.

(Reporting by Pavel Polityuk; Editing by Hugh Lawson) ((pavel.polityuk@tr.com; +380 44 2449150; Reuters Messaging:pavel.polityuk.thomsonreuters.com@reuters.net))