SA508　steels　are　typically　used　in　civil　nuclear　reactors　for　critical　components　such　as　the　reactor　pressure　vessel.　Nuclear　components　are　commonly　joined　using　arc　welding　processes,　but　with　design　lives　for　prospective　new　build　projects　exceeding　60　years,　new　welding　technologies　are　being　sought.　In　this　exploratory　study,　for　the　first　time,　autogenous　laser　welding　was　carried　out　on　6　mm　thick　SA508　CI.3　steel　sheets　using　a　16　kW　fiber　laser　system　operating　at　a　power　of　4　kW.　The　microstructure　and　mechanical　properties　(including　microhardness,　tensile　strength,　elongation,　and　Charpy　impact　toughness)　were　characterized　and　the　microstructures　were　compared　with　those　produced　through　arc　welding.　A　three-dimensional　transient　model　based　on　a　moving　volumetric　heat　source　model　was　also　developed　to　simulate　the　laser　welding　thermal　cycles　in　order　to　estimate　the　cooling　rates　included　by　the　process.　Preliminary　results　suggest　that　the　laser　welding　process　can　produce　welds　that　are　free　of　macroscopic　defects,　while　the　strength　and　toughness　of　the　laser　welded　joint　in　this　study　matched　the　values　that　were　obtained　for　the　parent　material　in　the　as-welded　condition.　(C)　2014　The　Authors.　Published　by　Elsevier　B.V.