UK's Costain tops smallcaps on $1.3 billion nuclear contract

** Infrastructure solutions firm Costain COST shares rise 10.9% to 155 pence, on track for biggest one-day pct gain since January if gains hold

** Top gainer on FTSE small cap index (.FTSC), which is up 0.3%

** Co secures contract by Sellafield , with COSG's share of order valued at up to 1 billion pounds ($1.33 billion)

** YTD, stock up 43%, while FTSC up 6.4%

($1 = 0.7501 pounds)