The lower house of parliament, the National Council, has declined to lift the ban on building new nuclear plants, sending the proposal back to the Federal Council for financing clarification.
After 10 hours of debate MPs voted 100 to 97 to return the file.
The centrist motion demanded guarantees on who will pay, noting no nuclear plant has ever been built anywhere without state support.
A 2017 vote backed phasing out nuclear with 58 percent in favour, banning new plant construction after Fukushima.
The right-wing "Stop Blackouts" initiative aims to lift that constitutional ban, but any return to nuclear power now depends on the financial answers the government can produce.
G7 underway in Evian
Police criticised after G7 protests
Bern clears path for Geneva for more parental leave
Geneva communes fight swimsuit law
Online shopping driving many to debt
G7 protest becomes violent
