Russia’s top nuclear official promised on Monday to succeed in developing new weapons as he paid tribute to five scientists killed.

The five scientists were buried in the closed city of Sarov on Monday. They died Thursday in what state nuclear agency Rosatom has said was an accident during a rocket test on a sea platform off northern Russia.

At memorial events in Sarov that included a gun salute, Rosatom head Alexei Likhachev praised the deceased nuclear experts as the “pride of the country” and the “pride of the atomic sector.”

“The best tribute to them will be our continued work on new models of weapons, which will definitely be carried out to the end,” Likhachev was quoted as saying by RIA news agency.

In a video interview published late on Sunday, an official at the scientists’ research institute in Sarov suggested that they had been working on a small nuclear reactor.

The official, Vyacheslav Solovyev, said the institute was working on “sources of thermal or electric energy using radioactive materials, including fissile materials and radioisotope materials.”

He said “these developments are also actually happening in many countries. The Americans last year … also tested a small-scale reactor … Our center also continues to work in this direction.”

The institute’s work serves both civilian and military ends, he said.

The city administration in Sarov, announced two days of mourning, saying the experts died while “performing a task of national importance,” RIA reported.