The blast reportedly went off at around 8am as staff were opening mail.
A statement by the company said that that two of its employees had been taken to hospital for treatment. One received minor injuries, and the other suffered moderately severe injuries, thought to be burns.
The group’s building was also damaged.
"Swissnuclear is shocked by this event and fiercely condemns this criminal action," the company's statement read.
Police in Switzerland have opened an investigation and are expected to release a statement soon.
Christian Taillebois, a spokesman for Foratom, the nuclear trade association in Brussels, told EurActiv that extra precautions were now being taken but that he did not expect any similar attacks here.
"We have informed our head of security and the person in charge of our mail in the building to be more vigilant," he said. "But there is no specific threat."
Such attacks were "unacceptable," he added.
No group has yet claimed responsibility.
The Guardian newspaper reported that Swiss police were investigating a possible link between a Greenpeace demonstration in Olten that was taking place a few hundred metres away at the time the bomb went off.
Greenpeace strongly denied any association. Their protest had been against plans by the power company Alpiq to build a new nuclear plant.
Alpiq is one of several companies represented by Swissnuclear in the company's dealings with national and international authorities.
Nuclear energy provides about 40% of Switzerland's electricity and makes up 10% of its overall energy production.
The country suspended last month its approvals process for three new nuclear power stations, in response to the disaster in Japan.
Nuclear issues are contentious in Switzerland, where the Greens and centre-left Social Democrats support abandoning nuclear power and every citizen must have access to a nuclear fall-out shelter by law.
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