The S-50 thermal diffusion process became the first stage of enrichment, achieving enrichment levels of less than 2% uranium-235.

The senior official here told journalists that the high enrichment level indicated that the equipment had come from Pakistan, the journalists said.

Enrichment can be used to produce uranium for reactor fuel or (at higher enrichment levels) for weapons.

Enriched uranium can be used to produce energy or, at high enrichment levels, as fuel for nuclear weapons.

Reports from Vienna last week talked of enrichment levels reaching 80 percent.

Inspectors have found such uranium, which at extreme enrichment levels can fuel bombs, twice in the past.

Nuclear submarines in the former Soviet Union use fuel of similar enrichment levels, they said.

The Shoreham fuel was made to several different enrichment levels.

Criticality is usually considered highly unlikely, owing to the low enrichment level used in light water reactors.

The enrichment level is 2% (0.4% for the end pellets of the assemblies).