As far as I know it comes down to enrichment levels of the nuclear material used (Uranium?). Nuclear power requires much lower enrichment levels.
> Most nuclear reactors that produce electricity only require fuel that is enriched to between 3-5% U-235.
> This fuel does not represent a proliferation threat primarily because of the critical mass issue — the amount of material necessary to maintain a self-sustaining neutron chain reaction. If the level of enrichment is low, then it holds that the amount of the material must go up in order for a chain reaction to be sustained. The size can quickly become impractical for weapons delivery, so low enriched uranium (LEU) is not a threat.
> Highly enriched uranium (HEU) is anything enriched above 20% and weapon-grade uranium is commonly considered to have been enriched above 90% U-235. However, some research reactors use 90% enriched U-235 to produce medical isotopes, so there are civilian applications for this fuel too.
> Most nuclear reactors that produce electricity only require fuel that is enriched to between 3-5% U-235.
> This fuel does not represent a proliferation threat primarily because of the critical mass issue — the amount of material necessary to maintain a self-sustaining neutron chain reaction. If the level of enrichment is low, then it holds that the amount of the material must go up in order for a chain reaction to be sustained. The size can quickly become impractical for weapons delivery, so low enriched uranium (LEU) is not a threat.
> Highly enriched uranium (HEU) is anything enriched above 20% and weapon-grade uranium is commonly considered to have been enriched above 90% U-235. However, some research reactors use 90% enriched U-235 to produce medical isotopes, so there are civilian applications for this fuel too.
From: https://armscontrolcenter.org/uranium-enrichment-for-peace-o...