For an exhaustive background, here are two declassified CIA reports on Soviet Civil Deefense.
Here and
Here.
For important industries, the key elements were supposed to be:
1) underground blast shelters for workers on site during their shift, and in some cases underground, blast-protected production facilties on site.
2) evacuation of off duty workers and their families to sites within a two-hour or so commute of the plant. (Many large employers maintained "workers holiday camps" and resorts nearby... when the USSR fell these sites were some of the burdens that made Russian industry so uncompetitive and they were mostly sold off). Those facilities provided fallout protection and basic shelter.
3) dispersal of sites around the USSR and even within a single site.
4) stockpiling of food and raw materials to allow continued production after a strike; and
5) civil defense training for workers - decontamination, first aid, clearing rubble and NBC recon, to allow those on site to quickly recover after a strike.
Of course this was the ideal. While fully implementing dispersal of sites made it more difficult for the US to wipe them out in wartime, it also increased demand for transportation from day one, and multiple sites (especially green-field ones developed in pristine areas) required additional resources to construct. Local officials made trade-offs, and the extra building materials for robust underground shelters were juicy targets for corrupt officials to skim off for private gain. The reports concluded that overall the Soviet civil defense system would protect a significant portion of the population from the immediate effects of an attack, but that the transportation and communications systems of the USSR, which barely functioned effectively in peacetime, would break down in the aftermath of a nuclear attack.
The CIA survey of 150-some sites found that they were pretty common, although not universal. Their consensus was that the non-Soviet Pact nations operated on a similar level.