In a recent Reddit thread about Chernobyl, someone wrote that young Red Army conscripts were given a choice: spend a few minutes shovelling radioactive debris from Chernobyl or spend two years fighting in Afghanistan. It sounds like an urban legend, but is there any truth to it?
No. There was no real choice offered to anyone in the matter. Soldiers from units specifically tasked with radioactive/chemical defence (РХБЗ) were sent to Chernobyl from all over the USSR, but their units were not in any way related to the 40th Army - the Soviet group of forces in Afghanistan. In fact ever reservists from Radio-Chemical units were mobilised for the liquidation - but soldiers assigned to these units weren't going to Afghanistan no matter what, though they were going to Chernobyl - no matter what.
Other than РХБЗ units, certain other forces, both specialised, such as Air Force under Major General Antoshkin for dropping firefighting compounds, as well as fairly random units - mostly being sent to the area of the catastrophe due to their physical proximity (being based in Kiev Oblast or a neighbouring one).
Altogether 340,000 soldiers were used as liquidators on the ground - the massive number being the result of the system where individual soldiers would only spend a limited amount of time in the "zone" before being replaced by a different colleague. Again - nobody was given any choice in the matter - existing units were simply deployed to the NPP through the direct order. It would be a bureaucratic nightmare to organise special units out of "volunteers" choosing liquidation over Afghanistan.
Source: various, but here's a pretty good description of legal and technical basis for the action.