A quick Googling gives this quote:

When coal is burned, all the uranium daughters accumulated by disintegration—radium, radon, polonium—are also released. The United Nations Scientific Committee on the Effects of Atomic Radiation evaluates the radiation exposure to the population from this source.2 Per gigawattyear (GWe-yr) of electrical energy produced by coal, using the current mix of technology throughout the world, the population exposure is estimated to be about 0.8 lethal cancers per plant-year distributed over the affected population. Table 7.2 summarizes these data. With 400 GWe of coal-fired power plants in the world, this amounts to some 320 deaths per year

https://rlg.fas.org/mwmt-p233.pdf

This estimate uses the Linear No Threshold (LNT) model [1], which is unsupported by science. The organization you mentioned, the United Nations Scientific Committee on the Effects of Atomic Radiation no longer uses this model.

The relevant wikipedia quote is

  One of the organizations for establishing recommendations on radiation protection guidelines internationally, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) that previously supported the LNT model, no longer supports the model for very low radiation doses.

The actual UNSCEAR paper is [2]. The relevant quote is on page 33:

  Projections of the absolute number of cancer cases in a population have less and less information value and can be increasingly misleading at lower and lower doses. As a matter of general practice, the Committee does not use the risks inferred from studies of populations following radiation exposure at moderate and high doses to project absolute numbers of radiation-induced cancers following exposure at low and very low doses.

[1] https://en.wikipedia.org/wiki/Linear_no-threshold_model

[2] https://www.unscear.org/docs/publications/2012/UNSCEAR_2012_...