Cumulative total number of deaths from Chernobyl, definitively the worst nuclear disaster in history, ranges from 4000 to 16000 (estimates, via Wikipedia). A dam bursting upstream of a few small towns will kill many more[0].
For comparison the Bhopal disaster (which is much less known in the West) that occurred on 3 December 1984 in Bhopal, Madhya Pradesh, India caused deaths in the range 3928 to 16000.
A government affidavit in 2006 stated the leak caused 558,125 injuries including 38,478 temporary partial injuries and approximately 3,900 severely and permanently disabling injuries.
Most of Europe drinks water from underground aquifers, which could not be affected by Chernobyl. Even breathing with air with radionuclides from Chernobyl in far distance from Chernobyl power plant didn't cause much radiation dose to the population. It was eating contaminated food and drinking contaminated milk that cause most radiation dose for population.
The precise mechanism was: radioactive particles fall to ground, or are washed to ground by rain, which concentrates them on vegetation with a lot of surface especially leafy vegetables, grass. Leafy vegetables are eaten directly by humans. Grass is eaten by cows, which again concentrates the radionuclides in milk. Humans drink milk, eat cheese concentrated from milk.
Not all radionuclides produced in nuclear fission have the same health impact on population in case of a nuclear disaster. To have a significant health hazard a radionuclide needs to have 3 properties: volatility, half-life, bioaccumulativity.
Volatility - some radioactive elements (heavy metals) are not moved far away by air, some radioactive elements like radioactive noble gases dilute very fast.
Stuff with a short half-life will transform into stable elements before migrating far. Stuff with with very long half-life will not produce much radiation during human lifetime.
Bioaccumulativity, radioactive stuff needs to stay in body to do damage. If it's eaten and then pooped out next day it usually doesn't cause much damage.
Most dangerous for general public in case of nuclear disasters are:
Iodine-131 (half-life 8 days): Iodine is stored in thyroid gland and stays in for long time in body. Especially children need a lot of iodine per kilogram of body weight. In regions where there is not enough of iodine in food (lacking seafood, table salt without added iodine), human body will try to get every bit of iodine from environment and hold it in body as long as possible.
Cesium-137 (half-time 30.04 years) : Alkali metal that forms salts. Has tendency to accumulate in soft tissues.
Strontium-90 (half-time 28.91 years) : Chemically similar to calcium. Has tendency to be incorporated into bones, teeth and stay in body for very long time.
Big part of radiation dose to the population could be prevented if the Soviet state didn't tried to cover up the Chernobyl and would prevent people from eating local food and milk, because most of the damage is done by eating iodine-131 in the first weeks after accident. Timely administration of potassium iodide tables would also help.
Chernobyl liquidators were affected with much broader range of radionuclides (radioactive stuff that did not migrate far) and with much high concentrations (radioactive stuff was not diluted much).
Direct deaths: 2 killed by debris (including 1 missing) and 28 killed by acute radiation sickness.
There many estimates about impact of Chernobyl disaster. I think the most comprehensive study is from Chernobyl Forum.
"On the death toll of the accident, the report states that 28 emergency workers died from acute radiation syndrome and 15 patients died from thyroid cancer. It roughly estimates that cancers deaths caused by the Chernobyl accident might eventually reach a total of up to 4,000 among the 600,000 cleanup workers or "liquidators" who received the greatest exposures."
Both are good sources of energy. If you're going to make the argument that "nuclear is unsafe so we shouldn't do it" though, it's relevant to keep in mind that since we've had nuclear power, dam failures have outpaced nuclear by many times in terms of deaths / TwH (1).
Edit to add: Before anyone jumps on for this it's important to note that without the Banquiao disaster the rates are about the same. Still means "nuclear is unsafe" is kind of a red herring.
"In August 1975, the Banqiao Dam and 61 others throughout Henan, China, collapsed following the landfall of Typhoon Nina. The dam collapse created the third-deadliest flood in history which affected 12,000 km2 (3 million acres) with a total population of 10.15 million, including around 30 cities and counties, with estimates of the death toll ranging from 26,000 to 240,000."
"After the disaster, the Chinese Communist Party and the Chinese government remained silent to the public, while no media were allowed to make reports."
"The official documents of this disaster were considered a state secret until 2005 when they were declassified."
The same holds for hydro. Even worse, there would be no time for evacuation. Yet nobody is considering banning dams.