Becquerels, Sieverts, Per Hour and Per Year — the Units Behind Nuclear Plant News

“Radiation levels at the plant remain normal.” The International Atomic Energy Agency (IAEA) has repeatedly issued statements like this about the Zaporizhzhia Nuclear Power Plant, which is under Russian control. For example, the August 29, 2026 statement, after reporting three drone attacks, said “Radiation levels at the site remain normal.” That sentence has no numbers and no units. Here is a guide to what to look for when you read a news report that does have numbers.

This article does not judge what a given radiation reading means for health. The number of reactors and how the occupation came about are covered in Ukraine has 15 reactors, and six of them are at an occupied nuclear plant.

Becquerels, grays, and sieverts

The Ministry of the Environment’s Unified Basic Materials on the Health Effects of Radiation and Other Matters (fiscal 2022 edition) explains units of radiation by dividing them into the emitting side and the receiving side.

According to the same materials, when expressed in sieverts, exposure from outside the body and exposure from radioactive material taken into the body can be compared and added using the same yardstick. Becquerel figures and sievert figures measure different things, so they cannot be compared directly.

“Per hour” and “per year”

Figures that often appear in the news, such as “0.1 microsievert per hour,” are dose rates per hour. One microsievert is one-thousandth of one millisievert, and one millisievert is one-thousandth of one sievert. A figure such as “1 millisievert per year,” on the other hand, is the total dose received over one year.

The same Ministry of the Environment materials give an example of the calculation linking the two. After the accident at Tokyo Electric Power Company’s Fukushima Daiichi Nuclear Power Plant, “0.23 microsievert per hour” — the figure used by municipalities as a guide for designating areas for decontamination — is derived from the value at which the additional exposure caused by the accident reaches 1 millisievert per year. Assuming 8 hours a day outdoors and 16 hours in a wooden house that reduces radiation to 0.4 times the outdoor level, 1 millisievert per year works out to 0.19 microsievert per hour. Adding the natural background radiation of 0.04 microsievert per hour gives 0.23. The materials state that this calculation is a simplified estimate made on the safe side, and that actual exposure could be lower.

What this shows is that even the same “per hour” figure means something different depending on whether it includes natural background radiation or represents only the additional amount from an accident or similar event. Whether a figure in the news is one or the other needs to be confirmed from the wording of the statement itself.

Older units that survive in Ukrainian law

Ukrainian materials sometimes use units that predate the sievert and the becquerel. The 1991 law defining the contaminated zones from the Chornobyl accident (No. 791a-XII) writes annual exposure as, for example, “1.0 millisievert (0.1 ber),” giving the sievert alongside the older unit ber (бер, the equivalent of the English “rem”). From this, you can tell that 1 millisievert equals 0.1 ber.

The same law expresses the degree of soil contamination in curies (Кі), for example as “cesium at 15.0 curies or more per square kilometer.” The curie is an older unit for the amount of radioactivity, and when placing a figure from this law alongside one given in becquerels, you need to convert them to the same unit before comparing. When reading Ukrainian-language materials, check the unit symbol before reading the number.

What an IAEA statement actually reports

According to the IAEA, as of September 2026 its staff are stationed at all of Ukraine’s nuclear power plants and at the Chornobyl plant. Most of the weekly statements report on the conditions that underpin nuclear safety rather than on radiation readings themselves: whether the plant is connected to an external power line, how many days’ worth of fuel the emergency diesel generators have, whether sounds of fighting or drones have been observed nearby, and whether staff are able to work without being pressured, among other things.

On radiation, the IAEA reports information that reaches it from the plant. The May 22, 2026 statement said that data from multiple radiation monitoring stations, both inside and outside the Zaporizhzhia plant’s site, is sent to the IAEA every day. But the July 15 statement said that, based on the data visible at IAEA headquarters, it appeared that more than two-thirds of the monitoring stations outside the site were not transmitting data. The plant explained that some stations were unable to transmit because of power outages or damage, and assessed that, while the current arrangement could still fulfill the purpose of monitoring in principle, the margin and reserves had shrunk. The IAEA says it will keep examining how this affects emergency monitoring.

The same is true for Chornobyl. When a fire burned 1,100 hectares in the exclusion zone on May 7, 2026, the IAEA reported that “the plant confirmed that no abnormal radiation levels were detected either during or after the fire.” A sentence like “remains normal” is a report of the fact that someone measured it and passed that on to the IAEA.

Reading whose statement it is

The Zaporizhzhia plant has been under Russian control since March 2022. The “(the) plant” or “plant representatives” that appear in IAEA statements refer to this Russian-controlled side of the plant. On the Ukrainian side, the state enterprise Energoatom, which operates the country’s nuclear plants, and the State Nuclear Regulatory Inspectorate of Ukraine (SNRIU) issue their own separate statements. The IAEA receives information from both and distinguishes in its wording where each piece came from.

When reading news that contains numbers, checking them in the following order reduces the chance of misreading them.

• Who measured the figure (the plant, a regulator, IAEA staff, or a third-party body)
• Where it was measured (inside or outside the site, and in which town)
• What the unit is (becquerels, sieverts, or an older unit)
• Whether it is “per hour” or “per year,” and whether it includes natural background radiation
• When the reading is from, and whether it has changed compared with other days

A general approach to reading the news is set out in Five things to check first when reading news about Ukraine.