Terabecquerel to Megabecquerel

The total release from Chernobyl Unit 4 is estimated at 5,200 petabecquerels (5.2 × 10⁶ TBq), though figures vary by source and isotope accounting. Of that, about 1,760 TBq was iodine-131 and 85 TBq was caesium-137. For perspective, the entire global nuclear weapons testing era released roughly 2.6 × 10⁸ TBq — so Chernobyl was devastating but still a fraction of Cold War fallout. Fukushima released about 520 TBq of Cs-137, roughly one-sixth of Chernobyl.

To sterilise food, you need to deliver 1–10 kilograys of absorbed dose in minutes across conveyor belts of product. That requires an enormous photon flux, which only a multi-hundred-TBq cobalt-60 source can provide. A typical facility starts with 500–1,000 TBq and replenishes as the Co-60 decays (5.27-year half-life). The food never becomes radioactive — gamma photons do not induce radioactivity in stable atoms at these energies. Over 60 countries have approved food irradiation for spices, meat, and produce.

Nuclear medicine staff literally call it a "moly cow." A generator arrives with ~150 TBq of Mo-99 adsorbed onto an alumina column. Mo-99 decays (66-hour half-life) into Tc-99m, which is washed off the column with saline — "milking" the generator. Fresh Tc-99m accumulates between milkings, reaching peak yield about every 23 hours. A single generator supplies a hospital for about a week before the parent Mo-99 activity drops too low. It is one of the cleverest supply chains in medicine.

Fresh spent fuel is extraordinarily active — a single assembly registers in the petabecquerel range, dominated by short-lived fission products like I-131, Xe-133, and Ba-140. Within a year, activity drops by about 99% as these burn out. After 10 years it drops another 90%, leaving mainly Cs-137 and Sr-90 (both ~30-year half-lives). After 300 years those are gone too, and the remaining activity comes from transuranics like plutonium — far less active per gram but with half-lives of thousands to millions of years.

Permanently, no — radioactivity decays by definition. Practically, it depends on the isotopes deposited and the cleanup threshold. Chernobyl's exclusion zone still restricts habitation 40 years later because Cs-137 (30-year half-life) contaminated the soil at levels above 1,480 TBq/km² in the worst spots. Parts of Fukushima were decontaminated and reopened within years because the deposition was lower. The real question is not whether an area recovers, but whether society is willing to wait — or pay for aggressive decontamination.

Diagnostic imaging doses fall neatly in the MBq range — a PET scan uses 185–370 MBq, a bone scan 500–800 MBq. Using becquerels would mean writing hundreds of millions; using gigabecquerels would mean awkward decimals like 0.37 GBq. MBq is the Goldilocks unit for the hospital pharmacy: large enough to avoid scientific notation, small enough to express a single patient dose as a tidy number on a syringe label.

That depends entirely on the isotope. Technetium-99m, the workhorse of diagnostic imaging, has a 6-hour half-life — so a 740 MBq injection drops to 370 MBq in 6 hours, 185 MBq in 12, and becomes negligible within 2 days. Fluorine-18 (used in PET) has a 110-minute half-life and is essentially gone in a day. Iodine-131 (used in therapy) lingers for about 8 days per half-life. Hospitals choose isotopes partly based on how fast they want the activity to vanish.

Most diagnostic isotopes (Tc-99m, F-18) have half-lives under a day, so hospitals simply store waste in shielded bins and let it decay. After 10 half-lives — about 3 days for Tc-99m — the activity is down to less than 0.1% of the original and can be disposed of as normal clinical waste. Longer-lived therapeutic isotopes like I-131 require weeks of decay storage. The vast majority of nuclear medicine waste is never shipped to a radioactive disposal site; it just sits in a locked closet until physics solves the problem.

A patient injected with 370 MBq of F-18 for a PET scan emits gamma rays at a dose rate of roughly 5–6 µSv/hr at one meter. That means sitting next to them for two hours gives you about 10–12 µSv — less than a chest X-ray. Staff handle dozens of patients daily so they follow time-and-distance protocols, but for family members the exposure from a single visit is trivially small. The activity halves every 110 minutes, so by evening the patient is barely distinguishable from background.

Molybdenum-99, which decays into the technetium-99m used in 30+ million scans per year worldwide, can only be produced in a handful of aging research reactors. It has a 66-hour half-life so it cannot be stockpiled — you have to make it, ship it, and use it within days. When a reactor goes down for maintenance (as happened in 2009 when both the Canadian NRU and Dutch HFR shut down simultaneously), hospitals worldwide face scan cancellations within a week. New production methods using particle accelerators and LEU targets are slowly diversifying supply.