A 1945 nuclear test in New Mexico sent fallout to 46 states within 10 days, a new reconstruction finds; Washington and Oregon showed no deposition in the model

Fallout from the Trinity test, the first nuclear detonation, reached 46 US states within 10 days, a new reconstruction in Scientific Reports finds. According to the Scientific Reports study, the device detonated at 5:29 AM local time on July 16, 1945, atop a tower about 30 m (roughly 100 feet) tall in New Mexico. Only Washington and Oregon showed no deposition in the model. The authors reconstructed the first 10 days of fission-product deposition for 94 of the 101 tests, using publicly available US Government test data, particle-size distribution benchmarked against National Cancer Institute iodine-31 estimates for 51 Nevada tests and NOAA’s HYSPLIT transport model driven by ERA5 weather analysis. It also says Trinity fallout alone reached 46 states within 10 days and an ensemble analysis was used to bound meteorological uncertainty despite its 1945 observations.The study, Reconstruction of fallout deposition from U.S. atmospheric nuclear tests conducted in New Mexico and Nevada, led by Sébastien Philippe, was published in Scientific Reports on August 24, 2026. His group had developed a model for the first 10 days of fission product deposition during 94 of 101 atmospheric tests conducted by the United States from 1945 through 1962. The Trinity test was the first one, while the other hundred tests occurred at the Nevada Test Site. Out of those hundred tests, seven were not associated with any fission product yield.The plume went Northeast first, then spread across the central USThe plume initially travelled northeast from the test site, depositing material across parts of New Mexico, the Midwest and the Great Lakes region. In 48 hours, the fallout spread to the Central part of the United States; also, there was some fallout even further south and west from ground zero. The maximum value of deposition at the Trinity site was 4×10^11 becquerels/m2. The yield of the test was 24.8 kilotons, up from the previous estimate of 21 kilotons, an increase of about 18%.A 1945 weather record and two years of CPU timeOlder Trinity simulations have used limited surface measures and low-resolution weather data, generating only short-range maps. The authors combined NOAA’s HYSPLIT transport model with the ERA5 weather analysis dataset, which was extended back to 1940 in 2023. For July 1945, ERA5 drew on about 2 million observations, split roughly evenly between surface and upper air measurements. To test how much the weather data affected the result, the authors ran the Trinity model with all 10 ERA5 and ensemble members. Deposition reached 46 states in six of the runs and 45 states in the other four. The authors ran HYSPLIT 104 times, once for each of the 94 tests plus 10 ensemble runs for Trinity, with 9 million particles per run. They put the total at about two years of CPU time, spread across 128 threads.Film packaging from Indiana and Iowa offers a check from 1945The reconstruction is consistent with the merge evidence available. The author's evidence of radioactive contamination discovered on strawboard used to package X-ray film from Indiana and Iowa during 1945 is one of the earliest known instances where it was found that fallout from the Trinity test reached great distances. It is also consistent with the 1987 NOAA study of fallout in the first 24 hours.According to the authors, the total deposited activity combined radionuclides that decay at different rates, so it could not be read directly as a measure of risk and converting the maps into doses would require additional modelling. The authors also said the model stopped at the US borders, although the plumes from several tests plausibly extended into Canada and Mexico.The National Cancer Institute modeled New Mexico doses separatelyThat dose work has been done for New Mexico alone. A 2021 technical note in Nuclear Technology, Estimated Radiation to Radioactive Fallout from the Trinity Nuclear Test, summarised the first dose, reconstruction and cancer risk projection for the state's population, which investigators from the National Cancer Institute published in 2020 after working for seven years. The Institute looked at Trinity fallout and cancer risk in New Mexico, whereas the new model covers the whole contiguous United States.A 2016 notice in the Federal Register laid out the plan. The goal of the institute was to calculate doses to the thyroid, stomach and red bone marrow and the population of each county in New Mexico, depending on age, gender and race. The plan included focus groups for people aged 70 and older who were alive at the time of the test. According to the National Cancer Institute’s assessment of the Trinity Test fallout, the highest radiation doses were estimated to have occurred in Torrance and Guadalupe counties. The researchers also projected that several hundred excess cancer cases may have resulted from the exposure, although the exact number remains uncertain.You use AI every day. Now get your AI Quotient. Take the AIQ test.