1. Background Radiation: Even in the absence of an atomic blast, all regions on Earth have a certain level of background radiation due to natural sources such as cosmic rays and radioactive elements in the ground. The average annual dose of natural background radiation in the United States, for instance, is about 3 millisieverts (mSv).
2. Radioactive Decay: Radioactive materials decay over time, meaning their radioactivity decreases. This process is characterized by the half-life, which represents the amount of time it takes for half of the radioactive material to decay. For the radioactive isotopes produced by nuclear explosions, such as cesium-137 and strontium-90, the half-lives are 30 and 28 years, respectively.
3. Cleanup Efforts: Following the atomic bombings in Hiroshima and Nagasaki, extensive cleanup and decontamination efforts were carried out to remove radioactive debris and materials. Contaminated soil and structures were removed and buried in secure locations, reducing the radiation levels in the affected areas.
4. Radiation Zones: Over time, the radiation levels in Hiroshima and Nagasaki have decreased due to decay and containment measures. However, the city governments have designated certain zones based on the residual radiation levels:
- Special Control Zones: These areas have the highest radiation levels and access is strictly restricted.
- Residential Zones: These zones have lower radiation levels and are safe for habitation, but residents may need to take precautions to minimize exposure.
- Released Zones: These areas have negligible radiation levels and are safe for unrestricted access and development.
So, if you were to visit the site of the first atomic bomb blast in Hiroshima today, you would likely be exposed to some amount of radiation, especially in the designated Special Control Zones. However, the radiation levels in the majority of the city, including the residential and released zones, would have decreased significantly over the past 60 years and would be within the range of background radiation.
Always remember that, safety guidelines and advice from local authorities or radiation experts are essential when visiting a site that may have been affected by nuclear activities or explosions.