Japanese follow trail of stray neutrons

2019-03-07 09:06:06

By Peter Hadfield in Tokyo RADIATION released by last month’s nuclear accident at Tokaimura in Japan may have spread more widely than was first realised, but at levels that are unlikely to have much impact on the local populace. The Japan Atomic Energy Research Institute had two radiation monitors 1.7 kilometres and 2 kilometres west of the accident, and they detected neutron emissions over a period of 17.5 hours between 30 September and 1 October. The monitors also recorded gamma radiation peaking at 9.5 microsieverts per hour. In contrast, the International Commission on Radiological Protection’s safety limit for nuclear workers is 20 millisieverts per year. Radioisotopes of caesium and strontium were detected immediately outside the Tokaimura facility, although investigators from the Science and Technology Agency say radiation levels in the area are now back to normal. But JCO, the company that runs the building where the accident took place, has shut down the ventilation system after radioactive iodine-131 was found leaking from air vents. JCO measured 2.1 × 10-5 becquerels of radioactivity per cubic centimetre at the vents on 8 October. Radiation is still leaking from the sedimentation tank where the critical reaction took place, but workers at the plant have built up a wall of aluminium-filled bags to keep any stray neutrons in the room. Although the failure to evacuate promptly led to at least 10 local residents being exposed to the radiation, local authorities do not believe any others have been affected. Tests on gold jewellery belonging to local people show little change from stable gold-197 to the heavier isotope gold-198,