ENSI Report on Fukushima IV: Radiological Effects
Nine months after the disaster at the Fukushima nuclear power plant, Japan is working to contain radiation exposure in the region of the accident. The challenges confronting the country in this endeavour are shown by the new ENSI report on the radiological effects of the accident on 11 March 2011. ENSI already deduced some “Lessons learned for Swiss radiation protection” at the end of October.
The Japanese authorities evacuated extensive tracts of the affected areas, enabling them to limit the exposure of the population to radiation. This is documented by the new report from the Swiss Federal Nuclear Safety Inspectorate (ENSI) on the radiological effects of the accident at the Fukushima Dai-ichi nuclear power plant.
The accident led to radiological contamination of people and the environment beyond the evacuation zone; this continues until the present day, and eliminating it confronts the Japanese authorities with a task that is as yet unaccomplished. The report is the work of an interdisciplinary team of experts at ENSI (the “Japan Analysis Team”) and is based on both Japanese and international sources. It complements the two analyses of the events during the accident which ENSI presented at the end of August.
Swiss radiation protection under review
ENSI has already derived lessons to be learned for Switzerland from the findings in the report that is now available. These are contained in the report entitled “Lessons Learned and Checkpoints based on the Nuclear Accidents at Fukushima” dated October 2011.
In that report, ENSI announced (among other measures) that personal dosimetry and operational radiation protection in Switzerland were to be reviewed in order to determine their reliability in case of a severe accident. Obvious failings emerged in this area at Fukushima, especially in the first hours and days after the accident. The progression of the accident suggests the conclusion that more reserves for incidents must be held in readiness in this area than was previously assumed.
Experience gained from Fukushima also indicates that even in case of a severe accident, there are still numerous options for reducing the dose and for the effective prevention of incorporations, provided that the protective equipment is used in a consistent manner. This underscores the fundamental importance of preventive measures in the area of radiation protection so as to protect the population throughout the world, as well as in Switzerland.
High levels of radioactivity even outside the evacuation zone
The Fukushima accident took place in several phases, with differing hazard potentials for the population. The first (acute) phase primarily entailed the discharge of rare gases. At this time, the doses for the population originating from direct cloud radiation were limited because the wind carried the radioactive substances mainly from the land towards the ocean at first, and because the population in a 20-km zone surrounding the nuclear power plant was evacuated at an early stage, on 12 March 2011.
In the second phase of the accident from 15 March 2011 onwards, large quantities of radioiodine and radioactive caesium reached the environment from unit 2 in particular. As the prevailing winds were from the south-east on this day and there was heavy rainfall, the area to the north-west of the power plant was contaminated by deposits of radioactive substances up to a distance of about 50 km from the site. In the affected area, this led in some cases to sharp increases in ambient dose rates due to direct radiation, but also to contamination of the food produced in these tracts of land.
The ambient dose rates are still at greatly increased levels today (November 2011) in a partial area of about 600 square kilometres outside the evacuation zone. After the accident, if a person were to remain constantly in the open air here for a period of twelve months, he or she would be subject to external radiation exposure of 20 mSv – a value that is specified as the limit in Switzerland for persons exposed to radiation in the course of their work. For persons not exposed to radiation in the course of their work, a limit of 1 mSv per year applies.
The additional dose due to the consumption of food is only of secondary importance for the Japanese population. The authorities have made advance provisions here by means of effective control and monitoring measures.
Radiation hinders clearance work
A significant discharge of radioactivity into the sea took place due to leakages in a connecting duct at the start of April 2011. Only a rough estimate of the quantity involved is possible because neither the durations of the outflows nor the leaked quantities are known with any accuracy.
Furthermore, radioactively contaminated water was deliberately fed into the sea in order to mitigate the consequences of the accident at the power plant site. The concentrations of activity in seawater in the area surrounding the stricken plant returned to normal again by the end of April 2011. Even today, however, isolated samples of fishery products which exceed the limit for Cs-137 are still being found in Japan.
The accident also led to increased dose rates on the power plant site itself. In places, these reached several Sv/h. There were no deaths due to radiation, and no cases of individuals with radiation syndrome. The increased dose rate (due to the accident) of 250 mSv was exceeded in six individuals. The increased values continue to represent an enormous obstacle to dealing with the consequences of the accident at present; in some places, for example, debris can still only be cleared by remote control.
According to the operating company’s current remediation plan, it will take another three years until the releases are fully contained and decontamination has been completed on the plant site.
Consequences of Chernobyl were more serious
One section of the ENSI report compares the accidents at Chernobyl (1986) and Fukushima (2011). According to the comparison of the ecological and radiological consequences of the two reactor accidents, the Chernobyl event should clearly be assessed as more serious. Due to explosions and fires in the interior of the Chernobyl reactor, about five to ten times more radioactive substances entered the atmosphere − and reached substantially greater distances − than at Fukushima, where a large percentage of the release was directed away from inhabited areas towards the Pacific Ocean.
A comparison of the long-term consequences of the two accidents is still difficult at present because not all the relevant information is available for Fukushima as yet. According to the data available at this time, the effective radiation doses due to Fukushima for the local and global populations will prove to be substantially lower than at Chernobyl.
The report also assesses the radiological impact of the accident at Fukushima on Switzerland. It was possible to detect traces of the released radioactivity in our country with the help of highly sensitive procedures. However, the contributions to the dose caused by direct radiation and inhalation were so small as to be negligible. The radioactivity from Fukushima ingested with food would only amount to 0.5 µSv, even if 25 kg of the most heavily contaminated vegetables were to be consumed. This corresponds to one ten-thousandth of the annual radiation dose for persons living in Switzerland (about 5 mSv). At no time was there any risk to the health of the population in Switzerland.