This diary reports on a recently published study of the levels of radiocesium contamination in rice grown in Fukushima Prefecture since the triple reactor meltdowns in March 2011. It is the most recent in an ongoing effort to report the results of peer reviewed, scientific research into the impacts of the disaster on the environment. An impressive monitoring/screening program has been put in place by the Japanese authorities to verify the safety of agricultural and fisheries products after the disaster under their Nuclear Disaster Act. Rice is the most culturally and economically important agricultural product in Fukushima Prefecture. Therefore, post accident inspections on rice were a focus of this monitoring program. The proportion of bags of rice with radiocesium concentrations exceeding the more stringent 100 Bq/kg limit put in place after the disaster was 0.8% in 2011, a mere 0.0007% (71 bags out of 10,338,000) in 2012 and 0.0003% (28 bags out of 11,001,000) in 2013. Not a single bag tested out of the more than 10,000,000 bags screened in 2014 exceeded the safety limit. This screening program has likely dramatically limited the exposure of the public to radionuclides deposited over the Japanese territory as a result of the disaster.
The study by Nihei and colleagues was published in the open access, peer reviewed journal Scientific Reports on March 3, 2015. They report on efforts to monitor for the presence of Cesium-137 (137Cs half life ~30 years) and Cesium-134 (134Cs half life ~2 years), two isotopes released in significant quantities from Fukushima Dai-ichi and representing a potential radiological health risk to the public. After screening of rice produced in the Prefecture in 2011 found product exceeding the 500 Bq kg-1 allowable limit at the time in areas most impacted by fallout a more comprehensive monitoring program was put in place with the ambitious goal of screening all rice produced in 2012. To accomplish this feat custom-built belt conveyor testers for rapid initial screening of rice bags were deployed:
(a) Flowchart showing the procedure to inspection rice produced in Fukushima. The weight of the rice bags are uniformly 30 kg. The screening level was 50–85 Bq/kg, which was dependent on the belt conveyor testers and the background level. (b) A picture of one of the belt conveyor testers. There are 5 belt conveyor testers certified by Fukushima Prefecture to perform the inspection of rice.
The flowchart in the figure describes how bags of rice are screened initially with the conveyor belt gamma detector with a detection limit of about ~85 Bq kg-1. If a given bag is below this level it is suitable for sale and certified. If the bag is above the initial screening level it is subjected to a much more sensitive screening with a high purity germanium gamma spectrometer. If it is above the 100 Bq kg-1 maximum allowable limit it is incinerated and if below it is certified for sale.
Results
Detailed results for the 2011-2013 screening program can be found in the studies results section but a brief summary is as follows:
- The emergency survey in 2011 examined 20,520 bags of rice and found that ~86% of sites in the Prefecture had radiocesium activities of 25 Bq kg-1 or lower, ~12% had measurements of 25–100 Bq kg-1 , and ~3% had measurements of 100–500 Bq kg-1,with the maximum value obtained being 1,154 Bq kg-1 (below the USA maximum allowable radiocesium activity).
- In 2012 10,338,291 bags were examined with 99.8% being 25 Bq kg-1 or lower, 0.2% had measurements higher than 25–100 Bq kg-1, and less than 0.001% of bags exceeded 100 Bq kg-1.
- In 2013 only 28 bags out of 11,001,000 screened exceeded 100 Bq kg-1 (0.0003%) while virtually all (99.9%) rice produced had radiocesium content 25 Bq kg-1 or lower.
- Not a single bag of the more than 10,000,000 screened in 2014 exceeded 100 Bq kg-1.
A map summarizing the 2011-2013 results is shown in the following figure:
Chart sorting the former municipalities (classifications as of February 1, 1950), including 374 sectors of the Prefecture, by color according to their maximum values of radiocesium activity concentrations in rice, measured by the preliminary survey, the main inspection, or the emergency survey in 2011 (a) and by detailed inspection using a germanium semiconductor detector employed after the screening of the inspection of all rice in 2012 (b) and 2013 (c). Blue area; Lake Inawashiro.
The map for 2014 where cultivation is allowed would of course show all green.
This rigorous monitoring program keeping contaminated rice out of the market and relatively low levels of contamination in a staple food like rice may explain the low levels of radiocesium found in residents (children and adults) of Fukushima Prefecture since the disaster. This screening program will be very useful to ensure food safety as previously closed areas to rice and other produce planting are opened in the future.