By Kapil Patil and P K Sundaram
The massive earthquake jolted the northeast coast of Japan on 11th March was followed by a killer tsunami, which damaged the reactors at the Fukushima prefecture and led to a widespread apprehensions about spread of radiation. The internet pages are filled with articles full of terminology obfuscation and are leading to an undue panic. Though the accident is serious, it is of paramount importance to pay heed to certain critical facts on the radiation release and the speculations of core melting. This article seeks to analyse the data released by the Tokyo Electric Power Corporation and Japan’s radioactivity safety survey on levels of radiation in Fukushima Dai-ichi site and in other prefectures.
In the immediate aftermath of earthquake at 14:46 hrs on 11th March, the control rods were automatically inserted into the reactor core effectively ceasing the fission process and using on-site power generators the decay heat removal started. The massive tsunami that followed the earth-quake at 15:41 hrs, destroyed the on-site generators that were powering the coolant pumps. The failure of generators resulted in pumps switching over to batteries which were also depleted after a short while. By the time mobile generators were managed, the reactors lost significant amount of core coolant. The coolant supply was re-established by 13.30 hrs on Saturday. In this period, there is possibility of reactor core being melted, which is being assumed primarily on the two contentions. First, the explosion caused by hydrogen on Saturday which destroyed the outer containment building of Unit 1, (the drywall “fourth” layer).
The explosion suggests that the vapour vented out of the reactor vessel included some amount of hydrogen. Hydrogen in such scenario can be formed due to steam reacting with the Zirconium claddings that hold fuel bundles. The Zirconium melts at about 1885 C, which indicates high rise in temperature inside the reactor vessel. Second, there is also a reported detection of Cesium and Iodine particles in the reactor vent. The detection of Cesium and Iodine has been attributed to the possible melting of the fuel rods. There are also widespread reports that the containment vessel of the reactor-2 is breached. However, this cannot be ascertained at this stage and is purely in the realm of speculation.
In order to avoid the over pressurization and eventual meltdown of the reactor containments of Unit 1, 2 and 3; the operator TEPCO released the radioactive vapour into the atmosphere. To maintain the manageable pressure inside the reactor vessel requires the operators to release the excessive steam in the atmosphere. Though the initial radioactive plume is blown towards sea by wind, the Japanese Government ordered a precautionary evacuation of people in the 30 Kilometres radius from the destabilized nuclear reactors. It would be important to note that the risks depends predominantly on how much radiation is released from the damaged reactors, how much people are exposed to and over how long a period, and in what direction the wind was blowing.
The TEPCO has been regularly releasing the radiation data since 11th March. The data is plotted in a figure 1 (click to enlarge). The data shows the normal levels of radiation on 11th and 12th March. With the first release of reactor vent on 12th March, the radiation levels at the Fukushima site spiked.
- On 12th march, the highest radiation levels measured were 646.0 µSv/hr at 14.10 hrs for a span of ten minutes. The subsequent ten minutes time-intervals measured the gradual decline in the radiation levels.
- The radiations measured on 12th March crossed the 100 µSv/hr only once and remained largely below the danger mark.
- On 13th March, the radiation pattern was not significantly different from the previous day. The highest measured spike was 905.1 µSv/hr at 14.10 hrs and subsequent readings shown a gradual decline in radiation levels.
- For next sixteen hours radiation maintained a steady level below 100 µSv/hr. On 14th March, radiation levels were well below the 100 µSv/hr spiking only twice at 293.5 µSv/hr and 3130 µSv/hr respectively.
- On 15th March, the radiation levels showed a steady rise and were in the range of 100 µSv/hr to 10000 µSv/hr.
- On 16th 2010, the radiation level was higher for significant period of time. After the earth-quake radiation level attained dangerous heights only on 16th March. The radiation levels were measured in the range of 4548 µSv/hr to 1312 µSv/hr.
- On 17th March the radiation level showed continuous rise and fall and continued to be in the range of 100 µSv/hr to10,000 µSv/hr. This range indicates a danger for the on-site workers. The radiation level of 100mSv/hr would cause radiation sickness after an exposure of nearly 2 ½ hours. The IAEA safety rules limit workers to exposure of 50 milli-sieverts per year.
- The data on 18th March, however, shows a steep decline in radiation levels from that of 17th. The highest radiation level measured on 18th was 287 µSv/hr.
The highest measured radiations on 16th March between 23.00hrs and 7.00hrs are a cause of concern for the people inhabiting in 10 Km radius, which is already evacuated. The data also shows that since 13th March the wind is largely blowing in North-West direction and the radiation levels measured in the chart (click to enlarge) are of North West of Fukushima.
It is learnt that the people have been moved in the opposite direction, i.e. South-West of Fukushima Dai-ichi. The levels of radiation in Tokyo are minimal, though modestly higher than normal levels. The radioactivity safety survey released by the government of Japan2 recorded the radiation levels in various states on 16th and 17th.
The survey shows that the radiation levels are nominal in all states except for Ibaraki and Tochigi where radiations were measured between 0.650 to 0.235 on 16th, where are the level declined on 17th.
Radiation levels in Tokyo were in the range of 0.028 µSv/hr to 0.079 µSv/hr. The lower reading of radiation in Tokyo shows the decreased intensity of the radiation due to dispersal. There is a reported release of Caesium and Iodine isotopes, however so far no traces are detected in the Tokyo region. As per the WHO study, an exposure of 500,000 µSv, can lead to nausea and fatigue within hours. A dose of 750,000 µSv causes hair loss within two or three weeks, and a dose of 1 million µSv will cause hemorrhage. Death usually occurs at a dose of 4 million µSv. In terms of long-term effects, experts estimate that if 10,000 people were each exposed to 10,000 µSv of ionizing radiation in small doses over a lifetime, about five or six more people in the group would die of cancer than would be expected without the radiation exposure.
On the basis of the radiation data released so far, we can infer that the radiation levels did not cross lethal levels for the initial 5 days. The radiation level spiked only on 16th March, which poses a significant danger for the on-site workers. However, the evacuation in 20 kms radius nearby Fukushima has precluded any major human catastrophe. In order to dispel the undue panic caused by radiation, it is imperative to constantly monitor the radiation levels till it reaches normal. The possibility of situation worsening can not be ruled out. However, it would be counter-productive to create panic by speculating only worst case scenario.
1. TEPCO Press release http://www.tepco.co.jp/index-j.html
2. See: Radioactivity Safety Survey, released by MEXT, Japan. http://www.mext.go.jp/
Originally published by Institute for Defence Studies and Analyses (www.idsa.in) at http://www.idsa.in/idsacomments/JapansNuclearCrisisandAnalysisofRadiationData_180311