Early Radiation Information

This was radiation information from the first few days after the situation in Fukushima began to develop. I essentially just tracked the numbers off the TV and picked up analysis where I could. This was long before MEXT started uploading the information regularly, and before translations became available. 

Radiation Readings



Measurements taken at the reactor entrance.

Date Time

Radiation Level at Fukushima Dai-ichi

(in microSieverts per hour, uSv/h) 

3/12 ~21:00* 70.5
3/13 ~11:00* ~1200*
3/13 13:52  1557.5
3/13 14:42 184.1
3/14 11:44 20
3/15 9:00 11,900
3/15 10:22**

#3 Reactor: 400,000

Btw #2 & #3: 30,000

#4 Reactor: 100,000

3/15 15:00 600

*Some times and levels above are estimates (~) based on my notes and memory. On 3/12, there was an early reading of 1015 uSv/h, but this was later said to be inaccurate and downgraded to 70.5. At a distance of 5 km, 1 uSv/h was measured at 11:36 on on 3/14.


**The spike at 10:22 was believed to have been caused by a fire at the #4 reactor. The fire burned out by 11:00 a.m. on 3/15.


For further data, check the following pdf from TEPCO: www.tepco.co.jp. The key column is the middle one ( 線). This lists radiation levels in microSieverts per hour (uSv/h). They are initially listed under a different measurement system, nanoGrays per hour (nGy/h), but they change over to uSv halfway into the morning of 3/12. As far as I can tell, Sieverts are just Grays multiplied by a constant indicative of biological effect.


Most of the readings in the above file were taken at the gate to the power plant (正門), while some were taken at other locations specified as MP-8 and such, but I'm not sure where these are.  

計測日  計測時間 計測場所  中性子線   風向  風速  







Radiation Level









 Location  Date Time 

Radiation Level

(in microSieverts per hour, uSv/h)

40 km from plant 3/15 4:00 23.72 (470x normal)
    6:00 3.94 (80x normal)
Tokai-mura, Ibaraki 3/15 7:46 5
    12:00 2

Tokyo, Kanagawa,

Chiba, Saitama,


3/15 Early morning

Single- to double-digit

multiples of normal levels

Miyagi, Yamagata,

Akita, Aomori,

Niigata, Gunma

3/15 Morning No significant change

While above normal, officials say that these off-site levels are not dangerous to human health.


What does this all mean? The situation is developing and there has been a lot of speculation. The following sources have been quite helpful.

Sir John Beddington

At 5pm Tokyo time (Tuesday 15th March 2011) a telephone briefing was given by Sir John Beddington the UK’s Chief Scientific adviser and Hilary Walker Deputy Director Emergency Preparedness at the Department of Health:


“Unequivocally, Tokyo will not be affected by the radiation fallout of explosions that have or may occur at the Fukushima nuclear power stations.”


More information at the UK embassy: http://ukinjapan.fco.gov.uk

Arclight (http://twitter.com/arclight)

This guy is a nuclear power plant risk management engineer, and he was tweeting as the situation developed. Sadly, he's since stopped, but the history is still valuable. 

Dr Josef Oehmen (http://bravenewclimate.com)

This person has been introduced as "a friend of a friend" on a brand-new blog, so the motives and motivations are a little sketchy. But he presents some very detailed information about the Fukushima Dai-ichi Reactor. 


The link above includes helpful diagrams. At the original post, knowledgable commenter Daniel notes that the Fukushima reactor likely does not actually have a core-catcher.