It is a very confusing time right now. There is so much information out there about the situation in Japan, but it is not always clear what applies to Tsukuba and what only applies to the area around the reactors. Here is my understanding of the situation.
Atmospheric Radioactivity and Radiation Exposure
The levels of atmospheric radioactivity in Tsukuba have continued to decrease since March 11. You can see it clearly by looking at the graph that KEK is maintaining (http://rcwww.kek.jp/norm/index-e.html). The level of atmospheric radiation around that institute right now (in Tsukuba) is 0.12 microSv/h. The “normal level” is between 0.07 and 0.09 microSv/h, so the level right now is extremely low. The US Nuclear Regulatory Commission says that the average annual radiation exposure from natural sources in the US is about 3.1 millisieverts, or 3000 microsieverts (http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/bio-effects-radiation.html). At the current rate of 0.12 microsieverts/h, we can calculate that if we receive this level of radiation for 24 hours over 365 days, our exposure will be 1051.2 microsieverts, or 1.1 millisieverts. Since that is less than half the annual rate in the US, I think we are currently safe in Tsukuba.
The question is, will we remain safe? Certainly we should not experience any acute radiation exposure situations. We may receive some low level exposure due to some low level contamination of vegetables, milk, or water, but likely not in excess of any levels set by the Japanese government. Long term risk to us will be small compared to the levels of risk that we take for granted in our every day activities.
Radiation exposure in extremely large doses over short periods of time (hours to a couple of days) can be fatal. This is known as an acute exposure. For example, if you received a dose of more than 4 Sieverts (=4,000 millisieverts = 4,000,000 microsieverts) in a short time, you would have a 50% chance of dying within 30 days if you received no medical treatment. At doses much lower than that, such as 0.25 Sieverts (=250 millisieverts = 250,000 microsieverts), it is highly unlikely you would feel any effects at all. Small blood changes might occur, but these could only be seen in specialized blood tests. The chance of us in Tsukuba receiving an acute dose of even 250,000 microsieverts is vanishingly small, especially when you consider that the highest external radiation level we have so far recorded in Tsukuba since the accident is 1.54 microsieverts/hour (AIST on March 15) and one of the highest levels recorded at the plant itself so far has been, I believe, around 11,930 microsieverts/hour at the gate of the plant on March 15, 2011 (source: http://www.tepco.co.jp/cc/press/betu11_j/images/110315e.pdf). The dose that you receive decreases as you get further away from the source (as you can see from the two readings just mentioned that were taken in Tsukuba and at the plant on the same day, March 15), so if the radiation levels at the plant boundary (gate) are nowhere near 250,000 microsieverts total than the radiation levels more than 150 km away (i.e. in Tsukuba) will not even get close to a level of concern. Keep in mind that the amount of radioactivity that reaches us in Tsukuba is also highly dependent on the weather, so even if a certain amount is emitted from the reactors, that doesn’t guarantee that an exact amount will then arrive in Tsukuba. Also, note that the 11,930 microsieverts/hour reading that was taken at the gate at 9am on March 15 was already under 1000 microsieverts/hour by 2pm that same day.
The actual radioactivity that is released from the plant becomes very dilute in the atmosphere, and by the time it reaches us (if the wind is blowing towards us), the levels are very small. We can possibly receive low level exposure from ingestion of food or water, but these exposures are not going to be at levels where acute exposure is a problem.
So, for us in Tsukuba, and for anyone who is not extremely close to the reactors, the issue is not acute doses, but chronic doses. A chronic dose means that you are receiving a constant dose at a certain level. The problem is that we do not really know what the limit is for a chronic dose. However, if we play out some scenarios like we did above, and we assume that, for example, a chronic dose of 9 millisieverts/year is safe (because this is the dose that an airline crew flying the New York to Tokyo polar route would receive in a year — source: http://www.bbc.co.uk/news/health-12722435), then the level in Tsukuba can increase 8 times (0.12 microsieverts/h x 24 x 365 / 1000 x 8 = 8.4 millisieverts/year) and still be safe. According to the same source, the current limit for nuclear industry employees is 20 millisieverts/year. In order to receive that annual dose, the current level in Tsukuba would have to increase 19 times (0.12 microsieverts/h x 24 x 365 / 1000 x 19 = 19.9 millisieverts/year). Also according to the same source, some places in Iran, India, and Europe have annual background doses in the 50 millisievert range. In order to receive that annual dose, which is also assumed to be safe, the current level in Tsukuba would have to increase 47 times (0.12 microsieverts/h x 24 x 365 / 1000 x 47 = 49.4 millisieverts/year). Also note that these increased levels could be maintained for a whole year and still be deemed safe.
I don’t have any data for the radiation levels in Tsukuba right after the earthquake and tsunami on March 11, but the highest level that I have seen detected in Tsukuba so far is 1.54 microsieverts/hour on March 15 (detected at the National Institute for Advanced Industrial Science and Technology). That would give an annual dose of 13.4 millisieverts/year (1.54 microsieverts/h x 24 x 365 / 1000 = 13.4 millisieverts/year) IF THE LEVEL REMAINED THAT HIGH FOR THE ENTIRE YEAR. However, as we have noticed, the levels do not tend to stay high, but decrease with time, so the likelihood of a spike like that being maintained for one year is unlikely. (As I said above, it is already down to 0.12 microsieverts/h, which is only around 1 millisievert/year.) And remember again that the spike is only 1.54 microsieverts, which would be totally harmless by itself as an acute dose, since we discussed above that an acute dose of 250,000 microsieverts might make you start to feel unwell.
So, we have to keep in mind that acute doses can cause radiation sickness if they are high enough, but that we would have to receive an acute dose of 250,000 microsieverts in order to get even temporarily sick from radiation. The chances of us receiving an instant dose at that level are extremely unlikely, since that would mean we would be getting a dose that is about 2,000,000 times higher than what we are receiving now. So I would like to propose that we not even think about acute doses as a problem in Tsukuba.
What we do potentially have to worry about are chronic doses, which do not cause radiation sickness, but are assumed at certain levels to be associated with an increased risk for certain kinds of cancers. The problem here is that we do not know the exact level at which the risk for cancers increases. From what I have read recently, it seems that this is a hotly debated question in nuclear research, and no one has come up with a definitive answer. However, if we look at the results that we have come up with above, we can see that the levels in Tsukuba can increase quite a bit (8 to 47 times, depending on where you draw the line) and still be considered safe by many experts. That is reassuring because I am certain that someone around here will give us a heads-up if the levels get anywhere near 5.64 microsieverts/hour (47 times what they are now).
Furthermore, even if the hourly levels did spike up to 5.64 microsieverts/hour, we could stay in Tsukuba for at least a year and still receive less than some people receive from natural background levels in several places in Iran, India, and Europe.
For reference, note that the radiation levels in most parts of Fukushima Prefecture currently range from 0.08 microsieverts/hour in Minamiaizu (about 115km west of the plant) to 1.80 microsieverts/hour in Koriyama, (about 58km west of the plant). The levels in Iitate (about 40km northwest of the plant) are at 5.21 microsieverts/hour, which is still below the threshold that we have identified above. (Source: http://www.worldvillage.org/fia/data/pd_e_692.pdf)
So, my advice is that it is fine to live in Tsukuba right now. The chances of us receiving an acute dose of radiation that will cause us to get radiation sickness are so extremely low as to be inconceivable. The chances of us receiving enough of a chronic dose of radiation eventually in the future to cause us to have an increased risk of cancer (note that “an increased risk of cancer” is not the same as “everyone who is exposed to this amount on a chronic basis gets cancer”), are higher, but not an issue right now. If the current levels go up 8 to 47 times higher than they are now, we can still stay in Tsukuba for one more year (or longer) and not expect our chances of developing cancer at some point in our lives to increase. (We may still get cancer, of course, but we would not be able to say that it was because of living in Tsukuba.)
(I asked a Board Certified Health Physicist in the US with 25+ years of experience in radiation safety to read this over and he agreed with my conclusion that it is unlikely that we would be evacuated at this distance.)
Food and Water contamination
The Ibaraki prefectural government has been testing the water from Kasumigaura on a daily basis. The levels of radioactive iodine and radioactive cesium are continuing to decrease on a daily basis. You can track the information here.
The levels of radioactive iodine and radioactive cesium in water from Kasumigaura (the main source of water for Tsukuba residents) are currently very low, and in fact, they were never high enough to warrant any restrictions on drinking tap water for infants or adults.
There are some foods (mainly vegetables) from Ibaraki that were restricted from being sold. Most of them are not showing high levels of radioactive iodine or radioactive cesium anymore. You can track this information here.
Basically, if they find high levels of radioactive iodine and radioactive cesium in any products, they will ban them from being sold, so the likelihood of consuming them in large enough quantities to be harmful is low.
We have had a lot of them, but they are expected to decrease over time. They can be mentally tiring, especially when they happen at night, but they are not necessarily a reason in themselves to stay away from Tsukuba, in my opinion.