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Posts Tagged ‘Fukushima

Radiation Measurements at the Campus of Fukushima Medical University

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….. through the 2011 off the Pacific Coast of Tohoku Earthquake and Subsequent Nuclear Power Plant crisis

Serial gamma radiation measurements in air with sodium iodide (NaI) scintillation counter before and after the magnitude 9.0 earthquake, tsunami, and subsequent nuclear power plant crisis. Results were stored every 4 hours as the average cps from the accumulated counts. Measured place was Fukushima Medical University Department of Natural Sciences (Physics) professor's office. Radiation surveillance is a routine activity of this department.

Tsuneo Kobayashi
An earthquake, Tohoku region Pacific Coast earthquake, occurred on the 11th of March, 2011, and subsequent Fukushima nuclear power plant accidents have been stirring natural radiation around the author’s office in Fukushima Medical University (FMU).
FMU is located in Fukushima city, and is 57 km (35 miles) away from northwest of the Fukushima Daiichi nuclear power plant. This paper presents three types of radiation survey undertaken through the unprecedented accidents at the campus and the hospital of FMU.
First, a group of interested people immediately began radiation surveillance; the group members were assembled from the faculty members of “Life Sciences and Social Medicine” and “Human and Natural Sciences”.
Second, the present author, regardless of the earthquake, had serially observed natural radiations such as gamma radiation in air with NaI scintillation counter, atmospheric radon with Lucas cell, and second cosmic rays with NaI scintillation.
Gamma radiation indicated most drastic change, i.e., peak value (9.3 times usual level) appeared on March 16, and decreased to 1.7 times usual level after two months. A nonlinear least squares regression to this decreasing data gave short half-life of 3.6 days and long half-life of 181 days.
These two apparent half-lives are attributed to two groups of radioisotopes, i.e., short half-life one of I-131 and long half-life ones of Cs-134, Cs-137 and Sr-90.
Also, atmospheric radon concentration became high since a stop of ventilation, while second cosmic rays did not show any response.
Third, late April, 2011, a team of radiation dosimetry under the direct control of Dean, School of Medicine, was established for the continuation of radiation survey in the campus and the hospital of Fukushima Medical University.
Read more: http://arxiv.org

Written by physicsgg

November 14, 2011 at 1:54 pm

Posted in NUCLEAR PHYSICS

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Fukushima Radiation In California

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We all remember the earthquake and ensuing tsunami that hit Japan (see figure 1) and caused major troubles in the Fukushima nuclear power plant, which was severely damaged. Problems with containing radiation followed, leading to the pumping of huge amounts of seawater into the reactor, in an attempt to cool it down.

Figure 1: Height of tsunami following earthquake. CLICK FOR LARGER SIZE. Source: Wikimedia Commons, derived from the Japan Meteorological Agency

The question on many people’s minds was: how much radiation was released?

Recently, a team of researchers at the University of California, San Diego, has released the first quantitative measurements of the amount of radiation that leaked from the damaged reactor. Their estimate is based on signals that were sent across the Pacific Ocean after seawater was used to cool the reactor. On March 28th, 15 days after the seawater pumping began, the team measured a great peak in radioactive sulfur in the air in La Jolla…. Read the rest of this entry »

Written by physicsgg

August 17, 2011 at 4:00 pm

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Chain Reactions Reignited At Fukushima After Tsunami

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Radioactive byproducts indicate that nuclear chain reactions must have been burning at the damaged nuclear reactors long after the disaster unfolded

Nuclear reactors produce radioactive by-products that decay at different rates. One common by-product is iodine-131 which has a half life of about 8 days while another is cesium-137 with a half life of about 30 years.

When a reactor switches off, the iodine decays more quickly so the ratio between these two isotopes changes rapidly over a period of days. That’s why measuring this ratio is a good way to work out when the nuclear reactions terminated.

There are some complicating factors, however. The most important of these is that the ratio of iodine-131 and cesium-137 to start with depends on how long the reactor has been operating and so is not constant.

That’s because, after a reactor has been switched on, the levels of iodine-131 reach an equilibrium on a timescale similar to its half life of about 8 days.

But cesium-137, with a half life of 30 years, takes much longer to reach equilibrium. To all intents and purposes, the levels of cesium-137 in a reactor continue to grow steadily during the timescales over which reactors are usually operated.

The Fukushima reactor was struck by a magnitude 9 earthquake at 14:46 local time on 11 March. The three operating reactors there were immediately shut down.

About an hour later, however, the facility was struck by a tsunami with waves up to 5 meters in height. This destroyed the reactors’ electric cooling ability and the reactors began to heat up. The reaction between water vapour and the nuclear fuels’ zirconium cladding generated hydrogen which exploded in reactors 1, 3 and 4.

The question on many people’s minds is whether the hot nuclear fuel then melted allowing a critical mass of molten fuel to form, allowing chain reactions to restart.

Today, Tetsuo Matsui at the University of Tokyo, says the limited data from Fukushima indicates that nuclear chain reactions must have reignited at Fuksuhima up to 12 days after the accident.

Matsui says the evidence comes from measurements of the ratio of cesium-137 and iodine-131 at several points around the facility and in the seawater nearby. He has calculated what the starting ratio must have been by assuming the reactors had been operating for between 7 and 12 months.

He says the ratios from drains at reactors 1 and 3 at Fukushima are consistent with the nuclear reactions having terminated at the time of the earthquake.

However, the data from the drain near reactor 2 and from the cooling pond at reactor 4, where spent fuel rods are stored, indicate that the reactions must have been burning much later.

“The data of the water samples from the unit-4 cooling pool and from the sub-drain near the unit-2 reactor show anomaly which may indicate, if they are correct, that some of these fission products were produced by chain nuclear reactions reignited after the earthquake,” he says.

These chain reactions must have occurred a significant time after the accident. “It would be difficult to understand the observed anomaly near the unit-2 reactor without assuming that a significant amount of fission products were produced at least 10 – 15 days after X-day,” says Matsui.

So things in reactor 2 must have been extremely dangerous right up to the end of March.

Matsui points out that there are some potential question marks about the data. One possibility is that the chemical properties of cesium and iodine might mean they are flushed away from the reactors at different rates, changing their ratios.

But it’s hard to see what chemical processes could be responsible for this and even harder to understand why they would occur in some places but not others at Fukushima.

Of course, it won’t be possible to determine exactly what went on in reactor 2 and in the spent fuel ponds at reactor 4 until the sites can be physically examined in detail.

But in the meantime, Matsui’s analysis gives us one of the best insights so far into the nature of the disaster that unfolded after the tsunami hit.

Ref: arxiv.org/abs/1105.0242: Deciphering The Measured Ratios Of Iodine-131 To Cesium-137 At The Fukushima Reactors
http://www.technologyreview.com/blog/arxiv/26738/

Written by physicsgg

May 9, 2011 at 8:13 am

Posted in NUCLEAR PHYSICS

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