What the radiation dose levels mean

Every nuclear site in Japan has in place a permanent radiation monitoring system. This includes many fixed measuring posts at site boundaries, mobile measuring equipment which checks environmental radiation levels inside and outside the site boundary and permanent radiation monitoring equipment at the ventilation stack exhaust and cooling water outlets of the plants (NucNet No 62a).

The earthquake did not damage this system seriously. Radiation levels at various monitoring posts at the boundary of Fukushima-1 nuclear power station began to rise with ups and downs depending on weather and activity levels and showed steadily increasing levels.

The background radiation levels were of the order of 0.07 microSv/hr; it rose to a few tens of microSv/hr and later to tens of milliSv/hr in the vicinity of the plant

At 11:00, 16 March, the dose at the Plant border was 3.3 milliSv/hr; evacuation area is up to 20 km from nuclear power plant.

People who stay in 20km to 30 km from the Fukushima- Daiichi are advised to stay indoors. For Fukushima-2 nuclear power station the radiation level at the plant border I was 0386 milliSv/hr at 4:00 16 March

For units 2, 3 &4, the dose levels at different locations continue to be a few tens or few hundreds of milliSv measured at different times.

Iodine-131, caesium-134, caesium-137, radioisotopes of xenon, krypton and other radionuclides released from the stricken nuclear reactors can cause exposure to the individuals. Among these, iodine-131 is very important during the early phase of the accident. Authorities distribute stable iodine tablets to the public if there is a possibility of exposure.

Generally, counter measures include sheltering, ad hoc respiratory protection, administration of stable iodine, evacuation, relocation, control of access, control of food and water and use of stored animal feed and decontamination of affected areas and buildings ( AERB guide SG/HS-1)

Iodine tablets

Stable iodine if present in the thyroid will prevent absorption of radioiodine by that organ. The dose reduction of 100 per cent is possible if stable iodine is administered before exposure. With administration one hour after exposure, the dose reduction will be 80 per cent if consumed after six hours of exposure stable iodine helps to lower dose to 50 per cent a day after it does not protect at all.

Impact on India

The radiological impact on India due to the radioactive releases from the Fukushima nuclear power plant will be an insignificant fraction of that, if any, due to natural background radiation. It will not cause any life-threatening doses to any one in India

Since we have state-of -the art radioactivity measuring procedures with ultra sensitive equipment, we may be, in the coming weeks, able to detect iodine -131, if the reactors released large amounts of the isotope.

Indian scientists measured traces of iodine-131 released from Chernobyl a few weeks after the accident.

Health physicists in every Indian nuclear power station routinely measure iodine -131 in goats' thyroid. Traces of iodine-131 may be deposited over grass in due course. Thyroid takes up the isotope. Extremely small amounts of radioiodine can thus be measured.

Indian scientists have measured iodine-131 on a few occasions over the past several decades. They detected the iodine-131 fall out from the Chinese and French atmospheric weapons testing and from the accident at the Chernobyl nuclear power station. In all these cases the doses to people were trivial and a negligible fraction of the dose due to background radiation.

We have reliable and sensitive techniques of measuring radioactivity in food stuffs. The administrative net work to collect food samples is also in place. This may be activated as a matter of abundant caution if there is any need. The accident at Fukushima is likely to have only local consequences.

Former Secretary, Atomic Energy Regulatory Board (

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Printable version | Oct 26, 2020 12:26:31 AM |

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