Background radiation level at any place depends mainly on terrestrial radiation from natural radionuclides such as uranium, thorium and their decay products and potassium-40 present in soil and on cosmic rays (radiation from outer space).
The top one metre soil in a land of area 0.1 acre anywhere in the country may contain 1.28 kg of potassium-40, 3.6 kg of thorium and one kg of uranium.
The soil factor
In different soils these values may be higher or lower. The dose due to natural radiation varies from place to place.
The releases from the Madras Atomic Power Station (MAPS) have not exceeded the limits prescribed by the regulatory agency. The data collected from 1982 to date by the Environmental Survey Laboratory (ESL) at MAPS do not indicate high increases in radiation levels. Nuclear Power Corporation Limited should invite students and public to the ESL to reassure them. BARC scientists have measured outdoor natural gamma radiation levels at 214 locations in different parts of India continuously over a year by using special dosimeters and got an average annual value of 775 microgray.
They got the highest value of 26730 microgray at Chavara, Kerala. (Gray is a unit of radiation dose.
Energy absorbed
When the dose is one gray, the energy from ionizing radiation absorbed is one joule per kg; since gray is a big unit, submutiples such as milli-one thousandth or micro -one millionth-of a gray are usually used).
Using the geological data on the types of rocks and the abundance of uranium, thorium and potassium in these rocks, BARC scientists prepared a radiation map of the country in 1986.
The natural background radiation doses at Mumbai, Kolkata, Chennai and Bangalore are 484, 810, 790 and 825 microgray per year respectively.
The annual cosmic ray dose itself varies from 280 microgray in Chennai to 440 microgray in Bangalore which is 921 metres above the sea level.
Variation of a few hundred microgray is not unusual.
What is the extra contribution, if any, to radiation dose made by a nuclear power station at its fence post?
Data from 1982 to 2007 indicate that for the MAPS, the lowest extra dose was 7.2 microsievert per year and the highest 86.3 microsievert per year. (Sievert — Sv — is another unit. For all practical purposes, background radiation expressed in sievert or gray may be considered numerically equal). These are much less than the AERB limit of 1,000 microsievert.
The status is similar in other power stations.
Activity can escape only if several barriers (ceramic fuel, its cladding, coolant, coolant tube etc) are broken. These are of proven quality. Fail safe devices including redundant and diverse safety systems ensure that such uncontrolled releases do not happen.
In April 1994, an “environmentalist” measured high radiation levels at the MLAs’ hostel and other locations in Lucknow. “Radiation levels at some points are higher than those at Chernobyl,” he claimed. Many newspapers published the story.
Recorded normal
A team of scientists sent by the Atomic Energy Regulatory Board (AERB) found that his Geiger-Mueller counter —based instrument was defective. It was light sensitive. When the detector was made light-proof, the radiation levels recorded normal!
In 2005, Padmanabhan and Nakul of the South Asians Against Nukes (SAAN) claimed that while monitoring radiation levels around the Madras Atomic Power Station in places such as Coimbatore, Chenglepet, Mahabalipuram and Kalpakkam they came across several spots with gamma radiation levels as high as a few millisievert to above ten millisieverts per year. The highest level measured was 54.5 millisieverts.
The speculation
They speculated that the wide variations between the readings taken from the same spot indicated that the source of radiation is not on the ground but are “picoparticles” from uranium fission floating and flying in air! There are simpler explanations.
If they were using a defect-free instrument, they were recording radiation from thorium rich soil in the region. This is nothing unusual.
Since over 70 per cent of their readings were above 2mSv, I believe that they were experiencing the problem faced by the “environmentalist” in Lucknow!
Former Secretary, AERB ( ksparth@yahoo.co.uk )
