Trends
Edge of a  black hole
NASA scientists and their international partners using the new Japanese Suzaku satellite have collected a startling new set of black hole observations, revealing details of twisted space and warped time never before seen with such precision.

Prof Yash Pal

Nuclear waste management
K.S. Parthasarathy

ON September 28 this year, the Minnesota Utility Commission unanimously approved Xcel Energy’s request to store highly radioactive nuclear waste in up to 30 above-ground containers outside its Monticello, Minnesota nuclear power plant.

A few lobbyists, environmental watchdogs and utility executives witnessed the event. There were no placard carrying, angry protesters!

Nuclear waste management is the most contentious issue in nuclear power generation. Patrick Klein, Deputy managing director of Ipsos Opinion, an opinion research agency believes that surveys and opinion polls regularly reinforce the belief that nuclear waste is one of the general public’s greatest fears. He feels that the media often handles this theme in a negative light.

Not even a single newspaper article from among the dozens I collected over the past four years, concede the fact that scholarly bodies have selected disposal in deep geological repositories as a favorable consensus choice for the safe disposal of high level wastes.

Scientists and engineers in the field must share the blame. They have not explained the waste management technologies to public at large.

Nuclear fuel, when introduced into the reactor, is mildly radioactive. As the reactor operates, the fuel becomes more active. After an year or two, when the operators remove the spent fuel remotely, it is so active that any one remaining near it can receive fatal radiation doses in minutes.

The spent fuel is about 3 per cent of the volume of all wastes but contains 95 per cent of the radioactivity. We must keep it away from the biosphere. For this, specialists select a multiple barrier approach. Initially, the spent fuel is very hot. It remains in specially designed pools for a few years to cool it.

Countries such as Canada, Finland, South Korea, Spain, Sweden and the US have accepted direct disposal of spent fuel. Others such as Belgium, China, France, Japan, Russia Switzerland, UK and India opted for reprocessing. Spent fuel is a resource. India developed technologies to recover plutonium and depleted uranium from the spent fuel.

The radioactive solution left after removing plutonium and uranium is very active and contains radio-nuclides such as cesium, strontium, etc. Most of these have half lives of the order of 30 years. After a few thousand years, its activity will be similar to that of the naturally occurring uranium ore from which the fuel originated.

Technologists immobilise the waste in an insoluble matrix such as borosilicate glass which is amazingly stable (This process, called vitrification, is done remotely). They seal the active glass in corrosion- resistant canisters.

According to the Uranium Information Centre, the vitrified waste from a 1000 MWe reactor for one year would fill about 12 canisters each of 1.3 metre height and 0.4 metre diameter and holding about 400kg of glass. (UIC, 2006).

Alloy C-22 and titanium may be used to make corrosion resistant canisters. The latter dissolves in flowing saline water at ambient temperature at a rate of about 0.0013 mm per year. A container of a few centimetres thick will last several thousand years. Scientists have estimated that 5 cm claddings of copper would take a million years to corrode.

Radioactive waste carrying non-corrosive containers are then kept surrounded by hundreds of metres of impermeable back-fill materials such as clay or granite or salt depending on the site. These layers retard the movement of radio-nuclides if they get released from their containers. In addition, the site chosen will be remote, seismically stable with very low rainfall. When the radio-nuclides reach the biosphere it will have negligible radioactivity.

Radioactive waste management involves many challenges. Scientists must inform the public how we can face these challenges with the technologies developed in India.

Dr K. S. Parthasarathy is a former Secretary, Atomic Energy Regulatory Board, Mumbai.

Trends
Edge of a black hole

NASA scientists and their international partners using the new Japanese Suzaku satellite have collected a startling new set of black hole observations, revealing details of twisted space and warped time never before seen with such precision.

The observations include clocking the speed of a black hole’s spin rate and measuring the angle at which matter pours into the void, as well as evidence for a wall of X-ray light pulled back and flattened by gravity.

The findings rely on a special feature in the light emitted close to the black hole, called the “broad iron K line,” once doubted by some scientists because of poor resolution in earlier observations, now unambiguously revealed as a true measure of a black hole’s crushing gravitational force. This technique can be exploited in future X-ray missions.

Seals can turn off shivering

Seals shiver when exposed to cold air but not when diving in chilly water, a finding that researchers believe allows the diving seal to conserve oxygen and minimise brain damage that could result from long dives.

The research provides insight into how seals allow their bodies to cool (become hypothermic) during a dive.

THIS UNIVERSE
Prof Yash Pal

We live in a very busy part of Jalandhar and sadly a lot of buses blow their pressure horns along the roads outside the house. The sound seems to travel through the walls and even jumps over the 20-foot high walls of our garden. Does it mean that sound penetrates our wall or does it travel in an arc over the wall?

I agree that sound pollution is a curse that we keep hurling on ourselves. This along with all other types of pollution is definitely a discomfort we are not civilised enough to avoid.

Coming now to the physics of your unsavoury experience, all optically opaque obstructions are not good blockers of sound. If they were we would not need such elaborate designs for sound recording studios.

I do not know about the thickness and composition of the material of your wall. Hard walls are not necessarily good sound insulators. The wall itself vibrates a little and transmits the sound on.

I suppose the wall does not surround you on all sides. Even if there are small gaps, or an entry door, lot of sound can come in.

We have to remember that sound is a wave and its wavelength might be of the order of a meter. Such a wave can diffract around edges, as you suspect.

We know this is true. For example if music is playing in the living room and the door to the bedroom is open we can hear the songs very well. The wave bends around to go through the door. Closing the door helps a lot.

If the building of your own house, or any other building close to your house is tall enough to oversee your compound wall then the sound of the pressure horn on the road could reach you after being reflected from these buildings.

Concrete or cement plastered surfaces are good reflectors of sound.

I might go on to advise you to cover outside walls of your house with sound absorbing material, but all this is getting rather ridiculous.

What you need to do is to file a public interest case in the High Court or the Supreme Court to make sounding of pressure horns within the city limit a cognisable offence. Get some of your neighbours with you and do that.