SCIENCE & TECHNOLOGY

N-energy with minimum waste
Amar Chandel
W
ORLD over, nuclear reactors are of the fission type, in which Uranium atoms are split to generate tremendous amount of energy. In these commercial ‘light water reactors’, uranium atoms are split by bombarding them with neutrons. The trouble is that the fission reactors also generate a lot of radioactive waste, disposing off which is a serious problem.

Interview
Fusion aided destruction of transuranics
Q: Are we talking here about fission or fusion energy?
A: Most of the energy is released from fission. Fusion is crucial for the destruction of the transuranic fission waste (producing energy in the process) that carries the greatest long term biohazard. The envisioned system is a fission-fusion hybrid in which fusion supplies neutrons crucial for the fission reactions.

Prof Yash Pal

Prof Yash Pal

THIS UNIVERSE
Prof Yash Pal
I travel very frequently by air. Most of time the pilots announce that the outside temperature is somewhere -30 to -50 degrees Centigrade at a height of 30,000 to 35,000 feet How is it so that on ground we have +20,+30 or even +35 but at this height which is closer to sun the temperature is so less?





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N-energy with minimum waste
Amar Chandel

Dr. Swadesh Mahajan
Dr. Swadesh Mahajan

WORLD over, nuclear reactors are of the fission type, in which Uranium atoms are split to generate tremendous amount of energy. In these commercial ‘light water reactors’, uranium atoms are split by bombarding them with neutrons. The trouble is that the fission reactors also generate a lot of radioactive waste, disposing off which is a serious problem.

Several types of nuclear waste are produced in the process. One variety is the ‘daughter atoms’, the leftover bits of the split atoms. These itself remain radioactive for centuries. That is irksome, but manageable. The material maybe stored and monitored above ground for a few hundred years till it becomes harmless.

The bigger worry is the new transuranic elements which get produced during fission, because they can stay radioactive for hundreds of thousands of years. This waste involves those atoms which absorb the bombarding neutrons but don’t split in the typical “light water reactor” the workhorse of nuclear industry. These highly toxic “transuranics” include several kinds of plutonium.  The standard fission reactors cannot split these new elements except for a small portion of the plutonium and they remain radioactive for hundreds of thousands of years. No wonder environmentalists are dead set against them, making nuclear energy hard to sell to a large section of the populace.

Fusion reactors in which atoms combine in place of splitting are still decades away, but pose no such problem. As we wait for pure fusion recators to become a reality, there is a great near-term potential use of fusion; the fusion neutrons can be exploited to clean up the “transuranics” generated in fission reactors.

According to a paper published in the journal Fusion Engineering and Design by Swadesh Mahajan (a senior research scientist at the Institute for Fusion Studies, University of Texas) and colleagues, a combined-cycle fission-fusion reactor could be efficiently harnessed to destroy the long-term biohazardous transuranics. The time needed for ~99 per cent transuranic waste destruction reduces from centuries (in the standard fission-only methods of waste destruction) to decades. And the number of reactors needed  for the job is reduced by a factor of 5-10. The fusion- fission hybrid , just like any other reactor,  will also generate net energy —  from transuranic waste providing the fuel .

One of the most important features of the fusion system devised  by Dr Mahajan and his colleagues, Mike Kotschenreuther, Prashant Valanju and Erich Schneider, is that it has high energy density and is highly compact (CFNS) — compact enough that it  can be slipped inside a typical fission reactor. The  core of the entire hybrid reactor is of a moderate size and can easily fit into a small room.

The high-power Compact Fusion Neutron Source (CFNS), in some sense, is the  centre-piece of the proposed  hybrid reactor.  Because of the compactness of the neutron source, there is very good coupling between  the source and  fission ‘blanket’  made of the waste material. Consequently,  the waste destruction is expected to take place rather efficiently.

This way, nuclear power could be made cleaner at an affordable cost, and become a more viable replacement for carbon-heavy sources like coal. One hybrid would be able to destroy the waste produced by 10 to 15 ‘light water reactors’ producing power.
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Interview
Fusion aided destruction of transuranics

Q: Are we talking here about fission or fusion energy?

A: Most of the energy is released from fission. Fusion is crucial for the destruction of the transuranic fission waste (producing energy in the process) that carries the greatest long term biohazard. The envisioned system is a fission-fusion hybrid in which fusion supplies neutrons crucial for the fission reactions.

Q: You are claiming that your Fusion-Fission Hybrid system can destroy transuranic waste, freeing fission energy of its most hazardous type of waste. It seems too good to be true. Are there any caveats? What about other wastes — is transuranic waste special?

A: Recent inventions have paved the way for designing a high power density compact fusion neutron sources without which such a claim could not have been. Much (75%) of the waste can and will be burned with standard fission reactors, but the remaining (25%) transuranic waste contains the worst of the long-lived hazardous material, and its incineration needs the huge numbers of neutrons that these new compact sources will produce at an affordable cost. Fission energy, however, can never be made fully waste free. The fission products (the smaller nuclei into which U, Pu etc breakup into) are radioactive; their radioactivity and toxicity in the long term is orders of magnitude lower than that of the transuranic elements.

Q: I see the Hybrid system destroys 99% of the bad waste that poses the greatest long-term environmental and biological hazard. What about the remaining 1%

A: Such small residue is realistically to be expected with any large system. It can be safely buried in a repository with a much smaller (by 10 times or more) capacity than is needed to hold fission waste in its current form. Moreover, there is no material constraint to using the systems to reduce transuranic inventories beyond 99%. The question at this level becomes one of the cost of operating the system to consume the final 1% versus the marginal benefit of reducing disposal requirements even further.

Q: What impact is your solution likely to have on the national debate on geological repositories like Yucca Mountain, and on the overall future of nuclear energy?

A: If the system proposed here is eventually shown to work, the need for Yucca-mountain-like repository will reduce to, perhaps, one (instead of 10) for this century. It is expected that such an efficient waste-destruction solution may remove the largest societal obstacle to major expansion of nuclear energy — a necessary but not yet widely accepted part of a multi-pronged strategy to avoid global warming.

Q: You believe that nuclear energy has to be a part of the near-term carbon-free energy mix— the wind, solar and other renewable sources can’t prove to be adequate. What about clean coal?

A: The short answer is no. Intermittency limits renewables to small (under 20%) fractions of total need. Like coal, nuclear power produces the base load energy that’s needed to power our cities and industries around the clock. Clean coal is at best decades in the future (if it is possible at all) — unlike nuclear fission, which is available today.

Q: You plan to use fusion to destroy waste- isn’t fusion supposed to be a technology of the future and not ripe for use in near-term. Are there any recent advances in fusion research that are responsible for opening up the doors for near-term applications of fusion?

A: producing net power from fusion is not a short term technology. However creating prolific neutrons in a fusion device is a much less severe task though one has to make these sources relatively cheap and compact to be efficient. Our invention allows a fivefold decrease in device volume while making the same number of neutrons. This key advance brings the size and cost of fusion neutrons into a range for this nearer term application. And this waste destruction system also allows the energy released by burning the waste to be converted into electricity.
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THIS UNIVERSE
Prof Yash Pal

I travel very frequently by air. Most of time the pilots announce that the outside temperature is somewhere -30 to -50 degrees Centigrade at a height of 30,000 to 35,000 feet How is it so that on ground we have +20,+30 or even +35 but at this height which is closer to sun the temperature is so less?

It would be nice if the pilot also announced the value of the atmospheric pressure outside. If he did you will find that this is about one fourth of the pressure at sea level. This is the first hint about the reason for the cold on high mountain tops. If one were to take a balloon containing one litre of air at sea level to an altitude where the pressure is one fourth of that at sea level the balloon will expand to a volume of four litres. During this expansion molecules of the gas move away from each other. In doing so they have to work against the force of mutual attraction between the molecules. As a result their random kinetic energy is reduced. Temperature is nothing but a measure of the same random kinetic energy of molecules. In other words the temperature of the gas substantially decreases. One can therefore say that it is cold on the mountains because the air at that altitude is cold. After I had typed this I sat down to estimate the temperature of air at an altitude where the atmospheric pressure would be reduced to one fifth its value on the ground. I found that it should be much lower than what the airline pilots announce! This baffled me for a while till I realised that I had been silly and neglected the effect of the earth albedo. In other words I had not taken into effect the fact that the infrared radiation of the earth would be absorbed by carbon dioxide and water vapour in the air and raise its temperature. Presence of these gases keeps the earth warm and if the level of carbon dioxide increases due to industrial activity we will definitely get into a regime of disastrous global warming.

Readers wanting to ask Prof Yash Pal a question can e-mail him at palyash.pal@gmail.com
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