CANDU HEAVY WATER REACTORS AND FISSION FUEL CONSERVATION

 by

 DUANE PENDERGAST 

Published in: 

Pendergast, D. R., "CANDU Heavy Water Reactors and Fission Fuel Conservation", Climate Change and Energy Policy, American Institute of Physics, Proceedings of the International Conference on Global Climate Change: Its Mitigation Through Improved Production and Use of Energy, Los Alamos, New Mexico, 21-24 October 1991.

INTRODUCTION 

Scientists have been predicting, for several decades[1], that rising carbon dioxide levels in the atmosphere will lead to increased average world surface temperature. Early articles on this "greenhouse" effect projected that the warming would be beneficial as plant growth would be enhanced and the onset of an anticipated ice age could be delayed indefinitely. This initial optimism about the consequences of the greenhouse effect then gave way to concerns that human interference with the atmosphere might be harmful[2]. Concern about global warming reached a crescendo during the summer of 1989. This initial concern is dampening. Recently publicized estimates of the magnitude of warming[3] are on the low side of widely ranging predictions. Fault is found[4] with modelling simulations. Expectations that climate change can easily be avoided by conservation and efficient energy use are leading to complacency. 

Nuclear power is acknowledged by most energy researchers to release very little carbon dioxide to the atmosphere. Nevertheless the large scale implementation of nuclear power is questioned by some[5],[6] as an appropriate response to the greenhouse "problem". Reasons given range from the ongoing debate about safety and nuclear proliferation to more recent doubts about the adequacy of nuclear fuel supplies. Some analyses suggest nuclear power simply can't be implemented quickly enough to turn around increasing carbon dioxide levels. 

This paper reviews the potential of nuclear power as a means of reducing carbon emissions. A role for the CANDU reactor in conservation of nuclear fuel supplies is explained. 

NUCLEAR ENERGY AND CLIMATE CHANGE - THE SCEPTICS VIEW 

Fossil fuels are believed by most climatologists to be the major cause of observed increasing carbon dioxide levels in the atmosphere. Nuclear industry visionaries have identified the tremendous potential[7],[8] to replace fossil fuel energy with nuclear derived energy[9]. The greenhouse issue has raised the possibility that the massive conversion to nuclear energy, envisaged by them, is essential now. The operation of nuclear power plants is acknowledged, even by those who believe nuclear power is not the way to achieve reduced global warming, to be relatively carbon dioxide free[10].  Why then would nuclear power be rejected as a solution?  

The two studies cited in the introduction raise lingering doubts about the safety of nuclear power. I will side step the safety issue. That issue is ongoing and well debated elsewhere. They then go on to discuss other aspects of the large scale deployment of nuclear power plants.  

One study[11] postulates increasing global energy use scenarios with a substantial nuclear energy component. Although nuclear power is nearly carbon dioxide free the models of energy use increase projected include a large component of fossil fuel energy. This inevitably leads to increasing carbon dioxide. Additional  analysis indicates to the authors that increased efficiency of energy use is the quickest most cost effective way to reduce carbon dioxide emissions for the scenarios chosen.  

Another evaluation[12] examines nuclear fuel supplies. The large scale conversion to nuclear power using current reactor designs is evaluated. This would quickly exhaust projected reserves of high grade uranium ore. Breeder reactors are rejected as being too slow to develop to the levels of fossil energy displacement needed to combat the greenhouse effect. The potential to utilize low uranium content ores in the quest for greater uranium supplies is then considered. The analysis assumes that fossil fuels would be the source of energy used to retrieve uranium. The energy derived from the uranium becomes less than the energy which could be derived directly from the fossil fuel assumed to be used in mining the low grade ores. 

The ultimate alternatives to fossil fuel energy envisaged by skeptics of the nuclear energy alternative are solar, wind and biomass energy. These are thought to be the source of energy for electricity production and, ultimately energy for transportation and other energy consuming processes. 

I believe the skeptics are excessively pessimistic about nuclear power's ability to play a role in providing low carbon dioxide energy. Two studies were selected for discussion. The first of these selects growth scenarios which preclude direct comparison of nuclear and fossil fuel derived energy. Assumed growth in fossil fuel use overwhelms the positive contribution of nuclear power. The second assumes continuing use of fossil fuel to recover low grade uranium ores. In the long run nuclear energy would be used for uranium recovery. A common theme of both studies is that action taken to ameliorate the greenhouse effect must be undertaken on an urgent basis due to projections of large temperature increases in the next century. These projections warrant re-examination. 

CLIMATE CHANGE AND THE NEED FOR URGENT ACTION 

Recent scientific articles, based on results from computer modeling of the climate, are predicting temperature increases at the low end of the broad range of uncertainty (1.5⁰C to 4.5⁰C increase[13] for a doubling of carbon dioxide) in this science. Some of the differences in results arise from such simple changes in the models as a change from an assumed sudden doubling of carbon dioxide to a time linear increase of carbon dioxide to the same end point[14]. Other major uncertainties include those of cloud[15] and ocean circulation modeling. In any case these lower temperature increase projections are beginning to be generally accepted as a genuine improvement in modeling. These new predictions allow time for a more careful and deliberate planned response to rising carbon dioxide levels. 

This current view of the lack of need to take quick action to compensate for the greenhouse effect is not very satisfying to those who want to undertake a massive response to reduce carbon dioxide emissions - right now! It appears that solid evidence of actual greenhouse induced warming, perhaps combined with a tax on carbon emissions to the atmosphere would be needed to initiate action in the near term. This isn't likely to be implemented in view of the developing consensus that climate changes are likely to be more modest than anticipated by the media a couple of years ago. The positive side of this is that we expect to have more time to implement efficiency improvements in energy use and production. This breathing space will allow for more measured development of the full potential of nuclear derived energy as well. 

THE POTENTIAL OF NUCLEAR ENERGY 

Climate warming has the potential to be the major  incentive over the next two or three centuries for further development of  nuclear energy as the major source of energy. Nuclear power plants, as they exist today, can be demonstrated to have minimal impact on the environment relative to fossil fuel derived energy. It is recognized by its friends and foes to produce very little carbon dioxide relative to other major means of energy production. As mentioned in the previous section, pioneers of the nuclear industry outlined the vast energy available from nuclear fission. This potential is worth reviewing. Newcomers to the industry, like me,  haven't been exposed to this. 

Nuclear fission derived electricity already makes a significant contribution to world energy supply. This proven technology substantially reduces the greenhouse effect relative to fossil fuel use which surely would have displaced it. Nuclear fission has the potential to provide a far greater fraction of world energy needs. It's possible widespread adoption raises additional questions of environmental and economic concern. It is the responsibility of the nuclear industry to ensure balanced understanding of these concerns related to the risks and benefits of alternative energy sources in the minds of the public. The industry will then be in a strong position for consideration as a major source of energy should climate change turn out to be as serious as many believe.  

Are there sufficient nuclear fuel reserves to support a massive conversion to nuclear power? Current global energy needs are roughly equal to the electrical output of seventy five hundred 1000 megawatt nuclear plants[16]. Quick calculations based on the fissile uranium recovery and consumption[17] of existing power reactors reveals that seventy five hundred 1000 megawatt plants would use up projected uranium reserves of 24 million tonnes[18] in one or two decades. That's not encouraging. However existing reactors use uranium inefficiently so there is a potential to extend the use of these reserves by a factor of about 50[19] through various forms of "breeding" additional fissile material. This would extend the energy content of these existing  "reserves" to last about 500 to 1000 years at current energy consumption rates. 

An extension of the human life style as we know it for another millennia may seem too short for some of us. Again the nuclear industry is holding a trump card. Estimates of uranium reserves given above are based on ores which allow profitable recovery at a selling price[20] of $130 per kg. The current price is at an all time low[21] of $20 per kilogram. This price is worthy of some reflection. Existing reactors consume about 20 to 30 mg of natural uranium per kWhr of electricity. This works out to a current market value uranium cost of about 1/20th of a cent per kWhr. This is a very tiny fraction of the current electricity selling price of 5 to 15 cents per kwhr in Canada and the United States. An increase in the price of uranium to $4000 per kgm would increase the uranium share of electricity cost to about 10 cents per kWhr effectively doubling its cost. We would complain of course but we could cope with such an increase easily through more efficient use and perhaps giving up just a little bit in our lifestyle. The incentive to producers to exploit lower grade uranium deposits would greatly expand usable reserves. Studies of deposits[22] indicate that the earth’s crust contains several orders of magnitude more uranium than is counted as a reserve or resource at current prices. This suggests uranium could serve as our sole source of energy for thousands of years even if used in the current wasteful fashion. The sea also contains about 4 billion tonnes of uranium[23] although it is quite dilute. Japan has undertaken a pilot project to recover it and has found that it would cost in excess of ten times current prices[24]. Although this is a bit indefinite it is well below the uranium cost of $4000 per kgm which we estimated would double electricity costs and suggests this source may be practical. Recovery of half of this uranium would thus supply all our current energy needs from existing reactor technology for about 2000 years.  

So far we have discussed fission power systems as we know them now. We've demonstrated nuclear fuel supplies are available for several thousand years for "once through" fuel use as reactors are currently operated. All of the uranium in natural uranium (140 times more than the original fissile content) is  potential reactor fuel if breeder reactors are used. In addition thorium, which is about 4 times as abundant in the earth's crust, can be used to create fission reactor fuel material. Experience shows that inherent losses in the conversion process reduce the net gain[25] to about a factor of 50. We can thus expect fission power to serve us with abundant energy for many tens or hundreds of thousands of years instead of the 10 or 20 years suggested by energy pessimists. Fifty years of research and development has provided access to this bounty. Workable techniques to begin extracting it are in place. 

CANDU REACTORS AND THE CONSERVATION OF NUCLEAR FUEL 

It seems that the political and economic climate at present precludes any massive nuclear power program to sharply reduce carbon emissions to the atmosphere. Nevertheless fossil fuels will become more expensive as supplies are depleted. Wind and solar power may not turn out to be an economic option. What can the nuclear industry do to preclude energy analysts concerns that nuclear fuel supplies are insufficient to be a significant source of energy? Perhaps incremental improvement to existing reactors to reduce uranium use is an attractive alternative. The development and deployment of reactors which breed additional fissile material in excess of that used could be delayed. The extraction of uranium from dilute sources such as low grade ores or sea water could be saved for the more distant future.  

Existing  reactor designs can be used to utilize the energy available from fissile material in used fuel stockpiles. I will focus on the Canadian CANDU (CANada Deuterium Uranium) system. A basic feature of the CANDU reactor is the heavy water moderator. Heavy water is particularly effective in conserving neutrons during moderation to the energy levels needed to generate fission. The fissile content of the fuel can thus be particularly low while an energy producing chain reaction is sustained. It is this property which allows a CANDU reactor to operate effectively with natural uranium fuel of 0.72% fissile content. Light water reactors are typically operated with the fuel enriched to about 3% fissile content. Since light water is poor at conserving neutrons, the fuel has to be discharged before the fissile material is depleted. 

In fact, the "spent" fuel from a light water reactor is a very good source of fuel for a CANDU reactor. Recent publications[26],[27] explore several ways of reusing spent fuel from light water reactors. These indicate that the energy from a given quantity of natural uranium can be nearly doubled by a second use of LWR fuel in a CANDU reactor. Most of the reuse-recycling processes described require some kind of chemical processing of the spent fuel. The goal is to extract the fissile from neutron absorbing materials. The studies have indicated that most of the benefits can be obtained by direct[28] use of LWR fuel in a CANDU reactor. The direct use process envisaged at present is to remove the fuel pellets from LWR fuel and reform them to a suitable form for fabrication into CANDU fuel bundles. (The fuel assemblies which have evolved for use in LWR and  CANDU reactors are similar in the materials used. The shape and size of the fuel assemblies prevent direct transfer from current light water reactors to CANDU reactors.) 

In addition to the greater energy extraction from uranium which would be realized from this reuse of spent fuel another benefit is obtained. The volume of spent fuel to be stored is reduced. 

Another scheme under study[29] at Atomic Energy of Canada indicates that a rearrangement of the geometry of the fuel and moderator by alternating close and far spaced fuel assemblies in the core has the potential to triple[30] the utilization of natural uranium in CANDU reactors without recycling. If proven practical this would be even more effective than the synergistic relationship between the LWR and the CANDU as a conserver of uranium supplies. Since it uses natural uranium as fuel the energy intensive uranium enrichment process would also be bypassed.  

SUMMARY AND CONCLUSIONS 

The enormous appetite for energy that man has developed in the past century has raised genuine concerns about climate changes postulated to result from increasing carbon dioxide in the atmosphere. A couple of years ago massive changes in energy use or supply were thought by many to be needed soon to sharply reduce carbon dioxide releases to the atmosphere. Some changes in modeling predictions towards lower warming have captured the heart of the media. The sense of urgency has worn off for now and more time is thought to be available for reasoned development of appropriate policy and response options. The nuclear industry which has developed over the past 50 years is a good candidate as an alternative source of energy which generates little carbon dioxide. Tremendous amounts of energy are available with full development of the nuclear fission option. Concerns  have been raised by energy analysts about the possibility of running short of nuclear fuel. These concerns can be alleviated in the short term through ingenious use of existing reactor designs. The examples cited include reuse of LWR fuel in CANDU reactors and a modification of the CANDU design which studies indicate would extend fuel supplies by a factor of three with a corresponding reduction in spent fuel.  

The youth of the nuclear power industry suggests fertile ground for considerably more innovations and improvements in reactor development. We can expect that many more will be discovered as the full potential of nuclear fission energy supply is realized in the decades to come.   

REFERENCES

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