The future shape of the nuclear industry world-wide will be strongly
influenced by three major considerations – economics, optimal use of
finite resources, environmental impact/waste management:
- Economics
There is generally increasing pressure on nuclear power to be genuinely economically competitive with other large scale energy sources, such as coal, oil and gas. Developments such as improved coal burning systems and combined gas cycle power plants are changing the economics of the fossil sector and increasing the pressure on nuclear utilities to reduce capital and operating costs while not compromising safety.
- Resource Utilisation
Another factor which is becoming increasingly important is that of making the best use of the world's finite energy resources. The important considerations here are increasing power plant efficiency, energy conservation, improved energy efficiency and improved fuel utilisation.
- Environmental Impact/Waste Management
There is increasing public awareness of the importance of proper accounting of the environmental impact of energy production and use. The emphasis is on global analyses which account for all of the environmental effects of energy production wherever they occur and not just on the impact on the area local to a power plant. This is an area where it can be very difficult to quantify all of the environmental costs and benefits, making comparisons between different strategies very difficult. A crucial element of minimising environmental impact is the responsible management and minimisation of all waste arisings and there is increasing understanding of the need for planners to take proper account of the full costs to society of waste management/disposal in intercomparing the costs of energy production among various alternatives. No matter what the energy source, past practice has often been to neglect the full environmental and health costs of waste disposal, an approach whose shortcomings are now recognised. For example, disposal on CO2 to the atmosphere carries no costs in most analyses of fossil plants, while decommissioning costs are often ignored for plants of all types.
Moreover, the issues of safety and non-proliferation,
while not quantifiable like economics, resource utilisation and
environmental impact, must be accounted for because of the necessity
for the widespread public acceptance of any options selected for the
future nuclear industry. It hardly needs saying that the safety of all
activities associated with nuclear power is of paramount importance.
Although some of the physics and engineering aspects referred to in
this report impinge on questions of safety, detailed discussion falls
outside the scope of this Study. As with safety issues, the question of
non-proliferation also falls outside the scope of this Study. All of
the plutonium recycle activities described in this report will need to
conform to the highest standards as regards non-proliferation and
verification.
The management of spent fuel from nuclear reactors is crucial to all of the above areas. There are a large number of options for spent fuel management, ranging from interim storage followed by direct disposal of the intact fuel assemblies to chemical reprocessing of spent fuel assemblies and recycling of some or all of the fissile materials recovered in either thermal or fast reactors or via a combination of both. There is at present no clearly accepted best option and there is a tendency for each country and indeed each nuclear utility within a country to have its own viewpoint and its own preferred strategy.
Regarding the economics of nuclear power, the choice of spent fuel management strategy has a modest but nevertheless significant impact on overall generation costs. Spent fuel management normally constitutes about 10% of the fuel cycle costs of a thermal reactor such as a Pressurised Water Reactor (PWR) or a Boiling Water Reactor (BWR), although it can be substantially larger in countries whose projected costs of waste management facilities are particularly high. Although spent fuel costs are very large in absolute terms, their impact on overall generation costs are very modest, bearing in mind that fuel cycle cost component of modern PWRs and BWRs typically only represents about 10% of the overall cost (the other 90% being made up of capital charges for the nuclear power plant, which is the dominant cost and operating and decommissioning provisions). A comprehensive and up to date account of fuel cycle costs can be found in a recent OECD publication (Economics of the Nuclear Fuel Cycle, OECD/NEA, 1994).
The spent fuel management strategy which a nuclear utility opts for in the context of a national policy has a direct influence on resource utilisation. On the one hand, a once-through thermal reactor fuel cycle with ultimate direct disposal of spent fuel makes the least efficient use of the original uranium ore. Some countries' and utilities' view is that with the current abundance of relatively cheap uranium reserves the direct disposal option is for them the most favourable. Such a judgement depends on a variety of strategic factors which vary both geographically and with time, such as the availability of indigenous uranium reserves and indigenous fossil fuel reserves and the price of alternative fuels on the world market.
An equally valid viewpoint is that in order to conserve uranium reserves attempts should be made to recycle fissile materials from spent fuel in order to extract increased amounts of energy from the uranium ore. This may extend to re-use of the residual U-235 and fissile plutonium in spent fuel in thermal reactors to recycling of these products in fast reactors. The impact of the various options available on resource utilisation ranges widely, from a modest figure of perhaps 30% increase in energy output per kg of uranium ore to the case of one-time recycle in thermal reactors to a factor of up to 100 times in the case of extended recycle in fast reactors. In terms of resource conservation, the impact of spent fuel management is therefore dramatic and may ultimately become the dominant consideration in a world with acute energy shortages and limited economic uranium reserves.
There is an obvious synergy between the areas of resource conservation and economics in that the cost of uranium ore affects the relative economics of once-through and reprocessing/recycle fuel management schemes. Future projections of nuclear fuel cycle economics depend heavily on projected uranium ore prices. Whereas these are universally accepted as likely to be stable for the foreseeable future, past experience of strategically important commodity prices warns against relying on such projections uncritically and nuclear utilities are well advised to keep in mind that the future may turn out very different to current expectations.
Finally, there is the issue of the environmental impact of spent fuel management. A nuclear power plant operating normally retains all but a minuscule fraction of the fission products and trans-uranics in the spent fuel. The spent fuel management option chosen by the utility determines the eventual fate of these radioactive by-products. In the once-through fuel cycle the full inventory is eventually committed to disposal in an underground repository and all nuclides that have not undergone radioactive decay have the potential to be dispersed in the natural environment on geological timescales. In the reprocessing/recycle fuel cycle, uranium and plutonium in the spent fuel are separated out for re-use and are largely excluded from the various waste streams which are ultimately intended for disposal.
As regards environmental impact, in addition to its importance as a fissile material and weapons proliferation issues, plutonium is one of the most important irradiation products to consider. Being very long lived, its presence or otherwise in spent fuel or radioactive waste intended for geologic disposal has a major impact on the long term toxic potential the waste repository.
Underlying all the issues touched on so far are the technical questions relating to the recycling of plutonium. In line with the discussion above, attitudes to plutonium management vary from country to country and from utility to utility. Some see plutonium as a constituent of spent fuel which is best committed to direct disposal along with the intact fuel assemblies. Others with past or future reprocessing commitments vary in their attitude to the plutonium. Some regard plutonium as a liability and others as a valuable strategic resource. The new issue of the use or disposal of ex-weapons plutonium is a further complicating factor with many wide ranging implications, although it forms only a small fraction of current world-wide reserves.
This Study was initiated with the intention of comprehensively reviewing the current status of the physics of plutonium recycle, as a contribution to a wider understanding of the broader issues discussed above. It complements another recent study being published by the OECD/NEA on “Plutonium Management”.
The physics of plutonium recycle is a broad and complicated technical area some aspects of which are well understood and well proven, while others may be uncertain to a greater or lesser extent or even very speculative. The purpose of this Study is to review the physics of plutonium recycle as it stands today and to identify what tasks remain to be done to support future plutonium recycle strategies. Although it is not the purpose of the Study to fully examine all aspects of plutonium recycle such as waste management, proliferation and risk, it is not practical nor is it appropriate to entirely divorce the physics issues from them. Consequently this Study will specifically address amounts, compositions and toxicities of plutonium and trans-uranic flows and inventories as they are impacted by various choices for managing the back-end of the fuel cycle. Also addressed explicitly are the impacts of reactor safety and safety of alternative systems.
The various chapters of this report consider the following
broad areas:
- Thermal Reactor Plutonium Recycle
With the large scale deployment of fast reactors a distant prospect in most countries, many nuclear utilities are planning, or have already implemented plutonium recycling schemes in thermal reactors, principally PWRs (though there is limited experience and plans for future plutonium recycle in BWRs). Plutonium is normally recycled as Mixed Oxide (MOX) fuel in which it is mixed with uranium dioxide (UO2) in the form of plutonium dioxide (PuO2).
Regarding the use of MOX derived from current reprocessing plants in PWRs at least, the physics aspects are essentially fully understood and give rise to no operational, licensing or safety concerns in those commercial power plants where it is already in use. Uncertainties are always present, of course, but their importance increases in significance when considering future MOX fuel cycles with higher discharge burnups and consequently higher initial plutonium contents. This is compounded when the plutonium itself may be recovered from spent MOX fuel assemblies which have themselves been reprocessed and the isotopic quality of the plutonium (meaning the ratio of thermally fissile isotopes Pu-239 and Pu-241 to total plutonium) is degraded. This is a situation which is likely to arise in some countries with mature programmes of thermal reactor recycle starting sometime beyond the turn of the century. Since this is the area where the greatest unknowns and uncertainties lie, the issue of multiple recycle of plutonium in thermal reactors is a central theme of this Study and forms the basis of the thermal MOX benchmark exercises described in Chapter 3.
In the context of multiple recycle of thermal MOX, the question most often posed is “how many times can thermal MOX be recycled?”. This question which is discussed in Chapter 2 is of particular relevance. In fact, each time thermal MOX is recycled, the isotopic quality of the plutonium degrades and there will come a point where its further recycle in thermal reactors is not practicable. Even if all the physics issues associated with multiple recycle were understood perfectly, this question could not be answered simply, because it depends on the particular scenario, especially on the ratio in which UO2 and MOX assemblies are blended in the reprocessing operations. In current thermal reprocessing plants it is not desirable to process campaigns made up entirely of MOX assemblies and the intentions are to admix MOX assemblies with UO2. This delays the degradation of the plutonium and allows an increased number of recycles to be achieved. Other factors affecting the number of thermal recycles are the reactor type, the fuel design and the discharge burnup, all of which would contrive to complicate the answer.
A simpler approach to the question is, however, possible. It involves asking not how many times MOX can be recycled, but what is the maximum concentration of plutonium in thermal MOX that is practicable? Although the exact answer is not yet known, this Study has concluded tentatively that the upper limit lies in the range 10 to 12 w/o total plutonium, assuming thermal MOX designs similar to those already in use today. For blending ratios foreseen in today’s reprocessing plants (typically three or four UO2 assemblies blended with every MOX assembly), this would permit at least two recycles of plutonium as thermal MOX. With a higher blending ratio or with a modified lattice geometry, an increased number of recycles could be possible, but the precise number is not yet determined. Moreover, actinide build up during multiple recycling, whatever the breeding ratio, will increase and can become a potential criterion for multi-recycling feasibility. This point is discussed in Chapter 4.
- Thermal Benchmark Exercises
Three benchmarks relevant to the recycling of plutonium in PWRs were specified as part of the Study. One of these was a wide comparison of lattice code predictions for a simple LWR pin cell with plutonium of good isotopic quality representative of that coming from today’s commercial reprocessing plants. For this case there exists a large amount of crucial validation evidence in the form of experimental measurements from zero power critical facilities and operating experience in power reactors. It was included so as to act as a baseline against which to compare a similar exercise in which very poor quality plutonium similar to that which might arise in future with multiple recycling in PWRs (i.e. when PWR MOX assemblies are themselves reprocessed and recycled) and for which there is no validation at present. Finally, a void effect benchmark was specified to determine whether the various physics codes available world-wide would agree as to the sign of the moderator void effect in a hypothetical and computationally challenging voiding pattern in a supercell containing a MOX assembly also containing poor quality plutonium. The analysis of these benchmarks is detailed in Chapter 3.
- Plutonium Recycling in Fast Reactors
Chapter 5 considers the physics of plutonium recycle in both oxide fuelled fast reactors (with conventional PUREX-based recycle) and metal alloy fuelled fast reactor systems (with Pyro-based recycle). Two topics of particular interest today are the potential of fast reactors for consuming plutonium and fissioning minor actinides, because burner reactors have not received the same level of focus in the past as breeder reactors. These two topics underlie the fast reactor benchmark exercises which were devised as part of this Study. A major driving force for the Study was the lack of international validation in this area.
- Fast Reactor Benchmark Exercises
Benchmark exercises were defined for both oxide fuelled fast reactors and metal alloy fuelled fast reactors. The purpose was to establish the level of agreement between different fast reactor code systems regarding the rate of destruction of actinides and plutonium in particular. A novel aspect of the benchmarks is the departure from the usual assumption of a fast reactor breeder - cases with non-breeding fast reactor systems (with conversion ratios in the range 0.5 to 1.0) were considered. At least as far as a breeding ratio of 0.5 is concerned, existing fast reactor designs could easily be re-designed to become non-breeders simply by omitting the radial breeder region.
In addition to the two main areas above, this report also
looks at two other aspects of plutonium recycle and also the topic of
uranium recycle:
- Waste and Radiotoxicity Reduction
Recycle of plutonium has an important impact on the waste arising and on its radiotoxicity. In the long term the radiotoxicity is dominated by the arisings of trans-uranics which can differ significantly depending on the plutonium recycle strategy. In particular, fast reactors can be used to achieve considerable reductions in the trans-uranic arisings per s of electricity production. Chapter 4 considers the physics basis underlying this.
- Plutonium Fuel Without Uranium
There has recently been increasing interest in using specially designed thermal reactors to optimise the management of civil plutonium stocks. The objective is to remove the plutonium from store and place it in-reactor where it is inherently more safeguardable. There is also a great deal of interest in the possibility of using ex-weapons plutonium for reactor fuel as a means to decrease the world stockpiles of plutonium pits. The concept of plutonium fuel with a carrier other than uranium has been extensively studied in the past and there is renewed interest since it offers the potential for incinerating plutonium by fission without at the same time generating any new Pu-239 from U-238 captures. Chapter 6 covers both thermal and fast reactors with plutonium fuel without uranium.
- Recycling of Plutonium in Advanced Converter
Reactors
Chapter 7 provides a brief review of advanced converter reactors and their relevance to plutonium recycle. These are thermal reactors which are optimised to achieve high conversion ratios and, in the case of heavy water moderated variants, low fissile inventories. The low critical mass can allow the fissile content of discharged fuel to be burned down to the point where recovery of the residual fissile material is not worthwhile. Advanced converter reactors therefore represent an intermediate step between today’s thermal reactors and fast breeders, either as the final irradiators before disposal, or as an “active store” for plutonium in the interim period.
- The Use of Recycled Uranium
Chapter 8 provides a brief review of the physics of recycled uranium. Although this is not directly relevant to the topic of the Study, this is included for completeness because not only plutonium but uranium as well is a product of recycle.
