Frieß, Friederike Renate (2017)
Neutron-Physical Simulation of Fast Nuclear Reactor Cores.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

According to a many publications and discussions, fast reactors hold promises to improve safety, non-proliferation, economic aspects, and reduce the nuclear waste problems. Consequently, several reactor designs advocated by the Generation IV Forum are fast reactors.

In reality, however, after decades of research and development and billions of dollars investment worldwide, there are only two fast breeders currently operational on a commercial basis: the Russian reactors BN-600 and BN-800. Energy generation alone is apparently not a sufficient selling point for fast breeder reactors. Therefore, other possible applications for fast nuclear reactors are advocated. Three relevant examples are investigated in this thesis.

The first one is the disposition of excess weapon-grade plutonium. Unlike for high enriched uranium that can be downblended for use in light water reactors, there exists no scientifically accepted solution for the disposition of weapon-grade plutonium.

One option is the use in fast reactors that are operated for energy production. In the course of burn-up, the plutonium is irradiated which intends to fulfill two objectives: the resulting isotopic composition of the plutonium is less suitable for nuclear weapons, while at the same time the build-up of fission products results in a radiation barrier. Appropriate reprocessing technology is in order to extract the plutonium from the spent fuel.

The second application is the use as so-called nuclear batteries, a special type of small modular reactors (SMRs). Nuclear batteries offer very long core lifetimes and have a very small energy output of sometimes only 10 MWe. They can supposedly be placed (almost) everywhere and supply energy without the need for refueling or shuffling of fuel elements for long periods. Since their cores remain sealed for several decades, nuclear batteries are claimed to have a higher proliferation resistance. The small output and the reduced maintenance and operating requirements should make them attractive for remote areas or electrical grids that are not large enough to support a standard-sized nuclear power plant.

The last application of fast reactors this thesis investigates promises a solution to the problem of the radioactive waste from nuclear energy production. The separation of the spent fuel in different material streams (partitioning) and the irradiation of minor actinides in a fast neutron spectrum (transmutation) is claimed to solve this problem. Implementation of partitioning and transmutation (P&T) would require centuries of dedicated efforts, since several irradiation cycles and repeated reprocessing of the spent fuel elements between the irradiation cycles would be necessary.

For all three applications, computer models of exemplary reactor systems were set up to perform criticality, depletion, and dose rate calculations. Based on the results, a specific critique on the viability of these fast reactor applications was conducted. Possible risks associated with their deployment were investigated.

A Super-Safe, Small and Simple reactor promises to meet the energy demand of remote, small energy grids. The discussion of the proliferation risks associated with the spread of this kind of reactors often addresses the sealed core. The fissile material produced in the core and the possibility of breaking a seal and extracting the fuel is neglected. To address these questions, the Toshiba 4S reactor was modeled as an example of a fast small reactor with a core lifetime of 30 years and an energy output of 10 MW.

The fast SMR core is said to have a high level of proliferation resistance. Depletion calculations, however, show a production rate of more than 5 kg plutonium per year. Furthermore, the plutonium-239 fraction in the fuel is higher than 90% even at planned discharge from the reactor, resulting in very attractive material for a possible proliferator. Several SMR characteristics complicate the unauthorized removal: the refueling intervals are extraordinary long and in-between the core does not have to be opened for reshuffling of fuel elements. It supposedly remains sealed for the whole time. Also, the machines needed to remove the spent fuel elements are not kept at the reactor side but will be transported there only for refueling. Still, the fissile material produced in the core poses a proliferation risk.

The dose rates emitted from fuel elements 30 years after discharge are higher than 1 Sv/hr. They fulfill what is currently considered to be an important part of the spent fuel standard. Yet, there is only a one year on-site cooling period planned before the spent fuel elements are transported back to a central facility. At this point, the spent fuel elements emit about 100 Sv/hr. This of course impedes diversion of the spent fuel from the reactor site, but also complicated transportation to the reprocessing facility.

Especially if these nuclear reactors are to be deployed on a global scale, the proliferation risks imposed by the material production in the core have to be addressed. The likely detection of unauthorized fissile material diversion might discourage some actors from this pathway. But for a state determined to acquire nuclear weapons and thus most likely willing to break its obligations under the Non-Proliferation Treaty and as a consequence to face corresponding reactions from the international community, the detection might not be a prohibiting factor. In the case of an open break-out, at nearly any point of the SMR operation cycle, the state has access to significant quantities of weapon-grade plutonium. After only two years of reactor operation already more than one significant quantity (8 kg) of weapon-grade plutonium has been produced in the core.

For a state opting for this type of nuclear batteries to power its remote small grid locations, the bottleneck for acquiring nuclear weapons is not the access to fissile material but reprocessing. Due to the modularity of small reactors, deployment of several of them in one country would not raise suspicions while the latent option of becoming a nuclear weapon state would emerge. The first generation of deployed SMRs would most likely be operated in a once-through fuel cycle and their number would be limited. In such a scenario, the concept of having only central facilities for (re)fueling might be realistic and the access to sensitive technology would be limited. But it is not yet clear by whom these facilities would be owned and how safe transportation to and from reactor sites can be ensured.

For the second generation of the SMRs, a closed fuel cycle is foreseen. With the projected possible high number of deployed SMRs, several reprocessing and fuel fabrication facilities would be needed. To reduce transportation efforts, those facilities might be decentralized as well. In these scenarios, the number of states that have access to key technologies needed to acquire fissile material and build nuclear weapons increases and the obstacles for non-state actors are reduced.

SMRs can only play their economic advantage caused by their modularity if they are produced and deployed in high numbers. Thus, for the proliferation risk assessment, this should be also taken into account. Even though providing enhanced features regarding the possible proliferation of nuclear material, the overall security case is not as easily made as suggested by its proponents.

The BN-800 breeder reactor was awarded Top Plant 2016 in the nuclear generation category by the POWER Magazine, the oldest American journal for the global power generation industry. This award is given to what are considered to be the most advanced and innovative projects. Among the winning attributes are the possibility to use the reactor for various purposes, including plutonium consumption. The BN-800 is essential for Russia's efforts to dispose of its excess weapon-grade plutonium as agreed-upon in the recently suspended Plutonium Management and Disposition Agreement (PMDA) signed between Russia and the United States. Depletion calculations for the BN-800 verify the viability of this disposition method, according to the requirements set by the PMDA. The ratio of plutonium-240 to plutonium-239 is 0.17 in the spent fuel, thus fulfilling the agreed-upon fraction of 0.1 or higher. Yet, depending on its position in the core, the plutonium content in the spent fuel amounts to 82%-88% and is very close to what is generally labeled weapon-grade (more than 93% plutonium-239). After a cooling period of 30 years, the spent BN-800 fuel elements emit more than 1Sv/hr and can therefore be considered to be self-protecting. According to IAEA regulation, they require less strict safeguards.

At the same time, in the blanket elements of the reactor attractive nuclear material is bred even when the reactor is operated with a breeding ratio below one. Not only is the plutonium produced in the blankets of weapon-grade quality with a plutonium-239 fraction significantly higher than 93%, the radiation barrier also deteriorates quickly. The elements cannot be considered to be self-protecting after a cooling period of 30 years. Currently, no separation and reprocessing of blanket material is planned, but it is not clear why the blankets are necessary at all. In particular for the purpose of plutonium disposition, it would be preferable if no new weapon-grade material be bred. Further research should be done to assess the possibility of operating the BN-800 without blankets. Additionally, the introduction of inert matrix fuel could further increase the rate of achievable plutonium reduction in the reactors. Unfortunately, with the PMDA suspended in September 2016, the issue lost its urgency.

The BN-800 is planned to play a key role in Russia's efforts to establish a closed nuclear fuel cycle in the future. A closed nuclear fuel cycle always implies reprocessing of spent fuel. In the case of the breeding blankets, weapon-grade plutonium will be separated at a certain stage in the fuel cycle, which contradicts the current efforts to dispose of such material. Once the BN-800 is exported to other countries for energy production, the possible proliferation of nuclear materials becomes of even greater concern. It is widely accepted that fast reactors are more suitable for the production of nuclear weapons material. Especially for newcomers to nuclear energy, the possible advantages of fast reactors, namely the option to close the nuclear fuel cycle, seem to be a distant option. On the other hand, the operating history and economic viability of fast reactors is far worse than for light water reactors, but they offer the option of access to nuclear weapons material.

Several measures could reduce the proliferation risk of the BN-800 in the case of export. The most obvious are of course IAEA safeguards, preferably including an Additional Protocol. Until now, only China showed interest in the BN-800.

It would be advisable to achieve transparency during all steps of building the nuclear power plant, operation, and decommissioning. Comprehensive monitoring and inspection mechanisms would increase trust among the different parties and could also act as an example for other countries. As a demonstration, the precise and continuous monitoring of the reactor power output and irradiation times would provide the basis for a reliable assessment of the amount of plutonium and fission products produced in the core and blanket.

The case of the BN-800 shows that especially in the light of several newcomer countries interested in buying nuclear technology the proliferation risk has to be assessed more comprehensively. Limiting the focus to the reactor itself and the country originally developing it is not sufficient. Disadvantages of fast reactors, such as the high costs and the proliferation and safety risks, have long been known. They should not be forgotten with the new reactor generation of reactors, even though some enhanced safety and security technologies are in place.

Under current economic circumstances, the implementation of a transmutation fuel cycle is not competitive compared to other means of energy production. The use of plutonium in MOX fuel alone brings an economic penalty compared to the once-trough fuel cycle and is motivated by other reasons such as a better resource utilization, which is necessary if nuclear power is to be used on a global scale. An objective of introducing a double-strata partitioning and transmutation fuel cycle using accelerator-driven systems for the transmutation of minor actinides is the treatment of high-level waste. The implementation of such a fuel cycles requires long-term dedication to the use of nuclear energy and the deployment of all facilities that make up a closed nuclear fuel cycle. Before taking such far reaching decisions, it should be ensured that the promised benefits will hold true in reality. To date, even the proof of concept of an accelerator-driven system is pending.

An analysis of the existing literature shows that some crucial points regarding a P&T scenario are not dealt with in sufficient detail. In this thesis, a closer look was taken at some of these issues. Burn-up calculations were performed based on computer models of the European proof-of-concept reactor MYRRHA and the facility explicitly designed to be used for transmutation of minor actinides (EFIT). Both are accelerator-driven systems (ADS). They consists of a sub-critical reactor core, a spallation target to provide extra source neutrons, and a particle accelerator to provide high-energy protons for the spallation reaction. Besides some general characteristics, such as the possible transmutation rate in those reactor systems, three key issues that might affect the implementation were investigated in detail: the change in the fuel composition, the characteristics of the spent fuel elements, and the concentration of long-lived fission products in the spent fuel.

The minor actinides have to be irradiated in the ADS for several cycles. For efficient transmutation, plutonium and minor actinides must be mixed in the fuel according to fixed fractions. After each cycle, the fuel has to be reprocessed and fresh fuel elements must be fabricated. It is noteworthy, that even today's nuclear reactor fuel is only reprocessed once and its use as MOX fuel is limited to a second cycle. Calculations of the effective neutron multiplication factor keff for various fuel compositions that depend on the number of previous cycles show the influence of the changing isotope vector. The claim that one initial load of plutonium is sufficient for several irradiation cycles can not be confirmed. Moreover, criticality calculations show that using fuel compositions as published for European implementation scenarios (PATEROS) result in keff = 1.6. This is a much too high figure, suggesting that P&T scenarios published so far are not feasible. Calculations were done with the EFIT reactor, the reference reactor in the PATEROS study. As a consequence, major adjustments of the fissile material content in the fuel are necessary to resolve the overall reactivity problematic. This in turn might lead to performance losses regarding the intended reduction of minor actinides within one reactor cycle.

The claimed benefits of a P&T scenario are the reduction of the minor actinide inventory in the deep geological repository. After each irradiation cycle, the spent fuel elements must be cooled and reprocessed before new fuel elements can be fabricated. Since transmutation requires several cycles, the necessary cooling periods before reprocessing of the spent fuel play an important role to assess P&T scenarios. The calculations show that due to the increased residual heat of the spent P&T-fuel elements, longer cooling periods than currently assumed would be necessary. The decay heat from the spent P&T-fuel elements after a 40 year cooling period is still higher than the decay heat from spent MOX and fast reactor fuel elements, although these contain significantly more fuel. Also, the dose rates and the activity of the spent fuel would pose challenges for the overall reprocessing and fuel fabrication scheme. The build-up of curium-242 with its high spontaneous fission rate causes a strong neutron background. Thus, heavy shielding would be necessary for the processing of the spent fuel elements. The high specific power makes permanent cooling of the tools and material unavoidable during all phases.

Finally, it is questions in this thesis that the benefit of minor actinide transmutation is as significant as claimed by the proponents of P&T. With regard to the risks emerging from a deep geological repository, several long-lived fission products dominate the dose rate released to the biosphere. The production of the relevant nuclides zirconium-93, technetium-99, and iodine-129 in an ADS is mostly comparable to their generation in light water reactors. However, the fraction of cesium-135 increases four-fold. For a German P&T scenario, the cesium-135 inventory in the deep geological repository would more than double as compared to the agreed-upon phase-out scenario in which the spent fuel elements are directly disposed. The overall inventory of long-lived fission products in a German deep geological repository would increase by more than 50% in a P&T scenario. It can be stated that the reduction of the minor actinide inventory would be bought in exchange for an increase of the inventory of long-lived fission products. These results question the benefits of the currently researched P&T strategy that claims to reduce the nuclear waste burden.

At the current stage of P&T research and development, there are several open questions that need to be answered before actual implementation. This includes not only technical challenges as the ones already discussed. Other crucial issues are the endurance of the cladding material in the core and the partitioning efficiency realistically achievable on industrial scale. Even with all these issues resolvable, the benefit of the technology remains uncertain. Over the years, the number of targeted isotopes published in P&T schemes has declined: while in the beginning the plan was to transmute long-lived fission products as well, it now seems that even curium must be left out of the minor actinide composition because of the challenges it poses to reprocessing and fuel fabrication.

Even though fast reactor research and development has a long history, operational experience of fast reactors is quite small. Since more suitable solutions exist for energy generation, in recent years additional applications have been discussed for new and emerging fast reactor designs. The examples above show that the use of fast reactors is not as straight forward and beneficial as the advocates of this technology would argue.

When looking at specific applications, fast reactors seem to offer solutions for various tasks, such as plutonium disposition, safe and secure energy supply in remote areas, and the treatment of radioactive waste. In a more comprehensive view, promises are fading and it turns out that suggested applications bear risks.

Critical fast reactors cause the spread of nuclear weapons material and, even more importantly, the technology and facilities to handle it. This is also true for fast sub-critical ADS, which would be deployed in a P&T fuel cycle. It is not yet clear in how far P&T technology can actually help to solve the nuclear waste problem. The argument in favor of nuclear waste treatment in an ADS is based on one simple index value: the radiotoxicity based on the total ingestion by humans. Besides, the development risks regarding P&T are high and it is not clear whether a P&T fuel cycle could actually be implemented in the near future. Several crucial technologies do not yet exist.

Moreover, nuclear reactors are first of all designed for energy production. Still, the vast majority of the current nuclear fleet are light water reactors and not fast breeder reactors. This might always be attributed to soft factors, such as political considerations and the public opinion. But maybe the reasons are intrinsic to the technology: fast reactors might just not be competitive for energy production. And it has not yet been proven that they are competitive in regard to emerging applications beyond power supply. In general, the new applications will lead to higher costs and risks and it sometimes seems puzzling why they are promoted by academia, industry, and policy. It also seems somehow contradictory to solve a problem, namely the excess plutonium stockpiles and the radioactive waste, by using the same technology that originally produced it.

Research efforts in this field have been going on for decades, and they have been substantially sponsored. Apart from the fact that this money is lost for other means, it could be argued that if even huge investments do not result in the desired outcome, other approaches should be tried. Critical assessment of the technology, however, is difficult as long as research is almost exclusively conducted by institutions that would benefit from a future implementation. Especially when official entities, such as the European Union, allocate funds for research and design efforts, they should take care that at least a fraction of the money also goes to independent researchers. This is the only way to guarantee that transparent and comprehensive data information and assessment is available. And only then, can society come to informed decisions on whether it supports fast reactor technologies - or not.

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