The Future of Nuclear Energy: Facts and Fiction - Part IV: Energy from Breeder Reactors and from Fusion?

Mon, 09 Nov 2009 22:51 GMT

This is the fourth part of a four-part guest post by Dr. Michael Dittmar. Dr. Dittmar is a researcher with the Institute of Particle Physics of ETH Zurich, and he also works at CERN in Geneva.

The accumulated knowledge and the prospects for commercial energy production from fission breeder and fusion reactors are analyzed in this report.

The publicly available data from past experimental breeder reactors indicate that a large number of unsolved technological problems exist and that the amount of "created" fissile material, either from the U238 → Pu239 or from the Th232 → U233 cycle, is still far below the breeder requirements and optimistic theoretical expectations. Thus huge efforts, including many basic research questions with an uncertain outcome, are needed before a large commercial breeder prototype can be designed. Even if such efforts are undertaken by the technologically most advanced countries, it will take several decades before such a prototype can be constructed. We conclude therefore, that ideas about near-future commercial fission breeder reactors are nothing but wishful thinking.

We further postulate that, no matter how far into the future we may look, nuclear fusion as an energy source is even less probable than large-scale breeder reactors, for the accumulated knowledge on this subject is already sufficient to say that commercial fusion power will never become a reality.

(Links to 1^st, 2^nd, and 3^rd parts)

1. Introduction

Over one hundred years ago, physicists began to understand that a huge amount of energy could be obtained from mastering nuclear fusion and fission energies. For example, the production of only 1 kg of helium from hydrogen "liberates" a thermal energy of about 200 million kWh. In the sun, this fusion reaction transforms about 600 million tons of hydrogen into helium every second, thus liberating 4 × 10²⁶ Joules per second.

The understanding of nuclear physics and its technological applications proceeded with breathtaking speed. It took only seven years from the discovery of the neutron in 1931 to the observation of the neutron induced fission of uranium at the end of 1938. This was followed, on the 2^nd of December 1942, by a sustained nuclear chain reaction with a power of 0.5 Watt (and up to 200 Watt at a later time) by E. Fermi and his team in a laboratory located below the Chicago University football stadium [1]. The next steps in using nuclear energy were the explosions of the Hiroshima and Nagasaki fission bombs, on the 6^th and 9^th of August 1945, resulting in more than 100,000 deaths and the beginning of the nuclear arms race. Only a few years after the first fission bombs exploded, the USA and the Soviet Union had constructed hydrogen fusion bombs. These bombs were up to 1000 times more powerful than the Hiroshima fission bomb.

Also the peaceful application of nuclear fission energy advanced very quickly: by 1954, the thermal energy from a controlled fission chain reaction could be used to produce commercial electric energy [2]. During the next 30-40 years, a large number of commercial nuclear power plants were constructed in most industrialized countries.

The rapid scientific and technical success in bringing this form of power into the production of commercial energy was impressive. Many nuclear pioneers expected that nuclear fission and fusion would provide their grandchildren with cheap, clean, and essentially unlimited energy. In fact, these successes led most of us to a euphoric and blind belief in continuous scientific and technological progress.

In contrast to such dreams, nuclear fission energy nowadays is not cheap, and even the most optimistic nuclear fusion believers do not expect the first commercial fusion reactor prototype until after 2050. One observes further that nuclear fission energy has been stagnating for about ten years and that its relative share in the worldwide electric energy production has decreased from about 18% during the nineties to only 13.8% currently [3].

Furthermore, the average age of the existing nuclear power plants, the limitations of primary and secondary uranium resources as well as the problems related to nuclear proliferation and nuclear waste all lead to doubts about the prospects of the standard water moderated nuclear fission reactors. In fact, it seems clear at this point that as fossil-fuel energy production declines, sufficient energy to ensure the survival of our highly industrialized civilization cannot come from a rapid growth of nuclear fission energy of this sort.

The problem with the limited amount of economically producible uranium resources can theoretically be addressed with the mastering of the technology of nuclear fission breeder reactors. It is claimed that this technology could increase the amount of fissile material from uranium by a factor of 60-100 and much more if the thorium breeder cycle can be realized [4]. It is believed that breeder technology will enable us to bridge the time gap before nuclear fusion energy, which would become the "final solution" to all energy worries, can be mastered [5].

In this fourth and final part of the Future of Nuclear Energy report, we discuss the experience with past and current breeder reactors in Section 3. We analyze how the remaining problems will be addressed with the worldwide Generation IV breeder reactor program and with thorium based breeder reactors (Section 4). The remaining obstacles towards a controlled and sustained nuclear fusion reaction chain are presented in Section 5. In order to simplify the discussion, we start in Section 2 with some facts and basic physics principles of nuclear fission and fusion energies.

2. Energy from nuclear fission and fusion, some facts and physics

As we have discussed in detail in parts I-III of this report [6], the publicly available data on long term worldwide natural uranium supply are in conflict with even a moderate annual 1% growth rate for conventional water moderated reactors.

Consequently, believers in a bright future of nuclear energy should concentrate their efforts on either (i) the realization of nuclear fuel breeder technology based on the uranium cycle, U238 to PU239, and/or the thorium cycle, TH232 to U233, or (ii) the mastering of commercial nuclear fusion reaction. In this section, an overview of the existing and planned nuclear reactor types and the experience with fast breeder reactors (FBR) is given (2.1). This is followed by a basic summary of the most important principles relevant to the use of nuclear fission and fusion energies (2.2 to 2.4).

2.1. Some facts concerning existing and planned nuclear reactor types

The worldwide nuclear fission reactors produced 2601 TWhe during the year 2008, or roughly 14% of the worldwide electric energy.

For the year 2009, one finds that commercial nuclear energy production will come from 436 nuclear fission reactors with a combined nominal electric power of 370,260 GWe [7].

Table 1: The evolution of different reactor types and their corresponding electric power ratings from the IAEA/PRIS data base (October 2009) [7]. Another five reactors are listed in the "Long Term Shutdown" category, four of which are PHWR's and the fifth is the 0.25 GWe Monju sodium cooled FBR reactor in Japan.

The PRIS data base of the International Atomic Energy Administration (IAEA) shows that the dominant reactor type today including reactors that are currently under construction is the water moderated fission reactor type. The abbreviation PWR (PHWR) stands for pressurized (heavy) water reactor whereas BWR denotes the boiling water reactor. As can be seen from Table 1, these reactors provide over 94% of the nuclear fission power worldwide. The remaining 6% of the nuclear fission power comes from graphite moderated and water or gas cooled older and smaller reactors. It seems that the PWR type has won the competition for the existing reactors and for the next generation of reactors by a large margin.

One observes that only two FBR's are declared operational. A third FBR has been in a "long term shutdown phase" since 1995. The two operational FBR's contribute together 0.2% of the world nuclear power. This tiny contribution from FBR's today is even smaller than it used to be. In the list of 122 decommissioned reactors, one finds 6 FBR's with a combined power of 1.6 GWe, or 4.3%. In the list of 53 reactors (October 2009) currently under construction, one finds only two relatively small FBR's.

These numbers indicate not only that FBR's play a negligible role today and during the next 10 years, but also that their operation experience is far from being an economical and technological success story. Some more details on the worldwide experience with various types of commercial FBR and thorium fuel breeder reactors and their operation are listed below:

The best operation experience comes from the Russian BN-600 FBR reactor with a rated power of 0.56 GWe. This reactor has been operated commercially for 28 years and is scheduled to close in 2010 [8]. Its average energy availability is given as 73.79%. In a document from the IAEA fast reactor data base [9], one finds that this reactor would be better called a "Fast Reactor," as it was designed to use more fuel than it could produce. A new BN-800 reactor with 0.8 GWe is currently under construction in Russia, and its scheduled start is now given as 2014. Like its smaller "brother," it is designed to consume Pu239 rather than breed surplus fissile material.
The other "operating" FBR is the Phenix reactor in France. Phenix originally started operation with a power of 0.233 GWe in 1974. Since 1997, it is rated with 0.13 GWe only, and an energy availability factor of 60.23% is given for 2008. According to the WNA (World Nuclear Association) data base, it ceased power production in March 2009 and will continue being operated as a research reactor until October 2009 [10]. The larger Super Phenix reactor, with a power rating of 1.2 GWe, achieved a maximal energy availability of 32.6% only. This very low performance, in comparison to PWR's, was achieved during the last operational year (1996) after a very short lifetime of only 10 years.
The Monju reactor in Japan was closed after a serious sodium leak in 1995. For many years now, the reactor is scheduled to "restart the subsequent year." Perhaps this time, it will really restart during the first few months of 2010 [11].
A next generation FBR reactor is currently under construction in India. According to the current plans, it will start producing electric energy during the year 2011 [12].
The KNK II reactor in Germany is listed in the IAEA data base [9] with a tiny capacity of 0.017 GWe. During its operational lifetime, 1978 to 1991, it achieved an average energy availability factor of 23.65%. A larger FBR, the SNR-300, with a rated power of 0.3 GWe was completed in 1985, but for various reasons never started. A large 1.5 GWe FBR, the SNR-2, never completed even the design phase.
A limited experience with a thorium admixture in the nuclear fuel in commercial prototype reactors exists as well. A WNA document mentions two THTR's (Thorium High Temperature Reactors) [13]: one with 0.3 GWe in Germany, which operated commercially between 1986 and 1989; the second was the Fort St. Vrain reactor with a power rating of 0.33 GWe in the USA. It is listed as the only commercial thorium-fuelled nuclear plant, following closely the German design. It was operated between 1976-1989.
The WNA document mentions further that the experimental Shippingport reactor in the USA, with a power rating of 0.06 GWe, has successfully demonstrated the concept of a Light Water Breeder Reactor (LWBR) using thorium. The Shippingport reactor began commercial electricity production in December 1957. In 1965, the Atomic Energy Commission started designing the uranium-233 / thorium core for the reactor. The reactor was operated as a LWBR between August 1977 and October 1982.

Several countries have so far managed to construct GWe water moderated slow neutron reactors, mostly of the PWR type. These reactors were operated safely and efficiently for many years, using U235 fuel enriched to 3-4%.

In contrast, large breeder reactors, based on a large amount of initial fissile material and the transformation of U238 and Th232 for breeding new reactor fuel, have so far not even successfully passed a prototype phase.

2.2. Energy from nuclear fission and fusion, some basics

Atoms consist of a nucleus, made of protons and neutrons, and electrons. The size and the chemical properties of atoms are defined by the number of electrons surrounding the nucleus. The combined mass of the protons and neutrons, each 2000 times heavier than the electrons, defines roughly the mass of the atoms. As the nucleus is 100,000 times smaller than the atom, it follows that its mass density is huge in comparison with that of the atom. The same chemical characteristics can be expected for atoms with a fixed number of protons and with different numbers of neutrons, and the energy in chemical reactions is of the order of 1 eV (1.6 × 10^-19 Joule). As the nuclear properties of an atom depend on the number of neutrons, the name isotope has been introduced to separate the chemically identical atoms according to their numbers of neutrons.

Without going into details, it is known today that the energy source of the sun and other stars is nuclear fusion. This fusion starts from the large number of hydrogen atoms present in the sun. The fusion reaction in stars is possible because of the enormous gravitational pressure that overcomes the electric repulsive force between positively charged protons. Fusion is the source of all heavier elements that were formed in super-novae explosions of super large early stars and shortly after the big bang. For our subsequent discussions on nuclear fusion, it is important to note that a relatively low fusion power density of about 0.3 Watt/m³, is found in the sun [14]. In contrast, the power density envisaged for a hypothetical fusion reactor must be at least one million times larger.

The nucleus is bound by the very strong nuclear force, which acts against the repulsive electrostatic force of the protons. Measurements have shown that the mass of the various atoms is almost 1% smaller than the mass of the individual protons and neutrons combined. Following Einstein's famous E = mc² formula, this mass defect corresponds to a huge amount of energy, about 8 MeV (8 million eV) per nucleon. This energy is liberated when one manages to fusion different nucleons together. Starting from the different hydrogen isotopes, e.g. one proton, deuterium (one proton plus one neutron), and tritium (one proton plus two neutrons), a binding energy of up to a few MeV is found. Further fusion of these hydrogen isotopes into the helium nucleus liberates another roughly 20 MeV.

Neutrons and protons in heavy atoms, such as uranium, are less strongly bound than in lighter atoms, such as iron, and energy can be released in the fission of such heavy atoms. For example, 1 MeV per nucleon, or 200 MeV in total, will be liberated in the fission processes of U233, U235, and U238, each containing 92 protons and 141, 143, and 146 neutrons, respectively. The energy liberated per fission reaction is at least 100 million times larger than in a chemical reaction.

It is therefore not surprising that this has created an enormous interest in subatomic physics and its application for ultimate weapons and/or for the commercial use of energy.

2.2.1. Civilian and military use of nuclear energy, some remarks

The focus of this report is the commercial use of nuclear energy. As the evolution of nuclear energy has always been strongly coupled with the military sector, we feel that a few remarks about the dangers of nuclear weapons and the ambiguity of the commercial use of nuclear energy are needed. First of all, governments wishing to have nuclear weapons were not faced with unsolvable problems related to the development of fission bombs based on Pu239 and U235. This is especially true if nuclear physics and engineering knowhow had been built up under the umbrella of peaceful and commercial use of nuclear fission energy.

Furthermore, it is interesting to notice that advocates of nuclear fission energy like to explain why the dangers from nuclear weapons are far less alarming than believed. This is usually followed by the statement that their praised future nuclear energy technology will avoid proliferation problems. A similar appeasement in their argumentation is found with respect to safety and radiation issues. The existing nuclear power plants are claimed to be very safe, and risks are small compared to many other dangers of modern life. Yet, when their favorite future nuclear energy system is being introduced, it is always pointed out that it further reduces the remaining risks by a large factor.

For example it is often argued that U233 produced in a future Th232 breeding cycle will be useless for nuclear weapons. This argument is certainly flawed as countries who want to have nuclear weapon capability will most likely choose the simpler way to make a bomb using Pu239 or U235. Yet, those who know how to breed and separate hundreds of kg's of U233 can easily replace Th232 with U238 and produce a few tenths of kg's of Pu239, sufficient to construct a few nuclear bombs.

Those not yet convinced of the mutual support of peaceful and military applications of nuclear energy technology should rethink their positions with respect to the Nuclear Proliferation Treaty, the NPT, and to the so-called "evil" government of Iran.

A careful reading of the treaty [15] reveals that Iran, at least so far, is in agreement with the NPT obligations. However one finds that NPT member countries should not exchange nuclear knowledge with nuclear weapon countries outside the treaty. It is also worth remembering that the official nuclear weapon states, Russia, USA, UK, France, and China, have declared in the treaty their intention to eliminate nuclear weapons as quickly as possible. Almost forty years after these countries signed the NPT, they still have more than 20,000 nuclear warheads.

The nuclear arms race at the end of the second world war and during the subsequent cold war is well documented in many reports, books, and movies, and we refer to the extensive literature largely available now on the internet. Especially for those who are not yet convinced about the dangers of nuclear weapons, we would like to recommend the short you-tube video on the largest explosion ever, the 60 Megaton hydrogen bomb in Siberia in 1961 [16] and to Stanley Kubric's masterpiece movie "Dr. Strangelove, or how I learned to stop worrying and love the bomb" from 1964 [17]. This film, even though almost 50 years old, presents many still relevant ideas related to the 20,000 remaining nuclear warheads.

2.3. Liberating the energy from nuclear fission and fusion

As we have seen in the previous section, a large amount of energy per reaction can be liberated from the fusion of light elements and from the fission of heavy elements like uranium. However at least two additional conditions must be satisfied before such a process can be considered for energy production.

In order to obtain a useful amount of energy from nuclear reactions, a continuous and controllable fission or fusion must be achieved for a large number of atoms. For example 10²⁰ U235 atoms, i.e., 0.05 gr, the amount of U235 found in 6 gr of natural uranium, need to be split every second in a 1 GWe nuclear fission reactor.
Enough raw material must be continuously available to sustain this chain reaction.

Only three relevant isotopes satisfy these conditions for the nuclear fission process. These are the two uranium isotopes U235 and U233 and the plutonium isotope Pu239. The energy liberated in the fission process is carried dominantly (about 80%) by the two daughter atoms. This energy is relatively easily transferred to a liquid or gas, and the heat can be used to operate a generator.

The chain reaction is possible as each neutron induced fission reaction produces on average between 2-3 neutrons. As one neutron is needed to initiate another fission reaction, 1-2 excess neutrons minus some inevitable losses are in principle available to increase the reactor power or perhaps to start a nuclear fuel breeding process. The introduction of neutron absorbers allows to control the reactivity of the nuclear reaction and thus to increase or decrease the reactor power.

As we have seen in Section 2.1, most of the large scale nuclear power plants of today are of the PWR (pressurized water reactor) type. They use dominantly U235 as primary reactor fuel. In these reactors, the prompt fission neutrons, with kinetic energies of 1 MeV, are slowed down (moderated) by elastic collisions with the hydrogen nuclei in the water molecules to subeV kinetic energies. The nuclear fission probability with such slow neutrons is increased by a factor of up to several hundred. As a consequence, a large reactor can be efficiently operated and controlled with a relatively low initial enrichment of U235, and large scale power production with moderated neutrons has been mastered by many countries. The combined running experience of such large scale reactors, currently more than 13,000 years, has resulted in stable electric energy production combined with small or negligible risks during regular operation up to an electric power output of more than 1 GWe.

In contrast, the neutron escape rate in smaller reactors and in unmoderated fast reactors is much higher. Therefore, a chain reaction in FBR's with comparable reactor power is more difficult to control, and a larger amount of initial fissile material with a higher density is needed. One consequence is that the required technology to make such highly enriched nuclear fuel will always be faced with the problem of its dual use for bomb making.

The use of the excess neutrons for the transformation of the U238 and Th232 isotopes into fissile Pu239 and U233 looks very promissing, as the amount of fissile material could be increased theoretically by a factor of more than one hundred. The breeding reactions considered would use the excess neutrons according the two reactions:

Some advantages and disadvantages for the U238 → Pu239 and the Th232 → U233 breeding cycles and some practical problems are listed in Table 2. Some of these problems and their proposed solutions will be discussed in detail in

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