Pergamon Progress in Nuclear Energy, Vol. 29 (Supplement), pp. 25-32, 1995
1995 Elsevier Science Ltd Printed in Great Britain
SELF-CONSISTENT NUCLEAR ENERGY SYSTEMS
A. Shimizu and Y. Fujii-e
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-10-okayama, Meguro-ku, Tokyo 152, Japan
A concept of self-consistent nuclear energy system (SCNES) has been proposed as an ultimate goal of the nuclear energy system in the coming centuries. SCNES should realize a stable and unlimited energy supply without endangering the human race and the global environment. It is defined as a system that reahzes at least the following four objectives simultaneously: a) Energy generation - attain high efficiency in the utilization of fission energy, b) Fuel production - secure inexhaustible energy source: breeding of fissile material with the breeding ratio greater than one and complete burning of transuranium through recycling, c) Burning of radionuclides - zero release of radionuclides from the system: complete burning of transuranium and elimination of radioactive fission products by neutron capture reactions through recycling, d) System safety - achieve system safety both for the public and experts: eliminate criticality-related safety issues by using natural laws and simple logic. This paper describes the concept o f SCNES and discusses the feasibility of the system. Both "neutron balance" and "energy balance" of the system are introduced as the necessary conditions to be satisfied at least by SCNES. Evaluations made so far indicate that both the neutron balance and the energy balance can be realized by fast reactors but not by thermal reactors. Concerning the system safety, two safety concepts: " self controllability" and "self-terminability" are introduced to eliminate the critieality-related safety issues in fast reactors.
self-consistent nuclear energy systems; zero release of radionuclides; burning of transuranium; elimination of radionuclides; fuel breeding; self-controllability; self-terminability; neutron balance.
It is only about 50 years ago, when the fission reaction of uranium by neutrons was discovered and the controlled neutron chain reaction was first achieved. The energy generated by the fission reactions, that is called "nuclear energy", has an outstanding feature that the energy generated per reaction is quite large, as large as by the factor of one million, compared with conventional energy generated by chemical reactions, such as burning fossil fuel. At that time we had a dream that we found a new and unlimited source of energy.
Peaceful use of nuclear energy has been developed for 50 years, and now it is making an important contribution to the world energy requirement, providing about 17% of its electricity. It is convinced in the nuclear community that the nuclear energy shall be one of the main energy sources in the coming centuries. The nuclear community, however, are challenged to examine the nuclear science and technology in relation to human values. We are challenged to examine not only the benefits of nuclear energy but also the risk to the human and the impact to the environment of nuclear energy. The ultimate goal of nuclear energy is to realize a stable and practically unlimited supply of energy without endangering the human and the environment.
A self-consistent nuclear energy system (SCNES) has been proposed by Fujii-e et al. of Research
26 A. Shimizu and Y. Fujii-e
Laboratory for Nuclear Reactors (Fujii-e et al., 1992a; Fujii-e, 1992b) as an ultimate goal of nuclear energy development. The present paper describes the definition and objectives, the concept, the feasibility, and the safety of SCNES. More details of the system will bepresented in this proceedings (Arie et al., 1995; Endo et al., 1995; Takagi et al., 1995; Akatsuka et al., 1995)
DEFINITION AND OBJECTIVES
In order to specify the ultimate goal of nuclear energy, four objectives are introduced as the fundamental conditions to be satisfied by the nuclear energy system. They are:
a. Energy generation b. Fuel production c. Burning of radionuclides d. System safety
The self-consistent nuclear energy system is defined as a system that realizes at least these four objectives simultaneously.
a) Energy generation
The energy generation is the most fundamental objective of a nuclear energy s~sNm. It is well known that a fission chain reaction generates high quality energy, about 200 MeV, per nSSlOn. The amount of fuel consumed to generate the energy of 1 MWd, that corresponds approximately to the amount of energy used per person per year in Japan, is 1 g of uranium in a nuclear energy system, whereas about 3 tons of coal must be burned to generate the same amount of energy. These amounts of fuel differ by the factor of one million. Quantitatively, the objective of the nuclear energy system concerning the energy generation is to attain high efficiency in the utihzation of fission energy.
b) Fuel Production
To secure stable and practically unlimited supply of nuclear energy, full utilization of nuclear energy embedded in natural uranium resources is inevitable. Is it possible to do so technically? The answer is definitely yes. We have already such technology in our hand. U238 can be changed to Pu239 by a capture reaction of neutron. Pu239 is fissile material like U235. The objective of the system concerning fuel production is expressed as "full utilization of nuclear energy embedded in natural uranium including U238 for peaceful use". The objective is expressed quantitatively in terms of breeding ratio. The breeding ratio is defined as the ratio of the amount of fissile produced in a reactor to that consumed. A projection of energy demand in future is assumed as follows. The energy demand will increase until some time in coming centuries. It is expected to reach an equilibrium. In a transition phase where the energy demand is increasing, the breeding ratio should be greater than unity. In an equilibrium phase, the breeding ratio should be equal to unity. At the equilibrium phase, the amount of fissile produced is identical to that consumed, that is, the self-sustaining condition is realized concerning fuel.
c) Burning of Radionuclides
It should be recognized that the production of energy is always accompanied by the transmutation of materials. That is to say, wastes are inevitably produced during energy generations. Concerning the amount of waste produced during the generation of energy of 1 Mwd, the fission of uranium produces fission products, the amount of which is only 1 g, and 1/3 of fission products are radioactive. On the other hand, the burning of coal produces carbon dioxide gas, ashes, SOx, and NOx. The amount of the wastes of a fossil energy system is as large as about 10 tons. It is larger than the amount of wastes of a nuclear energy system by the factor of 10 million. This is an outstanding feature of a nuclear energy system. The smaller amount of wastes makes it easier to treat them.
Although they are very small amount, the radioactive fission products and transuranium isotopes generated in a reactor have a potential for the risk to the human and the impact to the environment. The third objective of SCNES, i.e., "burning of radionuclides", means "zero release of radionuclides" from the system, in order to make zero radiological burden to be left for the next generations. This objective can be achieved both by complete burning of transurahium and elimination of radioactive fission products by the neutron capture reactions in reactors through recycling.
Self-consistent nuclear energy systems 27
d) System Safety
The system should be safe enough to be accepted not only by the experts in nuclear community but also by the public. Unfortunately, it should be recognized at present that there exists a gap between the perception of nuclear safety by the experts and that by the public. To minimize this gap, the objective of the system concerning safety is concentrated to eliminate the criticality-related safety issues by using natural laws. The objective oI system safety includes also to realize "zero release of radionuclides" at accidents. Two concepts concerning safety are introduced. They are "self-controllability" and "self-terminability". These concepts will be explained later. The objectives of SCNES are summarized as follows.
- The objective of energy generation is to achieve high efficiency. - The objective of fuel production is to secure unlimited energy source. Fuel breeding is required
with a breeding ratio greater than unity at the transition phase and that equal to unity at the equilibrium phase. - The objective of burning of radionuclide means "zero release of radionuclide" from the system, that
can be achieved by the complete burning of transuranium and radioactive fission products in reactors through recycling.
- The objective of system safety is concentrated to eliminate the criticality-related safety issues by natural laws, introducing the safety concepts of "self-controllability" and "self-terminability".
CONCEPT OF SCNES
Figure 1 indicates the input to the system and the output from the system. The input to the system is only natural uranium. The output from the system incluaes useful energies mostly in the form of electricity, thermal wastes, and stable isotopes. The effects to the society by the system are all positive. They include the generation of energy which is the main objective of the system, the production of useful stable isotopes such as platinum metals. Environmental effect of SCNES consists of both positive and negative ones. The consumption of natural uranium has the positive effect, since the natural uranium is hazardous material from the environmental point of view. The release of thermal waste has the negative environmental effect. It should be emphasized here again that SCNES releases no radioactive material.
Figure 2 shows the inside of the system. SCNES consists of a reactor, a recycle facility and a repository. The function of the reactor is as follows. It generates energy by fission reactions, that is, burning of uranium and transuranium. The fission reactions are accompanied by the generation of fission products. The reactor produces transuranium by neutron capture reactions. The reactor eliminates the radioactive fission products by neutron capture reactions. The outputs of the reactor are discharged fuel consisting of uranium, transuranium and the fission products generated, and the recycle fission products consisting of deactinized fission products and radioactive fission products. The recycle facility received the outputs f?om the reactor. It extracts uranium and transuranium, radioactive fission products and useful fission products from the discharged fuel. It also extracts deactinized fission products from the recycled fission products. It fabricates fresh fuel. The fresh fuel and the radioactive fission products, whose half lives are greater than one year are recycled to the reactor. The recycle facility has the following specific features as compared with the purex method. Firstly, it extracts uranium and transuranium collectively. There is no need to separate plutonium from uranium. Secondly it includes the isotope separation of fission products. The repository receives radioactive fission products whose half lives are less than one year from the
recycle facility and stores them until their radioactivity decrease to a level of effectively zero.
The feasibility of the overall system will be discussed here. "Neutron balance" and "energy balance" are introduced as the necessary conditions for SCNES to be satisfied at least. These conditions are evaluated based on scientific data as much as possible. Technologies, when their data are required in the evaluations, are assumed in idealistic states to be achieved in the future. Figure 3 indicates schematically the amount of accumulated radioactive fission products in the reactor through recycling. When its production rate is larger than its elimination rate, its amount increases with time (transition phase) and finally reaches an equilibrium state, where production rate is equal to elimination rate (equilibrium phase). The energy demand and the corresponding nuclear energy capacities are also separated in the transition phase and the equilibrium phase. In the evaluation of feasibility the transition phase and the equilibrium one are treated separately.
28 A, Shimizu and Y. Fujii-e
a) Neutron balance
The neutron balance is defined as the balance between the number of neutrons generated per fission and those consumed per fission in a reactor. Example of the neutron balance at the equilibrium phase are illustrated both for a fast reactor with metal fuel and a thermal reactor with mixed oxide fuel in Table 1. The numbers of neutrons generated per fission are almost same in both reactors with plutonium fuel. The number of neutrons consumed per fission to maintain fission chain reaction for energy generation is evidently one. The fission reaction is accompanied by capture reactions of fissile fuel. The number of neutrons consumed by fissile capture is 0.13 in a fast reactor and 0.4 in a thermal reactor. The number of neutrons consumed for breeding when breeding ratio is unity indicates also significant difference between a fast reactor and a thermal reactor. The table indicates the condition of neutron balance can be satisfied by a fast reactor, because the number of neutrons generated per fission is larger then the total number of neutrons consumed per fission. On the other hand, it is not satisfied in a thermal reactor. The difference between both reactors is mainly due to the differences in the two processes, that is, capture by fissile and fuel breeding. The main reason for these differences will be explained below.
The capture to fission cross-section ratio of Pu239, by which most fission occurs, is high in the thermal energy range and low in the fast energy range. Therefore, the capture to fission ratio averaged over the neutron energy spectrum in the core, ~', is high in thermal reactors and low in fast reactors. Concerning fuel breeding, the number of neutrons required for breeding per fission n8 R is given by the equation.
nsR = BR( I+ a---)/(1 + s) (1)
where BR represents the breeding ratio and s is the ratio of the fission by fertile to the fission by fissile. The value of s is about 0.3 in fast reactors and about 0.07 in thermal reactors. The difference in these two factors, U ande , makes the large difference in the number of neutrons for breeding per fission between fast reactors and thermal reactors. Concerning elimination of radioactive fission products, it can be shown that the number of neutrons required to eliminate them is dominantly determined by their yields in the fission reacti...