AN ALTERNATIVE, ‘NON-ELECTRICAL’ PATHWAY

FOR FUSION ENERGY DEVELOPMENT

Weston M. Stacey

Fusion Research Center

Georgia Institute of Technology

Atlanta, GA 30332

(404) 894-3758

June, 1998

ABSTRACT

The physics and technology that is being developed for and that will be demonstrated in ITER [1] will be sufficient to make a very good neutron source, there are a number of potential ‘national missions’ for a good neutron source, and the further technology advances beyond ITER that would be required for a neutron source facility are essentially the same as the advances that would be required for an electrical energy producing fusion demonstration reactor. Some preliminary considerations are presented for an alternative pathway for fusion energy development, proceeding from the present through an international test reactor (ITER) stage to a fusion neutron source facility (or non-electrical applications) stage and finally to the deployment of fusion electrical power reactors. Recent studies of two types of fusion neutron source facilities for ‘national missions’ are reviewed as representative examples.

I. INTRODUCTION

The ultimate promise of fusion is an environmentally benign and inexhaustible energy supply for the future of mankind, and it is this vision of the ultimate energy source which has guided the various national strategies for fusion development. However rational a justification this vision may be for the ultimate development of fusion energy, it clearly is not a compelling justification for the near-term development of fusion energy in a country that is awash in energy resources and not yet willing to face the environmental consequences of burning fossil fuels, as is evident from the low level of support for fusion energy development in the congress and in the administration. Clearly, it is a time to examine other justifications for the development of the physics and technology of fusion energy; the best path forward may not be the most direct path.

The D-T reaction, in addition to producing energy, produces a neutron. Often portrayed as a curse, this neutron could prove a blessing for fusion energy development. The physics and technology that is being developed for and that will be demonstrated in ITER [1] will be sufficient to make a very good neutron source, there are a number of potential ‘national missions’ for a good neutron source, and the further technology advances beyond ITER that would be required for a neutron source facility are essentially the same as the advances that would be required for an electrical energy producing fusion demonstration reactor. Needless to say, there are other possible neutron sources—nuclear reactors and accelerator driven spallation targets—whose proponents are well along in laying claim to these ‘national missions’, but fusion has some intrinsic advantages, not least of which is the fact that development of fusion neutron source facilities would also serve to develop the ultimate energy source for mankind.

The purpose of this paper, then, is to present some preliminary considerations for an alternative pathway for fusion energy development, proceeding from the present through an international test reactor (ITER) stage to a fusion neutron source facility (or non-electrical applications) stage and finally to the deployment of fusion electrical power reactors when the need for such is better appreciated. The standard electrical power pathway and an alternative non-electrical pathway for fusion energy development are discussed in section II, potential ‘national missions’ for neutron source facilities are described in section III, and the three candidate neutron sources are compared in section IV. Recent studies of two types of fusion neutron source facilities for ‘national missions’ are reviewed in section V as representative examples.

The ultimate objective of the government fusion energy development program is to bring one or more fusion reactor concepts to the stage at which that concept is sufficiently demonstrated that private industry is willing to take responsibility for the next step as a commercial venture, perhaps with some government subsidization. Demonstration of fusion power is generally agreed to require:

The collection of such sequential and parallel research programs and development steps as lead to these demonstrations is the fusion development pathway for a given fusion concept. The culmination of the development pathway for a given fusion reactor concept is generally agreed to be a demonstration plant (DEMO) in which the ultimate demonstration of most, if not all, of the above requirements is achieved. A recent study of likely tokamak DEMO characteristics is given in Ref. [2].

The ITER mission was defined with the objective that ITER, its supporting R&D programs, and the nuclear and materials testing program that would be carried out in ITER together with the supporting national nuclear and materials R&D programs would provide the design basis for a DEMO with respect to requirements 1-6. It was envisioned that advanced physics research would be carried out in and in parallel with ITER to develop an advanced tokamak physics concept for the DEMO in support of requirement 7. The development of advanced blanket, structural and other materials in parallel with ITER is necessary to provide the design data base for a DEMO that can satisfy requirements 2-8. These major elements, together with the base tokamak plasma physics and fusion engineering science research programs, constitute the standard fusion energy development pathway for tokamaks.

The above fusion energy development pathway is predicated on the implicit assumption that society will wish to deploy fusion as an energy source as soon as it is technically feasible to do so. In the absence of a strong government energy/environmental policy and in the regulatory environment in which the electrical utility industry presently operates, it is in fact unlikely that any new, capital cost intensive energy source will be introduced, no matter what the long-term environmental, economic and societal benefit. If this situation persists, then alternative fusion energy development pathways which proceed beyond the ITER stage to the government deployment of fusion neutron source facilities to meet various national needs (e.g. transmutation of spent nuclear reactor fuel, tritium production, hydrogen production) should be considered. The design data base that would be provided for DEMO in the tokamak development pathway would also constitute an adequate design data base for a tokamak neutron source. Such neutron source facilities should be able to satisfy the above eight requirements for fusion power demonstration. Thus, one or more tokamak neutron source facilities could replace the DEMO in an alternative, non-electrical fusion energy development pathway for tokamaks, as shown in Fig. 1.

III. POTENTIAL NATIONAL MISSIONS FOR

NEUTRON SOURCE FACILITIES

There are a number of potential national missions for neutron sources, as shown in Table 1. Neutron

TABLE 1

POSSIBLE APPLICATIONS OF FUSION NEUTRON

SOURCES

source facilities are required for fundamental and applied research and for the production of radioisotopes for medical and industrial applications. Replenishment

of decaying tritium (12.5 yr half-life) will become necessary within the next decade or two in order to maintain the nation’s weapons stockpile. The large-scale production of hydrogen will become attractive early in the next century when the technology for utilizing hydrogen for mobile energy production is developed. Extraction of the remaining energy content of spent nuclear reactor fuel by neutron fission of the remaining U-235 and of the transmuted Pu-239 and higher actinides would increase the nation’s utilization of its uranium resources by two orders of magnitude. Neutron transmutation of the long-lived actinides (primarily by fission) and fission products in spent fuel has the potential for dramatically decreasing the amount of nuclear waste that must be consigned to long-term (>10⁵ years) storage, and could possibly even eliminate the need for nuclear waste storage beyond a few hundred years. Surplus weapons grade plutonium resulting from the dismantling of nuclear weapons, which would be vulnerable to diversion to terrorist organizations or rogue states while awaiting utilization in nuclear power reactors, can be transmuted by neutrons into a form in which highly sophisticated technology was required for weapons assemblage, thus deterring such diversion.

IV. COMPARISON OF POSSIBLE NEUTRON

SOURCES

At present, there are three realistic possibilities for large-scale neutron sources—nuclear reactors, accelerator/spallation neutron sources and tokamak fusion neutron sources. The pros and cons of these neutron sources are shown in Table 2. Nuclear reactors have the very strong advantage of being an existing technology that provides a distributed neutron source, hence a large irradiation volume. The only intrinsic shortcoming of a nuclear reactor is that it needs to capture most of the neutrons that it produces in the fissile fuel in order to maintain the critical neutron chain reaction, and this limits the amount of parasitic absorber that can be tolerated, which has the effect of limiting the practical irradiation volume that can be devoted to any of the purposes shown in Table 1.

Both accelerators and fusion suffer from the disadvantage that the technology does not yet exist and must be developed. In both cases, the technology is being developed. The accelerator/spallation technology currently enjoys strong institutional and political support in the US government-laboratory system and is being developed as part of proposed projects for accelerator/spallation neutron sources for research and for tritium production. The tokamak fusion technology is being developed internationally in support of the ITER project and domestically in support of the US fusion program objective to develop the knowledge base for the demonstration of fusion power. Unlike the situation for the accelerator, the fusion technology is already being developed (with significant international leveraging) for another purpose and would not have to be developed specifically for the neutron source facility application.

Like the nuclear reactor, the fusion neutron source is a distributed, volumetric source, which allows geometrically for a large practical irradiation volume for any of the non-electrical applications shown in Table 1. The accelerator neutron source, on the other hand, is essentially a point neutron source, which not only places a severe limitation on the practical irradiation volume but also leads to very large local heat deposition and neutron damage rates in the target. Fusion intrinsically enjoys an advantage relative to fission in the number of neutrons produced per unit of heat produced (1 neutron per 17.6 MeV versus about 2.7 neutrons per 200 MeV).

Both the fusion and accelerator/spallation neutron sources would be able to achieve greater amplification of the original neutron source by n-2n, n-3, etc. reactions than would a nuclear fission reactor because of the higher energies of the neutrons emerging from the fusion and spallation reactions than from the fission reaction. This is particularly important for the fusion neutron source, in which tritium self-sufficiency imposes a constraint on neutron conservation similar to the criticality constraint in nuclear reactors.

TABLE 2

PROS & CONS OF POSSIBLE NEUTRON SOURCES

V. TOKAMAK FUSION NEUTRON SOURCE

FACILITY STUDIES

Three studies of neutron source facilities based on the fusion physics and technology which will be developed in the tokamak development pathway shown in Fig. 1 (up to the box labeled "Knowledge Base..") are summarized in this section. Two of these studies were for neutron source facilities for tritium production, which serve as examples of the less demanding neutron source applications, and the third study was for a neutron source facility for weapons grade plutonium disposition, which serves as an example of the more demanding neutron source applications.

The principal physics parameters for the two Tokamak Tritium Production Reactors [3,4] (TTPR-1 and TTPR-2) and the Tokamak Plutonium Transmutation Facility [5] (TPTF) are compared in Table 3 with the design parameters of the ITER EDA Final Design [1]. ITER is designed to achieve the indicated parameters operating in the ELMy H-mode, but also has the capability to operate in an advanced tokamak mode to achieve higher plasma performance parameters. Since a reduced-cost ITER (RC-ITER) design is also being developed, parameters for a representative RC-ITER [6] are also shown. The evolving advanced tokamak (AT) mode database is anticipated to support H_89P £ 3.0 and b _{N £} 3.5 within the next 2-4 years [7], and it may be expected that ITER will demonstrate operation at these or somewhat higher values. Thus, ITER should demonstrate the plasma performance needed for a Tokamak Tritium Production Reactor and has the possibility of demonstrating the somewhat more demanding performance requirements of a Tokamak Plutonium Transmutation Facility.

TABLE 3

COMPARISON OF PLASMA PHYSICS PARAMETERS

The technology parameters of the TTPRs, TPTF and ITER are compared in Table 4. Insofar as possible, the ITER technology was adopted for the TTPR and TPTF. The magnet systems were scaled-down versions of the ITER magnet systems. The same heating and current drive technology was used. The structure, coolant and breeding materials were selected from among those which are being developed in the international fusion program for testing in ITER, with the exception of the refractory metal structure which it was necessary to specify for TPTF in order to handle the high heat loads. The ITER R&D programs, the parallel national nuclear and materials R&D programs and the operation of ITER should demonstrate all of the fusion neutron source technologies required for a Tokamak Tritium Production Reactor and a Tokamak Plutonium Transmutation Facility, except the refractory metal structural material needed for the TPTF.

TABLE 4

COMPARISON OF TECHNOLOGY PARAMETERS

*ITER will test tritium breeding in several materials in test modules.

The technical details of the studies are published [3-5], and we limit further discussion to a summary of the performance capabilities of the facilities. The neutron source and power outputs of the three facilities are shown in Table 5.

TABLE 5

NEUTRON & POWER OUTPUTS

The surplus (in excess of the amount needed to refuel the fusion neutron source) tritium production capability of the Tokamak Tritium Production Reactors is shown in Fig. 2 as a function of plant factor and operating fusion power level. Decay loss rates were calculated assuming that the surplus tritium was stored for one year before being shipped. It is estimated [3] that 2 kg/yr of tritium will be needed for weapons replenishment after 2010. There is a wide range of rather modest operating parameters (plant factors as low as 15-20% at fusion powers of 500 MW or more and plant factors as low as 10-15% at fusion powers of 750 MW or more) for which 2 kg/yr of surplus tritium can be produced, and there is a good potential for producing substantially more excess tritium if the need should arise. We note that the ³He into which the weapons tritium decays has a very large thermal neutron capture cross section, as a result of which it is possible to efficiently transmute it back into tritium.

The performance of the Tokamak Plutonium Transmutation Facility is measured in terms of the rate at which weapons grade plutonium can be irradiated to the point at which highly sophisticated technology is required for weapons fabrication. We found that the most limiting deterrent would be the hard gamma dose associated with the fission products, and we calculated that irradiation of weapons grade plutonium to 115 MWd/kg, which would require 325 effective full power days in the TPTF, would lead to a gamma dose of 10 rad/min behind a 2 cm lead apron at a distance of 50 cm, which would be sufficient to incapacitate a would-be weapons fabricator in 30 minutes to an hour. The TPTF could process 10 metric tons of plutonium per batch. The entire world stockpile of weapons grade plutonium is estimated [5] to be about 170 metric tons. The rate at which a single TPTF could process weapons-grade plutonium into a diversion deterred form is shown in Fig. 3. For example, one-half of the estimated total inventory of weapons grade plutonium could be processed in a single facility operating at 25% plant factor for 40 years.

REFERENCES