This invention was made under, or during the course of, a contract with the U.S. Department of Energy.
The nuclear power industry today is mainly based on light water-cooled thermal burner reactors (LWR's) either of the pressurized water-type (PWR) or the boiling water-type (BWR). These reactors have proven performance in providing reliable and economical power and they actually supply more than ten percent of the electric power generated in the United States today.
The LWR's use low enriched U-235 fuel (LEU). Enrichment is obtained in government-owned gaseous diffusion plants where natural uranium containing seven tenth of a percent of U-235 is enriched to an approximately three percent U-235 content for use in LWR's. The enrichment concentration is chosen for safe, economical operation. It takes about six tons of natural uranium to produce one ton of LEU for use in the LWR power reactors. The depleted five tons of uranium contains about two tenths of a percent of U-235.
The one ton of LEU is fabricated into fuel elements and burned in the LWR at a conversion ratio of approximately six tenths (fissile material produced to fissile material burned) to a total burnup of thirty thousand MWD/ton of the LEU. The burnup limitation is not due to radiation damage to the cladding materials (zircaloy) of the fuel elements.
The spent fuel element upon removal from the LWR contains roughly two percent fissile material, about half of which is Pu-239 and half is the remaining unburned U-235. Assuming no recovery of this fissile material for reuse by reprocessing, the above-described nuclear fuel cycle requires a consumption of 6300 tons of natural uranium for each 1000 MW(e) LWR power reactor over a thirty-year production lifetime for the reactor. This includes initial core inventory. In effect only five tenths of a percent of the natural uranium is utilized to make power and the net burnup amounts to only five thousand MWD/ton of natural uranium. Approximately thirty percent less fuel is required if recovery of the Pu-239 is obtained from the spent fuel element by chemical reprocessing. The requirement would then decrease to about forty-three hundred tons of natural uranium.
The natural uranium resource in the United States has been estimated to be in the order of 3.times.10.sup.6 tons. This is for uranium which can be reasonably recovered at a cost of less than one hundred dollars per pound of yellow cake (U.sub.3 O.sub.8). This resource then can only support a maximum of 480,000 MW(e) of nuclear power. There are other estimates which fix the uranium reserves at only 1.5.times.10.sup.6. Therefore, only 240,000 MW(e) of nuclear power would be supportable. There are a number of conservative estimates which indicate that the United States will need somewhere in the neighborhood of 1,000,000 MW(e) of power at the turn of the century and that four hundred reactors (1000 MW(e) each) will share this requirement with other power sources such as coal, oil, and solar. Based on these values, LWR's supplied by the present nuclear fuel cycle cannot be considered as a long-term solution to the energy problem. In fact, utility executives today are quite concerned about whether to invest in another generation of LWR's. The fast breeder reactor (FBR) has thus been put forward as an absolute necessity in insuring the long-term establishment of a nuclear fueled economy.
The fast breeder reactor has a conversion ratio greater than 1.0 so that it allows converting essentially all the naturally occurring U-238 to fissile material for generating power. With the FBR, the uranium resource can ultimately be extended two hundred times the present value and essentially an unlimited energy source then becomes available. However, a number of drawbacks can be listed for FBR's. These include:
1. The fissile material concentration, being ten percent or more in FBR's, is much higher than in LWR's. In fact LWR's must supply the initial Pu inventory for the FBR's.
2. A new technology must be adopted for FBR's to replace the present LWR's. The FBR's are either liquid metal (Na) or gas cooled (He) which implies new and higher unit capital investments and new safety regulations and precautions.
3. Reprocessing of fuel from the FBR is an absolute necessity.
The present domestic policy on nuclear power is to postpone, indefinitely, reprocessing as a means towards impeding the possible proliferation of nuclear weapons. This policy, at once, further limits the nuclear fuel resource for nuclear power generation and tends to eliminate fast breeder reactor fuel cycles. Studies have, therefore, been initiated to investigate alternative nuclear fuel cycles which do not depend on nuclear fuel reprocessing. These studies indicate that without reprocessing, the best burner and convertor fuel cycles can do is stretch the nuclear fuel resource by not more than a factor of two and new HWR technology would have to be introduced. Further stretching in nuclear fuel resources would require safeguarded nuclear fuel reprocessing centers.
Relative to the above and the subject matter of the present invention, there has been found U.S. Pat. No. 3,349,001 which issued to R. M. Stanton on Oct. 24, 1967. This patent discloses a thermal nuclear apparatus having a molten metal proton target surrounded by a blanket of fertile material and a recirculating coolant. The molten metal passes through the assembly providing a changing exposure to accelerated protons for the generation of spallation neutrons.
More particularly, the Stanton patent reveals a target assembly for accelerated nuclear particles. The target comprises a housing with a conduit running symmetrically through and carried by the housing. Molten lead flows through this conduit and a recirculating apparatus is connected to the ends of the conduit for recycling the molten lead. An evacuated tube is longitudinally and symmetrically disposed within the conduit from within the housing to an externally disposed particle accelerator. The tube receives and guides a beam of accelerated nuclear particles emanating from the accelerator. A container is radially disposed around the conduit within the housing and fertile nuclear material is disposed within the container. A coolant passes through and in contact with the fertile material and is carried by the housing. The accelerated particles strike the molten lead which thereby emits nuclear particles which penetrate and react with the surrounding nuclear material causing additional nuclear reactions and generating heat.
As will be seen hereinafter, there are various fundamental differences between the present invention and that disclosed in the Stanton patent whereby the apparatus and methods of the instant invention are substantially more useful and practical. Stanton's approach would lead to excessively high local power densities and fluxes because of rapid beam attenuation.
Other patents which have been located include U.S. Pat. No. 3,993,910 (D. Parkin et al), U.S. Pat. No. 3,623,130 (D. Galrymple), and U.S. Pat. No. 3,500,098 (J. Fraser).
D. Parkin et al entitled their patent "Liquid Lithium Target as a High Intensity, High Energy, Neutron Source." This patent teaches the concept of generating neutrons by spallation of a cascading liquid metal by a high energy neutron beam.
The D. Galrymple patent is entitled "Target Assembly for Thermal Neutron Generator." This patent teaches the concept of utilizing liquid lead-bismuth as a proton beam target for generating neutrons.
J. Fraser entitles his patent "Intense Neutron Generator". This patent discloses a system wherein a molten metal flows past an ion beam.