Patent Number: 039768880
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to high energy neutron sources, particularly sources of about 14 MeV neutrons for simulating radiation exposures that may be encountered within controlled thermonuclear reactor (CTR) devices. Several areas of research have been identified as requiring neutron sources capable of providing large fluxes and fluences of 14 MeV neutrons. Applicants' recent report, "Fission Fragment Driven (d + t) Neutron Irradiation Source for CTR Materials Damage Irradiations", Aerojet Nuclear Co., ANCR-1134 (1974) lists a number of these applications respecting the testing of materials. This report is hereby expressly incorporated by reference. Those applications to which the present invention are thought to be particularly applicable include: Cross Sections and Related Nuclear Data The measurement of reaction cross sections, particularly for activation cross sections of reactions for which the target is scarce and/or the product has a long half life, and for the (n, .sup.4 He), (nn', .sup.4 He) etc. reactions by which helium is produced. PA0 Measurement Description PA0 Measurement Description PA0 Source Description PA0 Measurement Description Source Description Energy Range: 2-14 MeV PA1 Intensity: 10.sup.5 -10.sup.8 n/sec PA1 Geometry: point source, beam flux PA1 Radiation effect on hydrogen, deuterium and tritium permeabilities in vacuum wall and structural materials. PA1 Radiolysis of molten salt coolants and/or breeding blanket. PA1 Radiation decomposition of LiD, LiT, shielding and structural materials (e.g. borated water, organic materials for seals, etc.). PA1 Radiation effect in trapping efficiencies in diverter materials. PA1 Energy Range: keV -- 14 MeV PA1 Intensity: 10.sup.9 -10.sup.10 n/cm.sup.2 -sec PA1 Fluence: 10.sup.18 n/cm.sup.2 PA1 Geometry: beam flux, in-core irradiation PA1 Vacuum wall erosion PA1 Plasma contamination PA1 Energy Range: keV - 14 MeV PA1 Intensity (at sample surface): &gt;10.sup.12 n/cm.sup.2 -sec PA1 Fluence (yield 0.1 monolayer): 3 .times. 10.sup.17 n/cm.sup.2 PA1 High Fluence Effects: 10.sup.20 -10.sup.21 n/cm.sup.2 PA1 Geometry: point source, beam flux PA1 Neutron fluence effects on physical and mechanical properties: creep strength, and loss of ductility in vacuum wall and structural material at temperatures in range of 500.degree.-1000.degree.C. PA1 Synergistic effect of high gas generation and point defect production rates in a high flux of high energy neutrons on void formation at temperatures in the range of 500.degree.-1000.degree.C. PA1 Establish correlation between heavy-ion bombardment effects and neutron radiation effects at several energies (discrete or integral) in the range of 2-14 MeV. PA1 Transmutation effects on physical and mechanical properties of structural materials. Radiolysis in Materials Source Description Surface Physics Sputtering PA2 Radiation blistering by reaction products: (n, He), (n,p), etc. PA2 Particle desorption by direct neutron and reaction product interactions. PA2 Photo-decomposition of surface compounds by neutron-induced energetic photons and reaction products. PA2 Radiation damage in surface layers. PA2 Multiple backscattering PA2 Secondary particle emission PA2 Secondary electron emission (electron sheath formation). Material Radiation Damage ______________________________________ Source Description Energy Range: keV - 14 MeV Intensity (long term): &gt;10.sup.15 n/cm.sup.2 -sec High Fluence: 10.sup.23 -10.sup.24 n/cm.sup.2 Intensity (correlation 10.sup.13 -10.sup.14 n/cm.sup.2 -sec experiments): Minimum Fluence: 10.sup.20 n/cm.sup.2 Geometry: point source, beam flux in-core irradi- ation ______________________________________ From such studies predictions as to material swelling, transmutation of elements, void formation, changes in superconducting properties and various other materials' characteristics are to be obtained. For the studies relating to materials radiation damage extremely high neutron fluxes (up to 3 .times. 10.sup.12, 14 MeV n/cm.sup.2 -sec) and fluences (10.sup.20, 14 MeV n/cm.sup.2) may be required. Many facilities existing at present or presently proposed have various limitations either to these flux levels, to the required neutron energy or in that they are not conveniently available for materials testing. Table I below includes a partial list of existing neutron source facilities. A more comprehensive list is included within applicants' recent report ANCR-1134, (1974), cited above. TABLE I ______________________________________ NEUTRON SOURCE FACILITIES A. Nuclear Reactors Experimental Breeder Reactor EBR-II (ANL) ______________________________________ Fast neutron flux facility; in-core irradiaton capability. Neutron Flux Data: Radial Position (n/cm.sup.2 -sec) Energy Range Core Center Edge of Core ______________________________________ &gt;3.7 MeV 0.08 .times. 10.sup.15 0.04 .times. 10.sup.15 1.35 MeV-3.7 MeV 0.45 .times. 10.sup.15 0.20 .times. 10.sup.15 100 keV-1.35 MeV 1.70 .times. 10.sup.15 1.18 .times. 10.sup.15 1-100 keV 0.45 .times. 10.sup.15 0.33 .times. 10.sup.15 &lt;1 keV Total 2.68 .times. 10.sup.15 1.75 .times. 10.sup.15 High Flux Isotope Reactor HFIR (ORNL) ______________________________________ Thermal neutron flux facility; in-core irradiation capability. Neutron Flux data: Radial Position (n/cm.sup.2 -sec) Energy Range Central Region Fuel Region ______________________________________ &gt;1.35 MeV 5.21 .times. 10.sup.14 111 keV-1.35 MeV 6.80 .times. 10.sup.14 9 keV-111 keV 3.18 .times. 10.sup.14 0.4 eV-9 keV 1.06 .times. 10.sup.15 Nonthermal (max) 4.0 .times. 10.sup.15 Thermal 2.8 .times. 1.sup.15 ______________________________________ TABLE I ______________________________________ NEUTRON SOURCE FACILITIES B. Accelerator and Target Systems Los Alamos Meson Physics Facility LAMPF (LASL) High-current proton accelerator. Beam Energy, MeV 800 Ion Current (average), mA 1 Cycle Time, pps 120 Pulse Width (.THETA..sub.p), .mu.sec 500 Protons/sec 6 .times. 10.sup.15 Target Uranium Copper Neutron Source Intensity, n/sec 2 .times. 10.sup.17 7 .times. 10.sup.16 Neutron Flux (Target Cavity), n/cm.sup.2 -sec .about. 10.sup.14 &lt;10.sup.14 Energy Spectra Mean, MeV 2 4 Rotating Neutron Target System (LLL) 500-kV Insulated Core Transformer System. Beam Energy (Deuteron), keV 400 Current, mA 8 Target T Rotational Speed, rpm 1100 Neutron Source Intensity (Initial), n/sec 2 .times. 10.sup.12 Flux (1/2 cm from target), n/cm.sup.2 -sec .about. 10.sup.12 Target Half-life, mAh .about. 700 Neutron Energy, MeV 13-15 ______________________________________ One concept which offers immediate promise in producing an intense flux of 14 MeV neutrons is that of a thermal neutron converter. For this purpose materials such as LiOH--D.sub.2 O salts or LiD within converter plates have been suggested for installation within high flux, thermal neutron, reactor systems. The approximately 14 MeV neutrons of these concepts are produced by the following reactions: EQU .sup.6 Li + n thermal .fwdarw. t + .sup.4 He and EQU t + d .fwdarw. .sup.n 14 MeV + .sup.4 He Unfortunately only up to about 10.sup..sup.-4 neutrons of 14 MeV energy per thermal neutrons are produced by these reactions based on the interaction rate as represented by the cross section of the (d + t) reaction. SUMMARY OF THE INVENTION Therefore in view of the limitations of prior neutron source systems, it is an object of the present invention to provide an improved neutron source and method for producing approximately 14 MeV neutrons. It is a further object to provide a method of producing approximately 14 MeV neutrons from a thermal neutron source in which the rate of conversion is more than 10.sup..sup.-4 MeV neutrons per thermal neutron. It is a further object to provide a materials testing device that can be used to expose a sample material to a high flux of about 14 Mev neutrons produced from thermal neutron flux. In accordance with the present invention, fissionable material such as U.sup.235 is contacted with a deuterium - tritium gas mixture in the presence of a thermal neutron flux. As a result the following reactions occur to produce an intense flux of about 14 MeV neutrons: EQU .sup.235 U + n.sub.thermal .fwdarw. 2 fission fragments + 2.3 n.sub.fast EQU 2 fission fragments + (d or t) .fwdarw. (d or t).sub. 100 KeV EQU (d or t).sub. 100 KeV + (t or d) .fwdarw. 3.5 .times. 10.sup..sup.-4 (n.sub.14 MeV + .sup.4 He) A materials testing device employing these reactions includes a plurality of foils containing fissionable material enclosed within a vessel also containing a sample of the material to be tested. The vessel is filled with a mixture of deuterium - tritium gas and exposed to a thermal neutron flux. A coolant system utilizing the circulation of the deuterium - tritium gas mixture is included to remove the heat generated by the reactions.