Patent Number: 039716996
Section: summary

The present invention relates to neutronic reactors with solid moderators, and to methods of operating such reactors which minimize the deleterious effect of high energy neutron bombardment of the moderator over prolonged periods of time. It has been found that prolonged operation of a neutronic reactor with a solid moderator will cause changes in the physical properties of the material of the moderator, particularly when the reactor is operated at a relatively high power level. Among the changes which occur in the moderator, expansion presents the most immediate problems, since distortion of the shape of the reactor limits the life of the reactor. Extruded moderator materials expand normal to the axis of the extrusion of the material. For this reason, spaces have been provided adjacent to blocks of moderator material on the sides parallel to the axis of extrusion, as described in the copending application of John T. Carleton, Ser. No. 157,287, filed Apr. 21, 1950 now U.S. Pat. No. 3,200,046 dated Aug. 10, 1965. The changes in the physical properties of solid materials as a result of prolonged high energy neutron bombardment are believed to be due to distortion of the crystalline lattice of such materials. When a high energy neutron strikes one of the atoms of a crystal, the bonds which tie the atoms of the crystal together are placed under severe strain, and under certain conditions one of the atoms will be displaced from its position in the lattice structure. This may merely cause rearrangement of the atoms in the lattice structure of the crystal, or it may result in the bombarded atom entirely leaving the lattice structure and taking up a permanent interstitial position. This latter process is believed to be the one which occurs when crystalline materials are expanded by neutron bombardment. It has been found, that the expansion of crystalline materials under neutron bombardment may be minimized if the crystalline materials are maintained at a sufficiently high temperature in order to facilitate "self-healing." Self-healing is the process which occurs when the temperature at which the bombarded crystalline material is maintained sufficiently high to facilitate atoms in resuming the normal crystalline lattice structure. This method of curing the deformations in a crystalline substance was first discovered by Eugene P. Wigner, and described in the copending patent application, Ser. No. 605,958, filed July 19, 1945 now abandoned. A neutronic reactor constructed with a solid crystalline moderator which minimizes the moderator expansion by means of self-healing is disclosed in the copending patent application of Alfred A. Johnson and John T. Carleton, Ser. No. 253,908, filed Oct. 30, 1951. The present invention retards the expansion of the solid crystalline materials under neutron bombardment by maintaining the crystalline material in a fluid atmosphere of a fluid selected to have a thermal conductivity which reduces the heat transfer from the crystalline moderator material to whatever coolant medium is present, thereby increasing the temperature of the crystalline material. As a result, the rate of thermal self-healing will be increased. It is thus an object of the present invention to provide a neutronic reactor with a solid crystalline moderator in which expansion of the moderator is decreased or inhibited. It is also an object of the present invention to provide a method of operating neutronic reactors with crystalline moderators which will decrease or minimize the expansion of the moderator.