Induced nuclear reactions: processes – systems – and elements – Control component for a fission reactor – Particulate type
Reexamination Certificate
1954-03-02
2001-05-01
Behrend, Harvey E. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Control component for a fission reactor
Particulate type
C376S327000
Reexamination Certificate
active
06226341
ABSTRACT:
This invention relates generally to neutronic reactors, and more specifically to devices for preventing neutronic chain reactions from exceeding safe operating limits.
All neutronic reactors are constructed with as much excess reactivity as is possible considering the available reactor control system, the excess reactivity of a neutronic reactor being defined as the amount that the reproduction ratio of a neutronic reactor under most favorable conditions exceeds unity. The reproduction ratio of a neutronic reactor is the ratio of the number of neutrons in any given generation to the number of neutrons in the preceding generation within the actual pile structure. The excess reactivity of a neutronic reactor determines the magnitude of isotope production and other neutron-absorbing activities which the reactor may accomplish, and also determines the flexibility which is possible in operating the reactor.
The existence of excess reactivity in a reactor makes desirable both regulating control means and safety control means for the reactor. Regulating control means are necessary in order to maintain the reproduction ratio of the reactor at unity during constant power operation, and to make adjustments in the power level of the reactor. Safety control means are desirable in order to shut down the reactor more rapidly when unsafe operating conditions develop than is possible with the regulating control means.
There are many causes of unsafe operating conditions which make it desirable to shut down a neutronic reactor. If the reactor “period” becomes too short for any reason, it is desirable to shut down the reactor, the reactor period being the time required for the neutron flux density within the reactor to increase by a factor of e, or 2.718. In those reactors which employ cooling means and operate at substantial power levels, it is also desirable to shut down the reaction if there is a decrease in the flow of the coolant. There are also many other reasons for providing a safety control system to shut down a neutronic reactor, and the safety control system may be coupled to any of these dangerous conditions.
A number of safety control systems have been developed in the neutronic reactor art. In one of these systems, neutron-absorbing rods are disposed within channels which extend into the active portion of the reactor, the active portion being the region of the reactor in which the fissionable material is disposed. The rods of neutron-absorbing materials are mechanically biased to enter the active portion of the reactor when released, either by the attraction of gravity or some impelling force.
Another safety system provides a channel extending through the active portion of the reactor and means to impel bodies of neutron-absorbing material into the channel in response to an unsafe condition. The application of John J. Goett, Ser. No. 595,189, entitled “Reactor Control”, filed May 22, 1945, now U.S. Pat. No. 2,773,823, discloses such a system provided with a centrifugal impeller for driving balls of neutron-absorbing materials into a reactor.
It has been found that neither of these systems is entirely satisfactory. Both of the systems are complicated by the fact that it is desirable to keep the ambient atmosphere from the active portion of the reactor to as great an extent as possible, since both nitrogen and oxygen present in the ambient atmosphere have relatively large neutron capture cross-sections. For this reason, it is generally necessary to provide liners for all channels entering into the active portion of the reactor, and to seal the channels from the atmosphere within the reactor. It is also to be noted, that the liners themselves introduce added neutron losses into the reactor.
Considerations of neutron economy also dictate that the channels extending into the active portion of the reactor be confined to as small a cross-section as possible. As a result, the rod safety system permits relatively small clearance between the rods and the rod liners, and hence it is possible for some of the rods to jam within the channels in the reactor before the rod is effectively inserted into the active portion of the reactor. The problem of jamming is further complicated in neutronic reactors which use graphite or other solid crystalline moderator structures by the fact that distortion of the solid moderator structure results from the bombardment of the structure by the high energy neutrons which are present in neutronic chain reactions. The shifting of graphite blocks in reactors employing moderators thus constructed, may so distort the channels for the safety control devices, that rods could not be made to enter into the active portion of the reactor effectively, hence causing the entire safety system to fail.
The rod type of safety system, however, has some distinct advantages over other types of control, such as the impelled ball system described above. A rod may readily by recovered from its channel in the active portion of the reactor, whereas it is difficult to remove neutron-absorbing balls from the channels extending through the active portion of the reactor, both as a mechanical problem and as a health physics problem, since the neutron-absorbing balls will remain highly radio-active even after the neutronic reaction has ceased. Hence, it is an object of the present invention to provide a safety system for a neutronic reactor which is both neutronically safe and physically convenient.
Another object of the present invention is to provide a safety system which provides two separate safety operations, the failure of one operation actuating the second operation, thereby greatly reducing the probability of a failure of the safety system.
Further, it is an object of the present invention to accomplish the provision of a safety system having two separate operations without introducing into the neutronic reactor additional neutron-absorbing materials or additional neutronically deleterious channels.
REFERENCES:
patent: 2708656 (1955-05-01), Fermi et al.
patent: 2735811 (1956-02-01), Weinberg et al.
A.E.C.D. 3065, “High Power Water Boiler”, pp 10-13, 17-19, Feb. 27. 1951.
Behrend Harvey E.
Fisher Robert J.
Gottlieb Paul A.
Moser William R.
The United States of America as represented by the United States
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