Patent Number: 041585992
Section: summary

CROSS REFERENCES TO RELATED APPLICATIONS This application relates to the following applications filed concurrently herewith or filed previously as indicated and, to the extent necessary or desirable, incorporated herein by reference: U.S. Pat. No. 3,836,429 entitled "Means for Rapidly Exposing The Core of A Nuclear Reactor For Refueling" by Erling Frisch and Harry N. Andrews. U.S. Pat. No. 3,836,430 entitled "Cable Support Structure For Enabling A Nuclear Reactor To Be Refueled Rapidly" by Erling Frisch and Harry N. Andrews. U.S. Pat. No. 3,766,006 entitled "Rapidly Refuelable Nuclear Reactor" by Erling Frisch and Harry N. Andrews. U.S. Pat. No. 3,752,737 entitled "Combination of Nuclear Reactor and Missile Shield" by Erling Frisch and Harry N. Andrews. U.S. Pat. No. 3,685,123 entitled "Means For Retaining and Handling Reactor O-Ring Seals" by Erling Frisch. U.S. Pat. No. 3,837,694 entitled "Hydraulic Head Closure Mechanism" by Erling Frisch and Harry N. Andrews. U.S. Pat. No. 3,607,629 entitled "Reactor Refueling Method" by Erling Frisch and Harry N. Andrews. BACKGROUND OF THE INVENTION This invention relates to the nuclear reactor art and has particular relationship to nuclear reactors which serve as primary sources for power-supply facilities. A demand which is imposed on such reactors is that they be refueled periodically. Refueling operations carried out in accordance with the teachings of the prior art consume about 3 to 6 weeks. While this invention is applicable to reactors of other types, it is uniquely applicable to the refueling of reactors of the pressurized water type, P.W.R. In the interest of concreteness this application, in its descriptive text, confines itself to P.W.R.'s assuming the pressure vessel to be vertical. Such nuclear reactor (U.S. Pat. No. 3,607,629 above) includes a pressure vessel having a body sealed by a head. The body is typically 40 feet long and 15 feet in diameter and includes the fuel core, which typically may include 193 fuel assemblies and the upper and lower internals. The head is typically 15 feet in diameter and in the past has been sealed by about 52 studs. The reactor includes control rods which are inserted in, or retracted from, the fuel for control purposes by control rod control mechanisms. The mechanisms operate in housings which are sealed pressure tight to the head and extend above the head. There are typically 60 such mechanisms each including drive or drives, typically a piston, a control rod drive shaft connected to the drive and extending from the mechanism housing through the head, each shaft engaging associated control rods. In refueling, in accordance with the teachings of the prior art, the studs are detensioned and removed from the vessel flange and the head and mechanism housing are lifted and removed exposing the control rod drive shaft and the control rods. The control rod drive shafts are then disconnected from the control rods and removed with the upper internals and the refueling is carried out with the control rods in the core. After refueling the above described process is reversed. The long shut down of several weeks which this method of refueling demands renders refueling at frequent intervals not practicable and the refueling in accordance with the teachings of the prior art takes place approximately annually. It has been discovered in arriving at this invention that this annular refueling imposes severe restrictions on the initial or replacement fuel. Typically, the initial enrichment in fissionable material of the fuel must be sufficient to maintain the reactivity of the reactor for at least a year. Typically, this enrichment is of the order of 3.2%. Because of this higher initial enrichment the medium in which the fuel is immersed must have a higher concentration of neutron absorber such as boron. These conditions are imposed not only on the initial fuel but also on each replacement. In addition, the number of fuel assemblies replaced is based on the annual refueling cycle and must be a substantial fraction of the assemblies in the core. Evaluation of the economic effects of refueling time reveals that there is large economic incentive in reducing materially the refueling time. Not only can the loss, resulting from the reactor being out of operation for long intervals, be reduced, but, in addition, because the refueling can take place at short intervals advantages are available in feasibility of using fuel of lower enrichment and in frequent replacement of a relatively small portion of the fuel assemblies during each refueling. In addition the concentration of neutron absorbing material in the medium in which the core is immersed, for example boron, in water, may be lower. Typically in a pressurized water reactor annual refueling, demanding enrichment in fissionable material of the order of 3.2% requires a concentration in the water of about 1200 parts per million of natural boron typically including by weight 20% B.sup.10 and 80% B.sup.11 at the beginning of life with a consequent build-up of a high concentration of tritium, H.sup.3, during life while the concentration of boron is being reduced to about 10 p.p.m. In a typical example, for a semiannual refueling cycle the initial enrichment is reduced and the concentration of boron at the start of life is reduced to 650 p.p.m. with consequent reduction in the H.sup.3 generated; and for a three-months refueling cycle the enrichment is only 2.7% and the boron concentration at the start of life is reduced to 350 p.p.m. It has been found that by refueling during an interval of three days about every three months a fuel saving amounting to about $14 per kilowatt can be realized. It is an object of this invention to improve the economy of operation of a nuclear reactor and to provide a method of refueling a nuclear reactor which can be carried out in a short interval of only a few days permitting a short time cycle, of the order of three or six months between refuelings of the reactor and consequent reduction of initial enrichment in fissionable material of the fuel and low concentration of neutron absorbers, and with minimized reactor downtime for refueling. SUMMARY OF THE INVENTION In accordance with this invention the number of separate tasks to be performed in refueling are substantially reduced so that access to the pressure vessel is obtained rapidly. In addition cross transportation of the old and new assemblies is simplified and speeded up and refueling techniques are simplified and automated. The possibility of delays from maloperation is reduced by improving the reliability of each step of the refueling. The result is that refueling can be carried out in a few days conceivably under minimum electrical utility system load conditions, and the refueling cycle can be only three to six months. Specifically, the studs are detensioned rapidly by hydraulically operated detensioners, as disclosed in U.S. Pat. No. 3,837,694. The missile shield, disclosed in U.S. Pat. No. 3,752,737 is then displaced locking the control rod control mechanisms in the retracted position in their housings without dependence upon electrical current flow, as disclosed in U.S. Pat. No. 3,766,006. The whole upper package including the missile shield, head of the pressure vessel, the control rod drive mechanisms and their drive shafts, the control rods and the upper internals, is then lifted and placed out of the way in a single lifting operation. The integral structure including the missile shield is disclosed in U.S. Pat. No. 3,836,429. The cables are on a pivoted cable tray, as disclosed in U.S. Pat. No. 3,836,430, and are sufficiently long so that they need not be, and are not, disconnected during the whole refueling operation. The removal of the control rods during refueling requires that the boron concentration be increased, during refueling. It is of interest that the increase is to 2500 p.p.m. for an annular refueling cycle. For a six-month refueling cycle the boron concentration during refueling is reduced to 1880 p.p.m. and for a three-month cycle it is correspondingly further reduced. The new replacement fuel assemblies are transferred in one operation into the containment where the open vessel is disposed and are in position for refueling. The spent fuel assemblies are likewise transferred out of the containment in one operation. A fuel transfer pit is provided in this containment where the spent assemblies may be retained temporarily. Typically, in refueling, at 6 month intervals, one-fifth of the spent assemblies in the core are replaced. In this practice assemblies are removed from the center of the core and are replaced by assemblies outside of the center. The peripheral assemblies are then replaced. Alternatively, a refueling technique may be adopted, wherein no fuel assembly rearrangement occurs. Typically where there are 193 assemblies in the core, 20 may be replaced directly during each refueling. The cycle is three-months. The replacements are in each case identified so that in about 21/2 years all assemblies are replaced. After the refueling the boron concentration is reduced to the required magnitude (for example 350 p.p.m. for a three month's cycle).