cycle, central fuel assemblies have reached their maximal burn-up while peripheral fuel assemblies have no delivered completely their reactivity and energy potential.
c. Large reactivity margin must be formed in the central core region. Hence, control rods with appropriately large total reactivity worth must be placed in the core center. This can worsen neutron economy during the reactor operation cycle.
2.The “partial batch” refueling strategy presumes that only those fuel assemblies, which have reached their maximal acceptable fuel burn-up, must be removed after the irradiation cycle and replaced with fresh fuel assemblies. At the next refueling, fuel assemblies with the highest fuel burn-up are replaced again, and so on. If the reactor core is divided into several concentric layers, then each layer, starting from the innermost layer and proceeding outward, is replaced with fresh fuel in several successive refuelings.
Main advantage of the “partial batch” refueling str ategy is a fairly uniform fuel burn-up in each concentric layer. However, central layers are refueled more frequently than peripheral layers because maximal fuel burn-up is reached by central fuel assemblies for a relatively shorter time interval. Hence, spatial shape of heat generation rate can be shifted towards the core center, and the peaking factor increases.
3.The “scatter refueling” strategy was developed to solve the high peaking factor problem of the “partial batch” refue ling scheme. The “scatter refueling” technology presumes that the re actor core is divided into small local groups containing an equal number of fuel assemblies (four-assembly groups, for instance). At the first refueling, all fuel assemblies labeled 1 are removed and replaced with fresh fuel assemblies. At the second refueling, all fuel assemblies labeled 2 are removed and replaced, and so on. Thus, each fuel assembly is completely utilized for four irradiation cycles. Fresh fuel assemblies are not concentrated in the core center but they are scattered throughout the reactor core as a whole. That is why the peaking factor can be substantially reduced.
4.The “out-in” refueling strategy presumes that the reactor core is again divided into several concentric layers containing an equal number of fuel assemblies. At the refueling, only central fuel assemblies have reached their maximal fuel burn-up, and they must be removed from the reactor core. Fuel assemblies from the next outer layer are inserted into
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the central (inner) layer, and so on, i.e. fuel assemblies from the outer layer are moved to the neighboring inner layer in the common direction from the core periphery to the core center. Fresh fuel assemblies are loaded into the outermost layer. At the next refuelings, the same operations of central layer removal, inward movement of partially spent fuel assemblies and placement of fresh fuel assemblies into the outermost layer are repeated.
According to the “out-in” refueling strategy, fresh fuel assemblies with the highest reactivity potential are loaded into the core periphery. So, spatial shape of heat generation rate is depressed in the core center, and the peaking factor can increase.
5.The “modified scatter” refueling strategy is a combination of the “scatter” and “out-in” refueling schemes. The react or core is divided into an outermost layer containing one-fifth part of total fuel assemblies, and the inner zone containing four-fifths of total fuel assemblies. The inner zone is subdivided into small local groups, like in the “scatter refueling” strategy (four-assembly groups). At the first refueling, fuel assemblies with the highest fuel burn-up are removed from each fourassembly group and replaced with fuel assemblies from the outermost layer. Thus, the outermost layer is emptied and filled up with fresh fuel assemblies. The “modified scatter” scheme is charac terized by the flattened spatial shape of heat generation rate in central core region without high peaking factors in the “scatter refueling” str ategy and without the central depression of heat generation rate in the “ out-in” refueling strategy.
6.The “uniformly partial” refueling strategy is based on the following assumptions. Let spatial shape of heat generation rate be flat enough in the reactor core. It means that maximal values of fuel burn-up can be reached by all fuel assemblies simultaneously. The following case can be considered as an example. The time interval needed to reach maximal fuel burn-up equals 3 years, and the refueling is performed once a year. Then, the “uniformly partial” refuelin g strategy consists of the following steps:
a. At the first refueling, one-third fraction of fuel assemblies is replaced with fresh fuel assemblies, i.e. the discharged fuel has reached 33% of acceptable fuel burn-up.
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b.At the second refueling, one-third fraction of fuel assemblies is again replaced with fresh fuel assemblies, i.e. the discharged fuel has reached 67% of acceptable fuel burn-up.
c.At the third refueling, one-third fraction of fuel assemblies is again replaced with fresh fuel assemblies, i.e. the discharged fuel has reached 100% of acceptable fuel burn-up.
Beginning from the fourth refueling, an equilibrium refueling regime has been established. The equilibrium regime is characterized by quite similar compositions of the reactor core at the beginning and at the end of irradiation cycle:
a.At the beginning of each irradiation cycle, the reactor core contains one-third fraction of fresh fuel assemblies, one-third fraction of fuel assemblies with 33% of acceptable fuel burn-up and one-third fraction of fuel assemblies with 67% of acceptable fuel burn-up.
b.At the end of each irradiation cycle, the reactor core contains onethird fraction of fuel assemblies with 33% of acceptable fuel burn-up (the former fresh fuel assemblies), one-third fraction of fuel assemblies with 67% of acceptable fuel burn-up (the former 33%-fuel assemblies) and one-third fraction of fuel assemblies with 100% of acceptable fuel burn-up (the former 67%-fuel assemblies).
Main advantage of the “uniformly partial” refueling strategy is the same number of the discharged fuel assemblies with maximal acceptable fuel burn-up. Main drawback of the “uniformly partial” refueling strategy is a removal of only partially burnt up fuel assemblies at the first and second refuelings (at initial stage of the reactor operation, in general).
Unfortunately, real spatial shape of heat generation rate is not so flat that fuel assemblies in different core regions could reach maximal fuel burn-up for the same time interval. Under these conditions, the reactor core can be subdivided into several concentric layers, within each of them spatial shape of heat generation rate can be regarded as a flat enough. Then, basic ideology of the “uniformly par tial” refueling strategy can be applied to each concentric layer separately.
1.4.3.Technologies of nuclear reactor refueling
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All the refueling strategies listed above must be supplied with appropriate technological tools for conduction of the refueling operations. In principle, the reactor refueling can be performed:
a.after the reactor shutdown, cooldown, depressurization and removal of the reactor head;
b.after the reactor shutdown but without cooldown and removal of the reactor head;
c.at the reduced or full power level, i.e. without the reactor shutdown, cooldown, depressurization and removal of the reactor head.
In practice, light-water reactors are usually refueled only with application of the first scheme, i.e. after the reactor shutdown, cooldown, depressurization and removal of the reactor head. Once a year (or 18 months) the reactor is shutdown for 4-6 weeks, the reactor head is removed, some spent fuel assemblies are transferred to the fuel storage pool, the remaining fuel assemblies are reshuffled, and fresh fuel assemblies are introduced into the reactor core. All the refueling operations are performed under sufficiently thick water layer.
In contrast to LWR, refueling of liquid-metal fast breeder reactors (LMFBR) is done without removing the head of the reactor vessel. There are three areas involved into the refueling process: the reactor vessel, the fuel transfer chamber (FTC) and the ex-vessel storage tank (EVST). An in-vessel transfer machine (IVTM) can transfer fuel assemblies inside the reactor vessel only. Fuel assemblies are transferred between the reactor vessel and the EVST in a transfer bucket by means of a special hoist. The transfer ports are located between the reactor vessel and the FTC and between the FTC and the EVST. Fuel assemblies remain under sodium throughout the fuel transfer process.
Consider the replacement of spent fuel assembly with fresh fuel assembly starting with fresh fuel assembly. The fresh fuel assembly is lifted out by the EVST handling arm and placed into the transfer bucket. The transfer bucket has a space for one fresh fuel assembly and for one spent fuel assembly. At this step the transfer bucket contains only one fresh fuel assembly, in a vertical position. The transfer bucket is hoisted at an angle through the first fuel transfer port and guided by tracks up into the FTC. The FTC is then guided into the reactor vessel and down through the second fuel transfer port and placed in a vertical position in the region outside the reactor core.
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The IVTM is moved to the position directly above the spent fuel assembly to be replaced, and the assembly is grappled by the IVTM manipulator. The spent fuel assembly is raised above the remaining assemblies and transferred through the sodium pool to the open space in the transfer bucket, into which it is then lowered. The fresh fuel assembly is next withdrawn from the transfer bucket by the IVTM manipulator and transferred to the position in the reactor core from which the spent fuel assembly was just removed. The spent fuel assembly is then transferred through the FTC to the EVST, and the refueling process is ready to be repeated for the next fuel assemblies.
Contemporary LMFBR designs use a rotating plug concept, in which several (three, as a rule) rotating plugs are located in the reactor head, and the IVTM is mounted on the smallest plug. The largest plug is concentric with the reactor vessel while the smaller plugs are eccentric ones. Each plug can rotate independently so that the IVTM manipulator can be placed in any position above any fuel assembly inside the reactor vessel.
Heavy-water CANDU-type reactors have a distinct advantage over LWR of the same power due to their on-line, continuous refuelings. Natural uranium is used here as a fuel material thus elimination a need for uranium enriching services but excluding the use of light water as a coolant and neutron moderator material because of high neutron absorption cross-sections. Heavy water is substituted for light water. Annually, about one ton of expensive heavy water is needed per one megawatt of electrical output.
Fuel consists of 0,5-m-long fuel bundles inserted into horizontal pressure tubes that run through a thin-walled tank (calandria) filled up with heavy-water moderator. Each fuel channel contains twelve fuel bundles. The refueling process is done on a daily basis. Two refueling machines are connected to a fuel channel, one on each side of the horizontally placed reactor. Each refueling machine is equipped with a barrel that attaches onto a fuel channel, unlocks the end plug, removes and replaces it by itself. Up to twelve fuel bundles (one fuel channel) can be inserted or removed during one visit of the refueling machines to a fuel channel. One refueling machine inserts fresh fuel bundles while another refueling machine, at the opposite side, receives spent fuel bundles as they are ejected into its barrel. The fuel motion takes pun the direction
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