GB1604075A

GB1604075A - Fuel assemblies for use in nuclear reactors

Info

Publication number: GB1604075A
Application number: GB20769/78A
Authority: GB
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1977-07-18
Filing date: 1978-05-19
Publication date: 1981-12-02
Also published as: SE8406663D0; BE866444A; JPS5422090A; JPS5718599B2; CA1108316A; SE8406663L; AT364041B; LU79480A1; SE436809B; FR2398368B1; IL54460A; DE2825142A1; IT7809456A0; ES469174A1; CH639792A5; ATA324578A; IT1103093B; SE7806955L; FR2398368A1; NL7804259A

Abstract

The fuel rods (11), which are combined to form a bundle, have longitudinally finned sheaths (12) which contain the fuel. The continuous or interrupted fins of these rods are brazed either with fins of adjacent rods or directly to these rods (or both). The unit of rods soldered together can be used to fulfil the thermal and hydraulic requirements of a fuel unit whose moderator to fuel atomic ratios produce high conversion and breeding ratios. <IMAGE>

Description

(54) FUEL ASSEMBLIES FOR USE IN NUCLEAR REACTORS (71) We, THE BABCOCK & WILCOX COMPANY, a corporation organized and existing under the laws of the State of Delaware, United States of America, of 1010 Common Street, PO Box 60035, New Orleans, Louisiana 70160, United States of America, (formerly of 161 East 42nd Street, New York, New York 10017, United States of America), do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to fuel assemblies for use in nuclear reactors.

The advantages of utilizing nuclear breeder reactors which convert fertile material into fissile material and generate heat, e.g. for power generation, have been widely recognized in view of the limited known fissionable material resources of the world. Development of breeder reactors which convert the more abundant fertile uranium-238 into fissile plutonium-239 utilizing the latter as fuel, possibly in conjunction with plutonium generated m other known reactors, and breed more fissionable material than is consumed, is highly desirable. Since extensive technological development and experience exists in the design and construction of pressurized light and heavy water reactor plants, use of the pressurized water technology in a breeder application represents an attractive alternative to development of other breeder options.

Heavy water, deuterium oxide (D20), has essentially the same physical and chemical propertles as light water, H2O. Its nuclear properties, however, are different, the neutron absorption cross section and slowing down power of D2O being markedly lower than that of H2O. Hence, the use of D20 as a coolant in a fast breeder application is desirable due to its nuclear characteristics and the applicability of pressurized water technology. In a plutonium-uranium-deuterium oxide (Pu-U-D2O) reactor system, as the coolant to fuel atom ratio decreases, it is known that the conversion or breeding ratios increase. The breeding ratio is the ratio of the number of fissile atoms produced to those consumed. High breeding ratios, approaching a value of 1.40, may be realized in a Pu-U-D2O system if a fuel lattice geometry is developed wherein moderator to fuel volume ratios are adjusted to yield moderator to fuel atom ratios approaching 1.0 or less. As the selection of a moderator to fuel atom ratio defines the volume of coolant per unit mass of fuel, it can be appreciated that difficulties arise in designing a fuel lattice capable of passing adequate cooling flow rate at low moderator to fuel ratios. The high flow rates needed to ensure adequate reactor core cooling necessitate high velocities in flow channels that are significantly restricted when achieving a low moderator to fuel ratio. In the tightly packed fuel pin lattices, the use of conventional spacer grids is disadvantageous owing to inherent limits in fuel pin packing due to the interposed grids, a tendency to flow induced by spacer grid vibration, the parasitic absorption of the grid plate material, and the increase in hydraulic pressure loss resulting from introduction of grids within the restricted flow passages.

The prior art teaches heavy water moderated and cooled reactor designs for particular fuel "rod" diameters and spacings within a moderator to fuel atom ratio range from 0.35 to 4.0 and suggests that a moderator to fuel atom ratio of approximately 0.3 can be achieved in a fuel lattice utilizing touching fuel rods arranged in a triangular pitch. Reduction of heat flux to the degree necessary to avoid potentially destructive hot spots at fuel pin contact points, however, would severely limit the capability of operating such a core at pressurized water reactor conditions. Furthermore, close spacing of the fuel pins may lead to plugging by solid particles carried by the coolant and prohibitively high reactor coolant pumping power requirements. Other difficulties become readily apparent. On the one hand, elimination of spacer grids is desirable in order to permit the higher coolant flow velocities needed to approach the moderator to fuel atom ratios yielding the high conversion ratio of the touching fuel rod configuration. On the other hand, elimination of spacer grids may result in imprecise fuel pin spacing, flow induced vibration and unequal cooling.

According to a first aspect of the invention there is provided a fuel assembly for use in a pressurized water cooled nuclear fast breeder reactor, the fuel assembly comprising a plurality of fuel pins disposed with parallel longitudinal axes in closely packed array, each fuel pin comprising a generally tubular metal cladding bearing a nuclear fuel and at least one longitudinally extending fin formed as part of the surface of the cladding of the fuel pin, the extremity of said fin being fixedly joined by a brazed connection to the tubular cladding of a juxtaposed fuel pin to form an integral fuel assembly having a moderator to fuel atom ratio in the range from 0.624 to 0.82 sufficient to achieve high breeding ratios.

The water employed may be light water (H2O) or heavy water (D2O), the latter being preferred.

One form of fuel assembly embodying the invention utilizes longitudinally finned fuel pin cladding tubes arranged to form an integral fuel assembly by brazing together the continuous or interrupted fins of one fuel pin to the fins of other fuel pins. The integrally brazed fin fuel pin assembly is designed to satisfy the thermal and hydraulic requirements of the very tight lattice required to achieve high breeding ratios.

In an alternative embodiment the fins of some fuel pins are connected directly to the tubular section of other fuel pins so that the resulting assemblies have moderator to fuel volume ratios which tend to increase the breeding ratio in a Pu-U-D2O reactor core.

According to a second aspect of the invention there is provided a fuel assembly for use in a water cooled nuclear breeder reactor, the fuel assembly comprising a nuclear fuel, a metal block having a plurality of first and second transversely spaced parallel channels, said first channels containing the nuclear fuel and said second channels defining means for the flow of the coolant through the block, said first channels and said second channels being further disposed such that neutrons can pass between first channels without traversing the volume of one of the second channels.

Embodiments of the invention described hereinbelow may overcome or at least alleviate disadvantages of the prior art bproviding means for obtaining moderator to fuel ratios which are conducive to a highu-U-D2O reactor breeding ratio while ensuring accurate spacing of the fuel pins without the parasitic losses associated with the prior art's use of spacer grids. Furthermore, the arrangement of these embodiments of the invention eliminates or at least reduces hydraulic pressure losses associated with conventional spacer grids and reduces the tendency of fuel pin vibration. The finned fuel pin arrangements, moreover, increase the strength of the pins, increase the available heat transfer surface and improve the overall heat transfer coefficient.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which like reference numerals designate like or corresponding parts throughout, and in which: Figure 1 is a partial section in plan of a fuel assembly embodying the invention; Figure 2 is an elevational view of part of a number of finned fuel alements arranged to form an alternative embodiment of the invention; Figure 3 is an elevational view of part of a number of fuel elements arranged to form another alternative embodiment of the invention; Figure 4 is a partial sectional plan of a fuel assembly having fuel elements arranged in accordance with still another embodiment of the invention; and Figure 5 is a part plan of a block core arrangement for a fuel assembly, embodying the invention, for a low temperature reactor.

Figure 1 shows art of a nuclear fuel assembly 10 for use in a pressurized water cooled nuclear fast breeder reactor, the assembly 10 comprising closely packed fuel pins 11 arranged in an array with their longitudinal axes parallel. Each fuel pin 11 comprises a generally tubular cladding 12 which has a plurality of longitudinally extending fins 13 formed as part of the outer suface of the cladding and spaced circumferentially thereabout.

A nuclear fuel 14, consisting of a mixture of fissile and fertile material, is contained within the cladding 12. The fuel pins 11 in Figure 1 are arranged so that the extremity of each fin 13A abuts with the extremity of a fin 13B of a juxtaposed fuel pin; fins of peripheral fuel pins may abut the fuel assembly can structure 15. The extremities of the fins shown in Figure 1 are joined to each other and to the reactor can structure by means of brazing at 16 and 17, respectively, to form the integral fuel assembly 10.

The fins 13, in one embodiment, extend without interruption along the longitudinal surface of the fuel pin forming channels 20 in the interspaces of the fuel pins which direct reactor coolant flow (not shown) therewithin generally in parallel with the longitudinal axis of the pins. The fins 13, however, need not extend continuously along the length of the fuel pins but can be interrupted fins 21, as shown in Figures 2 and 3, so as to allow transverse flow and intermixing of the coolant through the fuel Din interspaces. The axially interrupted fins 21 of juxtaposed fuel pins may be brazed to each other at 22 (Figure 2) or, as shown in Figure 3, directly to the tubular portion of the fuel pin at 23. An assembly utilizing a combination of both arrangements shown in Figures 2 and 3, i.e., fin to fin contact and fin to tube contact, is also possible.

A finned fuel pin 26 design utilizing broad fins 24 brazed to each other at 25 is shown in Figure 4. Broad fins may be utilized to further limit the moderator volume fraction at some sacrifice of specific core power.

Elimination of conventional spacer grids and the formation of fins as part of the tube cladding permits reduction of the reactor core moderator volume fraction to values consistent with the achievement of the desired moderator to fuel atom ratios. Illustrative physical design parameters are set forth in Table 1.

TABLE I Example 1 2 3 Fuel Pin Diameter, inches .35 .40 .40 Fuel Pin Pitch, inches .39 .43 .43 Clad Thickness, inches .015 .02{) .020 Clad Material Incoloy Type 316 Type 316 (Trade Mark) Stainless Stainless 800 Steel Steel Pitch - Diameter, inches .040 .030 .030 Number of fins per Pin 6 3 3 Fin height, inches .020 .030 .030 Fin width, inches .020 1 .030 .030 Fin interruption, percent of length 0 0 30 Fuel Volume Fraction .6105 .6357 .6357 Structural Volume Fraction .1381 .1659 .1541 Coolant Volume Fraction .2514 .1984 .2102 FueVCoolant Volume Fraction Ratio 2.43 3.20 3.02 Moderator/Fuel Atom Ratio .82 .624 .66 The fuel pins in the examples of Table I are formed in the shapes of rods. The fuel pins of examples 1 and 2 are Provided with continuous fins along their length. Example 3 illustrates an alternative embodiment to example 2 wherein the fins traverse approximately thirtv percent of the length of the rods. The values for the moderator to fuel atom ratios shown in Table I approximate normal pressurized water reactor operating conditions including primary coolant temperature and pressure, fuel pellet shape, clearances between the fuel pellets and clad, and percent of theoretical UO2 density achieved in the pellet.

The fuel assemblies of Table I would be typically formed by furnace brazing in a hydrogen atmosphere at 1950 to 2000"F with a brazing alloy tradenamed "Nicrobraz 50" (available from the Wall-Colmonoy Corp., Detroit, Michigan, U.S.A., using jigs, fixtures and methods of braze alloy placement known in the furnace brazing art.

In still another embodiment, Figure 5 illustrates a design for low temperature reactors suitable for breeding plutonium and low heat generation purpose, e.g. residential heating.

In this embodiment a fuel assembly is fabricated from a block 32 of metal. e.g. aluminum alloy. Parallel channels are formed for flow passage 31 and for fuel 30. The surfaces of the flow channels may be roughened where needed to increase critical heat flux. Illustrative design parameters for a block type reactor are shown in Table II.

TABLE II Example 1 2 Fuel channel diameter, inches .40 .325 Fuel channel pitch, inches .500 .40 Coolant channel diameter, inches .156 .125 Coolant channel pitch, inches .500 .40 Fuel volume fraction .503 .518 Structure Volume fraction .421 .405 Coolant Volume fraction .076 .0766 FueVCoolant Volume Fraction Ratio 6.62 6.76 Moderator/Fuel Atom Ratio .44 .43 The moderator to fuel atom ratio of Table II corresponds to a primary coolant water temperature of about 250"C at low pressure. Other process parameters are similar to those assumed for Table I.

The geometry of the coolant and fuel channels in the block type fuel assembly will produce a degree of what might be termed "moderator escape probability" which will serve to harden the neutron spectrum and improve the core conversion or breeding ratio. This occurs because each fuel channel is not completely surrounded by moderator. Hence, some neutrons produced in a fuel channel can pass to another fuel channel without traversing a volume containing moderator, therebv improving the breeding or conversion ratio since the average neutron energy at which fission occurs is increased. This, combined with a moderator to fuel ratio less than that which can be achieved with touching fuel pins, should yield a uniquely high breeding ratio for either H2O or D2O cooling.

By virtue of the moderator to fuel atom ratios made possible by these approaches to fuel assembly design, fast reactor physics can be applied to pressurized water reactor tehnology.

This combination has important advantages including: a. Avoidance of gas or liquid metal coolants otherwise used for fast reactors.

b. Reduced clad operating temperature.

c. Availability of additional methods of reactivity control, namely, chemical shim and spectral shift control.

Availability of additional methods of reactivity control reduces the normal dependence of fast reactors on control rods. They allow a general reduction in required control rod worth and provide a means for continuous adjustment of excess reactivity to a minimum value, thereby greatly enhancing the safety of fast reactor cores. This would include operation with higher worth rods out of the core.

WHAT WE CLAIM IS: 1. A fuel assembly for use in a pressurized water cooled nuclear fast breeder reactor, the fuel assembly comprising a plurality of fuel pins disposed with parallel longitudinal axes in closely packed array, each fuel pin comprising a generally tubular metal cladding bearing a nuclear fuel and at least one longitudinally extending fin formed as part of the surface of the cladding of the fuel pin, the extremity of said an being fixedly joined by a brazed connection to the tubular cladding of a juxtaposed fuel pin to form an integral fuel assembly having a moderator to fuel atom ratio m the range from 0.624 to 0.82 sufficient to achieve high breeding ratios.

2. A fuel assembly according to claim 1, wherein at least one of the fuel pins includes a plurality of said fins extending continuously without interruption along the longitudinal

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. alloy. Parallel channels are formed for flow passage 31 and for fuel 30. The surfaces of the flow channels may be roughened where needed to increase critical heat flux. Illustrative design parameters for a block type reactor are shown in Table II. TABLE II Example 1 2 Fuel channel diameter, inches .40 .325 Fuel channel pitch, inches .500 .40 Coolant channel diameter, inches .156 .125 Coolant channel pitch, inches .500 .40 Fuel volume fraction .503 .518 Structure Volume fraction .421 .405 Coolant Volume fraction .076 .0766 FueVCoolant Volume Fraction Ratio 6.62 6.76 Moderator/Fuel Atom Ratio .44 .43 The moderator to fuel atom ratio of Table II corresponds to a primary coolant water temperature of about 250"C at low pressure. Other process parameters are similar to those assumed for Table I. The geometry of the coolant and fuel channels in the block type fuel assembly will produce a degree of what might be termed "moderator escape probability" which will serve to harden the neutron spectrum and improve the core conversion or breeding ratio. This occurs because each fuel channel is not completely surrounded by moderator. Hence, some neutrons produced in a fuel channel can pass to another fuel channel without traversing a volume containing moderator, therebv improving the breeding or conversion ratio since the average neutron energy at which fission occurs is increased. This, combined with a moderator to fuel ratio less than that which can be achieved with touching fuel pins, should yield a uniquely high breeding ratio for either H2O or D2O cooling. By virtue of the moderator to fuel atom ratios made possible by these approaches to fuel assembly design, fast reactor physics can be applied to pressurized water reactor tehnology. This combination has important advantages including: a. Avoidance of gas or liquid metal coolants otherwise used for fast reactors. b. Reduced clad operating temperature. c. Availability of additional methods of reactivity control, namely, chemical shim and spectral shift control. Availability of additional methods of reactivity control reduces the normal dependence of fast reactors on control rods. They allow a general reduction in required control rod worth and provide a means for continuous adjustment of excess reactivity to a minimum value, thereby greatly enhancing the safety of fast reactor cores. This would include operation with higher worth rods out of the core. WHAT WE CLAIM IS:

1. A fuel assembly for use in a pressurized water cooled nuclear fast breeder reactor, the fuel assembly comprising a plurality of fuel pins disposed with parallel longitudinal axes in closely packed array, each fuel pin comprising a generally tubular metal cladding bearing a nuclear fuel and at least one longitudinally extending fin formed as part of the surface of the cladding of the fuel pin, the extremity of said an being fixedly joined by a brazed connection to the tubular cladding of a juxtaposed fuel pin to form an integral fuel assembly having a moderator to fuel atom ratio m the range from 0.624 to 0.82 sufficient to achieve high breeding ratios.

surface of said fuel pin generally in parallel with the longitudinal axis of the pin.

3. A fuel assembly according to claim 1 or claim 2, wherein the nuclear fuel is plutonium.

4. A fuel assembly according to claim 3, installed in a reactor in which the pressurized water is heavy water.

5. A fuel assembly for useinaressurized water cooled nuclear fast breeder reactor, the fuel assembly being substantially as herein described with reference to any one of Figures 1 to 4 of the accompanying drawings.

6. A fuel assembly for use in a water cooled nuclear breeder reactor, the fuel assembly comprising a nuclear fuel, a metal block having a plurality of first and second transversely spaced parallel channels, said first channels containing the nuclear fuel and said second channels defining means for the flow of the coolant through the block, said first channels and said second channels being further disposed such that neutrons can pass between first channels without traversing the volume of one of the second channels.

7. A fuel assembly according to claim 6, wherein the block is formed from an aluminium alloy.

8. A fuel assembly according to claim 6 or claim 7, installed in a reactor in which the water is heavy water.

9. A fuel assembly according to claim 8, wherein the moderator to fuel atom ratio ranges from 0.43 to 0.44.

10. A fuel assembly for use in a water cooled nuclear breeder reactor, the fuel assembly being substantially as herein described with reference to Figure 5 of the accompanying drawings.