Patent Number: 059498374
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION A. The Nonproliferative Light Water Thorium Nuclear Reactor Turning now to a detailed consideration of a first preferred embodiment of the present invention known as the nonproliferative light water thorium nuclear reactor, FIG. 1 illustrates a nuclear reactor core 10 comprised of a plurality of fuel assemblies 12, known as seed-blanket units (SBUs), that are arranged in a generally hexagonal configuration, and are themselves hexagonal in cross section. The core 10 is of the same geometrical configuration and dimensions as a conventional Russian light water reactor known as the VVER-1000 so that it can be easily retrofitted into a VVER-1000, and is formed of 163 of the SBU fuel assemblies 12. The difference between the core 10 and the VVER-1000 reactor core lies in the composition of the SBUs 12 as will be discussed in greater detail below. It will be understood that the shape and arrangement of the core 10 and the SBUs 12 can be modified as necessary to facilitate retrofitting into any type of conventional light water pressurized water reactor (PWR). For example, conventional PWRs in the United States and other countries employ fuel assemblies having square cross sections, and the SBUs 12 would also have square cross sections if they were designed to be retrofitted into such a PWR. Surrounding the core 10 is a reflector 14 which is preferably comprised of a plurality of reflector assemblies 16 as illustrated in FIGS. 1 and 5.1-5.9. Each of the reflector assemblies 16 preferably contains a mixture of water and core barrel/pressure vessel metal. Alternatively, each of the reflector assemblies 16 could also be formed predominantly of thorium oxide. FIG. 2 illustrates the composition of each of the SBU fuel assemblies 12. Each of the SBUs 12 includes a centrally located seed region 18 and an annular blanket region 20 which surrounds the seed region 18. The seed region 18 is comprised of a plurality of seed fuel rods 22 which are preferably formed of uranium-zirconium alloy containing U-235/U-238 initially enriched to as high as 20% U-235, which is the maximum enrichment that is considered to be nonproliferative, i.e., incapable of being utilized to manufacture nuclear weapons. While it is not necessary to maximize the initial U-235 enrichment to 20%, it is preferable to employ this enrichment level to minimize plutonium production in the seed during reactor operation. Alternatively, the fuel rods 22 can be made of cermet fuel with uranium oxide particles embedded in a zirconium alloy matrix. The use of zirconium alloy (zircalloy) in the seed fuel rods 22 is preferred over oxide type fuel because the zirconium alloy fuel has a much higher thermal conductivity. As will be discussed in greater detail below, this is important because it reduces the amount of space needed in the SBU 12 for heat removal, and thereby increases the amount of space available for water moderator. The seed region 18 also contains a plurality of water tubes 24 for reception of water moderator (or conventional burnable poison rods and/or control rods as discussed in greater detail below) to control reactivity in the seed region 18. The blanket region 20 contains a plurality of blanket fuel rods 26 which are preferably formed of mixed thorium-uranium oxide. The initial uranium oxide volume content in the thorium-uranium mixture is preferably in the range of approximately 2-10%, and is employed to help fuel the blanket region 20 on start up before the thorium has had a chance to absorb neutrons from the seed, and generate the blanket's own fissile fuel, U-233. As in the seed fuel rods 22, the uranium oxide contained in the blanket fuel rods 24 is preferably U-235/U-238 enriched initially as high as the maximum nonproliferative ratio of 20:80. The seed-blanket core 10 operates in accordance with the following simplified equation for the power sharing between the seed 18 and the blanket 20: EQU P.sub.b /P.sub.s =.epsilon.(K.sub.b /(1-K.sub.b))(K.sub.s-1)/K.sub.s In the foregoing equation, K.sub.s and K.sub.b are the multiplication factors of the seed and blanket respectively. P.sub.s and P.sub.b are the powers generated in the seed and blanket respectively, while .epsilon. is the fast effect, which is slightly over 1. The seed multiplication factor, K.sub.s, is greater than 1, and the blanket multiplication factor, K.sub.b, is less than 1. Thus, the blanket is subcritical, and the seed acts as a source of neutrons for the blanket. In order to maximize the amount of energy produced from thorium, it is necessary to make the fraction of the core power produced in the blanket 20 as high as possible. This is accomplished by making K.sub.s as high as possible, and it has been determined that K.sub.s can be as high as 1.70, while K.sub.b is selected between approximately 0.85 and 1. The number of neutrons absorbed by U-238 in the seed 18 must be minimized. Most of the neutrons absorbed in U-238 are in what is called the resonance energy region marked by closely spaced energy intervals of extremely high absorption. On the other hand, most of the fissions in U-235 occur at lower energies in the thermal region where the average neutron energy is in near equilibrium with the ambient temperature of the light water moderator. By making the water content of the seed 18 as high as practicable, the number of neutrons in the resonance region is decreased, and thus, fewer neutrons are captured by the U-238. Reduction of U-238 captures produces two favorable effects. First, the multiplication factor of the seed, K.sub.s, is raised, thereby increasing the fraction of core power produced in the blanket as discussed above, and second, the formation of plutonium is minimized since it is the U-238 neutron captures which forms the plutonium. The amount of water that can be placed in the seed region 18 is limited by the need to have enough room for the fuel rods 22 to permit adequate heat removal from the same. The volume and surface area of the fuel rods must therefore not be reduced to the point where the power density in the core rises beyond operational limits dictated by the reactor's cooling system. By fabricating the seed fuel elements 22 out of uranium/zirconium alloy, which has a much higher thermal conductivity than does oxide fuel, the water moderator/fuel volume ratio in the seed 18 can be made as high as 4 or 5 to 1 as compared with less than 2 to 1 in a conventional uranium core. The water moderator/fuel ratio in the seed 18 should therefore be selected between approximately 2.5 and 5.0, and most preferably between 3.0 and 3.5. Another advantage to the high moderator/fuel volume ratio in the seed 18 is that it substantially reduces the quantity of high level radioactive waste generated in the seed 18. In particular, because the seed spectrum is very thermal due to the large water fraction, very few transuranic or minor actinides will be produced. It is these actinides, with half lives of millions of years, that require very long term storage in underground repositories. The 10 year blanket life coupled with the reduced actinide production from the nonproliferative core 10 therefore produces less radioactive waste materials and also less long term heat generation. This results in underground repository space requirements being significantly reduced. In addition, low level waste is also somewhat reduced because no boric acid is dissolved in the water moderator for normal operation, and thus no tritium is generated in the core. It should be noted that the reason boric acid is not employed in the water moderator is that it would unacceptably lower the multiplication factor in the blanket region 20. The moderator/fuel ratio in the blanket region 20 is also a very important parameter, however, it is governed by different constraints. In particular, the situation in the blanket 20 is more complex because too much water reduces K.sub.b by absorbing too many neutrons coming from the seed fuel elements, and thereby taking them away from the thorium. On the other hand, too little water in the blanket increases the loss to protactinium. When thorium absorbs a neutron, it forms protactinium, which after a 27.4 day half-life, decays into fissionable U-233. During this interval, protactinium is vulnerable to absorbing a neutron and thereby forming nonfissionable U-234. This is a double loss of both a neutron and a prospective U-233 nucleus. Research indicates that to minimize this loss, the optimum value of the water/fuel ratio in the blanket 20 should be selected between approximately 1.5 and 2.0, and preferably approximately 1.7. Preferably, the seed region 18 comprises between approximately 25 and 40 percent of the total volume in the SBU 12. This range of values is also determined based upon competing considerations. First, the core 10 is designed to burn as much thorium as possible, thus the blanket region 20 must be made as large as practical. On the other hand, the seed region 18 cannot be made so small that the power density therein rises too high for the reasons given previously. The range of 25-40 percent has been determined to provide the optimum balance of these competing in considerations. Still another important design aspect of the SBU 12 is the central seed/annular blanket configuration. In Applicant's previously published International Application, Publication No. WO85/01826, a seed-blanket core is disclosed which employs an annular seed with both an inner, central blanket section and an outer, surrounding blanket section. Such an arrangement cannot be made nonproliferative because the thin, annular seed has a correspondingly small "optical thickness" which causes the seed spectrum to be dominated by the much harder spectrum of the inner and outer blanket sections. This results in higher thermal neutron energies, and a resulting increased production of Pu-239 in the seed. The central seed arrangement of the SBU 12 overcomes this drawback by making the seed section 18 thick enough to avoid excessive interaction with thermal neutrons crossing from the blanket section 20 into the seed section 18. The referenced core and fuel assembly parameters for the core 10 and each of the SBUs 12 are presented in Tables 1 and 2, respectively, below. These parameters were selected to provide a complete compatibility of the SBU fuel assembly with an existing (typical) VVER-1000 plant. TABLE 1 ______________________________________ Core Parameters Parameter ______________________________________ Total Power (MWth) 3000 Average Power Density 107 (w/cm.sup.3) Average Moderator Temp., .degree.C. 306 Number of SBUs in Core 163 Number of Control Rod 61 Clusters (CRC) Number of Control Rods 12 per CRC Blanket Fuel U + Th(O.sub.2) Seed Fuel U/Zr Alloy Seed Reload Schedule 54 Seed/Cycle (.apprxeq.1 Year) Blanket Reload Schedule 163 Blankets/9 Cycles (.apprxeq.10 Years) ______________________________________ TABLE 2 ______________________________________ SBU Parameters Parameter Seed Blanket ______________________________________ Outer Radius of Fuel 0.310 0.380 Pellet, cm Outer Radius of Gas Gap, -- 0.3865 cm Outer Radius of Cladding, 0.370 0.4585 cm Cell Radius, cm 0.6652 0.6731 Pitch, cm 1.267 1.282 Moderator/Fuel Volume Ratio 3.18 1.68 Number of Fuel Rods 156 162 Number of Water Tubes 12 0 Number of Other Tubes 1 0 Seed Total Weight, tH.M. 6.71 -- Blanket Total Weight, tH.M. -- 35.82 U (In Blanket) t -- 3.11 ______________________________________ To provide additional reactivity control during each seed cycle, the SBU 12 can be modified as illustrated in FIG. 3 to include a plurality of burnable poison containing rods 28 and 30 which are positioned at spaced locations in the seed section 18. In the example illustrated in FIG. 3, the first group of burnable poison rods 28 comprise standard Westinghouse burnable poison rods known as WABAs as are presently utilized in conventional PWR fuel systems. These rods are formed of a composite material consisting of boron-10, boron-11, carbon, aluminum and oxygen. The second group of burnable poison containing rods 30 comprise uranium/zircalloy seed fuel rods which have been modified to contain a small percentage of natural gadolinium. Any number and combination of the burnable poison containing rods 28 and 30 can be employed as necessary. In the example illustrated in FIG. 3, each SBU 12 contains 12 of the WABAs 28 and 6 of the gadolinium/fuel rods 30. Both types of burnable poison rods have their advantages. The WABAs provide a more uniform control of reactivity until the end of each reactor fuel cycle, while the gadolinium/fuel rods 30 provide a large negative reactivity input for the first third of the reactor cycle life. FIG. 4 illustrates the reactivity level K in each of the SBUs 12 as a function of full power days for each of four seed control variations: no poison, gadolinium poison, boron poison and combined gadolinium and boron poison. As illustrated, the combination of both types of poison control results in the flattest reactivity curve. Conventional control rods are also preferably employed to compensate the excess reactivity in the reactor core. In addition, the control rods can be employed for emergency shutdown (scram) of the reactor and compensation for power transients resulting from Xe oscillations and moderator temperature transients. The control rods are assembled into control rods clusters (CRCs) with 12 control rods per CRC. As noted in Table 1, it is not necessary that each of the SBUs 12 include a CRC, and calculations indicate that it is sufficient to place place one CRC in each of 61 of the 163 SBUs in the core. In the operation of the nonproliferative light water thorium nuclear reactor core 10, a once-through fuel cycle is employed in which all of the fuel rods in both the seed and blanket regions 18 and 20 are used in the reactor core only once. However, a unique fuel management scheme is employed in which the seed and blanket fuel assemblies follow separate fuel management paths. In particular, each of the seed fuel rods 22 remains in the reactor core for more than one seed fuel cycle (approximately 13 months), preferably three cycles, however, only a fraction (preferably 1/3) of the seeds is replaced at the end of each seed fuel cycle. Preferably, the positions of the SBUs 12 in the core 10 are also shuffled at the end of each seed fuel cycle to improve the power distribution throughout the core. In contrast, each of the blanket fuel rods 24 remains in each SBU 12 for the entire life of the blanket 20, which is preferably 9 fuel cycles, or approximately 10 years. This fuel management scheme combined with the seed-blanket arrangement and associated core parameters allows approximately 80-90% of the uranium in the seed fuel elements 22 to be consumed before they are removed from the core 10. As a result, the spent seed fuel rods 22 are of no economic or nuclear value since so little of the original U-235 loading remains. In addition, this extended burn-up of the seed fuel rods causes a buildup of Pu-238 which is sufficiently high to completely denature the small amount (approximately 30 kg. per year) of Pu-239 that is produced in the seed 18. More specifically, approximately 8-9% of the total plutonium produced by the reactor core 10 is Pu-238. Since Pu-238 is a heat generator which produces approximately 300 times the amount of heat generated by Pu-239, weapons grade plutonium, such a high percentage of Pu-238 prevents the plutonium produced by the reactor core from being used for weapons purposes. In particular, numerous studies have determined that reactor grade plutonium cannot be used for weapons purposes, even by refrigerating the weapons down to 0.degree. F., where the content of Pu-238 equals or exceeds 4.9% by weight. At these concentrations, the heat generated by the Pu-238 causes the high explosives to melt and the plutonium to eventually melt also, or at least change phase from its normal Alpha Phase to Delta Phase. The phase change decreases its density and substantially increases its critical mass. Since the nonproliferative core 10 produces concentrations of Pu-238 well in excess of 4.9%, this effectively renders the discharged plutonium essentially nonproliferative. The multiple batch fuel management scheme is illustrated in greater detail in FIGS. 5.1 through 5.9 which show a pie slice section of approximately one-fifth of the SBUs 12 in the core 10. Each of the FIGS. 5.1-5.9 shows the fuel loading map for each of the nine seed fuel cycles which correspond to one blanket fuel cycle. The fuel loading maps reflect the basic approach adopted, i.e., a three batch fuel management scheme. This means that at all cycles, with the exception of the transient cycles one and two, there are three seed batches: fresh, once-burned and twice-burned. These are designated on the reload maps as F, O and T, respectively. Another major factor influencing the reload pattern is the heavy use of burnable poisons which are capable of suppressing local power peaks. It should also be noted that the majority of the fresh fuel is not loaded at the core periphery, but is distributed predominantly within the middle part of the core at positions 6, 8, 10 and 12, and near peripheral positions 20, 21, 23, 26 and 32. Additional information shown in FIGS. 5.1-5.9 shows the distribution of the U-Gd and WABA poison rods within the core. The elaborate burnable poison distribution reflects the complexity of the reload patterns and the low leakage configurations used in this design. Those SBUs having CRCs are also indicated by a C. At the beginning of core life, i.e., cycle one, all fresh seed fuel assemblies are loaded. In order to achieve a reasonable radial power distribution, three different uranium enrichments and weight fractions are used. As indicated in FIG. 5.1, a first third of the SBUs 12 contains seed fuel rods having 9.5% by volume uranium enriched to 12% by weight U-235, a second third of the SBUs 12 contain seed fuel rods having 14.5% by volume uranium enriched to 17% by weight U-235, and the remaining third of the SBUs 12 contain seed fuel rods having 17% by volume uranium enriched to 20% by weight U-235. The target fresh fuel enrichment of 20% by weight of U-235 was used thereafter for each of the following cycles 3-9. Thus, cycles one and two are transient cycles, while cycles 3-9 are quasi-equilibrium cycles. The fresh fuel enrichment was constant at 20% U-235 by weight, but the weight fraction of uranium in the U/Zr alloy was varied to assure 300 full power days of operation which correspond to one seed fuel cycle. Since the reactor is not usually operated at full power during the entire fuel cycle, it is estimated that the actual length of the seed fuel cycle is approximately 13 months. B. The Plutonium Incinerator The second preferred embodiment of the present invention is another seed-blanket reactor core design known as the plutonium incinerator. As the name implies, the goal of this embodiment of the invention is to consume as much weapons or reactor grade plutonium as possible. This is in contrast to the goal of the first preferred embodiment of the invention which is to derive as much energy as possible from the thorium fuel in the blanket. As will be discussed in greater detail below, the completely different goal of the plutonium incinerator dictates that completely different core parameters be employed. The preferred form of the plutonium incinerator embodiment is illustrated in FIG. 6, and comprises a reactor core 100, again formed from a plurality of SBUs 102. The core 100 has a generally circular cross section, and 89 of the SBUs 102, each of which has a square cross section. It should be noted once again that the size and shape of the reactor core is arbitrary, and can be varied as necessary to achieve a desired power output, and/or accommodate retrofitting into any type of conventional core. Each of the SBUs 102 includes a central seed region 104 and an annular blanket region 106. The total percentage of the SBU volume occupied by the seed region 104 is chosen in this embodiment to be as large as possible, preferably between approximately 45 and 55%, so that as much plutonium can be burned in the seed as possible. A reflector 108 made of any suitable material, such as thorium oxide, surrounds the core 102. One preferred form of the SBU 102 is illustrated in FIG. 7. As illustrated, the seed region 104 is comprised of a first plurality of seed fuel rods 110 formed of plutonium (weapons or reactor grade) and zirconium alloy, or alternatively, cermet fuel. A plurality of water holes 112 are uniformly spaced throughout the seed region 104 for reception of control rod pins. First and second pluralities of burnable poison containing rods 114 and 116 are also uniformly positioned throughout the seed region 104. The burnable poison containing rods 114 are preferably formed of a mixture of the seed fuel and gadolinium. These can be of two types, the first type having a gadolinium concentration of 0.36 g/cc, and the second type having a gadolinium concentration of 0.72 g/cc. The burnable poison containing rods 116 preferably comprise conventional WABA poison rods. Any combination of the two types of burnable poison containing rods 114 and 116 can be employed as desired. The blanket region 106 contains a plurality of blanket fuel rods 118 formed predominantly of thorium oxide. Preferably, a small percentage, less than approximately 1% by volume, of plutonium oxide is mixed with the thorium oxide in the blanket fuel rods 116 to keep the blanket multiplication factor high during initial reactor operation. In addition, it is very important that approximately 2-5% by volume uranium tailings (natural uranium with most of its U-235 isotope removed) are added to the thorium to denature the U-233 which is formed in the thorium during reactor operation by nonfissile isotopes, U-236 and U-238. This is necessary because, unlike in the first preferred embodiment in which a small amount of enriched uranium is added to the blanket fuel rods which itself can generate these nonfissile isotopes, the plutonium added to the blanket fuel rods in the plutonium incinerator embodiment is incapable of generating these nonfissile isotopes. The moderator/fuel volume ratio in the seed region 104 is selected to be much higher than in a conventional reactor core, however, the reasons for doing so are different than in the nonproliferative embodiment of the present invention. In particular, the moderator/fuel ratio is selected to be between approximately 2.5 and 3.5, and preferably between 2.5 and 3.0. This effect further increases the control poison reactivity worth therein, thereby making the reactor much easier to control. As in the nonproliferative core embodiment, the moderator/fuel ratio in the blanket region is selected to be between approximately 1.5 and 2.0. Example values for the main core and SBU parameters for the plutonium incinerator embodiment of the present invention are provided in Tables 3 and 4 below: TABLE 3 ______________________________________ Main Core Parameters Parameter Value ______________________________________ Power Level, MWth 3250 Number of SBUs in Core 89 Equivalent Diameter of Core, cm 380 Active Height of Core, cm 365 Average Power Density, w/cc 78.5 ______________________________________ TABLE 4 ______________________________________ Additional Core Parameters Parameter Seed Blanket ______________________________________ Number of Fuel Rods/SBU 264 384 Number of Water Holes/SBU 25 0 Distance Across Flats, cm 25.5 35.7 % of SBU Volume 51 49 Fuel Pin Diameter, mm 8.7 8.7 Fuel Rod Diameter, mm 9.7 9.7 Pitch, mm 15.0 12.75 Moderator/Fuel Volume 2.54 1.49 Ratio Fuel Type Metallic Oxide Composite Fuel Material 2.4 Vol 0.55 Vol % Pu % PuO.sub.2 97.6 Vol 94.45-97.45 % Zirc- Vol % ThO.sub.2 alloy 2.0-5.0 Vol % U tailings Core Heavy Metal Loading, 2300 Pu 60,700 Th kg 392 Pu 100 U tailings ______________________________________ In the operation of the plutonium incinerator core 100, the seed fuel rods 110 and the blanket fuel rods 118 reside in the core for two years, and are discharged simultaneously. This fuel reload scheme is optimal from the point of view of the plutonium inventory reduction rate, but probably is suboptimal from the thorium utilization point of view. However, this is not a concern since the goal of the plutonium incinerator core 100 is to maximize consumption of plutonium. Preferably, the fuel management scheme adopts a two-batch core with a standard out-in pattern. The reload configuration and accumulated burnup for the once and twice burnt fuel assemblies are illustrated in the core map of FIG. 8. The accumulated burnup for the once burnt assemblies is approximately 15 GWD/T and the discharge fuel averages approximately 31 GWD/T. Three different types of fuel assemblies are illustrated in the core map of FIG. 8. Type A assemblies employ 20 of the gadolinium based burnable poison rods 14, each having a gadolinium concentration of 0.36 g/cc, type B fuel assemblies also contain 20 of the gadolinium based burnable poison rods 114, however, these have a gadolinium concentration of 0.72 g/cc, and type C fuel assemblies contain 20 of the gadolinium based burnable poison rods 114 with a gadolinium concentration of 0.72 g/cc, as well as 20 of the WABA burnable poison rods 116. The annual charge of Pu-239 in the plutonium incinerator core 100 is approximately 1350 kg. Each year, 500 kg of plutonium are discharged from the reactor thus leaving a net destruction rate of approximately 850 kg of total plutonium, although only approximately 200 kg of Pu-239 remains since the rest of the remaining plutonium is in the form of the other plutonium isotopes, Pu-240, 241 and 242. An equilibrium cycle based on a standard sized LWR fuel assembly utilizing the seed-blanket concept will give the equivalent results. The advantages of using the thorium fuel cycle for incinerating Pu-239 in a seed-blanket reactor result from the neutronic properties of thorium, namely its high thermal absorption cross-section. This leads to a high initial Pu inventory, and therefore to high consumption of Pu per unit energy. Driving the thorium blanket with Pu fissile material causes a high Pu power share and therefore efficient Pu incineration. Use of a conventional homogenous light water reactor (LWR) core design presents a controllability problem. Excess reactivity of a fuel cycle based on Pu is of the same value of a similar uranium based cycle, while reactivity worth of a standard control mechanism is significantly lower. The Pu-based fuel is characterized by a very high thermal absorption cross-section, which is competing with control poison material for thermal neutrons. The results of a conventional homogeneous assembly design indicate that the effectiveness of control rods, soluble boron and burnable poisons is reduced by approximately a factor of 2 as compared with conventional LWR values. The obvious solutions to this problem are to improve the reactivity control worth of different control mechanisms, such as utilization of more potent absorbers and/or increasing moderator/fuel volume ratio of the core. Unfortunately, such solutions have a negative impact on safety and economic performance parameters of the reactor. The thorium based seed-blanket design provides a unique solution to this problem which does not carry economic or operational penalties. Since the control rods and/or burnable poison rods are only positioned in the seed region 104 of each SBU 102, the control effectiveness of these is substantially increased because the power density of the seed portion is much higher than that of the core average. Thus, the neutron importance function in the seed is very high, thereby increasing the reactivity worth of the control and poison rods. In addition, the high moderator/fuel volume ratio in the seed region improves power distribution within the SBU, thereby further increasing the control poison reactivity worth. C. Summary In summary, the present invention provides two novel thorium based seed-blanket reactor core arrangements which are particularly significant in that they provide economical, viable solutions to the problems of nuclear proliferation and weapons grade nuclear fuel destruction, while at the same time providing an economic reliable source of electrical power. The nonproliferative embodiment of the present invention is ideal for use by lesser developed countries because it eliminates any concern that the reactor fuel or waste materials will be used for making nuclear weapons, since neither of them can be used for this purpose. The plutonium incinerator embodiment is particularly attractive for use in providing an excellent means by which stockpiled weapons and reactor grade plutonium can be conveniently destroyed. In both embodiments, the seed-blanket core arrangement is necessary to provide the desired results. Without it, the nonproliferative embodiment would not work, i.e., would generate proliferative waste materials. In the plutonium incinerator, the seed-blanket arrangement is needed to insure proper reactor control, and prevent generation of significant new amounts of Pu-239. Although the invention has been disclosed in terms of a number of preferred embodiments, it will be understood that numerous other variations and modifications could be made thereto without departing from the scope of the invention as defined in the following claims.