**1. Introduction**

Nuclear reactors can be divided into twotypes: those used to produce electrical power and those for other purposes. Power reactors take advantage of the energy liberated in the fission of fissile nuclides (235U, <sup>239</sup>Pu) to produce electricity and nonpower reactors are used for training and

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

mainly uses the neutrons produced in the fission reactions for materials testing, fuel qualifica‐ tion, radioisotope production, neutron activation analysis, neutron diffraction, silicon trans‐ mutation, research, etc. Actually, there are 442 operating nuclear power reactors in 33 countries around the world, while 66 are being constructed [1]; those used for naval propulsion are not recorded. Modern nuclear power reactors can generate more than 1000 MW of electrical power andone important characteristic ofthis type ofreactors is thatthey work attemperatureshigher than 300°C and usually at very high pressures [2]. Nonpower reactors are nominated by their maximum thermal power generated, which is proportional to neutron density; thermal power rangesgofrompracticallyzerouptosome tensofMW,reachinginsome casespowersof several hundreds of MW and fluxes greater than 1014 neutrons.cm‐2.s‐1. In these nonpower reactors, working temperatures are normally near room temperature; for very high density powers, maximum temperatures at the center of these high surface to volume ratios, fuel units do not surpass 200°C. There are 248 operating nonpower reactors in 52 different countries around the world [3].

Considering the different requirements to which both kinds of reactors are subsumed, the corresponding nuclear fuels have different characteristics. Typical operating power densities in power reactors are usually higher than 100 kW/l; in the case of nonelectrical power reactors, operating power densities can reach values as high as 1000 kW/l. Power reactors have their fuel material in the form of oxides, basically UO2, with 235U enrichment lower than 5%, in pellet form filling a zirconium alloy tube. Nonpower reactors have usually plate‐like fuels, or pins, with some uranium compound powder dispersed in an aluminum matrix and with an aluminum alloy cladding. Seventy‐six research reactors use high enriched uranium fuels (HEU > 90% 235U) [4] and the remaining ones use low enriched uranium (LEU < 20% 235U) fuel.

Actually, there is an important interest that the reactors that are working with HEU fuels be converted to LEU, trying to not affect their performance. The reason is to diminish proliferation risks since HEU is used for the construction of nuclear weapons.

#### **1.1. The beginnings of testing reactors, 1948–1978**

One of the first necessities in the development of power reactors was to acquire knowledge on the behavior of irradiated materials and devices. For these purposes, materials testing reactors were constructed. The greatest interest for this purpose, by the beginning of the 1950s, was to obtain thermal neutron fluxes higher than 1014 n.cm‐2.s‐1, high specific power and high excess reactivity, with the possibility of configuring slab geometries of the nuclear core so as to obtain particular neutron spectra [5, 6]. The reactor nucleus had a volume of 100 liters with a total power of 30 megawatts. The fuel elements with 200 grams of 235U were assemblies of 19 hot colaminated curved plates brazed into slotted lateral plane plates [7]. The plates had inside a monolithic aluminum alloy with HEU shaping a dispersion of UAl<sup>4</sup> precipitates in a rich aluminum matrix and aluminum cladding. The monolithic meat had a total uranium density of 1.6 gU/cm3 . The typical size of the plates was 1.4 x 69 x 555 mm with a cladding thickness of 0.46 mm. Separation between plates was 2 to 3 mm with cooling water flowing between them at a velocity of 10 ms‐1.

Plates were also developed using powder metallurgy technology [8] with UO2 dispersed in aluminum and in stainless steel obtaining higher charges of uranium densities. Maximum heat fluxes were 32 W/cm2 in the case of aluminum plates working at 93°C and 70 W/cm2 in the case of stainless steel plates working at 290°C. These power densities are equivalent to a maximum specific power of 2 kW/g of 235U. Stainless steel plates had total and cladding thicknesses of 0.7 and 0.12 mm, respectively [9]. Plutonium and 233U charged fuel plates were also developed [10].

At those early times, reactors with LEU were proposed showing that the critical mass does not increase significantly over that obtained using HEU; some metallurgical problems have to be reconsidered to fabricate in the highly concentrated combination required [11]. Tubular geometries of plate‐like fuels were also used. Pool‐type reactors and pressurized vessels were constructed. Channel‐type reactors had different positions to obtain desired neutron fluxes and spectra avoiding fuel elements reconfigurations. Many other experiences were encour‐ aged but the ones mentioned have many of the basic elements that are used up today in the fuel nuclear industry.

#### **1.2. Enrichment reduction of nuclear fuels start up, 1978–1996**

mainly uses the neutrons produced in the fission reactions for materials testing, fuel qualifica‐ tion, radioisotope production, neutron activation analysis, neutron diffraction, silicon trans‐ mutation, research, etc. Actually, there are 442 operating nuclear power reactors in 33 countries around the world, while 66 are being constructed [1]; those used for naval propulsion are not recorded. Modern nuclear power reactors can generate more than 1000 MW of electrical power andone important characteristic ofthis type ofreactors is thatthey work attemperatureshigher than 300°C and usually at very high pressures [2]. Nonpower reactors are nominated by their maximum thermal power generated, which is proportional to neutron density; thermal power rangesgofrompracticallyzerouptosome tensofMW,reachinginsome casespowersof several hundreds of MW and fluxes greater than 1014 neutrons.cm‐2.s‐1. In these nonpower reactors, working temperatures are normally near room temperature; for very high density powers, maximum temperatures at the center of these high surface to volume ratios, fuel units do not surpass 200°C. There are 248 operating nonpower reactors in 52 different countries around the

Considering the different requirements to which both kinds of reactors are subsumed, the corresponding nuclear fuels have different characteristics. Typical operating power densities in power reactors are usually higher than 100 kW/l; in the case of nonelectrical power reactors, operating power densities can reach values as high as 1000 kW/l. Power reactors have their fuel material in the form of oxides, basically UO2, with 235U enrichment lower than 5%, in pellet form filling a zirconium alloy tube. Nonpower reactors have usually plate‐like fuels, or pins, with some uranium compound powder dispersed in an aluminum matrix and with an aluminum alloy cladding. Seventy‐six research reactors use high enriched uranium fuels (HEU > 90% 235U) [4] and the remaining ones use low enriched uranium (LEU < 20% 235U) fuel.

Actually, there is an important interest that the reactors that are working with HEU fuels be converted to LEU, trying to not affect their performance. The reason is to diminish proliferation

One of the first necessities in the development of power reactors was to acquire knowledge on the behavior of irradiated materials and devices. For these purposes, materials testing reactors were constructed. The greatest interest for this purpose, by the beginning of the 1950s, was to obtain thermal neutron fluxes higher than 1014 n.cm‐2.s‐1, high specific power and high excess reactivity, with the possibility of configuring slab geometries of the nuclear core so as to obtain particular neutron spectra [5, 6]. The reactor nucleus had a volume of 100 liters with a total power of 30 megawatts. The fuel elements with 200 grams of 235U were assemblies of 19 hot colaminated curved plates brazed into slotted lateral plane plates [7]. The plates had inside a monolithic aluminum alloy with HEU shaping a dispersion of UAl<sup>4</sup> precipitates in a rich aluminum matrix and aluminum cladding. The monolithic meat had a total uranium density

of 0.46 mm. Separation between plates was 2 to 3 mm with cooling water flowing between

. The typical size of the plates was 1.4 x 69 x 555 mm with a cladding thickness

risks since HEU is used for the construction of nuclear weapons.

**1.1. The beginnings of testing reactors, 1948–1978**

world [3].

90 Nuclear Material Performance

of 1.6 gU/cm3

them at a velocity of 10 ms‐1.

After India exploded a nuclear bomb in May 1974 a nonproliferation International Nuclear Fuel Cycle Evaluation (INFCE) was set up [12]. The Reduced Enrichment for Research and Test Reactors (RERTR) Program was released in 1978 to "develop technology necessary to enable the conversion of civilian facilities using HEU to LEU fuels and targets" [13]. The reason to adopt this criterion is that LEU is considered to be less of a proliferation concern than HEU because the critical mass—without neutron moderation—increases rapidly below 20% 235U. One of the first fuels to be developed as nonproliferate was the French Caramel that consisted in plates with thin square slabs of UO2—with uranium enrichment lower than 10% placed in a square pitch grid of zircaloy and welded between two zircaloy sheets [14]. The total uranium density of this fuel is 10.3 gU/cm3 and takes advantage of the experience gained in power reactors. Because of low conductivity of the oxide, thin square type slabs were developed. Fuels with uranium oxides and silicides dispersed in aluminum had at that time less known performance. It is interesting to revisit some of the initial review works that followed the consolidated policy of enrichment reduction [15]. In this guidebook of 1980, preferred high density fuels were mentioned, such as U3Si, UMo alloys and UO2.

Experience can also be gathered from the design of nuclear fuels for naval propulsion, where several uranium compounds and enrichments were reviewed. As an example it can be mentioned that uranium with 10 wt.% (21.6 at.%) molybdenum alloys were used in fast reactors without surpassing 2% burn up because of excessive fission gas induced swelling that occurs at temperatures greater than 400°C [16]. It is also mentioned that in comparison with uranium niobium and uranium zirconium alloys, at lower temperatures, UMo alloys are apparently more resistant to swelling.

It was inevitable to move on to powder metallurgy to incorporate to the fuel more uranium to compensate the 235U lower concentration. This technology was already known since dispersion fuel elements present several advantages over elements containing the fuel in homogeneous ceramic form [17]. Also it was in mind trying to use high density uranium compounds. In the case of uranium alloy worked as a monolithic meat with UAl4 precipitates, the uranium loading must be less than 35 wt.% uranium (4 at.% U)—to avoid inhomogeneous dispersions that could produce hot spots—rendering a final meat density of 1.35 gU/cm3 of total uranium. In the case of a UAlx (x = 2, 3 or 4) dispersed powder in an aluminum matrix with a concentration between 40 and 50 v/v%, densities of total uranium in the meat can reach values higher than 2 gU/cm3 .

One of the simplest choices to increase fuel density was to develop fuel plates with LEU U3O8 dispersed powder in an aluminum matrix. The density of U3O8 is 8.3 g/cm3 and the meat total uranium density can reach values higher than 3.1 gU/cm3 . Dispersed UO2 powder in an aluminum matrix is less stable than U3O8 since important swelling is formed due to low density reaction products and diffusion porosity formed [17].

Uranium silicide compounds such as U3Si and U3Si2 have different behavior under irradiation. While the first one presented in some cases break away swelling [18], U3Si2 was finally selected to rich total uranium densities at the fuel plate meat of 4.8 gU/cm3 [19]. This fuel is the last one that has been qualified for general uses in research reactors using LEU. It is worthwhile to comment that U3Si is a ductile compound and centrifugal atomization [20] was specially dedicated to obtain a ductile compound in powder form.

#### **1.3. Uranium molybdenum long run qualification**

By 1996, in a systematic study, several high density uranium compounds were revisited to be used as nuclear fuels with the intention of reducing even more the utilization of HEU in research reactor fuels and to look for a fuel more easily reprocessed than the U3Si2 dispersions [21, 22]. Some of these compounds to be qualified resulted to be gamma stabilized uranium alloys [23], in particular the metastable phase γ‐UxMo. Quickly, it was determined in irradi‐ ation experiments that the needed composition had to have more than 6% w/w Mo (x > 6) [24]; afterwards it was observed that γ‐UMo had an exceptional behavior in the allocation of fission gas bubbles [25]. UxMo alloys, with weights percent between seven and ten (7 ≤ x ≤ 10), are being tested since the stability of the gamma phase is enhanced as molybdenum concentration increases [26, 27], favoring intermediate fabrication processes. UMo presents interface incompatibilities with aluminum at high irradiation fluxes when fission gases generate undesired porosity [28–30]; this drawback is diminished by the incorporation of silicon to the aluminum matrix [31]. Fuel/cladding interaction can be minimized by covering the fuel particles with a diffusion barrier material or tailoring the fuel or matrix materials with the incorporation of additional alloying elements.

Since γ‐UMo is a ductile alloy, it can be used as a monolithic fuel; nevertheless it cannot be colaminated with aluminum because of the different thermomechanical properties of both materials [32]. Several alternatives have been proposed to obtain a γ‐UMo monolithic fuel with aluminum cladding [33]. They comprise a first step in which UMo foils are hot laminated to final dimensions and a second step is the incorporation of the aluminum cladding that can be performed by transient liquid phase bonding, friction stir welding or hot isostatic pressing. All of these three alternatives have UMo/Al interfaces where fission gas bubbles can coales‐ cence. To avoid a UMo/Al interface a zirconium diffusion barrier can be incorporated between both materials and afterwards hot pressed to complete an aluminum cladding [34]. Irradiation results can be obtained in more detailed overviews [35].
