As noted before, plutonium is a highly concentrated fissile material. Thus, it must be diluted before it can be used. Furthermore, its physical, chemical, and mechani­cal properties do not allow it to be used in unalloyed metallic form. Plutonium has a stronger tendency to form intermetallic compounds than uranium. However, they have similar behavior of alloy formation. There are a few elements that can form alloy with plutonium. A plutonium alloy with intended application must have the following characteristics: (i) plutonium required for criticality is kept to a minimum,

(ii) should have good fabricability features, (iii) good thermal and irradiation stabil­ity, (iv) high corrosion resistance, and (v) available alloying elements.

Alloying elements like Al, Ga, Mo, Th, and Zr can stabilize the delta phase, even though it could make it metastable much like the stabilization of gamma-uranium. Gschneider et al. [10] reported that the negative thermal expansion coefficient of delta-plutonium becomes less negative and eventually becomes positive with increased concentrations of alloying additions of Al, Zn, Zr, In, and Ce because of increased electron concentration.

One of the well-known alloys is Pu-3.5 at% Ga alloy. This alloy was developed dur­ing the days of the Manhattan project and was used as a fuel in the erstwhile Los Ala­mos fast reactor. In this alloy, delta phase is stabilized in a wider temperature range. It improves the corrosion resistance of plutonium by several times; for example, the weight loss was only 0.1 mg cm~2 during the test of exposure of27 000 h in moist air.

There has been a deep interest in developing metallic alloy fuels involving fissile — fertile combinations for a breeder reactor like LMFBR as the metallic fuels have the inherent advantages of high fissile atom density leading to higher breeding ratio along with shorter doubling time compared to ceramic fuels. The phase diagram of U-Pu is shown in Figure 7.15. Pu-U is a complex system with complicated phase relations. Plutonium content more than the solubility limit in gamma-uranium leads to an extremely brittle phase. This makes casting of this alloy very difficult and also results in alloys that are quite susceptible to thermal cycling and radiation damage. However, addition of molybdenum has some beneficial effects in that it suppresses the creation of the embrittlement phase. There are many different com­positions that have been studied. A single-phase alloy with better corrosion resist­ance has been found to be U-21Pu-16Mo (in atom%). Other ternary alloy systems like U-Pu-Th, U-Pu-Al, U-Pu-Fe, and so on have also been studied.

7.2.3