**5. Accident-tolerant fuel**

#### **5.1 General provisions**

Fast reactors of the new generation are focused on the use of ceramic mixed uranium-plutonium fuel. The immediate prospects are associated with the use of mixed oxide (MOX). A more promising in terms of possible exceptions of severe accidents in NPP can be considered mixed mononitride uranium-plutonium fuel (MN fuel, mixed nitride). It is proposed to use in BREST reactors with lead cooled and, in the future, perhaps in power sodium-cooled reactors.

Nanotechnology, in relation to the creation of new materials for nuclear technology, is widely used since the early 1960s.

Thanks to the introduction of nanotechnology the unique fuel materials, characterized by high density and thermal conductivity, can be obtained. The task of creating high-performance nuclear fuel (mostly fast reactors), which is achievable for the high burnup, isolated two problems [39]:


The porosity of the ceramic nuclear fuel consisting of micrograins is approximately 25%. When making fuel from nanopowder, it is possible to significantly reduce porosity (up to 5–10%). For example, in order to increase the density of MN fuel to 95% of the theoretical density by reducing porosity, the inventors [48, 49] propose a fuel based on MN nanopowder. In terms of increasing reactor power, reducing porosity is equivalent to increasing the volume of fuel in the core or increasing the volume of the core.

It is known that, compared to ceramic fuel (MOX, MN, etc.), cermet fuel has some advantages. It is more attractive due to increased thermal conductivity and density, increased BRC (up to 1). The main disadvantage preventing the use of such a fuel is the reduction of the melting point. The use of a nanopowder of metallic uranium in conjunction with ceramic fuel micrograins will provide the same advantages, eliminating the disadvantage. Ideally, the pores between the micrograins can be filled with nanopowder. In this case, the melting point of the fuel will be determined by the melting point of the ceramic (MOX, MN, etc.). The nanopowder in the fuel composition can melt in emergency modes; there will be droplets of molten metal between the ceramic microloans, which will have practically no impact on the reactor safety. As a result, it is possible to obtain fuel having high thermal conductivity and density, which contributes to self-protection of the reactor. When using such fuel, the condition BRC = 1 is easily achievable.

At the beginning of the twenty-first century, in Russia fast reactors with sodium cooling was considered vibrocompacted MOX-fuel with uranium (up to 10% by weight) getter of free oxygen [19, 20]. Since it is difficult to control the uniformity of mixing MOX and uranium metal powders in a relatively large volume of fuel element (in case of vibration compaction), it makes sense to switch to small volumes (pellet fuel) [50, 51].

The finer the uranium powder particles, the better it exhibits getter properties. Nanodisperse powder is one of the best free oxygen getter. The ideal getter is a powder ground to size when any atom in the nanoparticles can be considered surface. (The best getter is thorium [46]. However, its use will require a reorientation of Russian nuclear fuel cycle enterprises, which is not economically feasible).

#### **5.2 ATF based on ceramics and beryllium (a new look at the old concept)**

The free oxygen getter may be a material having a high chemical affinity for oxygen (see **Table 4**). The first steps towards solving the problem of free oxygen binding were taken in the late 1950s and early 1960s [52]. Beryllium (and Be2O) can also serve as a getter. The possibility of using oxide fuel with beryllium additives was considered at the dawn of the development of fast reactor technologies [4]. This was abandoned due to the decrease in BR.

Homogeneous placement of metal beryllium in MOX or MN fuel helps to solve the problem of corrosion of the inner surface of cladding and increase self-protection [51]. The spectral component of the VRE for an endless reactor with MN fuel and MN-5%Be fuel is \$23.6 and 12.7, respectively [51]. The Doppler reactivity coefficient for infinite array of fuel elements is −7.99 × 10<sup>−</sup><sup>6</sup> and −2.06 × 10<sup>−</sup><sup>5</sup> К−<sup>1</sup> , respectively.

### *Accident Tolerant Materials for LMFR DOI: http://dx.doi.org/10.5772/intechopen.90703*


**Table 4.**

*Chemical bond energy E for some getter and oxides formed (as reported by [46]).*

The increase in the thermal conductivity of the fuel (due to beryllium) leads to a decrease in the temperature of the fuel. As a result, the nature of the change in maximum fuel temperatures in medium or high power reactors (BN-800, 1200, 1800) in LOF WS and TOP WS is same. To increase self-protection against these accidents, a large absolute negative Doppler reactivity coefficient is required.

Additives of beryllium to MOX fuel contribute to the minimization of corrosion rate of fuel element shells from inside, the reduction of VRE, the elimination of known contradiction in requirement for Doppler reactivity coefficient in emergency modes LOF WS and TOP WS in high-power reactors, and the improvement of reactor self-protection. At the same time, beryllium supplements reduce BRC and BR. It's a major flaw.
