Ek30

Figure 7.4. Comparison between chemical and nuclear sources.

physically impossible to achieve. For instance, in solid-core reactors, the most common type, the nuclear fuel is alloyed for structural and neutronics reasons, and partitioned into individual modules, called fuel "bars" or "rods". Figure 7.5 shows a classic fuel bar design from one of the NERVA reactors mentioned in Section 7.5; there is literally no way the fuel can reach critical mass when distributed among bars and alloyed with a moderator material.

Because of the Chernobyl "accident" in 1986 there persists a certain amount of confusion among the general public between a nuclear explosion (that of an atomic

LOX/H2 Metastable Fission Fusion Annihilation combustion nuclei (183Ta) (235U) (D-T) (p+-p~)

H end

NbC cladding

19 hole fuel element 0.098 diameter holes

H2 propellant & Stainless tubing Cone support

Pyrolytic graphite

NbC cladding

Fuel element Center support (loaded) element (unloaded)

Stainless steel tie rod

Fuel element Center support (loaded) element (unloaded)

19 hole fuel element 0.098 diameter holes

H2 propellant & Stainless tubing Cone support

Pyrolytic graphite

Stainless steel tie rod

Pyrolytic graphite sleeve

Cluster support block

Fuel element cluster

Figure 7.5. Structure and size of a NERVA-type fuel bar [Gunn, 2001].

Pyrolytic graphite sleeve

Cluster support block

Fuel element cluster

Figure 7.5. Structure and size of a NERVA-type fuel bar [Gunn, 2001].

bomb), and a thermal explosion caused by reactor overheating and/or meltdown. What happened in Chernobyl was due to overheating following the deliberate (and foolhardy) shut-down of the cooling system to check the spin down time of the reactor turbine. Overheating caused a fire of the graphite moderator, not an atomic explosion [Del Rossi and Bruno, 2004, 2008].

So-called nuclear thermal rockets (NTR), one of the many propulsion systems based on fission, are to all practical effects miniature nuclear power stations, where solid 235U-enriched fuel fissions, releasing heat to a coolant fluid playing also the role of propellant. The heat release occurs inside the structure of the rod; so, maximum temperature is limited by what the rod can tolerate without cracking, breaking or melting. Solid temperatures higher than 3,000-3,500 K cannot be realistically foreseen with this strategy; in fact, they are remarkably very close to (or lower than) those of combustion gases in chemical rockets.

The third nuclear energy source mentioned is associated with so called "meta-stable'' nuclei, also called nuclear isomers. These are materials in which the atomic nucleus is "strained", that is, neutrons and protons are still bound by the nuclear force but their spatial structure, or arrangement, is not in its minimum energy state (for a general discussion of the nucleus shell structure and its consequences on nucleon energy see [Mukhin, 1987, Section 2.3.2]; the theory of deformed nuclei can be found in [Myers and Swiatecki, 1966]). Such nuclei can "snap", like a stretched rubber band, or a plastic bottle slightly crumpled, and in doing so they reach their stable configuration. During this relaxation their excess energy will be released. This is a very interesting nuclear process, since it does not fission nuclei, but simply rearranges their structure; accordingly, the energy release is intermediate between fission and chemical reactions, and neutrons are not emitted. So, radiation effects are limited to less dangerous high energy photons (mostly X- and gamma-rays). Radiation shielding is still necessary with this strategy, but is easier to deal with than in conventional fission.

Comparing energies, metastable nuclei (e.g., 178mHf, or 180mTa) have energies of order 2.4 MeV for hafnium, and about 75keV for tantalum. Per unit mass these energies are 100-10,000 times lower than in fission, but 1,000 times larger than possible in combustion: a cubic centimeter of pure 180mTa holds 300 MJ, or 10,000 times the energy released by a cubic centimeter of gasoline when burnt with air [Walker and Dracoulis, 1999]. Of course, such nuclear isomers are rare, in the case of tantalum about 100 ppm compared to the most common isotope of tantalum, and are quite stable.

The main issues in metastable nuclei are their natural scarcity or their breeding strategies, and thus the technology and cost of separating them from their stable brothers, their geographic provenience and geopolitical issues, and especially the need for ways of releasing their energy in a controlled way. Progress about this last issue seems at hand [Collins, 2005]. All these problems notwithstanding, this nuclear energy source is the object of much interest; applications, such as high-altitude, long-endurance (HALE) airplanes, have been openly discussed [Hamilton, 2002]. However, applications are still speculative, and must wait until many fundamental issues have been sorted out and resolved in an engineering sense.

Substantial theoretical and experimental work must be carried on before this source can become just as practicable as fission, so it will not be further discussed in this chapter.

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