Nuclear Reactors Basic Technology

Since it is the oldest technology, solid-core reactors are often taken as the baseline to gauge the performance of more advanced reactors. A nuclear propulsion system consists of a nuclear reactor (NR) coupled with a working fluid or propellant system. The fluid is heated inside a heat exchanger, the key engine component. Heat is produced by the primary fission reaction of (typically) 235U, or other fissionable material (the nuclear ''fuel'').

While fissioning, a reactor produces high-energy fission fragments, or FF: these are the nuclei formed by splitting of the fuel by neutrons. The FF are absorbed by the solid material encapsulating the fuel, meaning their kinetic energy is deposited as heat during their trajectory inside the core material (''thermalization''). Fission produces fission fragments at a rather low rate, of the order of kg/h; Section 7.2 showed that if the material where fission fragments thermalize is cooled by a much larger flowrate of propellant/coolant, the reactor core may be kept at temperatures that will not damage it or create structural problems.

In fact, this is the strategy of most nuclear reactor concepts: the heat deposited inside the core material is removed by a coolant fluid pumped through the reactor. The fluid will heat; in a simple NTR it will be expanded and accelerated in a nozzle, producing thrust just as in any chemical rocket engine; see Figure 7.18. In a NEP system instead the hot working fluid circulates in a closed loop, drives a thermo-dynamic cycle, and produces, eventually, electrical power.

The energy deposition rate (thermal power) of fission fragments in a solid material may be extremely high, in fact, as high as wanted; witness the application of fission to atomic weapons. Structural material may even melt or vaporize if fission is not ''moderated'' (controlled) by inserting or pulling bars (or drums, depending on design) made of neutron-absorbing material. In nuclear physics energy is conveniently measured in electronvolts (eV) rather than °F, °C or joules. For reference, FF can be released during fission with energies up to 102 MeV. On a per nucleon (neutron or proton) basis, average nuclear binding energy is 8 MeV/nucleon [Mukhin, 1987], and since FF may have an atomic weight of the order of 40 to 140 when using 235U as fuel, their energy may reach some hundreds of MeV, with an energy spectrum that depends on the particular fragment. Together with FF, neutrons are also emitted, with a spectrum centered around 5 MeV. To compare these energies with those in chemical rockets, note that 1 eV electron has a kinetic energy corresponding to ~ 11,300 K. Note that to dissociate H2 into two H atoms needs only ^0.2 eV, and to ionize H, ejecting its electron and producing H+, needs just 13.8 eV (eV, not MeV!).

What these numbers mean is that fission fragments can theoretically heat other (non-fissioning) propellant particles close to their own energy. Of course, if the mass rate of the propellant is much greater than that of FF and neutrons, the energy of the fragments will redistribute and the maximum propellant temperature will be accordingly much less than that of the fragments, but still capable, if not controlled, of melting or vaporizing all engineering materials. High temperatures are desirable in propulsion based on thermodynamics, but carry also structural risks.

The main problem with NTR is thus to slow down FF by transferring their kinetic energy to a fluid in a gradual manner, that is, one that will not cause intolerable thermal stresses or temperatures. Substances called ''moderators'' help in thermalizing FF. The choice of moderators is driven by the need to ''thermalize'' neutrons, from their ^5 MeV energy down to ~0.1 eV (1,000°C or so; see Section 7.4.1).

So, the maximum temperatures the heat exchanger can withstand limit the solid-core reactor performance. Thus structural materials and their reactivity with the fluid at high temperature (called also ''hot corrosion'') are paramount problems, witness the effort at LASL during the 1950s and 1960s to extend the life of fuel rods.

To place NTR with solid-core reactors in perspective, with modern materials their Isp can reach 1000 s, their mass/power ratio 10-3 to 10-1 kg/kW (a typical NASA-Glenn goal for a future 75,000 lbf thrust engine is 0.08 kg/kW), and their thrust/mass ratio 10-1 to 10 g. In this respect they are close relatives of chemical rockets, except their Isp is higher by a factor 2 to 3.

The working fluid par excellence is hydrogen, because it has the lowest molecular weight (MW = 2) of all species, and favorable specific heat ratio 7 = Cp/Cv; helium has a strong point in its lack of reactivity and has been considered, but is much costlier and its higher 7 and molecular weight (MW = 4) yields lower Isp than hydrogen.

In the following sections some of the most interesting NP technologies will be presented. They have been chosen on the basis of current or recent interest, and on the amount of public domain information available. In this vein some concepts have been omitted, because their stage of development is still unknown (as in the case of propulsion by nuclear microexplosions) or because they are simply ideas waiting to be even preliminarily analyzed (see also [Lawrence, 2008]).

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