Solid-fueled reactors

J. R. Dietrich and W. H. Zion, Solid Fuel Reactors, Addison-Wesley Publishing Co., Inc., Reading, Mass., 1958. 

There are three sources of contamination connected with the operation of solid fuel reactors, excluding uncontrolled nuclear reactions. The... 

Refueling any solid-fuel reactor involves primarily mechanical operations to replace spent nuclear fuel (SNF) from a reactor core. While no direct experience with solid-fuel refueling operations in a salt coolant exists, several molten salt reactors (MSRs) were built in which the fuel is dissolved in the coolant. Although there were no solid fuel assemblies and thus no traditional refueling operations associated with these MSRs, many t5qjes of mechanical operations were conducted with the equipment irmnersed in liquid salt that demonstrate mechanical operations in this enviromnent. Section 3 siunmarizes this experience base. 

Among solid-fuel reactors, the AHTR has potentially unique accident-mitigation capabilities that is, characteristics that limit the extent and scope of an accident and potential radioactive releases. 

MSRs have numerous potential advantages over solid fuel reactors. Most benefits relate to the fluid nature of the fuel form. It is this fluid nature which at first may be a point of concern for those solely familiar with solid fuels. The listed benefits below will of course vary between MSR concepts but are generally universal in nature. 

MSRs have safety advantages over solid fuel reactors in a number of different dimensions as next outlined ... 

The continuous removal of the noble gas xenon means that there is no reactor dead time after shutdown or a power decrease—a phenomenon that most solid-fueled reactors must deal with due to the production of 135Xe from the decay of 1351. As well, there needs to be no excess reactivity in place to deal with such situations. 

As breeders, such designs can have impressive doubling times but typically are rather impractical. They remove the large advantage of other MSR designs wherein the fuel is also the coolant. Thus, as for solid-fueled reactors, the flow of coolant fluid must be assured in all cases. As well, these designs typically would have large positive void coefficients of the coolant in a loss of coolant scenario. 

The major resource advantage over other burner designs may seem surprising given that a conversion ratio of 0.8 does not appear that much superior to LWR and PBMR (both 0.5-0.6) or CANDUs (0.7). However, conversion ratios do not take into account the limited residency time of fuel in solid-fueled reactors. Perhaps, a new term of effective conversion ratio would be to compare fissile consumption versus needed annual fissile additions. By this metric, most other reactors on a once-through cycle have effective conversion ratio of near zero since they consume about 1000 kg/GWe-year but need to add 1000 kg of fissile 235 U per year. Even with Pu recycle, they do not improve significantly. Thus, the great... 

MSRs Avoid Several Solid-Fuel-Reactor Problems with Burning Actinides (High-Burnup Pu, Am, Cm)... 

Compared with solid fuel reactors, MSFR systems have lower fissile inventories, no radiation damage constraint on attainable fuel bum-up, no requirement to fabricate and handle solid fuel, and a homogeneous isotopic composition of fuel in the reactor. These and other characteristics give MSFRs potentially unique capabilities for actinide burning and extending fuel resources. 

As a brief conclusion to this section, let us recall that the global safety objectives are fully transposable to the MSFR reactor. The difficulty lies, among other things, in the identification of severe accidents for this type of reactor. Thus a core melt in the case of solid-fueled reactors is central to present safety studies and has no immediate equivalent in a Uquid-fiieled reactor. A safety analysis for the MSFR must then proceed from the fundamentals of nuclear safety. 

The decay heat generation is represented versus time in Fig. 7.15. The MSFR design implies that FPs are present in two different places when the reactor is stopped. Some are in the liquid-fuel salt and some in the gas processing unit. Approximately one-third of the heat is produced in the gas processing unit and two-thirds in the liquid fuel. The power of both heat sources decreases rapidly (by a factor of 10 in 1 day) from the value at shutdown, which depends on the history of power generation. The total amount of power at shutdown is approximately 5% of the nominal power. This value is lower compared with solid fuel reactors because FPs are continuously removed in this concept. 

A direct transposition to liquid-fueled reactors of the traditionally identified accidents of solid-fueled reactors is not possible. In a liquid-fueled reactor, the fuel is also the coolant so that a loss of coolant accident implies the simultaneous loss of the fuel and of the coolant. We can study these initiators by equating the primary circuit coolant to the liquid fuel while keeping in mind that the phenomena related to the accidents will not necessarily he comparable to those of a solid-fueled reactor. Another interpretation could identify the MSFR s intermediate circuit with a solid-fueled reactor s primary circuit. To retain more clarity, we prefer to redefine the accident types as outUned in the following for the fuel circuit ... 

For power production, homogeneous burner reactors can be considered as possible competitors to the highly enriched solid-fuel reactors, such as the Submarine Thermal Reactor and the Army Package Power Reactor. By eliminating fuel-element fabrication, fuel costs in homogeneous burners with either DoO or H2O as the coolant-moderator are in the range of 4 mills/kwh at present Atomic Energy Commission prices for enriched uranium [25]. 

The HRE-2 instrument and control system [2.3]. The control system for a homogeneous reactor such as the TIRK-2 differs drastically from that for solid-fuel reactors because control rods and fast electronic... 

The essential difference between the processing cycle shown in Pig. 10-1 and that for solid-fuel reactors is associated with the continuous rcmoi al of fission-product gases and of insoluhle fission products (in solution reactors). Puel and fertile material processed by Thorex would undoubtedly be removed from the reactor on a semibateh basis. 

Comparison of fluid- and solid-fuel reactors. In order to better understand the development and characteristics of the Liquid Metal Fuel Reactor, fluid- and solid-fuel reactors should be (compared, and a distinction should be made between the features of fluid fuels in general and those of liquid metal fuels in particular. 

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