Critical zero power--------assembly

Approximately 180 research reactors and critical assemblies currently are under IAEA safeguards. The vast majority of the research reactors operate at relatively low power levels (10 megawatts-thermal or lower) and the critical assemblies at virtually zero power. 

Cylindrical reactor with source. An attempt is made here to describe the kinetics of a homogeneous cylindrical reactor with a point source of fast neutrons located at an arbitrary interior point on the axis of the reactor. The theory developed is quite general, at least to the extent that five delayed neutron groups are taken into consideration and appropriate ages are assigned to the source neutrons, the prompt fission neutrons, and each of five groups of delayed fission neutrons. Application of the theory is made to the problem of the determination of the power level in a so-called zero power critical assembly. 

The study at hand was motivated by the reporting of inconsistent experimental results by a group of experimental reactor physicists, in a well known industrial laboratory, working on a zero power critical assembly. The difficulties were resolved by the present study [12]. The experimentalists were in error primarily because of failure to take delayed neutrons into account and were getting results incorrect by a factor of 10. Although the... 

To examine the NF temperature coefficient, a detailed three-dimensional Monte Carlo model was made of the NF-Zepo (a zero-power LASL critical-assembly mockup of NF). This model contained 370 space cells and 15 material compositions. A hot temperature profile corre-, sponding to that in tiie transport calculations was used. The Monte Carlo calculations gave keff = 1.000 0.004 for a room-temperature core (measured keff = 1.000) and keff = 0.978 0.004 for the hot core, i.e., Ak = -0.022 9b 0.006 (- 3.1 0.8). This is consistent with the transport calculations. 

Originally, the idea for creating fast breeder reactors came in the post-WWII era and was intended to ensure the sufficient supply of uranium (U) for the rapidly expanding nuclear program of the USSR. Alexander Leypunsky was one of the initial proponents of the idea and received government support in 1949. By 1955, the USSR constructed its first fast reactor Bystryi Reactor or Fast Reactor (BR)-l at the Instimte of Physics and Power Engineering (IPPE) in Obninsk, Russia. This essentially was a critical assembly with a weapons Pu core and a U blanket. It had no coolant and essentially zero power. After the BR-1, a series of experimental sodium-cooled fast reactors, including BR-2, 5, and 10, was rapidly built (Pshakin, 2010). 

The acceptance tested spacecraft and the certified fueled reactor module were to be received by KSC. The electrically heated power unit was to be replaced with the fueled power unit. Cutting and seal welding these joints was considered a critical, complex task, requiring extensive preparation and training. The HeXe coolant is purified, loaded, and sealed into the RM. Final functional, electrical, and software checks were to be performed. A preference existed to perform slider motion checks and zero-power critical tests at KSC prior to launch if this is appropriate from a regulatory standpoint (this issue had not been resolved at the time of project termination). The assembled spacecraft was to be mated to the launch vehicle and prepared for launch. 

Classical percolation is a very familiar concept. Anyone who has tried to cross a stream by stepping from rock to rock knows that a minimum density of rocks is needed to prevent wet feet. A more relevant, but still easily visualized model system, is a collection of glass and metal spheres in a box. One asks, How does the electrical conductivity of such an assembly depend on the relative proportions of metal and nonmetal The answer is that the conductivity tends continuously to zero at a critical concentration of metal. This is the classical percolation transition. This behavior can be verified experimentally by simple physical models (see, e.g., Last and Thouless, 1971) or by computer simulations (Kirkpatrick, 1971,1973). Near the transition, the conductivity depends on the fraction of metal according to a power law... 

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