Spatial effects, fast reactors

The sodium cooled fast reactor JOYO has been operated more than 20 years (about 5 years of effective full power years) since its initial criticality and the cumulative reactor output achieved over 1.9E+5 MWd. Since JOYO has not yet experienced any operation with breached fuels, FP radioactive contamination has not become an issue in the plant system. To reduce the radiation dose from long-lived Na, all primary coolant sodium in the main circulating loops is drained into a storage tank during annual plant inspections. Under these conditions, the spatial gamma-ray dose rate distribution is dominated by the radioactive CPs deposited on inner surfaces of the primary piping and components. This means that most personnel dose was due to these CPs. 

The traditional way to calculate the physical characteristics of a fast reactor is to carry out the following steps (1) preparation of the effective cross sections for regions of the reactor (2) a three-dimensional calculation to obtain k-eff, and real and adjoint fluxes (3) edit the results of the previous steps to estimate the power and reaction rate distributions, neutron kinetics parameters, control rod effectiveness, etc., and (4) a bumup analysis, calculating the variation of the isotopic composition with time, and then recalculating the results obtained in the previous steps for particular bumup states. This scheme has been implemented, for example, in the TRIGEX code [4.49]. This code calculates k-eff, few group real and adjoint fluxes, power spatial distribution, dose factor and reaction rates distributions, breeding parameters, bumup effects, and kinetics parameters (effective delayed neutron Auction, etc.). 

PHWR safety analysis requires a comprehensive set of physical models. Reactor physics analysis may require a transient three dimensional model for the large PHWR cores. The most demanding application is a large LOCA, because of the relatively fast kinetics and the spatial effects associated with flux tilts and shut-off rod (or liquid absorber) insertion. Three dimensional effects are also important in slow loss of reactivity control starting from distorted flux shapes. 

The high heat transfer rates achievable in micro heat exchangers and reactors avoid unfavorable reaction conditions resulting from hot spots or thermal runaway effects. An optimum temperature or temperature profile for the reaction can be chosen with respect to spatial distribution and time. Thus, a fast-flowing fluid element can be cooled down or heated up very rapidly, in fractions of a millisecond. Because of the small thermal mass of microdevices, a periodic change of temperature of the reactor can be realized, with a typical time constant of some seconds. All these examples offer possibilities to improve yield and selectivity. 

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