Fuel, burnup critical concentration

Another class of time-dependent problems of concern to the reactor physicist are questions on fuel burnup, poison production and burnup, breeding ratio, and the like. These problems differ from those on reactor stability in that they involve time scales measured in hours (or years) in contrast to stability problems which are concerned with fractions of a second. Reactor-analysis problems, such as the determination of critical mass and neutron-density distributions, are based on the steady-state operating condition of the reactor. The day-to-day operation of the reactor at steady state involves, however, long-time changes in the fuel concentration. Except in the case of circulating-fuel reactors, the fuel is introduced into the reactor according to some predetermined cycle. As the fuel is consumed, some gradual adjustments can be made by means... 

Pressurized Water Reactor. The PWR contains three coolant systems (1) the primary system, which removes heat from the reactor and partially controls nuclear criticality (2) the secondary system, which transfers the heat from the primary system via the steam generator to the turbine-driven electric generator (3) the service water system (the heat sink), which dumps the residual coolant energy from the turbine condenser to the environment. The service water is recirculated from a river, lake, ocean, or cooling tower. In the primary system (Fig. 31.21), dissolved boron is present to control nuclear criticality. Fixed-bed ion exchange units are used to maintain the water quality in both the primary and the secondary systems. In addition, the chemical and volume control system reduces boron concentration during the power cycle to compensate for fuel burnup. These operations are carried out continuously though bypass systems. A more complete... 

A series of experiments has been completed with plutonium obtained from extreme high burnup fuel to establish the combined effects of the various isotopes of plutonium on criticality. The isotopic composition of the plutonium, as measured on August 11, 1971, was Pu-0.2 wt%, Pu-41.4 Wt%, Pu-42.9wt%, Pu-10.8wt%, and Pu-4.7 wt%. A nonnegligible amount of Am also was present to the extent of 1.08 wt% of the total Pu. The experiments were performed in a 24-ln.-diam, water-reflected, cylindrical vessel. Ten data points were obtained with the critical heights and masses de rmined for various plutonium concentrations and nitric acid molarities in the plutonium nitrate solutions. Plutonium concentrations covered the range of about 40 to 140 g Phi/liter with nitric acid molarities ranging from about 1.5 to 5. 

Basis for the investigation was UO2 fuel with an initial enrichment of 3.5% as PWR fuel elements of the Biblis tjrpe. This enrichment was chosen because a PWR foel element enriched to 4% is not yet clearly specified. The burnup of the fuel reaches 36,000 MWd/MTU, the subsequent cooling time is one year. Remaining contents of fissile isotopes and fission products were calculated with ORIGEN. Criticality calculations with HAMMER were made for lattices and homogeneous solutions with various concentrations of gadolinium. Similar calculations were performed for mixed-oxide fuel with 6 wt% PuOx (isotope vector 1.82 wt% Pu, 58.29 wt% Pu, 24.32 wt% Pu,... 

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