4. METHOD OF SOLUTION
RAFFLE can solve iteration type problems to obtain the multiplication factor(k) and normal-mode fluxes, or it can solve external-source problems. The external sources may be distributed volume sources of general shape or may be incoming current sources specified on particular surfaces. The current sources may have any energy and angular distributions. For external source problems, fission daughters can be subsequently generated from the source neutrons. For any type of problem, the neutron slowing-down treatment can be as detailed as desired. Inelastic discrete-level and evaporation-model scattering; resolved resonance absorption, scattering and fission; and elastic scattering with any degree of anisotropic angular scattering can be treated. Multiple thermal groups are optional and can be used for thermal systems. Also for resonance problems, temperature can be spatially dependent. RAFFLE allows three types of scoring techniques. The path technique uses a Richtmeyer-type estimator, which is essentially an averaged path length estimator and is the scoring technique well suited to most problems. The second scoring technique which may be used is the ray technique, where the uncollided neutron beam is followed, at each event, until the contribution falls below some cutoff weight.
The last scoring technique available is the KENO tehnique in which cross sections of all regions are made equal and delta-type scattering is allowed. This last technique is not commonly used in RAFFLE because it can not be used when resonance parameters are included in the cross section description.
The RAFFLE code uses both multigroup cross sections and pointwise cross sections. Pointwise data are used to determine the mode and angle of scatter. Pointwise data may also be used to describe the scalar absorption, fission and scatter cross section over specified energy ranges.
RAFFLE treats the resolved resonance interactions by use ofthe single-level Breit-Wigner formula for the pointwise cross sections which are then Doppler-broadened using a Maxwellian distribution for the nucleus velocity.
The RAFFLE Code contains standard biasing techniques which may be employed to decrease running time for some classes of problems. Weight reduction in which at a collision a neutron always undergoes a scattering, may be used. Neutron splitting may be used in regions where more accurate detailed information is desired. Russian roulette may be played in various degrees, dependent upon the particular group and region.