BLAST, Accident Conditions in Critical and Subcritical Thermal Reactor System

1 - Nature of the physical problem solved: BLAST has been developed to study accident conditions in critical and subcritical thermal multiplying systems. The programme computes the time behaviour of the thermal neutron density and the system temperature following a step change in reactivity. The integrated thermal neutron density is also computed, from which the total number of fissions during an excursion may be obtained. 2 - Method of solution: The computational method consists of computing the neutron flux over short time intervals during which the reactivity is considered to be constant. The fact that the reactivity does change during such time intervals is duly corrected for. The temperature rise during this time interval is calculated from the average neutron flux during the time interval, and the reactivity change due to the temperature coefficients of the system is calculated. The process is then repeated