The PArticle STress Analysis (PASTA) code was written to evaluate stresses in coated particle fuel embedded in graphite of high-temperature reactors (HTRs). Existing models for predicting stresses in coated particle fuels were extended with a treatment of stresses induced by dimensional change of the matrix graphite and stresses caused by neighboring particles.

PASTA was applied to two practical cases in order to evaluate the significance of this model extension. Thermal hydraulics, neutronics, and fuel depletion calculation tools were used to calculate the fuel conditions in these cases. Stresses in the first fuel loading of the High-Temperature Engineering Test Reactor (HTTR) and in the fuel of a 400-MW(thermal) pebble bed reactor were analyzed.

It is found that the presence of the matrix material plays a significant role in the determination of the stresses that apply to a single isolated TRISO particle as well as in the transmittal of the stresses between particles in actual pebble designs.