The paper describes work on irradiation embrittlement that has been undertaken in the United Kingdom since preparation of the Second Marshall Report on the Integrity of pressure water reactor (PWR) pressure vessels. Results from research programs concentrate specifically on work on steels for Magnox and PWR pressure vessel (PV) applications.

Models have been developed that describe the influence of copper precipitation on the yield stress changes in Magnox PV steels. Results from strength and microscopic examination confirm the significance of copper precipitation after long-term aging. The results provide support to the expression in the model to describe the time dependence of the copper precipitate contribution to the increase in yield strength.

Accelerated irradiation and surveillance data are present for both the Magnox and PWR PV steels. Modeling of data includes variations on neutron energy spectrum, neutron fluence, flux intensity, and irradiation temperature. The data for PWR PV materials have been used to examine the applicability of a recent draft revision of the U.S. Nuclear Regulatory Commission's Regulatory Guide 1.99 for estimating irradiation shifts on modern materials.

A study of mechanisms of embrittlement is reported including developments in positron annihilation studies and small-angle neutron scattering (both with and without the application of magnetic fields).