as enemy aliens, at least until peierls' naturalisation papers came through in february 1940, frisch and peierls were excluded from the most important–and secret–war work being done by oliphant's team at birmingham, that on radar. however, oliphant would ask peierls a theoretical question about, say, the solution to maxwell's equations in a hemispheric cavity. peierls knew that questions of this nature related to the work on microwave radar, and oliphant was doubtless aware of this too, but the façade of secrecy was maintained. the nuclear investigation was not yet secret, so frisch was available to work on it. he began experimenting with uranium enrichment through thermal diffusion, a process first demonstrated in germany by klaus clusius. progress was slow; the required equipment was not available, and the radar project had first call on the available resources.