He worked on low-temperature physics - through the demagnetization of salts - under German and Hungarian Jewish refugees Francis Simon and Nicholas Kurti at the Clarendon Laboratory.
A patent for the separation process (with Rudolf Peierls, Simon and Arms as the applicants) was filed in 1943 (and granted in the US in 1960).
In 1945, he joined John Cockcroft at the National Research Council Canada, experimenting with heat transfer in the Chalk River Laboratories NRX heavy water reactor.
The group returned to the UK the following year and began the Atomic Energy Research Establishment at Harwell; Arms was Principal Scientific Officer in charge of the engineering laboratory, developing the first British nuclear reactor and also working at Windscale to produce military plutonium.
The company gained the contract to build a 500MW nuclear power station at Hinckley Point, Somerset (which opened in 1966).