In parallel with work on GLEEP, design and construction of the BEPO reactor went on rapidly. A good deal of preparatory work had been carried on by the "graphite group" in Canada the production of high purity graphite had been established at Welland and over 800 tons of the BEPO graphite was shipped across the Atlantic in due course.
As early as April, 1945, the graphite group had calculated the basic lattice, dimensions and overall dimensions and predicted that criticality would be achieved when 28 tons of uranium metal was loaded. The actual figure turned out to be 30 tons.
By March, 1946, many detailed calculations such as the specification of shut-off rods, shield dimensions, ion Chambers for pile power measurement, etc was sent to Harwell will stop at this time a Harwell pile group took over the responsibility of providing and coordinating information to Risley and Leonard Owen chaired a BEPO design committee to take important decisions on design. We were told at an early meeting at the bottleneck in the construction would be an 18 month delivery date for the four exhausters which would suck the cooling air through the pile.
By the autumn of 1946 an enormous hole had been done in the floor of hangar 10 and by the onset of winter a Bailey bridge had been erected to lift the hangar roof. By first November, 1947, 2000 tons of graphite had been machined to shape in the Harwell graphite workshop and by first March, 1948, 11,000 aluminium sheathed uranium metal fuel elements had been delivered. Eight weeks of day and night work by the pile operated group brought the pile to within three stringers of divergency by 2:30 PM on Saturday, 3 July, 1948. A meeting of the Atomic Energy Technical Committee was held at Harwell that day and the final fuel elements were inserted one by one by members of the committee until the pile diverged at 3:55 PM, the safety rods going in with a crash at a power level of 35 watts to celebrate the achievement.
BEPO achieved high power operation in February, 1949, after the high power exhausters had been installed. Radioisotope production was then transferred from GLEEP and grew enormously in magnitude and specific activity. Radioisotopes not requiring processing were dispatched directly to customers by the shortest route by the isotope division, ably directed by Henry Seligman. Radioisotopes requiring processing and particularly requiring incorporation into labelled compounds were treated at the radiochemical centre at Amersham, under the direction of Doctor Grove. The centre developed from a process laboratory for radium operated by Thorium Limited during the war years. Doctor Grove who was a founder member of this organisation visited the Montréal laboratory during the last weeks of 1944 and became familiar with the prospects of producing radioisotopes in reactors…
The Ministry of Works were greatly handicapped by the severe winter of 1946/7 especially when snow came down through the open roof of the BEPO hangar. The construction workers were housed in a nearby housing estate at Kingston Bagpuize complained of low pay, which meant not enough overtime, poor food, poor accommodation. Small amount of overtime work was actually due to a trades union complaint that we had been infringing an agreement to limit overtime to "work of an urgent character". I was asked by the Ministry of Works to speak to the workers from a platform temporarily rigged up in the BEPO hangar. I told them about the importance and urgency of our program and as a result of this plea agreement was reached to work 10 hours a day whereupon our labour troubles disappear at. In spite of these difficulties the warm laboratory came into commission in February, 1948, and the hot laboratory in the summer of 1949 …
The coming into high power operation of BEPO in 1949 enabled as to amount up to 50 scientific and technological experiments in the reactor - one of the advantages of this very large core was to make BEPO continuously useful for over 18 years. A typical experiment studied the effect of radiation on the chemical reaction between hot CO2 and graphite - a reaction leading to the formation of carbon monoxide and which could lead to the mass transfer of carbon from one part of the reactor circuit to another. This was of special interest for the Calder Hall and later power reactors. So a "loop" was installed in the BEPO core and the rate of formation of CO measured as a function of temperature. From these results predictions were made of the magnitude of the corresponding reaction in the Calder Hall reactor core and the prediction was remarkably well borne out by results later.
Another experiment studied the effect of reactor irradiation on the behaviour of uranium metal fuel elements and showed how to minimise the distortion effects by suitable heat transfer treatment of the uranium metal which was worked out in the metallurgical laboratories.
The effect of radiation on the properties of graphite was another important problem. The changes in dimensions of graphite under irradiation were well-known in principle from US experience and was known as the "Wigner effect" after the distinguished Hungarian physicist, Eugene Wigner. The changes in dimensions are due to changes in the shape the individual graphite crystallites and the overall effects depend on crystal orientation and size. The individual crystallite changes due to carbon atoms being displaced from the crystal planes by neutron impact and they accumulate in interstitial positions. In these positions they cause changes of shape and also result in energy being stored after the irradiation. The energy can be released steadily if the graphite temperature rises beyond that at which it was irradiated. The displaced atoms then return to their crystal planes and release energy. We gradually realised that this could be a dangerous phenomenon and might under certain circumstances lead to overheating of the whole reactor core and melting of the fuel elements. So measurements of the stored energy after irradiation became of great importance in the early 1950s and its importance was again emphasised by the overheating of the Windscale pile during an operation designed to release the stored energy …