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Technical Meeting on Low-Power Critical Facilities and Small Reactors Ottawa, Ontario, Canada, 2010 November 1-2

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Technical Meeting on Low-Power Critical Facilities and Small Reactors Ottawa, Ontario, Canada, 2010 November 1-2 Technical Meeting on Low-Power Critical Facilities and Small Reactors Ottawa, Ontario, Canada, 2010 November 1-2 FIRST CRITICAL: The Original SLOWPOKE Reactor John.W. Hilborn, Peter D. Stevens-Guille Both retired ABSTRACT – The first approach to critical of the SLOWPOKE reactor is described together with many as yet unpublished photographs of the event. The development of the reactor and the overpower testing are discussed. 1. Introduction The approach to critical and the overpower testing of the original 1970 Slowpoke; a laboratory-sized reactor is described. SLOWPOKE was intended to provide an intense neutron flux for teaching, reactor physics experiments and to produce small amounts of radioisotopes. In 1967, the Los Alamos Laboratory reported a new, low mass of U-235 in a polythene core with a beryllium reflector. (Ref 1) That discovery, and an understanding of the influence of the temperature coefficient of reactivity, led to the SLOWPOKE concept: a self-regulating reactor relying on natural water convection cooling, which did not require complex and costly electromechanical safety systems, and allowed unattended operation with remote monitoring for an indefinite period. 2. Development and testing of SLOWPOKE In the 1970’s, Chalk River Nuclear Laboratories (CRNL) was at the forefront of Canadian nuclear technology. The first CANDU generating station, Douglas Point was in operation and Ontario Hydro’s 4-unit Pickering generation station was nearing completion. The time was ripe for innovation on the reactor front. The material to build a unique reactor was, by chance, at hand. Fuel, in the form of Aluminum clad highly enriched U-235 was available from NRX, and beryllium, a costly and toxic metal, was available in the form of a 9 in. inside diameter, 17 in. outside diameter cylindrical annulus from CRNL’s terminated ING project. These available materials came together in the construction of the original SLOWPOKE in the form of a tank- type low power assembly with the smallest known mass in the world of solid fuel (~ 760 gm) to reach criticality. A cross section of the reactor is shown in Fig 1. SLOWPOKE was the sixth reactor to go critical at CRNL and, apart from the ill-fated MAPLE reactors, the last. The conference presentation is illustrated with many yet unpublished photographs of the approach to critical on a sweltering insect-infested night in 1970 when hopes were as high as the outside temperature. Criticality was achieved by adding highly enriched uranium fuel elements to the core until the reactor was almost critical, and then adding beryllium plates above the core to reflect some of the escaping neutrons and thereby achieve a sustained chain reaction. “First Critical” was indicated by an exponential rise in neutron flux when the control absorbers were removed. Technically, the reactor multiplication factor, k-effective, was slightly greater then 1.0, and the reactor was slightly supercritical. Within a few days of achieving criticality, overpower tests were performed. These involved the rapid removal of a neutron absorbing control rod and observing the overpower transient. Reactor power rose to some 90 times normal and the tank glowed blue with Cerenkov radiation. However, the transient was self-regulating and after many hours, power approached normal. Figure 1 Cross section of a SLOWPOKE Installation 3. Conclusions Five of the original seven SLOWPOKEs in Canada are still licensed and operating, and there is one other SLOWPOKE at the University of the West Indies in Jamaica. However, if replication is a supreme source of flattery, SLOWPOKE enjoyed it. Since details of the original SLOWPOKE were published (Ref 2), the design was copied in China and designated by them as the “Miniature Neutron Source Reactor” or MNSR. Four of these reactors were installed in China, and five more exported to Syria, Iran, Pakistan, Nigeria and Ghana. What a missed AECL sales opportunity! 4. References [1] G.A.Jarvis and C.B.Mills, "Critical Mass Reduction", Los Alamos Scientific Laboratory, report LA-3651, (March 22, 1967) [2] R.E.Kay, P.D.Stevens-Guille, J.W.Hilborn, “SLOWPOKE; a New Low-Cost Laboratory Reactor,” International Journal of Applied Radiation and Isotopes, 1973, Vol 24, pp509-518. .
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