POSEIDON - Silicon Doping

POol-Side Equipment for Irradiation and Doping Of silicon by Neutrons

Silicon in nuclear reactors?

Under thermal neutron flux, ³⁰ Si activates into ³¹ Si, which decays in ³¹P in a few hours. As it happens, P is one of the impurities which are diffused in the ingot in order to control its electrical resistivity. P is tri-valent and generates the charge unbalance which raises the electrical conductivity of the semi-conductor material.
Because neutrons do not interact much with matter, they can penetrate deep into the ingots. Hence, the generation of P31 impurities is spread with great uniformity across the ingot. The uniformity of the doping realised by neutron irradiation is much better than the one obtained by the classic chemical diffusion method. It is a key benefit of the irradiation method. It is particularly valuable if components of large size are to be produced, as it is the case for power electronics.

Silicon doping activities started at SCK•CEN in1992, with the commissionning of the SIDONIE device. Because SIDONIE provides simultaneous up and down and rotation movements, it achieves consistently a final resistivity in the range of 2% of the target, despite being irradiated in strong axial and radial flux gradients.

Recently, concerns about global warming led car manufacturers to study reduced-emissions vehicles. Among those, hybrid cars seems to be a promising compromise. Production figures are anticipated at 1.000.000 cars by 2010. Those cars, with their dual power supply, fossile and electric, need power electronics components.

The anticipated increase for production led ingot manufacturers to seek for larger ingots, for which the SIDONIE installation was no longer suited. Even BR2 does not have irradiation positions large enough to accommodate the future ingots size. A new installation with a much higher capacity was in need.

POSEIDON

The core of POSEIDON is a graphite block enclosed in an aluminium box, fitted with 6 holes, in which baskets containing the ingots can be irradiated. The box is suspended at a parallelogram attached to working floor in BR2 pool. It can be pushed against the reactor vessel. Neutrons "escaping" through the vessel are "collected" in the graphite block and used for the doping.

One of the most distinctive features of BR2 is the hyperboloidal shape of its vessel. Whereas this shape allows optimum flux conditions in the core, BR2 vessel prevents up and

down motions – as in SIDONIE – to be used for the axial gradient compensation. Only remains a rotation movement for the compensation of the radial gradient. The axial gradient is compensated by the permutation between the upper and the lower ingot of the basket at mid-irradiation. Yes, this requires an additional handling at mid-irradiation.

Nevertheless, the nice thing with POSEIDON is that it does not lead to any reactivity change in BR2, nor does it cause any additional fuel costs, as would be the case if the installation was loaded in the reactor.

Parallel to the irradiation system, a complete rebuild of the ingots management system was made necessary by 1) the increase of production rates and 2) the increase of ingots size and weight.

Because of the weight increase, hand-processing is no more a viable option. Ingots can weight up to 18 kg and assistance must be provided to the operator. In addition, the capacity of existing equipment had to be raised to accommodate for the new dimensions, which led to the replacement of the ancillary equipments.

The management of the ingots during the operation of BR2 is a particularly challenging task because:

• Requested target resistivities – i.e. irradiation durations – are different according to different manufacturers and even can be different within batches produced for a given manufacturer
• The six irradiation positions are not stricly equivalent (there is up to 33% difference between the hottest and the coldest position), leading to different irradiation durations between the positions in order to achieve a given resistivity.
• An inversion top/bottom is requred at mid irradiation
• Once the irradiation is completed, the ingots must be stored 24 h for cooling and radioactive decay

This means that, at any given moment, there are at least 60 ingots – each one looking pretty much like the others - in BR2 pool, which have to get their own irradiation dose.

Because of the huge number of manipulations, the production in POSEIDON strongly intereferes with other productions at BR2, like the isotope production. Both productions are in direct competiton for the utilisation of the pool bridge.

A few figures

• Ingots: 6" and 8" O.D. Adaptations between 6" and 8" are made by means of a graphite sleeve.
• Ingots height: 250 mm
• Irradiation capacity: 12 ingots in 6 baskets
• BR2 pool silicon storage capacity: 75 baskets, i.e. 150 ingots.
• Irradiation block: width: 1400 mm – height: 1050 mm – depth: 700 mm - 6 holes, 214 mm I.D.
• Expected resistivity uniformity: 5% across a 8" ingot
• First operation: planned for cycle 2/2008

For further information please contact:

Dekeyser Jean