The section Physics of Nuclear Reactors of the department of Radiation, Radio-nuclides and Reactors (R³) used to operate a subcritical assembly containing fuel pins made of natural uranium (metal) in a hexagonal lattice. As a moderator, light water was used. However, for some practical reasons, like the heavy weight of each fuel pin (almost 7 kg), the rather low k_eff (» 0.85) and the fact that some fuel pins stuck to the grid plates, this assembly is not used anymore. Therefore, it was decided to build a new assembly, called Delphi, for training and research.

In principle, Delphi consists of two vessels one upon the other. The lower vessel is made of stainless steel and is filled with de-mineralized water before the start of an experiment. The upper acrylic air-filled vessel is used to store the fuel pins that can be lowered one after the other using a special handling tool. Below the steel vessel, a shielding box is positioned containing the neutron source that can pneumatically be inserted to its experimental position in the steel vessel.

The Delphi assembly contains 168 fuel pins made of 3.8% enriched UO₂ fuel with a total length of 66.5 cm. The fuel pins are positioned in a square lattice of 13x13 positions, with the central position being occupied by a water-filled tube. The pitch between the fuel pins is 23 mm, being the value at which the k_eff of the assembly is maximal. The resulting k_eff is expected to be close to 0.916. Attached to the top of each fuel pin, a special stainless steel head facilitates manipulation with a tailor-made handling tool that can hold of a fuel pin very much like a refillable lead pencil.

The cylindrical stainless steel vessel has an inner diameter and height of 100 cm. Before starting an experiment, the vessel is filled with water. On top of the steel vessel, a cylindrical transparent acrylic vessel with outer diameter of 50 cm, and height of 80 cm is mounted containing the fuel pins when they are in storage. The assembly is loaded by a controlled movement of each fuel pin from its upper position in the air-filled acrylic vessel to its lower position in the water-filled steel vessel using the handling tool.

The ²⁵²Cf-neutron source contained in a plastic capsule has an initial strength of 18.5 MBq corresponding to a neutron source emission rate of 2.4E6 s^-1 and a gamma-ray emission rate of 1.3E7 s^-1. It is stored in a stainless steel rectangular box shielded with paraffin and B₄C grains. Calculations with the Monte Carlo code MCNP-4C show that the dose rate at the outer side of this box is limited to 6.5 mSv/hr, which is about the same as the dose rate to experimentalists during measurements.

When Delphi is not in use, the source is stored in the shielding box. Before the start of an experiment, the box is pulled from its position under the steel vessel to remove the shielding plug, and pushed back under the vessel. Only if the reactor vessel is filled with water, the source tube can be lifted pneumatically and be connected to the steel vessel. Subsequently, the source capsule can be inserted and withdrawn from the vessel by means of air pressure.

From bottom to top, Delphi contains the shielding box, the steel vessel and the acrylic vessel. In total, the height reaches about 250 cm, which makes it necessary to manipulate the fuel while standing on a loading platform. This platform is made of stainless steel and sufficiently large to accommodate 8 to 10 persons.

Delphi is equipped with two 10-bar ³He proportional counter tubes with diameter of 6 mm and active length of 76 mm. The pulses from the detector are amplified and converted to TTL format and recorded in a PC equipped with a pulse-counter card and LABVIEW software to control the measurements. Besides the detector pulses, this software also records each passage of the source synchronized with the neutron pulses.

For more technical information, see the report IRI-131-2003-008, or the poster presentation.

For more information, please contact j.l.kloosterman@tudelft.nl.