Design of the 50 kW neutron converter for SPIRAL2 facility

doi:10.1016/j.nima.2010.01.038

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 618, Issues 1–3, 1–21 June 2010, Pages 1-15

https://doi.org/10.1016/j.nima.2010.01.038 Get rights and content

Abstract

SPIRAL2 is a facility for the study of fundamental nuclear physics and multidisciplinary research. SPIRAL2 represents a major advance for research on exotic nuclei. The radioactive ion beam (RIB) production system is comprised of a neutron converter, a target and an ion source. This paper is dedicated to the designing of the 50 kW neutron converter for the SPIRAL2 facility.

Among the different variants of the neutron converter, the one based on a rotating solid disk seems quite attractive due to its safety, ease in production and relatively low cost. Dense graphite used as the converter's material allows the production of high-intensity neutron flux and, at the same time, the heat removal from the converter by means of radiation cooling.

Thermo-mechanical simulations performed in order to determine the basic geometry and physical characteristics of the neutron production target for SPIRAL2 facility, to define the appropriate beam power distribution, and to predict the target behaviour under the deuteron beam of nominal parameters (40 MeV, 1.2 mA, 50 kW) are presented.

To study the main physical and mechanical properties and serviceability under operating conditions, several kinds of graphite have been analyzed and tested. The paper reports the results of such measurements.

Radiation damage is the most important issue for the application of graphite as neutron converter. It is well known that the thermal conductivity of the neutron-irradiated graphite is reduced by a factor of 10 from the initial value after irradiation. Difference in volume expansions between the matrix and the fiber results in serious damage of neutron-irradiated C/C composites. Calculations showed that at high temperature the effect of neutron radiation is not so critical and that the change in thermal conductivity does not prevent the use of graphite as neutron converter.

Introduction

The SPIRAL2 project aims at delivering high intensities of rare isotopes beams by adopting a suitable method for each desired radioactive beam. The RIBs will be produced by the ISOL “Isotope Separation On-Line” method via a converter, or by direct irradiation. The combination of both methods (i.e. via fission induced by fast neutrons in a uranium target or by direct bombardment of the fissile material) will allow covering broad areas of the nuclear chart. Moreover, it will allow carrying out promptly significant experiments and activities in both fundamental and applied nuclear Physics (Medicine, Biology, solid state, etc.) [1].

The neutron converter has to produce an intense flux of fast neutrons, mainly in the forward direction with respect to the incoming primary beam, inducing up to 10¹⁴ fissions per second in the uranium carbide target located upstream the converter. The primary beam is constituted by deuterons of energy 40 MeV and current up to 5 mA (up to 200 kW beam power). The neutron converter is conceived as a high-speed rotating target, which limits the peak surface temperature of converter materials well below 2000 °C. Nuclear graphite made of natural carbon is a very suitable material as a neutron converter. In fact, ^natC(d,n) reaction is very prolific, especially in the forward direction where the neutron yield is comparable to that generated by other light material converters [2].

The thermal properties of graphite (melting point of 3632 °C) allow a compact geometry and the power dissipation from the converter does not demand a sophisticated cooling system but simply the heat is exchanged by radiation with the water-cooled vacuum chamber wall. The thermal power deposit in the converter material is dissipated only by thermal radiation. Heat removal from vacuum chamber is carried out by water circulating inside cooling channels, fixed to the chamber's walls. In alternative to the water, liquid lead may provide a more efficient and safe power dissipation at full operation conditions.

The facility, at the beginning and for a relatively long period of time necessary to assess its performances, will be operated at reduced power, up to 50 kW. The suitable neutron converter has been studied for this first period of operations and was designed on the basis of a previous experience on the 70 kW prototype [3].

The neutron production target, as well as the uranium carbide target and the ion source, is placed inside a “production module”, which is surrounded by the biological shielding [4]. In practice, the “production module” is a shielded box that contains all the sub-systems dedicated to the production of radioactive ions and that became highly radioactive and contaminated. Removal of the “production module” has to be done only by remote handling device. The assembling and disassembling of the “production module”, part replacement or conditioning of elements has to be conducted inside hot-cell to ensure that the radioactivity is confined.

Section snippets

The neutron converter

The neutron converter design is based on a high-speed rotating wheel, which operates within the temperature range of 1850–2000 °C. Graphite made of natural carbon has been chosen as converter material to be employed with deuteron beam. The present design (Fig. 1) is based on the solid graphite disk with apertures and separation between the areas of beam position. The disk is clamped to the shaft by several spokes made of stainless steel. This design is similar to the one used in PSI [5]. The

Material for the neutron converter

Many materials can produce intense fluxes of fast neutron when bombarded with low-energy deuteron beams. Among them only a few can withstand high-power beam and work at temperatures around 2000 °C. Among several tested materials the graphite-brand turns out to be the most suitable one to produce intense neutron flux under rather severe operation conditions [9]. Several types of commercially available graphite have been investigated and experimentally tested. Two of them, the MPG fine-disperse

Determining the graphite lifetime

According to [13], [14] the predicted lifetime of the neutron target converter can be roughly approximated by the simple Zhurkov formula based on the representation of the thermal-fluctuation nature of solid fracture $τ = τ_{0} \exp [(U_{0} - γ σ) / kT] .$

Here k is the Boltzmann constant, τ₀ has an order of magnitude of 10^–13 s and is close to the period of the typical thermal vibrations of atoms in solids, U₀ is the initial activation energy of the destruction process that is reduced by the applied stress σ and γ

Measurement of the graphite evaporation rate

Different values for the evaporation rates of graphites have been published in the literature and frequently are in contradiction with each other. To clarify the situation, the evaporation rate, in function of temperature of four different commercial graphites, have been measured. Based on their thermo-mechanical properties, these graphite types have been identified as potential candidates for the converter material.

The selected graphites are: Carbone Lorraine 1116PT, POCO AF5 (ZFX-5Q),

Radiation effects on the graphite thermal conductivity

The minimum lifetime demanded for the neutron converter is of one operation cycle, corresponding to 90 days. During one operation cycle the neutron converter is irradiated by a neutron fluence of 3.1×10¹⁹ n/cm², corresponding to a graphite radiation damage of 0.06 dpa.

Radiation damage is the most important issue for the application of graphite as a neutron converter. Some authors [20], [21], [22] have shown that the thermal conductivity of neutron-irradiated graphite is reduced even by a factor

Conclusions

Based on the analysis carried out, the 50 kW target can be designed as a rotating unit (rotation frequency 10–20 Hz), which includes the graphite converter clamped to the shaft by means of metal rods. For the 1-cm-wide Gaussian beam the target diameter occurs to be 52 cm for the maximum converter temperature of 1850 °C. The converter should be 7 mm in thickness (which exceeds a little bit the 40 MeV deuterons stopping length). According to the numeric analysis performed, the maximum thermo-mechanical

Acknowledgements

We acknowledge the financial support of the European Community under the FP7–Infrastructures-2007-1 “Preparatory Phase of the SPIRAL2 Project” Grant Agreement N. 212692 referring to LNL and financial support of ISTC in the frames of ISTC Project No. 3682 referring to BINP. The EC is not liable for the use that can be made of the information contained herein.

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Design of the 50 kW neutron converter for SPIRAL2 facility

Abstract

Introduction

Section snippets

The neutron converter

Material for the neutron converter

Determining the graphite lifetime

Measurement of the graphite evaporation rate

Radiation effects on the graphite thermal conductivity

Conclusions

Acknowledgements

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