Neutron flux and minor actinide transmutation potential measurements at MEGAPIE

F. Marie, Ch. Blandin 2 , S. Chabod, A. Letourneau, F. Molinie 1 , D. Ridikas, JC. Toussaint 1

1 DAPNIA/SIS, 2 CEA/DEN/DER/SPEX

We propose to measure the neutron flux in the liquid Pb-Bi MEGAPIE 1 MW target at PSI. Micro fission-chambers will be placed inside the central rod of the target to determine both thermal and non-thermal components of the neutron flux. In principle, both time- and space-dependent variations of the flux will be on line monitored with a precision better than 5% .

Description of the project.
In the frame of the MEGAPIE (from MEGAwatt Pilot Experiment) project [1], a Pb-Bi liquid target will be installed at the SINQ spallation neutron source [2] of the Paul Scherrer Institut. It will assess experimentally the performance of a 1 MW liquid target during 4-9 months irradiation period. This will be an essential step for the further development of high power targets in the 20-50 MW range required for Accelerator Driven Systems and for high intensity spallation neutron sources.

Among all the uncertainties related to the operation of Pb-Bi spallation targets, three topics have kept our interest:

• The formation of 210 Po which generates a high radio-toxicity and which has been studied at ILL at thermal neutron energy [5].
• The knowledge of the neutron flux, both in energy and in absolute intensity and its evolution. This will validate neutron generation and transport codes.
• What performances in terms of transmutation potential could offer this innovative very intense spallation neutron source?

As far as the transmutation studies are concerned, the Mini-Inca group has developed, in collaboration with CEA/Cadarache, and in partnership with the PHOTONIS Company, a new type of micro fission chambers [3], which have been validated in the V4 channel of the High Flux Reactor at the Institut Laue Langevin in Grenoble. The intensity of the flux has been measured to be 1.6 10 15 n/s/cm 2 with a relative uncertainty of 7% [4]. The outstanding performances obtained for our new fission chambers in a very intense neutron flux have lead us naturally to propose a solution for measuring the neutron flux in the Pb-Bi liquid target at MEGAPIE in order to enlarge our transmutation studies at wider range of neutron energies [6,7].

Experimental Method and technical developments 
We propose to implement 8 fission chambers in the central rod of the target in order to measure on line the flux in the MEGAPIE target (see Fig 1). Due to the small place available in the central rod of the target, (i.e. 1.3 cm inner diameter) , detector of small size (4.7 cm external diameter and 8 cm length) have been chosen allowing the positioning of two detectors side by side for an optimal background subtraction. 235 U deposit fission chambers will be sensible to thermal neutrons whereas Gadolinium shielded 235 U fission chambers will be used to measure the epithermal component of the neutron flux. 242 Pu deposit fission chamber and threshold reaction flux monitors (Fe, Ni, Nb...) will measure the fast component of the flux. Monte Carlo simulations [8] show that at a distance of 35 cm from the Pb-Bi target entrance in the central rod the neutron flux is ~1.7x10 14 n/s/cm 2 with nearly 42% thermal and turned to almost 100% thermal at 85 cm from the target window. For this reason, (as seen in Fig 2) non thermal neutron detectors will be placed in the bottom part of the central rod whereas thermal neutron detectors will be equally placed in order to monitor space variations of the flux. 237 Np and 241 Am deposit detector will be also irradiated to study the transmutation-incineration potential of such a neutron source. Finally 2 detectors with no fissile deposit will be also equally distributed for background measurement. During all the operation of the MEGAPIE target, the fission rate of the detector should be low enough to allow a precise monitoring of the time variations of the neutron flux (at the level of 1%) whereas at the time scale of few months the 235 U burn up should be sufficient to allow a inline calibration of the detectors. The main technical challenge of our project is to built detectors with the capability to stand high temperature (600 o C) and frequent fluctuations of temperature (250 o to 420 o in several sec) corresponding to the normal operation of the target. Our signals ranging from 500 nA to 100 m A will be measured with a relative uncertainty of 1%. Thus, a special care has been brought to the electronic modules, cables, connectors and electromagnetic background together with Monte-Carlo simulation to assess the background level due to y and charged particles in the detector.

FIG1: Scheme of the detector implementation in the MEGAPIE target

Tests of prototype detectors at ILL


FIG2: Example of fast monitoring of the reactivity (blue curve) and gamma flux (red curve) of the ILL core (time scale in second).

After several months of study, four prototype detectors have been constructed and irradiated in the V4 channel at ILL by the end of 2003. The conditions of irradiation (43 days under a flux of 10 15 n/s/cm 2 with a maximum fission rate of 5.10 9 f/s and a total burn up of 90%) were corresponding to the expected MEGAPIE experimental conditions. In particular, temperature cycling between 250 o and 600 o has been performed on one detector, prior the irradiation, in order to test its electric resitivity. The total neutron flux has been also extracted from the burn up of the fissile deposit, and found to be 1.05 10 15 n/s/cm 2 at the level of 5%. Finally, detectors have been calibrated and their ageing studied. During these integral tests the following crucial points have been validated:

• the heat proof of the detectors (new design)
• the new high precision electronic modules for the current readout
• a new acquisition program
• the choice of mineral cable (among Al 2 O 3 and MgO): temperature versus radiation hardness
• detector functioning point and coefficient of calibration
• g background subtraction

 
Fig 2 and 3 show an example of fast and slow monitoring of the reactivity obtained with 235 U deposited chambers. In deed, relevant 0.1% fluctuations of the neutron flux have been measured confirming the potentiality of our detectors at MEGAPIE. Final detectors will be constructed by the end of 2004 and installed at PSI mid 2005.

This challenging program will provide highly requested quantitative data to test simulation codes, for neutron generation and transport in realistic geometry, and offer a unique opportunity to assess the neutron performances of the new generation of spallation targets. 

References
[1] M. Salvatores, G.S. Bauer and G. Heusener, The MEGAPIE Initiative , Report MPO-1-GB-6/0_GB, Paul Scherrer Institute, Zurich (1999).
[2] G.S. Bauer, The Swiss Spallation Neutron Source SINQ , Proc. Of ICONE 8, Baltimore (2000).
[3] M. Fadil et al., Nucl. Instr. & Meth. A 476 (2002) 313 [
4] M. Fadil , thesis INPG, Grenoble, DAPNIA -03-01-T (2003)
[5] A. Letourneau et al, Proc. of the 11 th Int. Symp. on Cap. g -Ray Spectroscopy, Sept. 2002, Prague, Czech Republic.
[6] F. Marie & al. Proposition au CSTS du DAPNIA/SPhN du 3-4 juin 2002
[7] F. Marie & al. Proposition au CSTS du DAPNIA/SPhN du 16-17 decembre 2002
[8] M. Fadil, & al., rapport DAPNIA-02-243 (2002)