Druck-Materie 20b.qxd - JUWEL - Forschungszentrum Jülich

ACoM - 6
6 th Meeting of the Collaboration on Advanced Cold Moderators
Jülich, 11 - 13 September 2002
Study of radiation effects in hydrogeneous moderator materials at low temperatures
(URAM-2 project)
E. Shabalin, V. Golikov, S. Kulikov, L. Golovanov,V. Melikhov, E. Kulagin, A. Belyakov, V.
Ermilov, V. Konstantinov, A. Androsov, Yu. Borzunov
Laboratory for Neutron Physics, Joint Institute for Nuclear Research
141980 Dubna, Russia
1. INTRODUCTION
The goal of the URAM-2 project was to study fast neutron irradiation effects in solid
methane, water ice, and methane hydrates which are of crucial importance for a design of cold
moderator of the ESS target station. Later on, tetrahydrofuran (THF) hydrate was added to be
studied because it is a material easy to be manipulated with. The URAM-2 programme was
expected to give knowledge about possibility of burping in ESS cold moderator beads, temperature
rise in burps, and maximum allowable residence time of beads until burping.
Results obtained on accomplishment of the program are presented in this report. Preliminary
research and development of a cryogenic irradiation facility at the channel N o 3 of
the IBR-2 reactor have taken two years (1999-2000). In 2001, the URAM-2 irradiation
facility has been constructed, tested, and mounted to the working position near the IBR-2
reactor. The irradiation rig consists of three principal parts: - irradiation rig head, charging
device, and cooling system. It enables to irradiate either previously prepared pelletized
samples, or to make samples by condensing gas into irradiation cavity. Description of the
facility and the project plan were reported earlier [1, 2]. Irradiation experiments were started
on November 13, 2001 and finished May 22, 2002.
2. SUMMARY OF THE RESULTS AND ANALYSIS.
2.1 Solid methane
27 runs of irradiation with 11 types of samples (layers of methane with thickness from 0.28
mm up to 5 mm condensed onto the walls of the chamber and spherical segments of 4.6, 7.5,
9, and 14.3 mm height) were performed. Irradiation time was from 3 hours up to 26 hours.
Main results:
Only one spontaneous burp occurred (in 5 mm shell sample after 11 hours). No other spontaneous
burp was observed even for bigger samples and longer irradiation. For irradiation time
≥ 4 hours and for size of samples ≥ 1 mm thermally induced burps were always registered; the
smallest sample 0.3 mm displayed a burp once of two attempts. Maximal rate of energy accumulation
in solid methane (at temperatures more than 20K) was estimated to be as high as
1.6%±0.2% of absorbed dose rate, that is, ~32mW/g for ESS dose rate of 2 W/g. Saturated
stored energy was observed to be dependent on irradiation temperature (as it was predicted
based on previous experience with solid methane irradiation) and was estimated to be as high
as 110-30 J/g in the range 21-28K.
Basing on a comparison analysis of URAM-2 results and experience with solid methane moderators
of IPNS, IBR-2, and KENS facilities, conclusion is made that for irradiation tempera-
129