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308 Materials and Nanotechnology | Progress Report

6 h. The samples were prepared metallograph- tor, KIH, is not exceeded. Hence, the threshold
ically and observed in an optical microscope, stress intensity factor KIH at 227 °C and 250
the micrographs indicated coalescing of the °C of zircaloy-4 tubes charged with 125 ppm of
copper particles and other metallic elements hydrogen was determined. This study, carried
and formation of porosity. out in CCTM, was part of a global round-robin
test involving 10 countries with PWR reactors
Optical micrographs indicated presence of and coordinated by the IAEA. The objective was
porosity inside the structure and partial ho- to determine whether zircaloy-4 clad spent in
mogeneity, due to the non-dissolution of the nuclear fuel is immune from Delayed Hydride
elements involved in the metal alloy (Figure Cracking during Dry Storage. The overall results
39). of this Coordinated Research Project suggested
that unirradiated Zircaloy-4 fuel cladding is
Control rod plates for the IEA-R1 reactor immune from DHC above about 320°C and this
temperature increases to 360°C by irradiation.
An innovative program to develop new control
rods for the IEA-R1 Research Reactor at IPEN Options for the Back-End of the Nuclear
was undertaken. Four rectangular Ag-In-Cd Fuel Cycle of research reactors
plates were inserted and sealed in stainless
steel envelopes using a laser welding process Research reactors (RR) are used worldwide for
(Figure 40). The sealed steel envelopes were education of nuclear scientist and engineers, to
then laser welded to stainless steel plates to produce medical and industrial radioisotopes,
form one of two forks of a control rod. for Silicon doping, to perform advanced fuel
and material testing to support life extension
of operating nuclear power plants, to qualify
new fuels (including studies on fuel behaviour
in incidental and accidental conditions), etc.
Presently, there are over 400 RRs worldwide
and of these 230 are operating and the rest
Figure 40: IEA-R1 reactor’s control rod plate assembled for laser
welding. are either in a stand by situation, decommis-
sioned or under decommissioning. A common
Hydrogen induced degradation of problem related to all RR is management of
zircaloy-4 the Back End of the Nuclear Fuel Cycle. To help

address this problem, the IAEA initiated in
Delayed Hydride Cracking (DHC) has been 2015 a Coordinated Research Project (CRP) to
responsible for cracking in zirconium alloy develop a comprehensive set of options for
pressure tubes and fuel cladding and is a con- the back end of the fuel cycle. The options
cern for spent fuel storage. A flaw in a loaded included the means and methods for storage,
component may extend by DHC because hy- transport, security, and disposal. In addition,
drogen diffuses up the tensile stress gradient the CRP is creating a tool to analyse scenarios
at the flaw tip. Sufficient hydrogen must be comprised of these options with a focus on
present for hydride to form at the flaw tip and their economic aspects. In this context, CCTM
the local tensile stress must be sufficiently has been participating to provide an additional
large to crack the hydride. A crack will not option to safely store spent nuclear fuel for
extend if the threshold in stress intensity fac- extended periods in wet basins.

Instituto de Pesquisas Energéticas e Nucleares