Petten research reactor converts to using low-enriched
On the 6th of May, NRG (the
Netherlands’ national nuclear research centre in Petten)
started up its High Flux Reactor (HFR) for the very first
time, with a core that consisted solely of low-enriched
nuclear fuel. The conversion from high-enriched to low-enriched
uranium was the result of a technical development programme
that lasted several years and required the finalising of
new licensing procedures and switching over to using a different
type of nuclear fuel. Thanks to the efforts of the many
employees involved, the conversion process went smoothly.
One objective was to avoid any disruption to Petten’s
isotope production and other ongoing research programmes.
With this conversion, Petten will offer an important contribution
global effort of diminishing the use of proliferation-sensitive
high-enriched uranium (HEU).
In order to make U-235 suitable for use in nuclear
power plants, the 235U content needs to be enriched. For nuclear
power plants, the enrichment percentage of 235U is normally approximately
4%. Until recently, the HFR used HEU containing 235U of 89–93%.
This high enrichment makes the HEU proliferation-sensitive, meaning
that the fuel, which the HFR used to use until recently for civil
purposes is also suitable for nuclear weapons. In order to prevent
even the remotest possibility of certain countries or groups from
obtaining this HEU to make nuclear weapons, NRG decided to switch
to using low-enriched uranium (LEU), in which the amount of fissionable
235U is less than 20%. The Joint Research Centre (JRC), a European
Commission Directorate General and formerly the HFR’s licensee,
made a concerted effort, together with NRG (the HFR’s operator/user),
to convert from HEU to LEU. This has now been achieved.
The JRC/NRG conversion project was planned in
three phases: a feasibility study, the technical qualification
of the conversion process and the licensing procedures.
The initial phase resulted in the formulation
of detailed calculation models so that the reactor core could
be optimised. The substantial percentage increase of the ‘non-active’
238U decreases the thermal flux of neutrons and the core optimization
made it possible to keep the reduction of the thermal neutron
flux to a minimum. By adapting the calculation model in this way,
the fission material can be optimized - by changing its density
– in order to compensate for the lower degree of enrichment.
During phase two, the conversion was granted
technical qualification following a comprehensive professional
study, the carrying out of safety analyses, the testing the new
nuclear fuel elements and the carrying out of thermal and hydraulic
Phase three focused on the licensing procedure.
JRC is the HFR’s owner but was also the licensee. Quite
apart from the decision to switch from HEU to LEU, the license
was in any case due for renewal. Since it was more logical that
NRG, as the reactor’s operator and user, should also become
its licensee, these outstanding issues were easily resolved by
obtaining the new license in NRG’s name. This license transfer
had already been recommended by the IAEA. When the license application
was made the conversion process was included in the application
request, so that when the authorities granted the license, in
February 2005, the targeted conversion became a reality, at least
Last October, the first LEU elements were placed
in the reactor’s core. Now, for the first time, the HFR
starts up using only LEU elements. Consequently, NRG and JRC are
actively contributing towards reducing the use of proliferation-sensitive
For more information about this development
and about the other activities of NRG and the JRC in Petten visit
the following web site: www.nrg-nl.com