On the 6th of May, NRG will start the High Flux Reactor (HFR) in Petten, for the very first time, with a core that solely consists of low-enriched nuclear fuel. Thus the conversion from high-enriched to low-enriched uranium will be a fact, and after several years a process of technical development, licensing procedures and switching to a new nuclear fuel will have been successfully completed. Thanks to the efforts of the many employees involved, the conversion went smoothly. They saw to it that any adverse effects to isotope production and the ongoing research programmes, have remained acceptable. With this conversion, Petten will offer an important contribution to the global effort of diminishing the use of proliferation-sensitive high-enriched uranium.

Natural uranium is extracted from uranium ore. It contains 0.7 percent fissionable U-235 and in order to make it 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 high-enriched uranium (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 the remote possibility of certain nations or groups from obtaining this high-enriched uranium to make weapons, it has been decided to start using low-enriched uranium (LEU), in which the amount of fissionable 235U is less than 20%. The JRC (Joint Research Centre, a European Commission service and formerly the HFR’s licensee) has therefore together with NRG (the HFR’s operator/user) made a concerted effort resulting in the conversion from HEU to LEU.

After NRG and JRC’s decision to bring about the conversion, preparation procedures started. In doing so, the conversion project was divided into three stages: a feasibility study, the conversion’s technical qualification and the licensing procedures.

The initial stage resulted in detailed calculation models so that the reactor core could be optimised. This made it possible to keep the reduction of the thermal neutron flux to a minimum, as the substantial percentage increase of the ‘non-active’ 238U decreases the thermal flux of neutrons. By adapting the calculation model, the fission material can be optimised, by changes in its density, to compensate for the lower degree of enrichment.

During the second stage, the conversion’s technical qualification included a comprehensive professional study, safety analyses, testing the new nuclear fuel elements and performing thermal and hydraulic calculations etcetera.

The third stage concerned the licensing procedure. JRC is the HFR’s owner but was also the licensee. Apart from the decision-making process in switching from HEU to LEU, the licence was due to be renewed. Since it was more logical that NRG, as the reactor’s operator and user, would also become its licensee, one could resolve these issues by obtaining the new licence in NRG’s name. This licence transfer had already been recommended by the IAEA. In applying for the licence, the conversion had been included, so that when the authorities granted the licence in February 2005, the intended conversion was a fact, at least in writing. Last year in October, the first LEU elements were placed in the reactor’s core. Now the HFR for the first time starts up entirely using LEU elements, so that NRG and JRC are contributing towards reducing the use of proliferation- sensitive material.

Frequently asked questions

What does HEU to LEU stand for?

HEU stands for ‘high-enriched uranium’. Natural uranium contains 0.7 percent fissionable 235U. After natural uranium is enriched, HEU contains 89 to 93% fissionable 235U. LEU means ‘low-enriched uranium’, in which the amount of fissionable 235U is less than 20%.

Why was this conversion necessary?

HEU is regarded as ‘weapons-grade nuclear materials’. This means that it can be used as basis for the production of nuclear weapons, and hence is proliferation- sensitive. Several organizations, notably the US government but also the IAEA, have urged research reactors such as the HFR to switch to LEU, in order to prevent the possibility that certain countries or groups obtain HEU material for nuclear weapons.

Doesn’t the use of LEU reduce the output considerably?

NRG’s was indeed worried about a lower output. Extensive testing of materials, the use of advanced calculation models and sophisticated fuel changes have now resulted in a minimal yield loss.

What happens with HEU waste?

According to agreements with the HEU-supplier (the U.S. Department of Energy), the Americans have supplied HEU and collected the irradiated HEU to safely handle and store it.

Where does NRG purchase its LEU, and what happens with the irradiated LEU waste?

NRG purchases the new fuel from its European partners. The used LEU fission elements are taken to COVRA (Centrale Organisatie voor Radioactief Afval = Central Organization for Radioactive Waste) in Vlissingen. COVRA is the only organization in the Netherlands in charge of the collection and safe storage of Dutch waste.

Additional Information:
For more information contact:

• NRG Communications, Juliette van der Laan-Jenniskens, tel 0224 - 56 4967, e-mail: j.vanderlaan@nrg.eu or visit our website: www.nrg-nl.com