MOLINOS DE CRUDO

The existing license for the HFR is obsolete. It was issued before the Nuclear Energy Act entered into force and revisions have been very fragmentary. In the past, the HFR received relatively little attention from the regulatory body because of the heavy workload it was then under with respect to the two nuclear power plants then in operation and the assumption that it presented a lower potential risk. The KFD wished to update and modernise the license but made little progress. The Ministry of Economic Affairs, which was at that time running the secretariat of the competent authorities for the licensing of nuclear installations, was heavily involved (both programmatically and financially) in the research programme of ECN and of the HFR in particular. In the late nineties, two events caused a change: 1. Due to persistently negative (public and political) attitudes towards the construction of new

nuclear power plants in the Netherlands, the Ministry of Economic Affairs shifted its attention from nuclear research to other energy research programmes.

2. The secretariat of the licensing authorities was transferred from the Ministry of Economic Affairs to the Ministry of Housing, Spatial Planning and the Environment.

These changes, together with the practice of complete 10-yearly safety reviews at the NPPs, enabled the regulatory authorities to embark on a modernisation plan for the HFR and its licence.

Scope

The scope of work for this safety review was agreed in discussions between the regulatory body and both the licensee (JRC-Petten) and the operating organisation (NRG). Firstly, a new Reference Licensing Basis (RLB) needed to be established in order to produce a state-of-the-art yardstick for nuclear safety. Secondly, a Risk Scoping Study was to be conducted for the identification of technical weaknesses which might be overlooked by the deterministic comparison with the RLB. A new set of safety analyses was to be made on the basis of a more complete set of Postulated Initiating Events (PIEs), including the assessment of fire, flooding and seismic events as well as ageing. Following recommendations from the analyses, a new safety concept was to be established, as well as a modification programme to achieve this safety concept.

Reference Licensing Basis

To produce a yardstick by which to measure a research reactor built to a very early design against the state of the art in nuclear safety, a new RLB was agreed between the regulatory body and both the JRC-Petten and NRG. The IAEA Safety Standards and Guides issued specially for research reactors (IAEA Safety Series 35) were taken as a basis for this. Because the HFR is a large research reactor, applicable parts of the IAEA Safety Standard for the design of NPPs were also used as a basis.

Risk Scoping Study

Since a full-scale Probabilistic Safety Assessment (PSA) as conducted for an NPP was thought to be too costly for a research organisation, it was decided to undertake only a limited PSA (a so-called Risk Scoping Study). Also, a full scope PSA for the HFR was considered very complicated due to the lack of reliable data for component failure as well as for operator handling.

The objective was to ensure that no potential occurrences presenting a substantial risk to the public would be overlooked in the deterministic safety analyses performed in relation to the HFR. Both the current plant configuration using HEU fuel and the future plant configuration using LEU fuel and with planned modifications had to be assessed. Because the initial objective was mainly to identify weaknesses and not to provide figures, the scope of the PSA was confined to hazards associated with the core. Internal initiators, including flooding and fire within the plant, had to be selected to:

1. identify those initiating events and sequences which might contribute to core damage or unusual releases of radioactivity and to estimate the core damage frequency (level 1);

2. identify and assess the containment failure sequences and associated source terms (level 2); 3. assess the off-site consequences in terms of public health risks of these source terms (level 3). The first level of the Risk Scoping Study was reviewed by an IAEA IPSART mission. The mission’s comments and remarks led to an upgrade of the study. A second review followed in 2002 with the emphasis on level 2 and level 3.

An important part of the Risk Scoping Study was the assessment of internal flooding and fire. Both the design review concerning fire protection and the fire hazard analysis proved to be very useful. They identified not only the presence of many unnecessary combustibles (such as filing cabinets), in particular in the control room area, but also a lack of spatial separation between redundancies and a lack of fire detectors.

Safety Evaluation

An important task during this first 10-yearly periodic safety review was the identification of deficiencies with respect to the Technical, Operational, Personnel and Administrative (TOPA) requirements laid down in the RLB. This TOPA review revealed several deficiencies relating both to the particular institutional situation (the separation between licensee and operating organisation) and to habits that had developed over time. In particular, there was inadequate definition of the management tasks, responsibilities and competences of JRC and NRG with respect to the operation of the HFR, and of the interfaces between the two organisations. That is to say, the scope of licensee control over the operating organisation was insufficient, and there were also problems regarding the safety culture. Another important issue was the need to improve operational feedback. There was a need to improve the systematic, periodic review of operating experience (including experiments and maintenance, health physics performance and training/retraining courses) and to ensure that the Reactor Safety Committee was involved in reviewing incidents.

Safety Analyses

On the basis of IAEA Safety Series 35, a set of comprehensive Postulated Initiating Events (PIEs) was established, incorporating all possible failure scenarios at the HFR, including LOCAs, loss of off-site power, start-up accidents and fuel channel blockage.

The large-break LOCA (guillotine break) was analysed as a beyond-design-basis accident (no conservatism as e.g. required by 10 CFR 50 appendix K). Different sets of PIEs were defined as a basis for thermo-hydraulic safety analyses or radiological analyses. In addition, both internal flooding and fire events and external events were analysed.

Assessment of Ageing

IAEA-TECDOC-792 “Management of research reactor ageing” was selected as a basis for the ageing evaluation, together with the relevant guidelines for NPPs. Three different studies were performed: 1. electrical components (cabling, wires and connectors);

Safety Concept and Modification Programme

The safety concept of the HFR is that the HFR must fulfil three safety functions: safe shut-down of the reactor, long-term decay-heat removal, and containment. This concept is based on the traditional principles of defence-in-depth and multiple safety barriers for all accident conditions. In addition, a 30-minute waiting period has been introduced during which no credit for operator intervention must be taken. The safety analyses and Risk Scoping Study conducted with this safety concept in mind have produced a number of recommendations for improvements, most of which will be implemented as part of a modification programme following the award of the new licence. Due to media and political attention, a measure to overcome the effects of a special large-break LOCA (installation of a vacuum breaker on the reactor vessel head) has been permitted separately and was implemented in late 2003. This will be discussed below. The major features of the modification programme are:

• installation of additional vacuum breakers on the primary system; • installation of Accident Pressure Equalisation lines;

• controlled use of pool water by installation of basin water injection valves and locking the convection flow valves;

• replacement of diesel driven decay heat removal pump by diverse electrical pump; • modification of Emergency Power System logic;

• limitation of the portal crane movement inhibiting hoisting above reactor vessel during reactor operation;

• installation of a manual operated alternative shutdown system for ATWS events.

HEU-LEU Conversion

Shortly after this modernisation project was commenced, the licensee decided to convert to the use of Low Enriched Uranium (HEU) as fuel instead of High Enriched Uranium (HEU). This is in line with the worldwide move to abandon use of HEU for non-proliferation reasons. It was decided that the necessary work to obtain a licence for this conversion should coincide with the modernisation project. Consequently, all the relevant neutron-physics core calculations and all the safety analyses had to be done again on the basis of the new fuel.

Transfer of the licence from JRC to NRG

Influenced by an IAEA Safety Culture Review (INSARR mission), the licensee (JRC) announced during the discussions on the progress of the licence renewal process that it would like to see the licence transferred to the operating organisation (NRG). JRC would remain the owner but NRG would be given full responsibility. In addition, JRC would guarantee the continuation of the current research programme for the next three to four years, thereby guaranteeing a good financial basis. JRC would also remain the owner of the reactor, so providing a guarantee for its future decommissioning.