THE HIGH FLUX REACTOR CASE 1. Characteristics of the HFR

The HFR (Fig. 2) is a 45 MW(th) ‘tank in pool’ type reactor, similar to the Safari reactor in South Africa and the (closed) Studsvik reactor in Sweden.

The Joint Research Centre of the European Commission owns the reactor.

The operator as well as the licensee is the Nuclear Research and Consultancy Group (NRG) located in Petten, Netherlands. The reactor was built in the late 1950s and has been in operation since 1961. In 1984, the reactor vessel was replaced. The reactor was not fully refurbished: most of the wiring, piping and instrumentation were not renewed.

For the last four years, the reactor fuel is low enriched uranium (LEU). The targets for producing ⁹⁹Mo are made of HEU.

FIG. 2. High Flux Reactor (HFR) in Petten, Netherlands.

2.2. The HFR problem

The first signs of deformation and decrease of thickness of the wall in a part of the primary cooling circuit were detected during a regular in-service inspection, as required by the license, in spring 2006.

These phenomena were carefully monitored during the following maintenance outages. In June 2008, after several in-service inspections, the Dutch nuclear supervisory authority (Kernfysische Dienst, KFD) and NRG recognized the existence of a safety problem with regard to the wall of the primary coolant system, which needed further actions.

In the course of the maintenance activities during the summer outage of August 2008, no significant progress in deformation and decrease of thickness was detected. A few hours before the reactor was to be restarted, camera images showed a gas bubble jet in the ‘reducer’ of the bottom plug liner (Fig. 3), revealing a pinhole at one of the inward deformations of the reducer.

FIG. 3. Position of the reducer in the bottom plug liner of the HFR.

At that time both NRG and KFD assessed the safety of the reactor as not sufficiently assured. Therefore NRG decided not to restart the reactor, but rather to analyse the situation and come up with a proposal for actions and repair.

In the following months, non-destructive research of the reducer wall was performed, analysis of the released gas bubbles was attempted and possible leak paths were investigated. It is important to notice that no leakage was found, and no increased radiation level was measured in the reactor hall. Further investigation lead to the assumption that corrosion is the most probable cause, and that the bubble gas consists of hydrogen.

2.3. The repair studies

Starting in September 2008, NRG studied repair methods. Two alternative solutions were considered:

— The ‘sleeve’ alternative: From within the cooling circuit an aluminium sleeve would cover the damaged part. After careful investigation it was concluded in November 2008 that this alternative was not feasible, neither with rubber seals (radiation too high) nor with epoxy seals (irremovable because it sticks to the wall).

— The ‘concrete’ alternative: After removal of the concrete, in which the bottom plug liner is embedded (Fig. 4), the deformed part will be cut out and replaced. This alternative, remaining the only feasible one, would take at least one year to prepare and three months to carry out. This would mean that the HFR had to be put out of service for about fifteen months.

2.4. Shortage of ⁹⁹Mo production and consequences for health care

Together with the NRU reactor at Chalk River, the HFR in Petten contributes about 70% to the ⁹⁹Mo production worldwide (Fig. 5). Bearing in mind the state of the Chalk River facility at that time, loss of Petten production would have meant considerable shortage (up to 70%) of the worldwide supply of the medical isotope ⁹⁹Mo for several months in 2009. This would have also meant that healthcare quality would be severely affected and that, as a consequence, waiting lists would have increased and patients could even be deprived of proper treatment.

The Dutch medical authorities estimated that the expected shortage of radioisotopes following prolonged outage of the HFR could lead to decreased healthcare quality for several thousands of patients in the Netherlands and that

possibly patients could die as a consequence. These consequences of prolonged outages of the HFR were deemed undesirable by the Dutch medical authorities.

2.5. Interim solution

NRG took the initiative to propose an interim solution to the KFD and the Dutch Government. On the basis of the results of the performed analysis and considering the fact that no leakage had been found, NRG suggested to restart the HFR before repairing it for a limited period of time (one year at the most) to allow for production of ⁹⁹Mo during the preparation of the repair of the HFR. During reactor operation, extra safety precautions would be taken with respect to possible leakage paths and monitoring of leakages.

NRG based its proposal on a safety case, which was also submitted for review by a team of renowned Dutch nuclear safety experts. The conclusions of NRG supported by the experts were:

FIG. 4. View of bottom plug liner before embedding in concrete.

— The overall risk as well as the core damage frequency from operating the reactor in this condition are only slightly higher than in the normal condition (without degradation of the integrity of the cooling circuit);

— The degradation is caused by a slow (corrosion) process; no sudden large leakage is expected; and

— There is enough time to allow for a safe shutdown of the reactor, according to existing procedures, in case a leakage is detected.

NRG concluded that operation of the reactor in this situation for a limited period of time would pose a limited safety issue that could be dealt with.