Measurement of pH and conductivity in the HFR research reactor, Petten (Netherlands)

9.4.1. Introduction

In the summer of 2007, a new analytical system for the measurement of the pH and conductivity water was installed to monitor the quality of primary cooling and basin waters of the HFR research reactor, Petten. A description of the new hardware is presented in this report.

9.4.2. The High Flux Reactor

The High Flux Reactor (HFR) is a 45 MW research reactor, located in Petten, Netherlands. The reactor, shown in Figs 61 and 62, is owned by the European Commission, and the licence holder is Nuclear Research &

Consultancy Group (NRG).

FIG. 60. Pitting induced by dust on an aluminium corrosion coupon immersed in the RA6 spent fuel pool. Disc diameter: 100 mm.

In 1961, the reactor became critical for the first time, and in 1962, it started regular operation at 20 MW. In 1966, the reactor power was increased to 30 MW, and in 1970, to 45 MW. In 1984, the reactor vessel was replaced.

Typically, an annual reactor programme comprises 290 full power days distributed over 11 cycles of four weeks, including a three day stop, and two maintenance periods.

Two main water bearing systems can be distinguished: the primary cooling system for cooling the reactor core and transfer the heat to the secondary cooling circuit, and the basin system. The primary cooling circuit has a volume of 151m³. The basin system comprises two storage basins, one for storage of in-core components, experiments and isotope capsules (106m³), and one for storage of spent fuel elements and experiments waiting for dismantling in hot cells (84m³). Table 24 shows the operational limits for the two systems, and Figs 63 and 64 show the respective diagrams. Demineralized feed water is produced on-site using a facility that produces water with conductivity at around 0.055 µS/cm. Water quality is monitored by measurement of pH and conductivity.

Measurements are performed every 8 hours for both systems.

TABLE 24. OPERATIONAL LIMITS FOR THE PRIMARY COOLING CIRCUIT

AND BASIN SYSTEM OF THE HFR

pH Conductivity (µS/cm)

Primary cooling system 5.5–7.5 0.1–0.2

Basin water 5.5–7.5 1–2

FIG. 61. The HFR research reactor building.

FIG. 61. The HFR research reactor building.

FIG. 62. View of HFR core.

The old pH/conductivity measuring system was first installed to check the performance of the demineralization system. Later on, it was decided to also use it to monitor the quality of the water in the basin and primary cooling system. It was an ‘off-line’ system, with samples being discharged to the drain system. When the equipment became too old (more than ten years), it was decided to replace it using a more modern system.

9.4.3. The new pH and conductivity measuring station

The new measuring station, shown in Fig. 65, consists of a combined pH–conductivity meter installed in an industrial housing. The pH electrode is equipped with a flat glass membrane and a reference electrode, which is suited for measurements under high pressures. The pH electrode is suitable for pH measurements in demineralized water. It is provided with an extra platinum grounding electrode, which enables measurement of the pH and reference electrode by means of separated, high impedance amplifiers. This is important because pure demineralized water has a relatively high electrical resistance (and thus low conductivity), complicating

FIG. 63. Primary cooling system.

FIG. 64. HFR basin configuration.

performance of measurement with a pH meter using a high impedance glass membrane as a sensor. Some features of the measuring station are:

— It has a user-friendly operation with limited maintenance, because it can operate in a stand-alone mode;

— All relevant data is displayed in a clear format, with capability to show results in the reactor control room;

— There is disturbance of the sample during sampling, viz. interaction with the atmosphere;

— Tubing and connections are made of stainless steel, so that contact with open air is avoided;

— There is a large capacity for storing and retrieving historic data, through the use of a memory card for storage of measurements and log data;

— The pH/conductivity system is designed for operation in low conductivity water under pressure;

— The pH electrode is a gel type reference electrode, max 8 bar, equipped with a glass membrane with a relative low resistance. pH amplifier with a sufficient high input impedance;

— The conductivity sensor is especially suitable for measurements at 4 bar (max 10 bar);

— Temperature compensation: pH and conductivity measurements;

— Electrodes and electronics are designed to allow performing self tests of the system;

— Simulation sensors allow to verify the working conditions of the electronics and the data acquisition module;

— In order to check functioning of the pH electrode, a set of pH buffers (pH4, pH7) are included. These standards are based on NIST standards;

— The equipment has an independent pump to allow significant measurements when the reactor is not in operation;

— Water flow is about 5 L/min, considered more than sufficient to guarantee an appropriate response speed, and representative measurements;

— During operation, the pressure of the incoming water is 2 bar; dropping to 1 bar when leaving the system, always well below the maximum pressures to which the pH and conductivity electrodes may be exposed, i.e.

8 bar and 10 bar, respectively;

— The conductivity meter is frequently checked by measuring a calibrated solution, prepared by an accredited laboratory (in this case, ECN, the Department of Engineering and Services);

— The pH meter is frequently checked by having the buffer solutions used analyzed and controlled by an accredited laboratory (in this case, ECN, the Department of Engineering and Services).

FIG. 65. The new pH and conductivity measuring station.

9.5. OPERATIONAL EXPERIENCE AT CSF, ARGENTINA: INFLUENCE OF WATER QUALITY IN