The primary circuit oil spill

The PFR primary sodium circulation system is shown in Figs 1 and 15. Figure 16 shows details of a primary sodium pump (PSP). The sodium from each PSP flows through filters and a stop valve to the diagrid, and thence to the fuel subassemblies. Each subassembly has a filter at its inlet. Figure 15 shows the relationship between the pump and subassembly filters. In 1974 primary sodium pump 2 (PSP 2) was removed from the reactor for modifications to its instrumentation and was noticed to be heavily contaminated by a black sooty deposit. In the same year the charge machine was removed, revealing that its immersed surface was black

with adherent tarry lumps. During this period of operation some 65 L of oil had been lost from the pump upper seal oil systems, part of which is believed to have entered the reactor vessel. When the reactor was taken critical no effects of the oil were observed. It is suspected, however, that as a result of the spill the filter on PSP 2 valve failed due to high differential pressure because it became blocked by oil-sodium reaction products (“O” in Fig. 15). It is also thought that partial blockage of the pump casing overflow pipe was to lead to the major problem in 1991 (Fig. 16).

FIG. 15. PFR primary sodium pump and its associated valve and filter assembly.

FIG. 16. Detail of a PFR primary sodium pump housing.

At the end of 1991, PFR had been shut down since 29 June when the reactor had been manually tripped following observation of overheating of the top bearing of PSP 2.

An extensive examination of the circumstances which led to the leakage of primary pump bearing oil into the primary sodium circuit, has concluded that it involved two stages, the first on 25 June 1991 and the second on 29 June 1991. Prior to these events:

— There was a maximum indicated level of 17 L of oil in the pump drains tank above a in layer of sodium/oil sludge;

— On 24 June 1991 the main argon gas blanket cover gas flow to the reactor dropped to zero due to blockage or partial blockage of the two absolute filters and/or the drain line associated with the aerosol filter;

— On 25 June 1991 efforts were made by the operators to restore the cover gas flow by venting the system through the gaseous effluent system.

Venting the reactor cover gas system to the effluent system reduced the cover gas pressure in the pump support vessel, allowing sodium to rise up the annulus between the pump drive shaft and the drains tanks inner wall and enter the drains tank. This sodium would displace oil to a level above the upper lip of the drains tank. Restoration of the cover gas blanket pressure on 25 June then forced the 17 L of oil down the annulus between the pump shaft and the drains tank into the pump support vessel sodium. Over the next few days, a further 5.5 L of oil which were added to the seal oil sump could have reached the sodium in the pump vessel. A sudden loss of 12.7 L of oil from the pump lubrication system sump occurred on 29 June. This activated the loss-of-sump oil level alarms in the control room and led to the observed overheating of the top bearing which initiated the decision to manually trip the reactor.

Examination showed no oil external to the system leading to an estimated total of 35.2 L of oil ingress into the primary sodium.

The remainder of 1991 had been spent in cleaning the primary sodium and in making preparations for examination of the three primary pump filters and the inlet filters on some of the fuel assemblies which had shown temperature increases at the time of the spillage.

Removal of the pump filters was a major task which had never been attempted before.

All three primary pump filters were removed for examination. Deposits incorporating some carbon were found on all three. Some difficulty was experienced in refitting them because of distortion of some thermocouple guide tubes and it was necessary to cut these away. The success of the overall operation showed that the opacity (and, as it was the primary circuit, the activity) of the sodium coolant is not an insurmountable impediment to maintenance work.

Examination of a number of fuel assemblies also showed deposits on the inlet filters of those which had shown temperature increases prior to shutdown but none on assemblies which had shown no temperature increases. However, the deposits were slight (about 0.5 g) and mainly of sodium. It was improbable that these were sufficient (even allowing for the possibility of some loss during handling) to cause the observed temperature rises and it is possible that roughening of the cladding surfaces by adsorption of oil degradation products might have been a contributory factor.

In parallel with these out of reactor examinations of fuel assemblies, a device was developed which would measure flow through each of the remaining fuel assemblies and through some of the blanket assemblies. Use of this device showed acceptable flows through all of them.

Work on the reactor was supported by laboratory studies to examine the effects of temperature, sodium and irradiation on both new and degraded oil. These included tests on the PROTVA rig at the IPPE at Obninsk where oil was injected into sodium upstream of a PFR fuel assembly inlet filter, and studies on a National Nuclear Corporation (NNC) water rig to examine the blockage-forming potential of material similar to that observed in PFR.

A major effort was required to remove all three valve and filter assemblies from the reactor for examination. These were the longest components in the reactor vessel, at 12 meters, and required considerable care in handling. Examination showed that at least one panel of each valve filter had failed, and oil-related debris was found on all the filters. A number of the fuel subassemblies which had showed outlet temperature rises during the incident were removed, and oil-related debris was found on their inlet filters and wrappers. The result of the oil ingress was an 18-month shutdown while PSP valve and filter assemblies were removed and new filters were fitted. The pump seal oil systems were modified to prevent any further possibility of oil ingress, and alarm and trip systems were added to prevent blockage of the pump filters in order to protect the subassembly filters.

Very fine particles of carbon were found in primary sodium samples after 1974. These are believed to have come from the 1974 oil ingress, and it appears that in the long term oil debris breaks down into finely-divided carbon particles which are dispersed in the sodium, pass through the filters, and circulate without obvious effect.

Oil ingress into the primary circuits of an LMFR is undesirable because of the potential release of methane gas through the core causing reactivity effects, and possible blockage of the subassemblies by solid carbon debris. In the case of PFR no reactivity effects were seen, possibly because the oil was retained in the pump cone for a prolonged period and broken down slowly without the formation of large bubbles. In the long term oil bearings are probably best avoided. The EFR design was changed following the PFR oil ingress incident by the introduction of the innovative features of magnetic bearing and ‘ferro-fluid’ seals to eliminate oil completely and remove the potential hazard of its ingress into the sodium.