Reactor core

ISSUE TITLE: Inadvertent dilution or precipitation of poison under low power and shutdown conditions (RC 1)

ISSUE CLARIFICATION:

Description of issue

This issue is also applicable to NPPs with LWR.

In some units, during shut down conditions, poison is added to the moderator system to maintain the necessary sub-criticality. During extended shut downs it is required to have purification through ion-exchangers. It must be ensured that the poison is not removed under these circumstances.

It should also be noted that inadequate pH control in the moderator system could lead to gadolinium precipitation, and therefore an inadvertent addition of reactivity.

Safety significance

Inadvertent removal of poison would reduce the margin of sub-criticality.

Source of issue (check as appropriate)

• ____xx____ operational experience

• __________ deviation from current standards and practices

• __________ potential weakness identified by deterministic or probabilistic (PSA) analyses

MEASURES TAKEN BY MEMBER STATES:

Canada

Gadolinium poison is usually added to the moderator to maintain the reactor in a “guaranteed shutdown” state. Strict administrative controls are maintained to ensure that inadvertent removal via the purification circuit does not occur. Gadolinium precipitation is prevented by maintaining an adequate pH level.

India

Originally the moderator purification system was kept isolated in Indian Nuclear Power Plants during shutdown conditions to prevent removal of poison (Boron is used as poison in 220 MWe Reactor and Boron/Gadolinium is used as poison in 540MWe Reactor). However, due to buildup of Cobalt and subsequent contamination of the equipment, the design modification in moderator purification system was carried out. This modification included two Boron saturated Ion Exchange columns which would be operated in boron saturated mode only during reactor shutdown conditions. Interlocks have been provided to prevent valving in of fresh Ion exchange columns during shutdown. Also, strict monitoring of purification flows and Neutron Log Power is done by the operator. In addition, the frequency of sampling for boron during shutdown is increased keeping in mind purification removal capacity (in case of inadvertent valving in of fresh ion exchangers). Make up of heavy water is avoided during reactor shutdown or done with appropriate boron addition, if absolutely necessary. For 540MWe Reactor, during shutdown, moderator purification system gets isolated. Continuous monitoring of Neutron signal with alarms set suitably above the current readings is done and on observation of any rise in the neutron signal unintentionally, addition of boron equivalent of 5 ppm is

made mandatory as a part of Technical Specification. To avoid precipitation of Gadolinium in the moderator, if present, the pH control of moderator at 5.0-5.5 is always ensured.

Korea, Republic of

During the outages gadolinium poison (or boric acid) is used to maintain the reactor in shutdown state with sufficient negative reactivity. Inadvertent purification of boric acid is prevented by operating procedures which state that the valve connected to the ion exchanger and D2O supply line should be lock closed during the shutdown period. Opening of these valves requires the permission of the senior reactor operator.

Romania

The document on Guaranteed Shutdown State is a type "A" document, is directly referenced in the license and it is approved by CNCAN. Gadolinium poison is added to the moderator to maintain the reactor in a "Guaranteed Shutdown" state. There are in place, as per this procedure, very clear and rigurous administrative controls to ensure that inadvertent removal via the purification circuit does not occur. Gadolinium precipitation is prevented by maintaining an adequate pH level.

All these requirements are part of the regulatory practice to have hold points as part of the licensisng process for main activities (including restart after outages) during outages. The monitoring of the heat sinks and guaranteed shutdown states are important parts of this process.

ADDITIONAL SOURCES:

• Cernavoda NPP Unit 1 Guaranteed Shutdown State procedure, 1999.

ISSUE TITLE: Fuel cladding corrosion and fretting (RC 2) ISSUE CLARIFICATION:

Description of issue

This issue is also applicable to NPPs with LWR.

The condition of a certain very small number of fuel bundles irradiated in CANDU reactors has been observed to differ from that predicted and accounted for in design, operation, and safety analysis documentation. The fuel bundles in question have shown signs of more-than-expected degradation such as end plate cracking, spacer-pad wear, element bowing, sheath wear, bearing-pad wear, oxidation of defective fuel, and fission produce release. Element bowing, sheath strain and, disappearance of the CANLUB layer, are other fuel conditions whose impact on safety assessment require quantification.

Fuel bundle degradation depends on the reactor design, fuel channel, fuel design, fuel manufacturer, and operating conditions. Since theoretical models have been unable to correlate adequately the fuel condition to these factors, fuel and pressure tube inspections are necessary. Owing to the number of factors upon which the degradation depends, the inspection program must be extended beyond inspection of defective fuel to observe these changes.

Fuel bundle degradation is sometimes also accompanied by fretting and scratching of the pressure tube.

The effect(s) of some of this bundle degradation on reactor safety is (are) not known partially because of a lack of experiments and safety analysis methods. As such, the important fuel and fuel channel parameters to measure are difficult to identify.

Safety significance

Damaged fuel bundles can be a source of debris and lead to increased coolant contamination and can further cause increased concentrations of fission products in the off-gas. Loose fragments of fuel elements in the core can induce further cladding damages, for instance by cooling channel blockage.

Source of issue (check as appropriate)

• ____xx____ operational experience

• __________ deviation from current standards and practices

• __________ potential weakness identified by deterministic or probabilistic (PSA) analyses MEASURES TAKEN BY MEMBER STATES:

Argentina

The main failure causes were problems related with plug – sheet welds (manufacture problems). In the sheets inspected due to be failure’s suspected, fretting evidences were not observed. Nevertheless, in most of cases only were possible to determine the secondary fault but not the primary fault.

Canada 1. CNSC

Some fuel inspections have been conducted and the results submitted to the CNSC (Canada), some Canadian licensees do not have a formal process to ensure that the fuel and fuel channel conditions are identified and accounted for.

The CNSC has identified this issue as a “Generic Action Item”, and issued a Position Statement to licensees giving the closure criteria and the required completion schedule.

To achieve closure, licensees are required to perform the following:

1. Implement an action plan to eliminate excessive fuel and channel degradation in acoustically active channels (where applicable).

2. Response to this closure criterion should include a rationale showing that the situation at all reactors is acceptable for long-term operation. If this cannot be shown, plans and schedules should be submitted, showing how this problem will be resolved.

3. Implement an effective formal and systematic process for integrating fuel design, fuel and channel inspection (in situ), fuel and channel laboratory examination, research, operating limits and safety analysis. This process must have the following features:

(a) annual review (by the licensee) to demonstrate effective implementation and adequate corrective actions taken for deficiencies identified in the review;

(b) sufficient resources for each participating group (design, inspection, examination, research, safety analysis, and operation) to ensure that the fuel condition is known and accounted for adequately;

(c) clearly defined maximum allowable limits, under normal operation, on fuel condition in terms of sheath strain, element bowing, wear (spacer pad, bearing pad, end plate), pressure tube scratching and wear, burnup and residence time; design documentation and pressure tube fitness-for-service guidelines should be updated accordingly;

(d) a determination, for the full range of the operating envelope, the power boost sheath failure threshold for CANLUB fuel and the chemistry effects of CANLUB on centerline temperature and fission product release;

(e) assurance that the safety analysis accounts for the allowable fuel condition when combined with aging effects such as pressure tube creep, the effect of CANLUB in the fuel, and any chemistry effects on temperature and fission product release, including a calculation of the number of sheath failures resulting from a bounded loss of power control;

(f) a surveillance program that demonstrates compliance with identified limits, e.g., detection of significant changes in fuel condition caused by changes in fuel fabrication and factors affecting acoustic resonance; and

(g) allowance for AECB audits.

2. Industry

Several work packages to address the CNSC’s generic action item on the Impact of Fuel Condition on Safety Analyses have been undertaken within the CANDU Owners Group (COG). In-reactor irradiation and post irradiation examination (PIE) have been used to quantify the impact of selected fuel parameters on fuel performance. These include:

(a) In-bay inspection techniques to measure fuel dimensions, non-destructive burnup determination, and improvements to visual inspection systems;

(b) Irradiation of fuels manufactured using (a) pore formers to assess the effect of fuel density, (b) high and low diameter clearances between fuel and clad, and (c) CANLUB/non-CANLUB elements; and

(c) PIE of power reactor fuels to assess the impact of reactor ageing (e.g. pressure tube creep and sag).

This series of experiments have found no evidence that these conditions are detrimental to fuel performance or safety related issues. This COG program is continuing and will continue to assess special manufacture fuel bundles and fuel from the ongoing fuel surveillance program.

COG has also funded the review and update of the Technical Specifications for 37-element CANDU fuel to ensure the design baseline is complete and open to audit.

Data from manufacture, irradiation and PIE are routinely entered into an Irradiation Fuels Database to ensure well characterized information is available to validate the computer codes used for safety analysis.

One of the Canadian utilities has developed a plan to achieve resolution and closure of all significant fuel performance issues by 2005. The plan involves verifying/establishing the basis for fuel limits, developing methods to check fuel condition, and developing a process to assess fuel condition and identify problem precursors.

India

Fuel cladding fretting does not appear to be a serious problem in India even though one failure has been observed due to this. The iodine and fission products in heat transport system and fuel failure rates are satisfactory and much below limits. Volumetric examination of more than 500 fuel channels in various Indian reactors have not indicated any defects of generic nature. However, in view of the issues raised in this document, it is planned to have a fresh review.

Korea, Republic of

Systematic cladding failure as a result of fretting induced by spacer or bearing pad is not occurred in Korea.

However, review is to be performed on the problem described in this document , if necessary.

Romania

The problems were subject of commissioning tests and verifications. There are no apparent problems on this topic in this moment and no events with safety significance encountered. However this is an aspect of possible future review based on the feedback from operation.

During operation the Licensee is performing mandatory test programs and preventive maintenance actions to support the maintaining of the control rods insertion characteristics, as confirmed by the commissioning results; we may notice also that there were no events related to this aspects for the operating period.

ADDITIONAL SOURCES:

• INTERNATIONAL ATOMIC ENERGY AGENCY, Nuclear Power Plant Life Management Processes: Guidelines and Practices for Heavy Water Reactors, IAEA-TECDOC-1503, IAEA, Vienna (2006).

• CNSC Position Statement 94G02, “Impact of Fuel Bundle Condition on Reactor Safety”.