Degradation of Fuel Elements in Temelin NPP

Actualised version of September 3rd, 2007

Throughout the recent days we have experienced, once again, a surge of interest some domestic and foreign media took in the issue of geometric degradation of fuel assemblies at the Temelín Nuclear Power Plant.

The entire design of the Nuclear Power Plant rests on a number of safety principles. One of the major ones is the principle of in-depth protection intended to separate the ionizing radiation and radioactive substances from environment using what is known as the safety barriers. The first such barrier is created by the cladding of fuel rods. The cladding aims to prevent the fission products from penetrating into the primary circuit of the Nuclear Power Plant. The second barrier is represented by the pressure boundary of the primary circuit and it keeps the radioactive medium from being released into the inner premises of the Nuclear Power Plant. The third barrier consists of the containment designed to avert any possibility of the radioactivity reaching the environment.

The nuclear reactor installed at the Temelín NPP contains 163 fuel assemblies and each assembly holds 312 fuel rods. All nuclear power plants are designed on assumption that a certain number of the rods develop leaks. The number of fuel rods supposed to become leaky is determined with reliance on safety analyses and detailed in Technical Specifications in the form of Safety Limits & Conditions; the law requires that Safety L&C are subject to approval by SÚJB (State Authority for Nuclear Safety). At the Temelín NPP the fuel rods are checked for tightness through monitoring the specific activity in the primary circuit, using the A 3.4.10 Limit Conditions. The conditions stipulate that the specific activity never exceeds these values:

A. Equivalent of I-131 ≤ 2,6 . 10⁷ Bq/kg

B. Overall specific activity of ≤ 3,7 . 10⁹ Bq/l

To inspect the cladding of irradiated fuel, and to seek for any leaky spots, the NPP employs two mutually independent systems:

a) "On-line sipping" - a system used to perform qualitative measurements and search for damaged fuel assemblies. The system applies a gas method of continuous changes in the Xe 133 concentration.

b) "Off-line sipping" - a system used to conduct qualitative measurements of a fuel assembly found damaged in an attempt to reveal the scope of damage done. While the measurement is in progress, the fuel assembly is encapsulated in a special fuel enclosure.

An overview of leaks disclosed in fuel at the Temelín NPP

FA – fuel assembly; FR - fuel rod

Main Production Unit 1
During the 2nd campaign of the 1st Unit we had some signals hinting at gas escaping out of a fuel assembly. The On-Line Sipping system singled out two assemblies as possibly leaky, specifically AB31 and AC03; the Off-Line Sipping system confirmed leaks only at the AC03 assembly. Both fuel assemblies have been removed and deposited in the spent fuel storage pool. The Temelín NPP is the globally first VVER reactor using fuel that can be disassembled and a specific rod revealed ruptured removed and replaced. In May 2003 the ultrasonic testing equipment used to search for the leaky fuel rods was not operable and, consequently, the AC03 fuel assembly was decided not to be reinserted and a new assembly was used instead. The pattern of fuel assemblies in the core for the 3rd campaign, 1st Unit, has been reshuffled.

Main Production Unit 2
The increased activities of Iodine 131 disclosed early in August 2004 and the subsequent rise in the Xenon activities indicated leaky fuel - zircon and niobium were not detected. To find the leaky fuel assembly, we have, once more, relied on the On-Line Sipping system, but it failed and became unable to check beyond the 53rd assembly.

With the campaign still under way we have determined the probable number of leaky fuel assemblies and when the campaign terminated the figure was specified to 1 through 3. The follow-up checks conducted on the Nuclear Fuel Inspection & Repair Stand confirmed the number.

Moreover, above and beyond the expected scope of operation checks, 48 fuel assemblies have been selected and inspected. Three assemblies in the group of 48 (BC33, BC08, BC10) have been found leaky and then transferred also to the Repair Stand where the ultrasonic method was applied to disclose the offending fuel rods.

When repaired the assemblies have been retested for tightness and two of them have been returned to the core; the third one (BC10) proved to still contain a leaky rod and, therefore, it has been declared unfit for further operation.

Summary of maximum activities found in the primary circuit coolant

Comparison of the values tabled above with the Limit Condition value of 3.7 . 10⁹ Bq/l makes it obvious that there is a reserve of 2 to 3 orders and hence the impact of the event on nuclear safety is negligible. The Government watchdog agency had no reason to impose any restrictions on the operation of Units exhibiting symptoms of gas leaking out of fuel.

Mechanical deformation

The mechanical deformations of fuel rods and assemblies, i.e. the deviations from their ideal geometric shapes and the elongation of fuel assemblies (radiation growth), come as a natural phenomenon accompanying the production of energy inside the cores of nuclear reactors.

a) Fuel rods

The safety analyses of reactor operation rely on certain assumptions of mechanical deformations experienced in the fuel rods. Such deformations are caused by uneven compressive forces exerted by spacer grids used to align the fuel assemblies in the required geometry. The magnitude of mechanical deformations is limited by the cross sectional area left available for the coolant to flow through. The first cores in the Temelín NPP have been designed to tolerate the flow area reductions by as many as 51%.

As determined by the inspections and measurements performed after the third fuel campaign of Unit 1, the area constriction was close to 49%. That was the reason why the Operator and the fuel Manufacturer designed the 4th campaign core under the conservative assumption that the fuel rods could touch.

b) Fuel assemblies

Like the fuel rods the fuel assemblies are also prone to geometry changes due mostly to the radiation growth experienced in the assembly skeletons. The changes translate themselves into bending and twisting. The largest radiation growth measured at the Temelín NPP affected the third campaign at Unit 1 and reached the length of 10.18 mm, while the attendant buckling was 29 mm.

The deformation figures themselves are irrelevant, since the nuclear safety will depend on the proper performance of the mechanical control of the reactor, i.e. on the clusters possibly affected by the deformation. The safe function of clusters is defined by the A.3.1.4 Limit Condition requiring that the clusters are capable of gravity dropping to the core, and that the time needed to elapse between the moment when the clusters are released in their top end positions and the moment when they enter the hydraulic dampers of guide tubes is 3.5 seconds or shorter.

As identified, the geometry changes in the fuel of the 1st and 2nd Unit resulted in an incomplete touch down of some clusters having fallen down through the core. Since the problem became known, the function of cluster dropping has been repeatedly tested at intervals specified by SÚJB.

The results of the last test of cluster dropping performed at Unit 1 on June 2 testify of further deterioration of this Unit's core. The number of clusters unable to touch down has increased by two, and, what is worse, two clusters came to a halt above the level of the hydraulic dampers, in other words they have failed to meet the Limit Condition quoted above. In view of the results the Operator has resolved, in keeping with the SÚJB requirement, to send Unit 1 to early unscheduled outage for refueling.

The results arrived at through the fuel monitoring and testing are used to improve the fuel assembly design and to make sure that the design principles and safety criteria are duly met. To remedy the problem of clusters not touching down, some modifications to the fuel assembly designs have been suggested and approved and while shut down for outages both the Units have been treated to the 1st stage of the modification (the suspension rods). The trend in the number of clusters failing to touch down is being analyzed primarily for any signs of the situation worsening. The analyses have brought up for consideration the idea of curtailing the fourth campaign of Unit 1 by approximately 2 months and to replace the deformed fuel assemblies with new ones having undergone the 2nd stage of modification (the hydraulic damper). Starting from 2007 new fuel with new structural material is to be put to use.

The numbers of clusters failing to fully touch down in tests conducted in 2005, 2006 and 2007:

Main Production Unit 1

Main production unit 2

When used in operation, the nuclear fuel entails the above-specified problems, potentially impacting on the nuclear safety. Nevertheless the solutions adopted and the modifications & checks performed make sure that the reactors are operated in conformity to the stipulated requirements and with sufficient margin of nuclear safety.

By no means we can accept the allegations that we make light of the problems and fail to keep them under proper control. The SÚJB Institute pays a long-term attention to these issues and its requirements binding upon the Operator never allow that the nuclear safety be compromised.