Shoulder to shoulder with Westinghouse

TVSA–the Russian-made fuel assembly with a rigid frame that is comprised of six elements and spacer grids–has been used for the successful operation of 17 VVER-1000 units, including Kalinin NPP, and Ukrainian and Bulgarian NPPs. So far more than 2500 TVSA assemblies have been produced.

Operational experience has shown that TVSA is highly reliable. During operation, it has achieved a validated maximum fuel burn-up of 61.8 MW·day/kgU, a validated maximum fuel element burn-up of 71.9 MW·day/kgU, a validated effective operational lifespan at nominal output of 49800 eff.hrs and a validated operational life span of 6-7 years. Thermal and mechanical features of TVSA make bending or twisting of the assembly almost impossible. They also help reduce the unit’s shutdown time through fast refuelling and through smooth, correction-free refuelling. TVSA has improved the technical, economic and operation-related characteristics of fuel for VVER-1000 reactors.

TVSA VVER-1000 fuel has better thermal control and is easier to dismantle and overhaul than previous fuels. Pre-reactor and reactor tests have confirmed the rigidity of the TVSA fuel assembly over its entire service life. After the implementation of TVSA, maximum flexion and clearance between fuel assemblies in the reactor core dropped from 20mm to 5 mm.

We have carried out a battery of experimental tests with fuel assembly models and electric heaters to validate the assembly’s thermohydraulic characteristics and reliability. More than 20 TVSA models have been tested, including simulations with different spacer grid clearances, and with uneven heating in radial and vertical dimensions. We have carried out coolant preheating tests, tests in characteristic bundle cells, departure from nucleate boiling, and post-CHF (post-critical heat flux) heat transfer in stationary and non-stationary conditions. The distribution of local velocities and consumption of a heat-transfer agent in cells along the fuel assembly’s cross-section, and in inter-assembly clearance, were studied on a model containing fragments of three adjacent fuel assemblies. We have carried out computational and experimental validation of the hydraulic compatibility of TVSA with other designs of VVER-1000 fuel assembly. Thermohydraulic computations and experimental studies confirm that the fuel bundles carried by TVSA can be cooled reliably. The internal thermal technical specifications of TVSA allow an output capacity increase of 4% within the reactor core, in addition to the existing spare capacity within the VVER-1000 reactor core.

Operational Results

Visual inspections and measurements have shown that TVSA assemblies operating in Kalinin NPP retained their structural rigidity and geometry. The flexion of fuel assemblies the reactor core in Unit 1 stabilised starting from cycle 18, and remains low — the mean flexion value for TVSA, measured in the 2006 outage was 3.8 mm. A post reactor study of used TVSA assemblies after four and six years of operation has confirmed the efficiency of the TVSA fuel assembly, rigidity of the frame, geometrical stability, and normal fuel assembly cladding condition.

Post-reactor tests of used TVSA carried out at FGUP GNC RF NIIAR (Federal State Unitary Enterprise - Research Institute of Nuclear Reactors) have lead to improvements aimed to simplify the design of the fuel assembly, and enhance the operation of spacer grids.

We have implemented a four-year fuel cycle at the Kalinin NPP, with three temporary shut-downs, and annual refuelling with 42 TVSA assemblies of uranium-gadolinium fuel with a maximum enrichment of 4.4%. We are implementing a transition to an effective five-year fuel cycle with 36 TVSA fuel assemblies (with maximum enrichment 4.95%), lasting up to 320 effective days with enhanced energy release fuel assemblies (Kr=1.6). In the 2007-2015 period, we are implementing a three-year cycle with refuelling every 18 months, in accordance with Rosenergoatom’s power generation boosting programme (see also Table 1).

Enhancing the TVSA design

We are currently testing modifications that would improve the technical and economic characteristics and competitiveness of the Russian fuel for VVER-1000. Modified TVSA assemblies have been successfully operated in Unit 1 of the Kalinin NPP for four years. There are three types: TVSA-T with a reduced number of spacer grids, TVSA-y for a taller reactor core, and TVSA-5 with 4.95% enrichment and greater uranium holding capacity.

A fourth modified design, TVSA-ALPHA, is the result of evolutionary development of the basic TVSA design. Its distinguishing feature is a reduced number of spacer grids–eight (as is the case in NPP Temelín, and in PWRs generally), higher uranium loading capacity, and inclusion of disturbed flow grids. TVSA-ALPHA also features heat-dissipating elements, and pellets with an external diameter of 7.8mm without a central opening (7.8/0).

TVSA-ALPHA has been validated by reactor tests of TVSA modifications, as well as by direct operational experience. There are currently 72 operating TVSA-ALPHA assemblies in Kalinin 1, with different service lives. Six assemblies of this prototype achieved burn-up performance 47 MW·day/kgU for fuel assembly, and 53 MW·day/kgU for fuel elements.

Inclusion of agitating disturbed flow grids in TVSA-ALPHA allows the temperatures along the fuel assembly cross section to be evened out, reducing the local steam content, increasing the operating reliability and heat-engineering reserves, and providing the possibility of additional output capacity in the reactor core. The grids comprise a slab lattice with flow deflectors. The mixing grids do not touch the fuel. They are installed between the basic spacer grids in the upper part of the reactor core.

Validation of agitating disturbed flow grids has included a study of agitating characteristics on stands, thermohydraulic characteristics, and departure from nucleate boiling, and also mechanical and vibration survival tests and endurance tests. Experimental results indicate that the design of the grids is efficient in terms of mixing, and so has improved critical heat flow.

As a part of the effort to increase output capacities of existing NPP units to 104% of nominal, TVSA-ALPHA fuel assemblies with agitating disturbed flow grids enable an enhanced operating environment for the fuel elements, and enhanced operating reliability of the fuel in efficient fuel cycles.

The TVSA-y design features a shorter fuel assembly bottom nozzle and fuel assembly head piece, and a fuel assembly column that is 150mm longer than standard TVSA assemblies.

Two TVSA-y fuel assemblies have been operating for four years at Kalinin 1 NPP. The maximum achieved fuel burn-up for the fuel assembly was 54.9 MW·day/kgU, and 64.9 MW·day/kgU for the fuel element.

The implementation of TVSA-ALPHA and agitating disturbed flow grids from validated solutions and extended length of the fuel elements can increase the output capacity of the reactor core. This fuel element arrangement improves the performance of VVER-1000 reactors, and meets current efficiency and heat output requirements (up to 3300MW).

TVSA-T for Temelín

For a fuel supply contract for Temelín NPP in the Czech Republic we have proposed the fuel assembly TVSA-T, based on TVSA-ALPHA solutions.

Compared to the TVSA-ALPHA design, the TVSA-T design uses combined grids and has a longer fuel column (by 150 mm). The combined two-tier grid consists of a cellular spacer grid and the TVSA-ALPHA laminar agitating disturbed flow grid, placed within a single rim. The combined grids even out the hydraulic resistance with fuel elements manufactured by Westinghouse within a combined reactor core, and they stir the heat-transfer fluid across the TVSA-T cross section. The basic characteristics of TVSA-T are shown in Table 2.

The validation work with TVSA-T include thermohydraulic tests and studies, and also comprehensive mechanical, vibration survival tests and endurance tests.

Thermohydraulic tests of TVSA-T have included:

- hydraulic tests using fragments and models

- hydraulic brake tests

- testing and optimisation of agitating grids

- study and validation of TVSA-T compatibility with neighbouring fuel assemblies (using a model consisting of two fragments of fuel assemblies of different types);

- study of thermohydraulic characteristics and departure from nucleate boiling using the TVSA-T fuel assembly models equipped with electric heaters, and validation of correlation of critical heat flow.

The scope of work for the fuel assembly development has included neutronic and thermohydraulic analyses, a fuel element project, a fuel assembly mechanical project, and development of a safety analysis report to the required extent.

The project validation and accident analysis have been carried out based on the methodology applied in Russia for safety validation of VVER, Rostekhnadzor regulatory documents, and in compliance with regulatory documents applicable in the Czech Republic. In the project validation we have applied computation codes certified in Rostekhnadzor, which are currently being licensed in the Czech Nuclear Regulatory Authority.

V.L. Molchanov, V.B. Ionov and A.A. Shishkin, JSC TVEL Fuel Company, 49 Kashirskoe shosse, Moscow 115409. O.B. Samoylov, V.B. Kaydalov, A.I. Romanov and A.A. Falkov, OKBM Special Design Bureau, Burnakovsky proezd, 15, Nizhny Novgorod, 603074, Russia. I.V. Petrov, P.M. Aksyonov and E.A. Minyashkina, JSC Mashinostroitelny Zavod, Electrostal City, Russia.