NUCLEAR FUEL FOR RBMK-1000 REACTORS

RBMK-1000 ‑ high-power channel type water-cooled graphite-moderated reactor – was developed in the USSR as an alternative for the pressurised water reactor. RBMK-1000 core comprises 1693 channels loaded with FAs. The key feature distinguishing RBMK-1000 reactor from all others is the possibility to discharge a spent FA and replace it with the fresh one without reactor shutdown. This reactor has considerable capacities for industrial-scale production of isotopes of different chemical elements.

RBMK-1000 fuel is manufactured and supplied in FAs. FAs are designed to generate thermal power and and pass it over to the coolant flow in reactor core.

During reactor operation FAs are held in fixed position. Control of the nuclear reaction, maintaining the set reactor power, power-level-to-power-level transition as well as reactor shutdown – all these actions are performed by vertical movement of RCCAs (rod cluster control assemblies) in the core.

RBMK-1000 reactors use two FA types: Working FA (WFA) and WFA for γ-chamber. Different types of FAs have some distinguishing design features.

Design of RBMK-1000 FA with burnable absorber (BA) – erbia integrated in the fuel ‑ and spacer grids made of corrosion-resistance steel possesses geometrical stability at burnups 30‑35 MWt·day/kgU, ensures high safety margin as well as good economic indicators for RBMK reactor cores. Typically RВMK FAs use reprocessed fuel.

RMBK FA is comprised of two FR bundles, two tailpieces, central rod with a bar (for WFAs) or a supporting tube with central void for sensors accomodation (WFAs for γ-chamber), fasteners and retainers.

The upper FR bundle in an FA is connected with the lower FR bundle either by the central rod with a bar or by the supporting tube and the appropriate fasteners. Total length of an RBMK FA equals 10 m including meat (7 m); in cross section FA is of circular shape of Ø 79 mm; one FA weighs approximately 185 kg. RBMK FAs are shroudless.

An FA bundle consists of 18 FRs, a skeleton complete with spacer grids as well as 18 clamping rings designed to secure FRs in the end grid.

FRs are the key functional elements of FAs: one of the rod ends is fixed in the end grid, while the other stays loose. By design FRs are tubes made of zirconium alloy loaded with pellets manufactured from sintered uranium dioxide-erbia powder, hermetically sealed with end plugs by welding. The use of FRs with erbia integrated in the fuel made it possible to improve power distribution across the reactor, increase safety margin as well as technical-and-economic indicators of RBMK reactor cores.

Specifications

Core	RBMK-1000
Power, MWt:
- electric	1000
- thermal	3200
Number of fuel channels, pieces	1693
Coolant	steam-water mixture
Steam parameters upstream from turbine:
- pressure, MPa	6.38
- temperature ,°С	280
Coolant temperature:
- at core inlet	270
- at core outlet	290
Water flow rate across reactor, t/h	37500
FA specifications
Maximum channel power with FAs, MWt	3.3
FA dimensions:
- length, mm	10014 (WFA) 10033 (WFA for γ-chamber)
- diameter, mm	79
FA mass, kg	185
U enrichment by U235, %	2.8
Number of FRs per FA, pieces	36
Average burnup, MWt·day/kgU	30
Maximum burnup, MWt·day/kgU	34.5

The upgraded new-generation RBMK-1000 FA designes have been developed, launched into manufacture and currently undergo test operation. These upgraded designs allow a more efficient use of the existing NPP facilities, they also give higher perfomance and reliability indicators; that makes it possible to increase an FA operating period inside the core up to 8 years.

Upgraded FAs:

- FA with centrally fixed FRs;

- FAs with axial blankets (fuel and burnable absorber);

- FA with tailpiece – debris filter.