HTGR Knowledge Base

Conference Article: Experimental investigations of reactivity effects caused by water ingress in a LEU-LHTR pebble-bed core

Abstract

A series of new critical experiments, in the framework of an IAEA Coordinated Research Programme on ''Validation of Safety Related Reactor Physics Calculations for Low Enriched HTRs'', is being carried out at the zero power facility, PROTEUS. The main aim of the experiments is the generation of safety related physics data to reduce the design and licensing uncertainties for small- and medium-sized, highly reflected, helium cooled reactors. The main parameters being investigated in the programme are critical masses, streaming effects, and changes in reactivity, neutron balance components and control rod effectiveness caused by water ingress into the reactor. Different deterministic pebble loading arrangements are being investigated in PROTEUS. This allows access to the core centre and the insertion of polyethylene rods for the simulation of water ingress. LEU fuel elements (German AVR type) of 16.7% ²³⁵U enrichment and graphite elements, are being used to cover a range of moderator-to-fuel ratios. A first water ingress experiment was carried out with small amounts of plastic in HTR-PROTEUS Core 1 (an under-moderated close-packed hexagonal lattice with a carbon-to-uranium atom ratio of about 960). The reactivity increase in this case was small enough to be compensated by movement of the control rods. Good agreement was obtained between measured and calculated values. Investigations in progress include a case with a much larger amount of plastic in this same type of core with criticality maintained by reduction of the core height. Experiments with plastic contents up to the amount corresponding to a maximal reactivity increase are planned in subsequent HTR-PROTEUS cores (a different core lattice arrangement is required to allow the introduction of the required amount of plastic in the form of rods).

view the full text of this article (7 pages, format: PDF, size= 1182kB)

Reference:

Technical committee meeting on response of fuel, fuel elements and gas cooled reactor cores under accidental air or water ingress conditions. Beijing (China). 25-27 Oct 1993

International Atomic Energy Agency, Vienna (Austria)

IAEA-TECDOC--784, pp:44-50