HTGR Knowledge Base

Conference Article: Fission-gas release during hydrolysis of exposed fuel kernels.

Richards, M.B.; Hanson, D.L.; Turner, R.F. (General Atomics, San Diego, CA (United States))

Abstract

A potential mechanism for fission-gas release from the Modular High-Temperature Gas-Cooled Reactor (MHTGR) results from the reaction of failed fuel with water vapor during hypothetical accident scenarios. During this reaction, known as fuel hydrolysis, water vapor reacts with the small fraction of exposed fuel kernels to release fission gases that have accumulated in the kernel pore space. Experimental work has been performed to obtain data for quantifying the release of fission products during fuel hydrolysis. The most recent experiments were the irradiations of capsule HRB-17 at Oak Ridge National Laboratory and capsule HFR-B1 at the Petten Establishment in the Netherlands. An analysis of the HRB-17 data has shown that the fractional release of stored fission gases is proportional to approximately the square of the H2O partial pressure over the range 200 to 2000 muatm. During MHTGR-accident scenarios, H2O) partial pressures are expected to be much higher (approx3.5 atm). During the HFR-B1 irradiation, the water-vapor injections covered an expanded range of 200 to 10,600 muatm H2O. In this paper, a preliminary evaluation of the HFR-B1 data is described. The HFR-B1 data indicate that the fractional release from failed fuel becomes independent of H2O partial pressure for the range 5000 to 10,000 muatm. This result is consistent with the Langmuir-type mechanism observed in most gas-solid reactions and is also in agreement with previous measurements on unirradiated UC2 kernels. At high H2O partial pressures, the fractional release appears to level off at 20-30% of the exposed kernel inventory for reaction times on the order of one day. For typical MHTGR-accident scenarios, the reaction times are expected to be about one hour, and the observed fractional release is less than 0.1%, or more than an order of magnitude below the current model prediction of 6% of exposed kernel inventory. To further verify the conclusions drawn from this analysis, new hydrolysis experiments should be performed using higher H2O partial pressures and additional modeling work should be performed.

view the full text of this article (5 pages, format: PDF, size= 383kB)

key words: Gas Cooled Reactor, Nuclear Technology
Reference:
Behaviour of gas cooled reactor fuel under accident conditions. Proceedings of a specialists meeting held in Oak Ridge, 5-8 November 1990
International Atomic Energy Agency, Vienna (Austria). International Working Group on Gas-Cooled Reactors
IWGGCR--25, pp:139-143