APR: your source for nuclear news and analysis since April 16, 2010

Saturday, March 12, 2011

FACTS: Core damage and atmospheric release

-Ever since the beginning of the Atomic Age, there have been incidents wherein fuel elements in reactors (whether research reactors, test reactors or power reactors) have failed in one way or another

-Fuel element failure describes, generally, a degradation of a fuel element either in terms of its structural integrity or its ability to contain fuel and fission products and prevent their entry into the primary coolant

-Fuel elements can fail for a variety of reasons, including stress, corrosion, and manufacturing flaws but the most highly studied type of failure results from overheating

-Overheating of a fuel element results from loss of cooling by action of the primary coolant, which in most reactors is water (which is very pure, having been filtered and deionized and deaereated) and this can occur with the reactor developing power or after the reactor is shut down since the fuel still develops decay heat

-When fuel elements are so badly overheated that melting occurs, there is the chance, however slight, that a metal-water reaction can occur that develops not only further heat but hydrogen gas as well

-There have actually been a number of reactor incidents and accidents worldwide, far more than Three Mile Island and Chernobyl that include fuel element failures; some of these and a few details are given below.

1952 NRX Reactor, Canada. 30 MW research reactor experienced fuel damage when accidental continuous rod withdrawal occurred, and no manual scram was ordered

1955 EBR-1 Reactor, USA 1.4 MW sodium cooled experimental reactor experienced fuel damage during operational tests when a planned scram was performed too late

1957 Windscale England. One of the largest reactor accidents ever occurred when this large plutonium-producing reactor experienced a Wigner energy release from the graphite moderator during attempts to anneal it. Core destroyed totally by graphite fire, with massive releases of fuel and fission products

1958 HTRE-3 32 MW experimental aircraft power plant suffered fuel damage when hydraulic failure resulted in continuous control rod withdrawal and overheating

1959 SRE Sodium Cooled Reactor Experiment experienced a blockage in cooling channels with resultant fuel overheat locally and fuel element failure

1960 WTR Westinghouse Test Reactor, Pa experienced a single fuel element failure (melted) due to coolant impurities blocking flow and fuel element bowing

1961 SL-1 3 MW test reactor operated by US Army at NRTS, Idaho Falls, destroyed by reactivity excursion after inadvertent rod withdrawal. Three killed. Plant scrapped, decontaminated as much as possible, buried.

1966 Fermi-1 Sodium cooled power reactor plant near Detroit, Michigan rated 200 MW experienced cooling channel blockage. Plant pictured below; photo from APRA files.

1979 TMI-2 Three Mile Island No. 2, a 2772 MWt pressurized water reactor, experienced a loss of cooling due to multiple equipment failures and personnel errors. Worst US Accident.

-The Chernobyl Accident, in which an RBMK-1000 boiling water, graphite moderated reactor experienced a runaway reactivity excursion, steam explosion disarranging the core and exposing it to atmosphere, and subsequent graphite moderator fire, remains the world's worst nuclear accident

-Because of not only the nature of nuclear energy and its inherent engineering challenges but also the experiences with actual fuel element failure the industry is continuously in mind to prevent it if at all possible, and to mitigate effects if it does occur, and provide cleanup and decontamination if it should release to the environs

-In a serious accident, not only Uranium that has not fissioned, and fission products like Iodine and Cesium can be released; there can also be impurities in the coolant system, such as wear products from pump bearings that are in suspension in the primary system and are radioactive

Finally, from our files a quote from AEC Pub. TID-14844. This document discusses the siting of nuclear plants, and credible accident scenarios and releases to the environment in major accidents. The following information may be generally helpful in understanding release during a major reactor accident (which has NOT repeat NOT been confirmed or supposed yet in Japan):

"In accidents of the 'maximum credible' type, it is usually assumed that the radioactive materials, along with erosion and corrosion products [AUTHOR NOTE: THESE ARE CALLED "CRUD" IN THE INDUSTRY] would be dispersed in the coolant through melting or rupture of fuel elements, and then find passage to the outer containment barrier through breaches in the coolant system. On breaching, the expansion to a larger volume and a lower pressure in the containment vessel would result in steam, in addition to the gaseous fission products, and production of vapors as well as liquid and solid aerosols of a wide range of sizes. Some ejected materials may conceivably burn on contact with air, and thus increase the volatiles and fractions of fine particles. At the same time, a certain amount of the airborne fission products would be removed by such phenomena as adsorption, deposition, plate-out and steam condensation within the reactor building or containment structure. The removal process would be complicated by conversion of normally gaseous fission products into solids by decay, and condensation of volatiles. Removal by adsorption and settling processes would be affected by turbulence. Superimposed upon these factors is the radioactive decay resulting in reduction of source strength with time by conversion to more stable isotopes. All of these factors make it difficult to determine with any exactness the radioactive content of the air which might leak out of the containment vessel."

... This applies well in a general sense to questions being asked the world over about the releases specifically from Fukushima Daiichi No. 1 and all affected plants in Japan in general if we wish to assume, purely for theoretical discussion, the presence of fuel element failure. However, it does appear now that fuel element failure or in other words a partial melt of the core at Daiichi No. 1 has actually occurred.

This article written by Will Davis for Atomic Power Review. Do not copy, or quote without credit.

8:55 PM Saturday 3/12

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