Reactor excursion

The purpose of this test was to ascertain the hazard that would result from a rapid reactivity insertion into a Kiwi reactor. The test provided an occasion to study radionuclide fractionation in debris from a reactor excursion. Because fractionation processes distribute hazardous radionuclides among debris particles in different manners, their effects require documentation and study. Chapter 17 by Crocker and Freiling in this volume will provide background for the reader who is unfamiliar with fractionation phenomena. 

FALLOUT (Radioactive . The term fallout generally has been used to refer to particulate mutter that is thrown into the atmosphere by a nuclear process of short time duration. Primary examples are nuclear weapon debris and effluents from a nuclear reactor excursion. The name fallout is applied both to matter that is aloll and to matter that has been deposited on the surface of the earfh. Depending on the conditions of formation, this material ranges in texture from an aerosol to granules uf considerable size. The aerodynamic principles governing tls deposition are the same as for any Other material of comparable physical nature that is thrown into the air. such as volcanic ash or particles from chimneys. Therefore, many of the principles learned in. studies of fallout from nuclear weapons can be applied lo studies of other particulate pollution in the atmosphere. 

P. Greebler, D. B. Sherer, and N. H. Walton, A Computational Program for the Analysis of Fast Reactor Excursions—FORE, GEAP-4090 (1962). 

In order to assess the degree to which the safety systems meet both these speed-of-control requirements, it is first necessary to consider the sources and mag nltudes of the reactor excursions Then, peurtlcular exciursions are examined to determine whether the criteria are indeed satisfied. 

RELAP Reactor Excursion and Leak Analysis Program... 

The first experimental breeder reactor (EBR-1), which was the first reactor to generate electricity on a practical basis, went into operation in 1951 at the National Reactor Testing Station in Idaho. After the first reactor was damaged by a power excursion, EBR-11 was put into operation in 1961 (57). As of early 1995 it continued to operate very well. 

The homogeneous reactor experiment-2 (HRE-2) was tested as a power-breeder in the late 1950s. The core contained highly enriched uranyl sulfate in heavy water and the reflector contained a slurry of thorium oxide [1314-20-1J, Th02, in D2O. The reactor thus produced fissile uranium-233 by absorption of neutrons in thorium-232 [7440-29-1J, the essentially stable single isotope of thorium. Local deposits of uranium caused reactivity excursions and intense sources of heat that melted holes in the container (18), and the project was terrninated. 

The accident at the Three Mile Island (TMI) plant in Pennsylvania in 1979 led to many safety and environmental improvements (4—6). No harm from radiation resulted to TMI workers, to the pubHc, or to the environment (7,8), although the accident caused the loss of a 2 x 10 investment. The accident at the Chernobyl plant in the Ukraine in 1986, on the other hand, caused the deaths of 31 workers from high doses of radiation, increased the chance of cancer later in life for thousands of people, and led to radioactive contamination of large areas. This latter accident was unique to Soviet-sponsored nuclear power. The Soviet-designed Chemobyl-type reactors did not have the intrinsic protection against a mnaway power excursion that is requited in the test of the world, not was there a containment building (9—11). 

Heat Release and Reactor Stability. Highly exothermic reactions, such as with phthaHc anhydride manufacture or Fischer-Tropsch synthesis, compounded with the low thermal conductivity of catalyst peUets, make fixed-bed reactors vulnerable to temperature excursions and mnaways. The larger fixed-bed reactors are more difficult to control and thus may limit the reactions to jacketed bundles of tubes with diameters under - 5 cm. The concerns may even be sufficiently large to favor the more complex but back-mixed slurry reactors. 

High reactor temperature due to failure of temperature control. Temperature excursion outside the safe operating envelope. 

Trend Reactor Temp, cattoil Ratio and Dispersion Steam Rate ChMk recent temp, and/or press, excursions Verity accuracy of VRaaotortetnp, ... 

The computer investigation can also yield a more definable relationship with fewer parameter excursions since the output will be free of scatter. In addition, excursions in reactor parameters can be taken which might be considered unsafe on or beyond the equipment limitations of an existing real reactor. 

On the other hand, the limiting conversion in a reactor of fixed size is dependent on the temperature and the radical concentration in the reactor and results from a predominating radical-radical interaction precipitated by an increased initiator concentration and the accompanying temperature excursion. At this point the solvent concentrations have little effect on the molecular... 

The reaction is exothermic and so to avoid serious temperature excursions the reactor consists of a bundle of narrow tubes, each a few centimeters in diameter, surrounded by a heat transfer medium. The catalyst consists of relatively large silver particles on an inert a-Al203 support. The surface area is below 1 m g". Promoters such as potassium and chlorine help to boost the selectivity from typically 60% for the unpromoted catalysts to around 90%, at ethylene conversion levels of the order of50%. 

Batch operation requires a larger inventory than the corresponding continuous reactor. Thus, there may be a safety incentive to change from batch to continuous operation. Alternatively, the batch operation can be changed to semibatch in which one (or more) of the reactants is added over a period. The advantage of semibatch operation is that the feed can be switched off in the event of a temperature (or pressure) excursion. This minimizes the chemical energy stored up for a subsequent exotherm. 

However, the most complex analysis is that in which heat transfer through the reactor walls is taken into account. This type of operation must be employed when it is necessary to supply or remove energy from the system so as to moderate the temperature excursions that would otherwise follow. It is frequently employed in industrial reactors and, to model such systems, one must often resort to two-dimensional models of the reactor that allow the concentration and temperature to vary in both the radial and axial directions. In the analysis of such systems, we make incremental calculations across the diameter of a given longitudinal segment of the packed bed reactor, and then proceed to repeat the process for successive longitudinal increments. 

For the reactor of Example 11-3 a high-pressure incident is expected once every 14 months. Compute the MTBC for a high-pressure excursion and a failure in the emergency shutdown device. Assume that a maintenance inspection occurs every month. 

Guide for the Identification and Control of Exothermic Chemical Reactions" (TAA-GS-05 1994). A document in German by the Technischer Ausschuss fur Anlagensicherheit (Technical Committee for Plant Safety) of the Federal Ministry of the Environment, Nature Conservation and Reactor Safety. Addresses safety assessment of reactions during both normal operations and excursions, as well as selection and extent of measures to be adopted. An English translation of this document is provided on the CD-ROM included with this publication. 

Preliminary residence time distribution studies should be conducted on the reactor to test this assumption. Although in many cases it may be desirable to increase the radial aspect ratio (possibly by crushing the catalyst), this may be difficult with highly exothermic solid-catalyzed reactions that can lead to excessive temperature excursions near the center of the bed. Carberry (1976) recommends reducing the radial aspect ratio to minimize these temperature gradients. If the velocity profile in the reactor is significantly nonuniform, the mathematical model developed here allows predictive equations such as those by Fahien and Stankovic (1979) to be easily incorporated. 

The disruption and fragmentation of the reactor core as a result of the excursion hurled all the capsules from both locations at least 500 feet away. The Pb styphnate sample which had been mounted on the pressure vessel showed evidenoe of a high-order detonation, which was assumed to be the result of the high rate of energy input from the KIWI—TNT test. This assumption was based upon several premises. Primarily, the possibility of the material detonating as a result of impact rather than by radiation did not seem reasonable, since the other sample... 

---