Fission products accidents

Safety. A large inventory of radioactive fission products is present in any reactor fuel where the reactor has been operated for times on the order of months. In steady state, radioactive decay heat amounts to about 5% of fission heat, and continues after a reactor is shut down. If cooling is not provided, decay heat can melt fuel rods, causing release of the contents. Protection against a loss-of-coolant accident (LOCA), eg, a primary coolant pipe break, is required. Power reactors have an emergency core cooling system (ECCS) that comes into play upon initiation of a LOCA. 

Rodgers, S. J., Udaveak, R. J. and Mausteller, J. W. In International Symposium of Fission Product Release and Transport under Accident Conditions, Oak Ridge National Laboratory, TN, USA, 1965, pp. 1204 1215. 

Code of Federal Regulations (lOCFR) part 100 provides reactor siting criteria. It specifies that the fission product release calculated for major hypothetical accidents shall produce a w hole... 

Cooling water - The cooling water, that provides cooling and moderation, also retains fission products - e.specially the chemically active semivolatiles, as demonstrated at the TMI-2 accident. 

Guides 1.3 and 1.4 which were used by the RSS. The RSS presented results of accident progression and fission products in release categories associated with various plant damage states. 

An accident sequence source term requires calculating temperatures, pressures, and fluid flow rates in the reactor coolant system and the containment to determine the chemical environment to which fission products are exposed to determine the rates of fission product release and deposition and to assess the performance of the containment. All of these features are addressed in the... 

Of these phenomena, the first three in particular, involve thermal hydraulics beginning with the pre-accident conditions. Items 4 through 7 address the meltdown of the core and its influence on (1) hydrogen production, which affects containment loads, (2) fuel temperatures, which affect in-vessel fission product releases, (3) thermal-... 

This section reflects on the limitations of the PSA process and draws extensively from NUREG-1050. These subjects are discussed as plant modeling and evaluation, data, human errors, accident processes, containment, fission product transport, consequence analysis, external events, and a perspective on the meaning of risk. 

The inventory of long-lived fission products is far less than an LWR because of the short exposure of the fuel to minimize Pu-240 production. But the health effects from an accident are comparable because it primarily results from short lived radionuclides. 

Technical Bases for Estimating Fission Product Behavior during LWR Accidents, June... 

In considering the operational safety and accident analyses of sodium-cooled fast reactors, similar information on the release of fission products from sodium is needed. Although the extent of vaporization can often be calculated from thermodynamic considerations (3, 4), appropriate transport models are required to describe the rate phenomena. In this chapter the results of an analytical and experimental investigation of cesium transport from sodium into flowing inert gases are presented. The limiting case of maximum release is also considered. 

The third principal component of environmental radioactivity is that due to the activities of humans, the anthropogenic radionuclides. This group of nuclides includes the previously discussed cases of 3H and 14C along with the fission products and the transuranium elements. The primary sources of these nuclides are nuclear weapons tests and nuclear power plant accidents. These events and the gross nuclide releases associated with them are shown in Table 3.1. Except for 14C and... 

Table 2.3. Release of fission products in weapons tests and accidents...

It is convenient to consider reactor accidents alongside weapon explosions so that the release of fission products can be compared, but the mode of dispersion is quite different. The configuration and thermal capacity of power reactors are such that bomb-like explosions are not possible. In the Chernobyl accident, nuclear overheating, a steam explosion and steam/zirconium reactions all contributed to the disruption of the reactor (U.S.S.R. State Committee, 1986), but the longdistance environmental effects were due to the subsequent releases of fission products from the damaged reactor. 

The total activity on the stack filter is not known accurately (Chamberlain, 1981), but its collection efficiency was not high, which is not surprising since it was not designed for the conditions which prevailed in the accident. The filter was certainly not responsible for the separation of volatile from refractory fission products. Most of the latter, together with the uranium and plutonium, remained in the fuel channels. 

Emission of fission products and other activities during the accident to Windscale Pile No. 1 in October 1957. AERE Report M-3194. Harwell, Oxon. 

Osborne, M.F., Collins, T.L., Lorenz, R.A. Strain, R.V. (1986) Fission product release and fuel behaviour in tests of LWR fuel under accident conditions. In Source Term Evaluation for Accident Conditions, IAEA, Vienna, pp. 89-104. 

Ionizing Radiation Sources and Biological Effects. New York, United Nations. U.S. Nuclear Regulatory Commission (1981) Technical basis for estimating fission product behaviour during LWR accidents. Report NUREG-0772, Washington, D.C. 

The release of 131I and other fission products in reactor accidents has been considered in the previous chapter. In the Windscale accident, the temperature in the fire zone reached an estimated 1300°C and 8 tonne of uranium metal melted. Over 25% of the 1311 in the melted fuel escaped to atmosphere. In the Chernobyl accident, the fuel was U02, the temperature exceeded 2000°C, and about 25% of the total reactor inventory of 131I was released to atmosphere, as vapour or particulate aerosol. In the Three Mile Island accident, 131I remained almost completely in the reactor coolant. The activities of 131I released in reactor accidents, including that at Chernobyl, have totalled much less than the activities released from weapons tests (Table 2.3). 

Fig. 3.8. Gamma dose rate from deposited fission products, normalised to initial deposit of 1 Bq m-2 of 131I. A, Instantaneous fission products B, reactor fission products C, volatile reactor fission products D, 131I only E, as measured at Munich after Chernobyl accident.

Public interest in radioactive aerosols began in the mid-1950s, when world-wide fallout of fission products from bomb tests was first observed. The H-bomb test at Bikini Atoll in 1954 had tragic consequences for the Japanese fisherman, and the inhabitants of the Ronge-lap Atoll, who were in the path of the fallout. In 1957, radio-iodine and other fission products, released in the accident to the Windscale reactor, were tracked over much of Europe, and these events were repeated on a much larger scale after the Chernobyl accident. 

Despite the safety regulations, accidents have occurred with nuclear reactors and reprocessing plants, primarily due to mistakes of the operators. By these accidents parts of the radioactive inventory have entered the environment. Mainly gaseous fission products and aerosols have been emitted, but solutions have also been given off. In the Chernobyl accident, gaseous fission products and aerosols were transported through the air over large distances. Even molten particles from the reactor core were carried with the air over distances of several hundred kilometres. 

Fission products of uranium and other actinides are released to the environment during weapons production and testing, and by nuclear accidents. Because of their relatively short half-lives, they commonly account for a large fraction of the activity in radioactive waste for the first several hundred years. Important fission products are shown in Table 3. Many of these have very short half-lives and do not represent a long-term hazard in the environment, but they do constitute a significant fraction of the total released in a nuclear accident. Only radionuclides with half-lives of several years or longer represent a persistent environmental or disposal problem. Of primary interest are °Sr, Tc, and... 

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