Iodine severe accident conditions

J.M. Ball, et Al., INTERNATIONAL STANDARD PROBLEM (ISP) NO. 41 Computer Code Exercise Based on a Radioiodine Test Facility (RTF) E5q)eriment on Iodine Behaviour in Containment under Severe Accident Conditions , In Proceedings of OECD Workshop on Iodine Aspects of Severe Accident Management, NEA/CSNI/R(99)7, Vantaa, Finland, 1999. 

E. Krausmann, Y. Drossinos, A model of silver-iodine reactions in a light water reactor containment sump under severe accident conditions , J. Nuclear Materials, 264, 113, (1999). 

One of the most important parameters controlling iodine volatility is sump water pH not only will the I2 hydrolysis equilibrium and the iodine partition coefficient be affected by this parameter, but the product yields of radiolytic reactions and the extent of formation of organoiodine compounds as well. Because of the lack of practical experience, the sump water pH to be expected under severe accident conditions has to be calculated on the basis of assumed concentrations of potential sump water ingredients. In Table 7.17. (according to Beahm et al., 1992) an overview of substances to be expected in the sump water, which would effect a shift in solution pH either to lower or to higher values, is given. Besides these chemical substances, radiation may also affect sump water pH irradiation of trisodium phosphate solution (5.3 kGy/h) was reported to decrease the pH from an initial value of 9.0 to about 4.0 after 60 hours of irradiation (Beahm et al., 1992). It is obvious that in such a complicated system definition of the sump water pH to be expected in a real severe reactor accident is a difficult task. Nonetheless, a model for calculation has been developed by Weber et al. (1992). 

Funke, F., Greger, G.-U., Hellmann, S., Bleier, A., Morell, W. lodine/steel reactions under severe accident conditions in LWRs. Proc. 3. Internal. Conf. on Contaiiunent Design and Operation, Toronto, Ontario, Can, 1994, V0I.I Funke, F., Hellmann, S. Reaction of iodine with steel surfaces. Final Report—Part 1 Literature study. Report EUR 15668/1 EN (1994)... 

Richter, R., Neeb, K. H. Laboratory-scale and technical-scale investigations concerning iodine water-vapour phase partitioning under severe accident conditions. Proc. OECD Specialists Workshop on Iodine Chemistry in Reactor Safety, Harwell, UK, 1985 Report AERE-R-11974 (1986), p. 209-223... 

The studies of this period led to a definition of severe accident protection criteria (see Section 1-2 and Chapter 18) similar to those already in force in Italy and to those developed in Sweden. In Italy, it was thought possible to provide a defence against severe accidents by accident management provisions and by some reasonable plant modification, up to the point of limiting iodine and caesium releases to 0.1 per cent with a probability higher than 95 per cent in the case of core melt (conditioned probability). 

Early reactor safety assessments [S-1] hypothesised that severe accidents would entail the prompt release of a significant fraction of a bounding radionuclide (t5q)ically iodine) to the reactor containment. Safety systems were designed, then, for massive, immediate response to this release. Now, it is understood that radionuclide releases will take place by multiple processes over protracted periods and will involve many different radionuclides in different chemical and physical forms. Mitigation methods will have to operate for long periods and may have to change as the sources of radionuclides vary. The inventories of radionuclides available for release from reactor fuel under accident conditions and the processes that lead to releases of these radionuclides are discussed in the next subsections of this report. 

S. Hellmaim, F. Funke, G-U. Gieger, A. Bleier and W. Morell. The Reaction Between Iodine and Organic Coatings Under Severe PWR Accident Conditions - An Experimental Parameter Study , In Proceedings of the Fourth CSNI Workshop on the Chemistry of Iodine in Reactor Safety, Wiirenlingen, Switzerland, 1996, p. 345 (1997). 

Collins, J. L., Osborne, M. F, Lorenz, R. A. Fission product tellurium release behavior under severe light water reactor accident conditions. Nucl. Technology 77, 18-31 (1987) Collins, J. L., Osborne, M. F., Lorenz, R. A., Malinauskas, A. R Fission product iodine and cesium behavior under light water reactor accident conditions. Nucl. Technology 81, 78-94 (1988)... 

Long after the onset of a severe accident (more than about 3 weeks), gas-phase iodine is expected to be dominated by organic iodide, with a small contribution from I2 the conclusions drawn from the TMI-2 accident are highly consistent with these results of model calculations. Iodine behavior and distribution, in the long run, are expected to have little relationship to the chemical forms or amounts released into the containment, because the iodine will have had enough time to deposit onto surfaces or in water pools, so that the environmental conditions in the containment will prevail in determining the chemical forms. 

It is probably not very far from reality to assume that under steady-state conditions the volatile iodine in the annuli atmosphere would consist of 50% each of I2 and CH3I, and that this ratio would be established within about 10 hours. It is important to note that errors in these estimates do not seriously affect the evaluation of the accident consequences, since, on the one hand, I2 is retained in the emergency filters with high efficiency, whereas, on the other, the radiotoxicity attributed to organoiodine compounds (which are retained in the filters to a lesser degree) is significantly lower than that of I2. In addition, calculations have shown that the consequences of a severe accident in the environment will be determined by the released aerosol iodide. 

Because of these differences in the conditions compared with an LWR severe accident, and because of the limited number of measurements performed, the data obtained here have not been introduced into the evaluations of the consequences of severe light water reactor accidents, so that the high degree of retention of fission product iodine within the plant which was observed in the TMI-2 accident turned out to be an unexpected and surprising fact for the accident analysts. 

J. L. Collins, M. F. Osborne, R. A. Lorenz, and A P. Malinauskas, "Fission Product Iodine and Cesium Release Behavior Under Severe LWR Accident Conditions," Nucl TechnoL 81(10), 78-94 (1987). 

Under extreme filtration conditions (temperature >150°C), impregnated charcoals suffer from an increasing desorption of radioiodine. For such severe conditions other iodine filter materials have been developed such as silver-containing inorganic sorbents which exhibit a very low desorption of iodine even under extreme conditions, since the adsorbed iodine is bound as the very stable chemical compound Agl. However, such conditions are far more severe than those to be expected in design basis accidents there is no need, therefore, to replace the impregnated charcoals by these expensive materials. 

Tellurium exhibits a chemical behavior which is nearly as complex as that of iodine, but only little is known about its reactions under the conditions prevailing in the primary system during the course of a severe reactor accident. As was discussed in Section 7.3.1.1., the formation of temperature-stable Zr-Te compounds retains tellurium for a certain period of time and results in a significant release from the reactor core only after extensive oxidation of the Zircaloy cladding. According to... 

Further, the impact of the pH of the aqueous solution, as well as of the temperature and the iodine concentration, on the yields of the different iodine species in the equilibrium state have been widely investigated. However, the results of these classic chemical investigations proved to be not sufficiently detailed with regard to the range of ambient conditions that have to be taken into consideration in the course of a severe reactor accident. 

In recent years, several attempts have been made to estimate the magnitude of the airborne iodine fraction in the contaimnent in a highly simplified manner, mostly based on the results of laboratory experiments. But because of the multitude of potentially influencing parameters, no well-founded results can be expected from such a procedure. On the contrary, the relative significance of the individual parameters for the specific conditions of the accident sequence under consideration has to be evaluated in detail. 

---