Iodine severe accidents

M. Furrer, R.C. Cripps, and E. Frick, Iodine Severe Accident Behavior Code IMPAIR 2, PSI Beiicht Nr. 25, Paul Scherrer Institut, SH-5232 Villigen/PSI, Switzerland. 

Cripps, R., Furrer, M., Hellmann, S., Funke, F. User s handbook for the iodine severe accident behaviour code Impair 2.2. Report PSI Nr. 114 (1992)... 

Answer Use the plant s PSA to determine the risk of accidents that include containment failure from overpressurization. Then make a preliminary design of a vented containment that has sufficiently low impedance to the gas at the pressure predicted for the most severe accident sequences such that the containment is not damaged. This containment bypass will include iodine and HEPA filters as well as scrubbers and a discharge through a stack. Estimate the dose that the population would get using this bypass for comparison with the PSA result for ruptured containment sequences. 

Beahm, E.C., Weber, C.R, Hress, T.S. and Parker, G.W (1992). Iodine Chemical Forms in LWR Sever Accidents, Final Report, NUREG/CR-5731. 

In a rather indicative way, it can be assumed that if in an uncontrolled (severe) accident X per cent of the noble gases inventory is released, the releases of iodine and of caesium may reach O.IX per cent, and the releases of other products roughly the 0.0 IX per cent. Each conceivable accident, however, has specific aspects which may strongly alter these indicative percentages, here mentioned in order to give an average measure of the natural release potential of the various isotopes. 

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). 

It is deemed realistic to ensure, by additional provisions of accident management, with a confidence limit of the order of 95 per cent, that the external iodine or caesium releases, in situations which otherwise would lead to uncontrolled severe accidents (core melt, and so on), be kept within the limit of the 0.1 per cent of the core inventory. 

A release of 40 TBq of caesium and strontium is, however, serious (when compared to the maximum acceptable releases from future European reactors, even in a severe accident, which might be expected of the order of terabecquerels of iodine-131, corresponding to fractions of terabecquerels of caesium-137). 

INSIGHTS INTO THE CONTROL OF THE RELEASE OF IODINE, CESIUM, STRONTIUM AND OTHER FISSION PRODUCTS IN THE CONTAINMENT BY SEVERE ACCIDENT MANAGEMENT... 

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. 

This report examines the severe accident sequences and radionuclide source terms at the Sizewell pressurised water reactor with a piestressed concrete containment, the Konvoi pressurized water reactor with a steel primary contaimnent, the European Pressurised water Reactor (EPR) and a boiling water reactor with a Mark 2 containment. The report concludes that the key accident sequences for European plant designs are transient events and small loss-of-coolant accidents, loss of cooling during shutdown, and containment bypass sequences. The most important chemical and transport phenomena are found to be revaporisation of volatile radionuclides from the reactor coolant system, iodine chemistry, and release paths through the plant. Additional research is recommended on release of fission products from the fuel, release of fission products from the reactor coolant system, ehemistry of iodine, and transport of radionuclide through plants. 

The technical discussions of the chemical and physical behaviour of iodine in containment presented in the previous sections of this chapter give a summary of the key processes and fectors that can influence the airborne iodine concentration during a severe accident. Based on this material, six options that could be used to limit the airborne iodine concentration are outlined below. 

A questionnaire was prepared to collect the information from the Member coimtries on the current practices on the management of gaseous iodine during a postulated severe accident. This section compiles the information provided in responses to the questionnaire. 

Weber, C.F., Kress, T.S., Beahm, E.C., Shockley, W.E., Darsh, S.R, "TRENDS A Code for Modelling Iodine Behaviour in Containment During Severe Accidents", In Proceedings of the International Cent. Heat Mass Transfer, 30,665-674 (1990). 

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). 

L.E. Herranz and F. Robledo, A Potential Strategies to Control Iodine Released into the Containment in the Case of a Severe Reactor Accident , In the Proceedings of OECD Specialist Meeting on Selected Containment Severe Accident Management Strategies,... 

Beznau 1 + II PWR (2 loops) W 1969 1130 large dry 36 380 427000 l/h In injection and recirculation phase borated water no additives no special filters for severe accidents Sulzer-wet aerosol scrubber additives sodium carbonate and sodium thiosulfate Capacity 5.2 kg/s of saturated steam DF (Aerosol) minimum 1000 DF (elementary iodine) minimum 100... 

Muhleberg BWR/4 GE 1972 1097 Mark 1 Drywell 3 100 Torus pool 2 200 Torus free volume 2 050 above the drywell floor, 25 l/s, no additives no additives no special filters for severe accidents Multi Venturi scrubber DF(Aerosol) 1000 DF (elementary iodine) 100... 

In general, it can be assumed that the reaction between silver and iodine species in the gas phase, as well as the reaction of iodine vapor with silver aerosol or with silver deposited on the primary circuit surfaces, is only of minor significance for iodine behavior in the course of a severe accident. The main reasons are the rather short residence time of the silver aerosols in the gas phase, the fact that iodine and silver volatilization from the reactor core may differ considerably over time and, finally, the small proportion of elemental I2 and of HI (compared with the Csl fraction) assumed to be present in the gas phase during transport through the primary circuit. In contrast, Agl formation is expected to proceed to a significant extent later on in the containment sump water (see Section 7.3.3.3.3.). 

Beard, A. M., Bowsher, B. R., Nichols, A. L. (a) Interaction of molecular iodine vapour with silver—indiiun—cadmium control rod aerosol. Proc. Intemat. Symposium Severe Accidents in Nuclear Power Plants, Sorrento, Ital., 1988 IAEA-SM-298/108, Vol. 2, p. 201-213... 

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). 

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. 

Beahm, E. C., Weber, C. F., Kress, T. S., Shockley, W. E., Daish, S. R. Chemistry and mass transport of iodine in containment. Proc. 2. CSNI Workshop on Iodine Chemistry in Reactor Safety, Toronto, Can., 1988 Report AECL-9923 (1989), p. 251—266 Beard, A. M., Bowsher, B. R., Nichols, A. L. Interaction of molecular iodine vapour with silver—indium—cadmium aerosol. Proc. International Symposium Severe Accidents in Nuclear Power Plants, Sorrento, Italy, 1988 IAEA-SM-298/108, Vol. 2, p. 201—213 Bell, J. T. Chemistry of iodine and cesium, in M. Silberberg (Report Coordinator) Technical Bases for Estimating Fission Product Behavior during LWR Accidents. Report NUREG-0772 (1981), Chapter 5... 

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)... 

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