Retention of radionuclides in the primary cooling system

3.6.1. Retention of fission products

The transport of fission products is dependent on their chemical and physical forms. The chemical form is determined by the chemical properties of the species; the presence of other fission products as well as structural materials and impurities; and the environmental conditions. The equilibrium chemical composition of a mixture of fission products can be readily calculated, for example, using the computer code SOLGASMIX [44]. For temperatures above about 1000 K, chemical equilibrium is achieved very quickly and the kinetics of the reactions can usually be ignored. At lower temperatures, the equilibrium takes longer to become established and the effect of chemical kinetics may need to be considered.

Accident sequences where radionuclides are released into water will be characterized by a substantially reduced or delayed release of the fission product to the containment structure.

For noble gases, it is usually assumed that 100% of the fraction released from the fuel to the primary coolant system is released into the containment structure; however, experimental data [45], data from an accident with core degradation under water [46] and results of destructive reactor tests [14] indicate that not all of the noble gas content in water is released to the atmosphere. This may be due to bubble entrainment in the primary cooling system and dissolution of non-condensables within the coolant according to Henry’s law. In the case of dissolution within the primary coolant, the dissolved gases may come out of solution and be transferred to the containment volume at a later time.

In a tank type reactor, fission products may be deposited in the piping of the primary cooling system. Volatile fission products in the form of vapours or aerosols, excluding the noble gases, will deposit on the cooling system surfaces.

The predominant mechanisms are vapour condensation and vapour chemi-sorption, and aerosol deposition by sedimentation, impact, thermophoresis, turbulent motion and Brownian diffusion. Resuspension can also occur and is influenced principally by temperature, flow, gas concentration, etc. The transport of fission products through the primary cooling system can be

analysed by modelling the transport and deposition of aerosols, as is done by computer codes such as TRAP-MELT [47].

In pool type reactors, a significant reduction of the source term can be achieved for fission product aerosols through the process of pool scrubbing. The decontamination factor due to pool scrubbing is dependent on various factors such as bubble size, aerosol particle size, steam fraction, subcooling and water depth. The scrubbing efficiency of pools also depends on the mode of gas/vapour injection, and the presence and type of non-condensable gases. Aerosol retention in subcooled water pools has been studied extensively for the following conditions: a low carrier gas flow rate, a single orifice type (~1 cm diameter) injector and a two-phase bubbly flow regime. Computer codes such as SPARC and BUSCA predict decontamination factors (DFs) under such situations quite well. In general, the DF strongly depends on the steam content of the carrier gas.

Steam condensation promotes particle removal by diffusiophoresis through the bubble–water interface. Thus, the DF increases with the mass fraction of the steam. The DF typically shows a minimum particle size in the range of 10^–1 to 1 µm. Larger particles (>1 µm) are removed mainly by centrifugal deposition, whereas smaller particles (<10^–1 µm) are retained by diffusion. An example of a calculation of radionuclide behaviour in water pools is given in Ref. [48].

The chemistry of iodine under accident conditions is complex and is still under investigation in power reactor source term studies. The iodine released from the fuel is most likely to be in the form of elemental iodine. Upon its release into gaseous media, iodine is likely to react with caesium, forming CsI, or with hydrogen, forming HI, depending on the relative timings of the releases of iodine and caesium, the temperature of the fission products and the length of time they have to react. In either case, once in the water, CsI or HI will decompose to form I^–.

Under accident conditions, iodine is known to exist in the atmosphere in several volatile chemical forms, including I₂ and organic iodine (e.g. CH₃I).

Several chemical processes may be responsible [49]. The airborne organic compounds of iodine are less likely to be affected by removal processes that are effective for aerosols or elemental iodine (e.g. plate-out or spray system removal). The process of producing volatile iodine is significantly affected by radiolysis and involves the interaction of iodine with various organic compounds (such as paints and coolant impurities) to form volatile organic species. Both these processes are enhanced at a pH of less than about 8. A number of complex models exist that allow modelling of the complex iodine chemistry in the coolant and atmosphere, which can be used for realistic best estimate assessment of iodine chemistry for research reactor accidents.

As has been mentioned, one major factor that affects the rate of volatile iodine production in the aqueous phase is the pH. Another important factor is

the rate of mass transfer between the aqueous and gaseous phases, especially for situations where pool water boiling takes place. An upper bound value can be obtained by assuming equilibrium between iodine in the steam and iodinein the liquid phase. An effective rate constant for iodine removal from the water is given by:

P_evap = (steam/V_liq × pc) (4)

where

steam is the boiling rate (m³/s);

V_liq is the volume (m³);

pc is the partition coefficient.

The partition coefficient is used as a measure of volatility and is defined as the ratio of equilibrium concentration in aqueous solution to equilibrium concentration in gas.

It follows, therefore, that small partition coefficient values are correlated with high volatility, and vice versa. Reference [50] provides data on iodine partition coefficients in water for temperatures up to about 185°C (the boiling point of iodine), which should cover most conditions of practical interest for research reactor situations. For more updated results, Ref. [51] gives an assessment of the partition coefficient for trace and high concentration solutions at high pressures.

3.6.2. Retention of other radionuclides

In general, the transuranic elements are retained in the fuel up to very high temperatures and are not usually released to the coolant (except under steam explosion type conditions).

For heavy water and liquid metal reactors, the coolant materials can be activated, forming tritiated heavy water (DTO), and ²⁴Na and ⁴²K, respectively.

These remain dissolved in the coolant but can be released into the containment atmosphere with the evaporating coolant. Activated corrosion products from reactor piping and core structures are entrained and remain in the coolant.

Irradiated materials from experimental facilities can contain fission products, in the case of a fuel test, or neutron activation products from other experimental facilities. (See Section 3.6.1 for a discussion of the retention of fission products in the coolant.) The retention of activated material from experimental facilities within the primary cooling system is case dependent.

Radioactive liquids, which are miscible in water, are generally retained within

the cooling system with some release due to evaporation. Activated gases, such as ⁴¹Ar, which is produced by the irradiation of air contained within experi-mental facilities, can be treated in a manner similar to that used for noble gases, as described in Section 3.6.1.