2.1 Fission Products

The SiC layer of the TRISO particle provides retention to fission products both in operation and in accident. However, fission products could still penetrate the coating layer by diffusion or release caused by coating failure. Besides, uranium contamination outside the coating during manufacturing is another dominant source of raidonuclides release.

The diffusion of radionuclides can be modelled with physical laws [7], namely the conservation of mass and the Fick’s law.

The conservation of mass can be expressed with the following formula

where V is volume, S is surface area, p is volume specific generation rate, λ is decay constant, c is nuclide concentration, and j is transport flux through outer surfaces.

The diffusion flux and time-dependent change of concentration can be described with the Fick’s law [8] in spherical coordinates as shown below

where c is nuclide concentration, D is diffusion coefficient, r is position or radius, λ is decay constant, and Q is fission production rate.

At the surface of the fuel pebble, the boundary condition can be described as [7]

where r_s is radius of the pebble, β is mass transfer coefficient from the surface to the coolant, c_bl is concentration in the coolant right above the surface of the pebble, an c_cl is the mean concentration in the coolant.

The diffusion of fission products is a structure dependent process, and effective diffusion coefficients have been applied with the Fick’s law to describe the overall diffusion process [7].

In the Jülich Research Center, Germany, the STACY code [9] was developed to model the diffusion behaviour in coated particles and fuel pebbles and to estimate the release rate and fraction of fission products. The failed fuel particles are treated as bare UO₂ kernel, while uranium contamination is directly in the graphite matrix. Free evaporation assumption (β→∞) is applied to account for the release rate from fuel surface to the coolant.

2.2 Activation Products

The chemical components of coolant salt, ⁶Li, ⁷Li, and ¹⁹F could be activated by neutrons through reactions including ⁶Li(n, α)³H, ⁷Li(n, nα)³H, ¹⁹F(n, γ)²⁰F, ¹⁹F(n, α)¹⁶N, and ¹⁹F(n, p)¹⁹O. Activation products of salt include ³H and short-lived nuclides, such as ¹⁶N, ¹⁹O, and ²⁰F, which will decay and emit gammas. For example, ²⁰F emits gammas of 1.64 MeV, 2.2 MeV and 2.45 MeV [10] while 16N emits gammas of 6.13 MeV and 4.12 MeV [11]. ²⁰F and ¹⁶N have short half lives of about 10 seconds while it is about 30 s for ¹⁹O.

During manufacturing of the coolant salt, impurities including carbon, nitrogen, and oxygen could be introduced. The reactions of coolant impurities activation include ¹³C(n, γ)¹⁴C, ¹⁴N(n, p)¹⁴C, and ¹³C(n, α)¹⁴C.

The activation of those impurities could produce a certain amount of ¹⁴C, which would exist mostly in the chemical form of oxides, oxycarbides and so on [12].

4.1 Results

After 250 FPDs, calculated inventory of key radionuclides, that could be potentially released from fuel pebbles, in the fuels are listed in Table 2. The calculations were performed using TRITON and ORIGEN-S modules of SCALE 6.1 package [13].

TRITON produced problem dependent burn-up library for the test FHR, and ORIGEN-S has been used to calculate inventory and for activation analysis.

STACY codes have been used to calculate the release rate (Table 3) according to the design parameters of the test FHR. The calculated release rate is a sum of effects including diffusion, coating failure, and uranium contamination. Effective diffusion coefficients of cesium, stronium, silver, and noble gases are taken from Ref. [7, 14]. For those nuclides lacking diffusion coefficients, the data of chemically similar nuclides have been applied. For example, the diffusion coefficients of cesium was used for rubidium. Since the pebble fuel produced in China, which was originally developed for high temperature gas-cooled demonstration reactor, is planned to be used in the test FHR. For the amount of uranium contamination (5.0×10^-6) and the particle failure rate (6.0×10^-5) refer to the data of HTR-10 and HTR-PM fuel elements (Ref. [15, 16]).

Based on the results shown in Table 2 and Table 3, the activity of radionuclides released from fuel pebbles to the coolant can be calculated with Eq. (4), Eq. (5), and Eq. (6). After 250 full power days, the amount of fission products are listed in Table 4.

Based on the experiences of MSRE, 100% of noble gases, 10% of iodine, and 10% of solid fission products (such as metallic fission products) would be released from the salt [17], and the rest of the nuclides remain in the primary coolant. Noble gases are in the form of elementary gases. Iodine after released from the salt is in the form of iodine ion or I₂, and metallic fission products are in the form of the fluorides [18].

Activation products of coolant and impurities are shown in Table 5. In the calculation, the contents of nitrogen, carbon, and oxygen impurities were selected as 100 ppm wt according to reactor design requirements.

4.2 Discussions

During operation of the reactor, the activity of activation products in the primary coolant system is several orders of magnitude higher than that of fission products (Table 4 and Table 5). As shown in Fig. 2, gammas are mainly from activation products, and those short-lived activation products (16N, 20F) emitting high energy gammas should be considered in shielding design work of the test FHR. After shutdown, short-lived nuclides decay quickly and disappear shortly, while the activity of long-lived nuclides decreases slowly.

Figure 2:Full-screen View

Gamma flux spectra of radionuclides in primary coolant at the end of the refueling cycle (250 FPDs)

Among those radionuclides accumulated in the primary coolant system, the release of ³H and ¹⁴C need to be paid more attention during normal operation as they can be released to the environment and result in exposure to surrounding residents through ground-shine, immersion, and food chain. ³H production rate of the test FHR is about 0.69 TBq/MWd while it is about 0.05 TBq/MWd for CANDU 6 reactor [19]. ¹⁴C production rate of the test FHR is about 1.15×10⁷ Bq/MWd. Normalized to 3000 MWth, both ³H and ¹⁴C produced in the test FHR exceed the release limits in "Regulations for environmental radiation protection of nuclear power plant" (GB 6249-2011). Therefore, treatment of ³H and ¹⁴C should be considered.

³H originates mainly from the activation of ⁶Li and ⁷Li [20]. According to the results in Table 6, which were calculated according to the neutron spectrum of the test FHR obtained with Monte Carlo simulation, ³H production cross section of ⁶Li is about 2.0×10⁵ times higher than that of ⁷Li. Although the amount of ⁶Li in the coolant is much lower (Table 1), it is still the dominant ³H production source. ¹⁴C has several sources including the activation of ¹³C, ¹⁴N, and ¹⁷O. Since the cross section of ¹⁴N(n,p)¹⁴C is the highest, it is the dominant source of ¹⁴C.