Bq/year

Fiscal year

Figure 6.8.6 Trends in discharged radioactivity of radioactive liquid wastes (excluding tritium) (total discharge for each reactor type)

Fiscal year

Figure 6.8.7 Trends in discharge of liquid tritium (for each reactor type)

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Chapter 6 Radiation Control Radioactive Gaseous Wastes Gaseous radioactive wastes are classified, as shown in Table 6.8.2, into waste arising from nuclear fission, such as radioactive noble gases and volatile iodine, and waste containing a small amount of radioactive particulates like exhaust gas from the ventilation system of the reactor building etc. Treatment methods are also listed in the table. Table 6.8.2 Treatment methods of radioactive gaseous wastes according to their natures Classification	Composition	Treatment method
Radioactive noble gases (inert gases)	Xenon, krypton, etc.	Passed through activated carbon for temporary adsorption or temporarily stored in a tank for attenuation
Other gases	13N, I6N
Volatile materials	Iodine, fluoride	Adsorbed onto activated carbon
Particulates	Cobalt, iron etc. (corrosion product particles)	Filtered with HEPA filters

Figure 6.8.8 shows the trends for discharged amount of radioactive noble gases.

Pin-hole defects of fuel occurred in the early days, from which radioactive noble gases and iodine migrated into the reactor water and were discharged from the exhaust stack, but the amount of the discharge has decreased since then as a result of improvements in fuel cladding tubes and pellets, and operation practice (PCIOMR). BWRs and PWRs together discharged radioactive noble gases in the amount exceeding 10¹³ Bq/year in the 1970s, but for BWRs, as a result of effective countermeasures, such as activated carbon filters (charcoal beds) installed to attenuate the radioactivity of the gases extracted from the turbines and purification of the turbine gland seal, the amount of the discharge has decreased year after year, and nowadays, there are many plants without any detectable discharge throughout the year.*³

Figure 6.8.8 Trends in the radioactive noble gases discharged (for each reactor type)

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Figure 6.8.9 shows the yearly trends of radioactive iodine C^3II) discharge.

In recent years, similar to the case of radioactive noble gases, improvements of fuel integrity have resulted in low level discharge for both BWRs and PWRs, and there are many plants without detectable discharge throughout the year.

For discharge of radioactive gaseous waste to the environment, the discharge is controlled so as not to exceed annual discharge control targets as well as the concentration limits provided by law in the same way as for radioactive liquid waste.

Fiscal year

Figure 6.8.9 Trends in the radioactive iodine (^{131 *}l) discharge (for each reactor type)

⁽*³⁾ Generally in PWRs, after holding reactor-generated

gases extracted during operation for 30 days for radioactive decay, the gases are discharged to the environment; they mainly contain nuclides (¹³³Xe, ¹³⁸Xe, ⁸⁵Kr) and comparatively weak y-ray emitters, that remain in the course of radioactive decay. Their measurement is performed with an ionization chamber with high /7-ray sensitivity (Nal (Tl) scintillator for

y rays in BWRs), etc., and as the air flow rate at the exhaust stack of PWRs is approximately one half of that of BWRs, the dilution rate is low. Due to these differences, the discharged radioactivity concentration collected and measured at the exhaust stack is evaluated to be higher for PWRs, even if the amount of leakage from fuel is the same as that of BWRs.

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