Radiation risk analysis of tritium in PWR nuclear power plant Yang maochun (DNMC, Shenzhen, Guangdong, PRC)

Radiation risk analysis of tritium in PWR nuclear power plant 1. Radiation risk of tritium Tritium is a common radio nuclide in PWR plant existing mainly in the manner of HTO. U-235+nH-3+X+Y , yearly yielding of 560~740TBq for 1000MW unit, about 1% enter the primary circuit Li-6+nH-3+а H-2+nH-3 B-10+nH-3+2а The yearly production of the above 3 manor is about 37TBq for a 1000MWunit q its radiation risk is mainly internal exposure when ingested : Absorption from skin. Intake from month. Intake from inhalation: the main way of HTO ingestion.

Radiation risk analysis of tritium in PWR nuclear power plant In the PWR plant, HTO radiation risk exist in : The spent fuel pool (in the Fuel Building): with surface of around 106m3. Reactor pit and fuel transfer pool (in the Reactor Building): with a surface of around 150 m3. Liquid waste sumps: in isolated rooms with small surface. The reactor building during power operation In summary, the radiation risk of Tritium in RWR plant is mainly exist in the Fuel Building, and the Reactor Building during outage.

Radiation risk analysis of tritium in PWR nuclear power plant 2. Relationship between SHCA and HTO concentration in water (ATW) When water is exposed to air, there shall be a permanent exchange of water molecules between Liquid and gas phases, and tend to reach equalization. The HTO concentration in air （SHCA） shall be saturated and reach the highest value when the equalization is set up. At lower temperature, the property of vapor is similar to theoretical gas, as a result, the proportion of HTO in vapor shall be the same as in water. For respiratory intake, the DAC of HTO is 8×105 Bq/m3 . According to this principle, the relationship among SHCA , the HTO concentration in water ATW(Bq/m3) and the water temperature could be derived:

Radiation risk analysis of tritium in PWR nuclear power plant For a PWR plant, the designed highest water temperature the reactor pit and spent fuel pool is normally 50oc, the practical temperature is normally less than 30oc. At the typical temperatures, there are: SHCA 30 C = 3.04  10–5 ATW SHCA 50 C = 8.30  10–5 ATW when the water temperature is 30oc and the HTO concentration (ATW) is 3GBq/m3, the SHCA reaches 0.1DAC, when ATW is 8GBq/m3, SHCA reaches 0.3DAC, when ATW is as high as 28GBq, SHCA reaches 1DAC.

Radiation risk analysis of tritium in PWR nuclear power plant

Radiation risk analysis of tritium in PWR nuclear power plant At different ATW, for example, when ATW=4GBq/m3, the relationship between SHCA and water temperature is: SHCA increase evidently with water temperature, when water temperature is 50oc, the SHCA is 2.7 times to that of 30oc. when ATW=8GBq/m3, t=30oc, SHCA =2.43×105Bq/m3, when temperature reach 50 oc, SHCA shall reach 1DAC.

Radiation risk analysis of tritium in PWR nuclear power plant

Radiation risk analysis of tritium in PWR nuclear power plant 3. The practical radiation risk analysis of HTO Because of the following reasons, the equalization shall never be reached, so the practical HTO concentration in air shall be lower than the saturated HTO concentration. The limited water area exposed to air and the tranquilized surface. The humidity in air. The operation of ventilation system The following practical situations make the HTO radiation risk even lower: Relatively low HTO concentration in water. Relatively low water temperature (generally lower than30℃) The risk exists only in limited number of areas. The limited exposure time to the risk.

Radiation risk analysis of tritium in PWR nuclear power plant Our practical monitoring results of HTO in the air of reactor and fuel building are about 3 × 103Bq/m3 in the outage periods. According to our practical monitoring results of ATW, the ATW of spent fuel pool was around 3GBq/m3, while the reactor pit was around 6GBq/m3. If the water temperature is 30oc, the SHCA of the 2 buildings shall be 9.12×104Bq/m3 and 1.82×105Bq/m3. The practical result was around 3.23% and 1.65% of the SHCA, about 50 times lower than its SHCA . In summary, at 30 oC, the practical result shall not reach 1DAC till the ATW reaches 1.3TBq/m3.

Radiation risk analysis of tritium in PWR nuclear power plant

Radiation risk analysis of tritium in PWR nuclear power plant Our individual urine monitoring results to most of the people worked in the reactor building before ventilation started revealed that the dose from HTO exposure are lower than 5μSv, the dose from HTO exposure is lower than 1μSv by the routine urine sampling for all the people monitored. According to the practical HTO air concentration measurement, the total collective dose from HTO exposure for an outage is around 1.8mSv, which are normally less than 0.5% of the whole collective dose.

Radiation risk analysis of tritium in PWR nuclear power plant 4. Conclusion and proposals to HTO monitoring and protection There is only limited areas existing radiation risk in RWR Nuclear Power Plant, with the practical situation of ventilation , low water temperature and lower ATW, the radiation risk of HTO is quite low. For individual protection, no special protection is needed for HTO in PWR plants. For individual dose monitoring, except the monitoring for selected samples of workers, no routine monitoring is needed. For the area monitoring, except the special monitoring, no routine monitoring is needed. Maintain normal operation of the ventilation system and the spent fuel pool cooling system is needed and effective to limit HTO in air.