0NRA presentation
- C Authorization Process
C-2-3 REIA – Potential exposure
OTSUJI YAMAMOTO Ayako
Nuclear Regulation Authority JAPAN
IAEA Regulatory Review Mission on ALPS Treated Water Handling
23 March 2022
1. NRA’s view on potential exposure assessment
2. Review on potential exposure assessment
2-1. Potential exposure scenarios
2-2. Source term
2-3. Exposure pathways
2-4. The representative person
2-5. Comparison of estimated doses and risks with criteria
Contents12
1. NRA’s view on potential exposure assessment31. NRA’s view on potential exposure assessment
Source: IAEA Safety Standards GSG-10 "Prospective Radiological Environmental Impact Assessment for Facilities and Activities"
Identification and Selection of Potential exposure
scenarios
• Safety assessment is conducted by TEPCO in
"validity assessment of facility design upon
unusual occurrences" assuming a single failure of
the component.
• The assessed unintended discharge amount is
approximately 1.1m3, greatly smaller than the
planned discharge amount per day (up to
500m3/day).
• In addition, safety assessment to identify
potential exposure scenarios should be done
assuming failures more than a single failure or
external hazards.41. NRA’s view on potential exposure assessment
Approach to identification and selection of potential exposure scenarios
• No event progression is expected for the Discharge Facility, except continuation of
unintended discharge.
Deterministic approach is sufficient.
• Not usual affiliated facility for nuclear reactor, but rather unique facility for this
discharge
The likelihood does not have to be determined, and the impact of the identified
event without the likelihood should be compared with the criteria 5mSv/event. The
NRA has set this criteria referring to the value specified for a simple assessment with
small inventories in GSG-10.52.Review on potential exposure assessment62. Review on potential exposure assessment
Major points to be clarified by TEPCO
• Concept of the assessment which does not follow the flow shown in Fig. 3 of GSG-10Comments from the NRA for further clarification
• Consideration on possible internal exposure assuming the situations that unusual
occurrences continue without being detected or countermeasures are delayed
TEPCO explained the revised assessment at the NRA Review Meeting on 18 March 2022.72-1. Potential exposure scenarios
During the review meeting on March 18, TEPCO explained:
• Identified scenario is:
ALPS treated water discharge without mixing with seawater, directly to the
ocean through pipe, assuming seawater intake pumps are stopped and
emergency isolation valves are not closed.
The whole amount of one group of K4 tanks, approx. 10,000m3 ALPS treated
water is discharged within 2 days.
Points to be further clarified
• The postulated scenario to derive the amount of discharge, approx.10,000m3, is
unclear.82-1. Source term
During the review meeting on March 18, TEPCO explained:
• The ratio of the nuclides other than tritium contained in ALPS treated water varies.
Calculation assuming each of 63 nuclides is contained up to concentration limit
corresponding to the operational limit* (*the ratios of the radionuclides other than tritium
to each concentration limit stipulated in the regulation is confirmed to be less than 1)
The largest impact among 63 nuclides is regarded as the assessment result, i.e.,
the assessment result of Te-127 which is the biggest contributer to external
exposure.
Points to be further clarified
• The assumption that 63 nuclides are represented by Te-127 is extreme considering
its half-life period.
• To be based on more realistic assumptions and then consider uncertainties92-3. Exposure pathways
During the review meeting on March 18, TEPCO explained:
• In addition to one exposure pathway in the initial REIA (external exposure from
the sea surface) , adding the other exposure pathways to be on a conservative
side:
External exposure during underwater work
Internal exposure during underwater work
External exposure from ship hull, beach sand and fishing net
Internal exposure due to intake of seafood
Points to be further clarified
• None102-4. The representative person
During the review meeting on March 18, TEPCO explained:
• The representative person is identified as a shipman exposed to radiation through
all the exposure pathways identified.
• The end point: 2days later considering emergency protective actions
Points to be further clarified
• None
Source: TEPCO’s material at the review meeting on 2022年03月14日
Evaluation
point112-5. Comparison of estimated doses and risks with criteria
During the review meeting on March 18, TEPCO explained:
• Effective dose is calculated using the largest concentration value within 1km from
the discharge point: 2.4E-10Bq/L
• External exposure is larger than internal exposure. Among 63 nuclides, Te-127
gives the biggest impact to external exposure.
• Assessment result with Te-127: 0.26mSv/event < the criteria 5mSv/event
Points to be further clarified
• The assumption that 63 nuclides are represented by Te-127 is extreme considering
its half-life period.
• To be based on more realistic assumptions and then consider uncertainties