用語 せ/D

線量拘束値
dose constraint

最適化ツール

防護の最適化を達成するためのツール

どのような線量か？

予測的で着目する線源に関連した個人に対する線量（リスク拘束値として用いる場合もある（潜在被ばくに対して））

線量限度との関係

管理下にある線源全てからの曝露が線量限度を超えないようにすることを意図している。

適用状況

計画被ばく状況に適用

職業被ばく

事業者により策定され用いられる。

For occupational exposure, the dose constraint is a tool to be established and used in the optimization of protection and safety by the person or organization responsible for a facility or an activity.

IAEA GSR part3

公衆被ばく

規制当局により策定される。

医療被ばく

carers and comfortersや生物医学研究の被験者を対象とする。

導入提案例

放射線審議会（第98回）

資料第98‐5号： 獣医療法施行規則及び関連告示に係る放射線障害の防止に関する技術的基準の改正等について

最大値を使うとしている例

1 6 3 - 3 - 2 号 ：放射性物質汚染対処特別措置法の規定に基づく放射線障害の防止に関する技術的基準について（説明資料等）（環境省提出資料）第163回放射線審議会総会 令和6年10月29日

参考資料等

IAEA

IRRS

Optimization

（略）
"With regard a proposal to introduce the ICRP/IAEA concept of dose constraints into the legislation to serve as an optimization tool, the Radiation Council reports its view that the dose constraints are not necessary since the introduction of specific dose constraints for occupational exposure may hinder the flexible and optimized management of licensees’ radiation protection measures. The previously mentioned Article 3.2 of the Ordinance on Prevention of Ionizing Radiation Hazards also precludes optimization. There are no regulatory mechanisms that allow the NRA to evaluate the effectiveness of optimization actions such as access to a national dose database or occupational radiation protection benchmarking between similar nuclear power plants. There are no guidelines for the Licensees to follow as to how to implement optimization and how to evaluate optimization initiatives (see Recommendation R3)."

IRRSフォローアップミッション報告書

IRRSフォローアップミッション報告書の公開

RF5

Recommendation: The NRA should strengthen its approach to optimization, including the use of dose (or risk) constraints as appropriate, and promote consistent application of the optimization principle across all facilities and activities.

IAEA Comprehensive Report on the Safety Review of the ALPS Treated Water at TEPCO’s Fukushima Daiichi Nuclear Power Station (FDNPS)

The relevant international safety standards require the establishment of dose constraints as part of the process for optimization of protection for any planned exposure situation. For the discharge of the ALPS treated water, the establishment of a prospective and discharge-related restriction on the individual dose attributable to the discharge, provides a basic level of protection for the most highly exposed individuals due to the discharge, and serves as an upper bound on the dose in the optimization of protection for the discharge.

The IAEA notes that dose constraints are not dose limits (see fundamental safety principle 6: limitation of risk to individuals) in the international safety standards: exceeding a dose constraint does not represent non-compliance with regulatory requirements, but it could result in follow-up actions. TEPCO carried out a radiological environmental impact assessment (REIA) to estimate the dose to the representative person resulting from the discharges of ALPS treated water and to evaluate the compliance with the dose constraint of 0.05 mSv in a year. The process carried out by TEPCO for the elaboration of the REIA is consistent with relevant international safety standards. Further detailed information regarding the REIA can be found in Part 3 (Section 3.4).

After discussions between the Task Force and METI/TEPCO during the first mission, the Task Force noted that the concept of a dose constraint does not exist in Japanese law. Following discussions with the Task Force, NRA confirmed that the criterion of 0.05 mSv per year established by NRA for the discharge of ALPS treated water – which is the operational target for nuclear power stations in Japan – could be interpreted as a dose constraint. Paragraph 5.16 of GSG-9 [9] states that “in practical terms, dose constraints should be selected within the range of 0.1 to less than 1 mSv in a year”. With this in mind, the Task Force noted that the selected dose constraint of 0.05 mSv per year is below this range and is therefore conservative.

The Task Force understands why these two criteria are used by NRA and noted that the use of the two dose criteria is not an issue of consistency with the international safety standards. However, the Task Force believes that having two dose criteria, apparently both relating to the discharge of ALPS treated water but calculated in very different ways, could be a source of confusion for interested parties. Therefore, the Task Force viewed it as important for NRA to devote effort towards explaining this difference to the public to avoid unnecessary confusion.

The NRA explained that optimization of the radiation protection of workers at FDNPS is conducted using dose limits and concentration limits for radioactive materials in the air inhaled. Some Task Force members highlighted that there is no single way to implement optimization of occupational exposure and added that the approach followed by the NRA is well documented.

TEPCO benefits from the implementation of optimization of protection and the use of dose constraints for the radiation protection of workers in addition to their own long-term operational experience. TEPCO effectively utilizes safety measures such as target values, daily dose follow-ups, and work permits related to workplace characteristics (including ALPS treated water discharge facility).

The NRA described their role during the March 2022 mission in the establishment of dose limits for occupational exposure, and also in the approval of an operational safety programme (including arrangements for monitoring and recording of occupational exposures). The NRA explained that optimization of the radiation protection of workers at Fukushima Daiichi Nuclear Power Station (FDNPS) is conducted using dose limits and concentration limits for radioactive materials in the air inhaled by workers with limits prescribed by the NRA.

Report

ALPS処理水への対応での適用

線量限度を用いているところ

『敷地境界の実効線量は、海水で希釈したＡＬＰＳ処理水の排水を考慮しても基準値の１mSv/年未満となること』

ただし、IAEAの包括報告書ではこの論理は記述されていません（もっとも評価値が線量限度を上回らないことは確認されている(2.6. Limitation of Risks to Individuals)）。

The REIA provides an estimate of the committed effective dose to the representative person (for adult, child and an infant), ranging from 0.000002 (2E-06) to 0.00004 (4E-05) mSv/year. The results were well below the dose limit of 1 mSv/year for the general public and the dose constraint of 0.05 mSv/year.

線量拘束値を用いているところ

通常時のＡＬＰＳ処理水の海洋放出による人への放射線影響の評価結果は、判断基準値50μSv/年を十分に下回ること

多核種除去設備等処理水（ALPS 処理水）の海洋放出に係る放射線影響評価報告書（案）

わが国の原子力規制体系には、厳密には線量拘束値は設定されておらず、代わりに通常運転時の発電用軽水型原子炉には線量目標値として0.05mSv/年が設定されている。

このような中、2022 年 2 月 16 日、原子力規制委員会より、放射線影響評価の確認における考え方と評価の目安として、「代表的個人について、評価結果が地域や生活環境等による人の年間被ばく量の変動範囲に比べ十分に小さいものであること、すなわち 50μSv/年を下回ることを確認する。50μSv/年は、通常運転時の発電用軽水型原子炉に適用される線量目標値であり、IAEA 安全基準における線量拘束値に相当する。」との見解が示された [12]。

放射線影響評価の確認における考え方及び評価の目安, 原子力規制庁, 2022.

NRA selected a dose constraint of 0.05 mSv in a year to the representative person for the discharge of ALPS treated water. Refer to Part 3 (Section 3.1) for additional information on the relevant dose criterion set by NRA.

The relevant international safety standards require the establishment of dose constraints as part of the process for optimization of protection for any planned exposure situation. For the discharge of the ALPS treated water, the establishment of a prospective and discharge-related restriction on the individual dose attributable to the discharge, provides a basic level of protection for the most highly exposed individuals due to the discharge, and serves as an upper bound on the dose in the optimization of protection for the discharge.

After discussions between the Task Force and METI/TEPCO during the first mission, the Task Force noted that the concept of a dose constraint does not exist in Japanese law. Following discussions with the Task Force, NRA confirmed that the criterion of 0.05 mSv per year established by NRA for the discharge of ALPS treated water – which is the operational target for nuclear power stations in Japan – could be interpreted as a dose constraint.

最適化の考え方の適用例

自己遮蔽型X線装置では、線量限度の担保だけでなく、2.6 µSv/hを超えないような設計を工業会では申し合わせしているそうです。

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・シングル・ボイス
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