NRC is (sort of) getting rid of “as low as reasonably achievable” standard

美国核管理委员会（NRC）正（某种程度上）废除“合理可行尽量低”（ALARA）标准

Last week, just before the US started its break for the July Fourth holiday, the Nuclear Regulatory Commission (NRC) proposed a new rule that would change how it regulated exposure to radiation. The Trump administration has been pushing to restart construction of nuclear power plants in the US, and many pro-nuclear advocates have been complaining about the US’s existing regulations, portraying them as the main barrier to the flourishing of the industry. So, it had seemed likely that major revisions were coming. Instead, the NRC’s proposed new rules endorse the science behind its current rules and suggest that any problems are largely in the vagueness of the terminology that it has been using. So, instead, it’s endorsing standards that are meant to accomplish the same thing, but avoid using some of the language it had relied on. Probably the clearest indication of the evolutionary change at play is that the NRC estimates the changing rules will save industry—not just power, but also medical and research applications—only about $9.5 million a year.

上周，就在美国开始庆祝独立日假期之前，美国核管理委员会（NRC）提出了一项新规，旨在改变其对辐射暴露的监管方式。特朗普政府一直致力于重启美国核电站的建设，许多拥核倡导者也一直在抱怨美国现行的监管规定，将其描述为行业发展的最大障碍。因此，外界曾认为重大修订即将到来。然而，NRC 提出的新规实际上认可了其现行规则背后的科学依据，并指出目前存在的问题主要源于所用术语的模糊性。因此，该委员会转而支持那些旨在实现相同目标、但避免使用其以往所依赖的某些措辞的标准。此次变革最明显的迹象或许在于，NRC 预计这些新规每年仅能为行业（不仅是电力，还包括医疗和研究应用）节省约 950 万美元。

LNT and ALARA

LNT 与 ALARA

There are two technical abbreviations at the center of US nuclear regulations. The first is LNT, which stands for “linear non-threshold.” It’s in reference to the issue of whether there’s any level of radiation that is so low that it no longer produces harmful biological effects—the “threshold” in LNT. The “non-threshold” implies that it doesn’t, and that’s in keeping with biology, which has demonstrated that even single particles or photons of radiation can damage DNA and that the mechanisms cells have for repairing that damage are inherently error-prone. The “linear” in LNT simply describes how the impact of radiation scales directly with the dose.

美国核监管的核心有两个技术缩写。第一个是 LNT，即“线性无阈值模型”。它涉及的问题是：是否存在一个足够低的辐射水平，使其不再产生有害的生物效应——即 LNT 中的“阈值”。“无阈值”意味着不存在这样的水平，这与生物学研究相符，因为研究表明，即使是单个粒子或光子辐射也可能损伤 DNA，而细胞修复这种损伤的机制本身就容易出错。LNT 中的“线性”则简单地描述了辐射影响如何随剂量直接成比例变化。

Despite the solid foundation in basic biology, LNT has been difficult to demonstrate in the real world. Humans are exposed to many factors that can influence the development of cancer, including naturally occurring radiation. Teasing out the impact of a small dose of radiation that occurs in addition to all those other exposures is extremely challenging, and the impact of extremely low exposures has not been decisively demonstrated. Complicating matters, a number of people have advocated for something called hormesis, in which small doses of radiation purportedly promote the cellular repair of damage from other sources. The evidence for this is even spottier, and when the NRC was petitioned to adopt hormesis as part of its scientific framework during Trump’s first term, it rejected the petition.

尽管在基础生物学上有坚实的依据，但 LNT 在现实世界中很难得到证实。人类暴露在许多可能影响癌症发展的因素中，包括自然界存在的辐射。要从所有这些其他暴露因素中剥离出小剂量辐射的影响极具挑战性，且极低剂量暴露的影响尚未得到确凿证明。更复杂的是，一些人主张所谓的“毒物兴奋效应”（hormesis），即声称小剂量辐射可以促进细胞修复来自其他来源的损伤。这一理论的证据更为零散，当特朗普第一任期内有人请愿要求 NRC 将毒物兴奋效应纳入其科学框架时，NRC 拒绝了该请愿。

Given its acceptance of an LNT model of exposure risks, the NRC had chosen exposure standards that fell under the general term of ALARA: as low as reasonably achievable. If any exposure to radiation poses a risk, then minimizing it is the clearest way to protect the health of people who work with radioactive substances. The challenge there is that it’s possible to set exposure limits that people outside the industry regularly exceed each time they board a commercial aircraft. So, the word “reasonable” plays an outsized role while remaining highly subjective. Critics have charged that it precipitates an endless cycle of reasonable exposure limits leading to searches for additional ways to lower them further, or of adoption without cost considerations.

鉴于 NRC 接受了 LNT 辐射风险模型，它选择了归类于 ALARA（合理可行尽量低）这一总称下的暴露标准。如果任何辐射暴露都存在风险，那么将其降至最低就是保护放射性物质从业人员健康的最明确途径。其中的挑战在于，所设定的暴露限值可能低于人们在乘坐商业航班时经常受到的辐射量。因此，“合理”一词发挥了过大的作用，却又极具主观性。批评者指责称，这导致了一个无休止的循环：合理的暴露限值会引发对进一步降低限值的追求，或者在不考虑成本的情况下盲目采用更严苛的标准。

And here, the NRC is acknowledging that there have been issues. “In essence, the reasonableness test that is supposed to be inherent to ALARA-related decision-making has gradually become an expectation that if a means of dose reduction is available, regardless of its reasonableness in relation to the total dose and the amount of reduction, it should be applied without further consideration,” its new proposal suggests. In the past, the NRC has attempted to address this by attaching a financial value to each unit of exposure based on estimates of the value of healthy life developed elsewhere. But in the new proposal, it accepts that “there have been challenges in the implementation of the ALARA requirement, namely a lack of clarity of when dose reduction is deemed sufficient, excessive subjectivity, and susceptibility for selective or inconsistent enforcement.” So, it’s giving up on a term that it now views as a source of confusion.

在此，NRC 承认确实存在问题。其新提案指出：“本质上，本应作为 ALARA 相关决策核心的‘合理性测试’，已逐渐演变成一种预期：只要有减少剂量的手段，无论其相对于总剂量和减少量是否合理，都应不加考虑地予以应用。”过去，NRC 曾试图通过根据其他机构估算的健康生命价值，为每个单位的暴露量赋予财务价值来解决这一问题。但在新提案中，它承认“在执行 ALARA 要求方面存在挑战，即对于何时认为剂量减少已足够缺乏明确性，存在过度的主观性，且容易导致选择性或不一致的执法。”因此，它决定放弃这一现已被视为混乱根源的术语。

What’s different

有何不同

One of the key things here is that the LNT model of exposure risks isn’t going away. In its earlier denial of petitions that it change its standards, the NRC had concluded that “in the absence of convincing evidence that there is a dose threshold or that the health effects of low levels of radiation are fully understood, the LNT model for cancers and genetic effects was appropriate for formulating radiation protection standards and planning radiation protection programs.” In the proposed new rules, that logic is left intact. “The NRC finds that no consensus-supported, regulation-ready alternative model to the LNT model exists at this time,” it states. More specifically, it states, “It is unlikely there might be a threshold level of exposure below which biological response does not occur. Such a threshold could only occur if DNA repair processes were totally effective in that dose range or if a single radiation track were unable to produce an effect.”

关键点之一是，LNT 辐射风险模型并未被废除。在早先拒绝修改标准的请愿时，NRC 曾得出结论：“在缺乏令人信服的证据证明存在剂量阈值，或对低水平辐射的健康影响尚未完全了解的情况下，用于癌症和遗传效应的 LNT 模型对于制定辐射防护标准和规划辐射防护计划是合适的。”在拟议的新规中，这一逻辑保持不变。委员会表示：“NRC 认为，目前尚不存在任何得到共识支持且可用于监管的 LNT 模型替代方案。”更具体地说，它指出：“不太可能存在一个低于该水平就不会产生生物反应的暴露阈值。只有在 DNA 修复过程在该剂量范围内完全有效，或者单个辐射轨迹无法产生影响的情况下，才可能存在这样的阈值。”

It’s worth noting that the NRC is making that decision despite the fact that Trump issued an executive order that describes LNT and then calls it irrational. “The NRC utilizes safety models that posit there is no safe threshold of radiation exposure and that harm is directly proportional to the amount of exposure,” the order reads. “Those models lack sound scientific basis and produce irrational results.” The agency is keeping LNT in place despite being specifically ordered to reconsider it. With LNT intact, the scientific backing for ALARA remains in place. So, the new proposed regulations largely focus on calling it so.

值得注意的是，尽管特朗普曾发布行政命令描述并称 LNT 为“非理性”，但 NRC 依然做出了上述决定。该行政命令写道：“NRC 使用的安全模型假设不存在安全的辐射暴露阈值，且伤害与暴露量直接成正比。这些模型缺乏可靠的科学依据，并产生非理性的结果。”尽管被明确要求重新考虑，该机构仍保留了 LNT 模型。随着 LNT 的保留，ALARA 的科学支撑依然存在。因此，新拟议的法规主要集中在对其称呼的调整上。