Nuclear reactors are controlled by shaping the population of neutrons within the core1. This distribution needs to be measured regularly and accurately to infer power and to ensure the safe and efficient burnup of reactor fuel. The fission process produces highly penetrating neutrons and gamma rays inside the reactor, some of which can be detected far from their point of origin. These radiations therefore can be imaged to yield information of the internal reactor conditions, independently from instrumentation within the core. Several levels of redundancy are needed for reactor monitoring due to the harsh environment associated with the fission process. Monitoring systems, such as fission chambers and rhodium detector systems are expensive and have limited lifespans. They face severe challenges associated with the in-core environment to remain viable, including high temperatures, corrosion, extreme radiation levels and miniaturization. In-core monitoring systems often have short lifespans relative to the expected life of modern reactors, are expensive, difficult to replace and can be destroyed easily in the event of a core-damage accident2.

In the context of major reactor accidents, the most significant of late is that of the Fukushima Daiichi disaster. Much research has been reported, and a variety of imaging methods have been used to image contamination from this incident in the natural environment3. These reports frequently exploit the use of Compton cameras that only respond to gamma radiation and are not resilient to high-intensity fields. Thus they are not easily transferable to power-resolved imaging or operational reactors that require assay of the neutron emission, let alone the immediate aftermath post-accident. There have been complementary reports of systems for internal inspection of failed reactor plant4 but these are intrusive and provide for the survey of fuel debris inside reactor plant once access to a stricken reactor is achieved. In terms of imaging the stricken reactor itself, the most evolved technique is the use of cosmic-ray muons5 that can non-intrusively reveal the distribution of fuel debris in a reactor, though not the reactivity profile. However, the major disadvantage of this approach is the extensive exposure time of the order of months. This precludes the assay of reactor systems in operation or power-resolved images as presented here, which require timescales of the order of minutes to hours. Most importantly, analogous to the comparison of X-ray transmission imaging with single-photon emission computerized tomography in the medical field, our report highlights the benefit of in situ radiation imaging over long-exposure, transmission imaging. The former yields reactivity information in addition to fuel layout whereas the latter only yields information on the distribution of material in the core. Subtle asymmetries in the power distribution can lead to economic under-performance in commercial power reactors, and can influence experimental facilities that rely on small-scale test reactors for sample irradiation or neutron radiography6,7. Accounting for the axial offset anomaly experienced with the operation of pressurized water reactors8, in which the neutron distribution differs from predicted estimates, could be aided by high-intensity imaging technology.

We present an approach to reactor monitoring using a stand-off imaging system, able to image fast-neutron and gamma-ray fields emitted by a reactor core. This method is passive, non-intrusive and does not require penetration into the core containment. The system is lightweight and portable, weighing a total of 20 kg, and will fit inside a small suitcase. The simplicity and compactness of this single-detector system9 overcomes the problems of other imagers reported in the literature that are not portable and use large arrays of detectors10,11, and therefore are not easily transferable to situations described here. Imaging high-intensity radiation fields currently has challenges, some of which have been addressed in nature. Slit pupils in the eyes of some terrestrial vertebrates, including some cats, are known to be oriented orthogonal to the visual streak12. This adaptation extends the useful visual range into high-intensity light levels13. In analogy this approach has been applied here by using a slit collimator that creates a thin band of spatial sensitivity, which prevents detector saturation and additionally preserves angular resolution. Slit collimators of similar designs have been used to image gamma-ray fields14,15 though have not been applied to more highly penetrating fast neutrons. This system provides a method of spatially dependent monitoring that is uniquely not in-core. Here we show the first images of a functioning nuclear reactor produced from radiation emitted directly from the core as a result of the fission process. In these images the isolation of the neutron field almost exclusively discriminates information about nuclear fuel material. Further, the results indicate that this imaging approach is power-resolved, such that the images reveal information of the reaction rates, that is, the rate of fuel burnup within the core. This capability may lead to a new widely used method of reactor monitoring with relevance spanning small medical isotope reactors through to the large power reactors currently in build worldwide, many of which emit a sufficient fast-neutron component16. This approach may be of merit to the new generation of small modular reactor designs17 where longer lifecycles might require lifetimes beyond that of today’s monitoring systems. We foresee the provision of continuous insight throughout the lifetime of a reactor, post-defueling to include decommissioning. This approach could also inform our view of internal conditions in the case of a nuclear accident where other monitoring systems will often have been destroyed. Further applications of this mixed-field imaging system are relevant to any other neutron-emitting systems and materials, including nuclear fusion research, and could herald the way for stand-off, non-intrusive enrichment assessment.