Thin neutron detector on a chip utilising silicon carbide

Following the Tohoku earthquake of the 11th of March 2011, the resultant tsunami initiated an INES level 7 nuclear accident at the Fukushima Daiichi Nuclear Power Plant. Following this event, debris was distributed in the bottom of the Primary Containment Vessel (PCV) and the housing areas within units 1-3 of the plant. This was a mixture of fuel, fissile material, activated isotopes and structural materials such as concrete. This presented both a hazard to the restoration teams and also a challenge in the longer term decommissioning and dismantling procedures. The particular challenges faced within the plant can be summarised as:

-An unknown mixture of fuel and activated waste emitting a variety of radiation types.
-An environment of extremes in temperature or humidity, and flooded in parts.
-High gamma background radiation - estimated to be up to 1,000 Gy/hr.
-Limited access in terms of physical size and weight for tools to aid remedial work.
-Limited access in terms of time due to worker dose limits.

Monitoring systems have confirmed that the fuel debris is not currently in a critical state, although this may change over time due to the fuel debris shape and water levels changing. There is significant interest in sub-criticality monitoring technology and criticality prevention technology to ensure this scenario does not occur however. In order to function within this harsh environment, instrumentation and electronics need to be radiation hardened beyond anything that exists currently. Operations within the plant require a detector that works in harsh environments - physical and radiological. Also required of such a device is that it be small and thin thus it can fit into small gaps or can be used atop a peripheral such as a robotic device which may be used to enter the reactor. The PCV's have been flooded in Fukushima leading to a preponderance of thermal neutrons and a temperature in excess of 60 degrees Celsius, thus both temperature and radiation tolerance is crucial. It is anticipated that background radiation within this reactor in everyday conditions will be of the order of 10^7 n/cm^2/s with a Gamma effective dose rate of between 1 to 100 mSv/hr. However, peaks are thought to reach approximately 10^13 n/cm^2/s and 1,000 Sv/hr.

The work to develop the Thin Neutron Detector System (TNDS) will encompass the development of a 3mm thick neutron detector using a Silicon Carbide fabrication process, deposition of a converter material, implementation of a signal processing chain to support the application to the Fukushima process and a development of concept of operations (CONOPS) for the use of the device in The Fukushima nuclear power plant.

This work will be undertaken over three sites internationally. This will begin with a Concept of Operations stage where the exact design specification will be determined via workshops in Kyoto. Once this has been completed, the practical work will begin at Lancaster University where proto-type versions of the detector on a chip device will be designed, constructed and tested using various software, and radiological sources. The work will continue at the SME 'Innovative Physics' based on the Isle of Wight, where in collaboration with the Japanese partner, a silicon carbide version of the detector will be designed and developed.

The final stage of the work involves the testing of the devices, beginning in Lancaster with a Cf-252 neutron source and CS-137 gamma sources. Assuming success, the detectors will be tested for extreme radiation tolerance at the Cobalt-60 irradiator at the Dalton Cumbrian Facility in Cumbria. The devices will then be tested in Japan at the 5MW Kyoto University Reactor (KUR) and the 100W Kyoto University Criticality Assembly (KUCA), before hopefully on to testing at the Fukushima site itself.

A substantial proportion of the world's electricity supply is provided by the 450 or so nuclear reactors around the world including the 15 operational nuclear reactors in the UK. As these reactors age, failure due to corrosion and fatigue and other forms of degradation must be monitored to prevent catastrophic failures and extended reactor shut-downs, which can be very expensive for both industry and the consumer, as well as the obvious safety concerns. However, the environment within reactor pressure vessels, pressurised water reactors and the associated piping is a very confined and hazardous environment. The high temperatures, high gamma and neutron radiation levels, and sometimes wet conditions makes this a very difficult environment for detectors to function in. The proposed research will develop a novel detector class, especially silicon carbide-based detector-on-chip technologies, which will operate in these hazardous environments, and perhaps more importantly in very confined spaces owing to their very small size.
This will provide for the first time the capacity to detect problems as they develop right by the reactor, i.e. within cracks in cladding or in the fuel assembly, where the greatest danger is. The technology will also allow for a greater level of integrity assessment in structures with restricted space using tracer methodologies. By allowing faults to be caught earlier, it will reduce the extent of damage due to aging and faults and hence the cost and time of repair. The resilient and nonintrusive nature of the technology means it can also form the basis of proactive ageing management in operating nuclear power plants. This is very important given the UKs intent of building several Generation III light water reactors in the near future. By having the level of scrutiny that the proposed technology will allow, it will help alleviate some of the publics concerns about nuclear power.
As well as being potentially a key technology for monitoring and ensuring the health of operational reactors, the technology will have significant use in the decommissioning of retired reactors and in the clean-up of damaged reactors such as Fukushima Daiichi. The decommissioning of such structures poses a significant health risk as well as significant costs. This technology will allow the identification of fissile fuel from other waste material facilitating the removal of the fuel first in a safe and expeditious way. As before, these environments are hazardous, being hot, wet and having high radiation fields. A key benefit of the technology is its low weight and size, which means that it will be easily integrated into autonomous and semi-autonomous waste sorting and decommissioning robotic systems, reducing the exposure to workers.
As part of the development of the detector-on-chip technology, highly resilient silicon carbide-based circuitry will be developed. Many of the components will be common to many systems, e.g. amplifiers, digital processing circuits, etc. This will enable many other technologies to be developed, especially in the world of robotics where circuitry needs to be close to the sensors and actuators for smart control. Currently this cannot be achieved in robotic systems to be operated in hazardous environments, due to the need for high levels of protection, especially shielding, which limits their functionality and robustness, a limitation removed by this technology.
Given the number of new reactors being developed and the number being retired and subsequently decommissioned, as well as the amount of fuel to be reprocessed, stored and disposed of, the market for detectors of this class is substantial. Development of this technology will be of significant commercial interest to many in the nuclear and related industries worldwide.