Technology Readiness Levels (TRLs) give a good idea of maturity of new technology and are used by industry and government organisations worldwide. Nine TRLs exist ranging from initial ideas at Level 1 where basic principles are observed to fully robust technologies validated for application in industry at Level 9. TRLs can be further subdivided into Readiness Levels for system, materials, methods, manufacturing and instrumentation as shown in Figure 1. The main TRLs were originally devised by NASA and also used by ESA for aerospace industry. DOE 1 in USA produced a guide on TRL assessment for wider application. EU H2020 2 and NDA (UK)  also produced TRL guides out which the one defined by the NDA(UK) is perhaps the most relevant to the nuclear industry but it was specifically written for nuclear decommissioning sector. This paper presents a case to develop definitions of fusion specific TRLs.
The 1 to 9 TRL scale is an ordinal scale. The effort or time needed to move from one point to another may not be linear. TRLs are time specific and most importantly context specific. TRLs should not be used to measure progress, risk, cost or duration. Technologies with low TRL can mature more quickly and technologies with high TRL can stagnate and never mature as is shown in Figure 2. TRLs of individual items say nothing about whether the whole system will work together. The integration and interfaces need to be assessed separately. TRLs are for individual technologies not for a system requiring integration of many. There will be a need to breakdown a complex system into sufficient number of sub-systems to apply TRLs. Furthermore, a technology which is mature in one sector may not be mature for fusion application and vice versa. All this has to be analysed in the case of Fusion technologies
Benefits of TRLs are that they can help decide whether a technology is ready for implementation and can help plan its further development. For cost-effective development of a technology, a better definition of TRLs is needed as it will help focus the resources. The example shown in Figure 1, indicates the target TRLs to be achieved by a project and a stage gate review indicates the current status which highlights the areas where development effort needs to be invested. A good TRL definition and assessment will help produce a cost-effective development plan, and it not available for fusion so far.
The need for fusion specific TRLs arises from the fact that technology requirements in the nuclear sector are different from other industry sectors. Furthermore, there are some notable differences between the fission and fusion. The IAEA Tecdoc 1851  highlights these differences. For example, in the fusion applications, there
is no reactivity control or emergency cooling requirements and no core melt conditions to be addressed. Prevention of core meltdown is not a safety related function for fusion reactors. Most of the main parameters like the safety functions, consequences of failure, fault frequencies, contributing to the safety classification process in fission applications are different for the fusion applications. Yet, the fusion industry has mostly relied on existing codes, standards and industrial practices developed for fission. Fusion technology challenges are also different if not more demanding. In the fusion nuclear facilities there are many kinds of accidents that can be postulated due the fact that a tokamak is a complex and dense zone with many different energy source terms. In addition, the first confinement barrier, which is surrounded by those energy source terms, has a very complex boundary and is subjected to a combination of extreme loads not seen in other industry sectors. E Surrey et al  have acknowledged inadequacy of the existing TRLs for fusion and proposed an alternative methodology that allows a quasi-numerical analysis by a combination of three quantities: unmitigated probability of failure, severity and probability of failure detection.
To help fusion technology achieve full industrialisation, it is prudent to consider internationally harmonised definitions for fusion specific TRLs and provide fusion specific definitions for all the 9 levels of TRL for system, materials, methods, manufacturing and instrumentation.
The new definitions can also take into account new approaches to design and validate a system, structure or component. For example, risk informed performance based probabilistic design methods can help produce cost-effective designs without compromising safety. The current design approach is to use allowable stress codes to design components that rely on experience-based safety factors which gives no idea of reliability or risk in design of the full system. The new approach can produce cost-effective design and give a good idea of the risk and probability of failure. This new approach is going to have wider scope and will be applicable to next generation of reactors.
- U.S. Department of Energy, Technology Readiness Assessment Guide, DOE 413.3-4, 2009.
- Technology Readiness Levels (TRL), European Commission, HORIZON 2020, Work Programme 2014-2015, General Annexes, Extract from Part 19 – Commission Decision C (2014) 4995.
- Guide to Technology Readiness Levels for the NDA Estate and its Supply Chain, Nuclear Decommissioning Authority (NDA), 2014.
- Integrated Approach to Safety Classification of Mechanical Components for Fusion Applications, IAEA Tecdoc 1851.
- Assessing component suitability and optimizing fusion plant design – Alternative approaches to TRLs, E Surrey, J. Linton and M. Sadler, TPS 10398.
|Affiliation||Wood Group (Nuclear)|
|Country or International Organization||United Kingdom|