How NFCFs compare with nuclear reactors and chemical plants

As stated in Section 3 of Ref [14], NFCFs imply a great diversity of technologies and processes.

They differ from nuclear power plants (NPPs) in several important aspects, as discussed in the following paragraphs.

First, fissile materials and wastes are handled, processed, treated, and stored throughout NFCF mostly in dispersible (open) forms. Consequently, materials of interest to nuclear safety are more distributed throughout NFCF¹¹ in contrast to NPP, where the bulk of nuclear material is located in the reactor core or fuel storage areas. For example, nuclear materials in current reprocessing plants are present for most or part of the process in solutions that are transferred between vessels used for different parts of the processes, whereas in most¹² NPPs nuclear material is present in concentrated form as solid fuel.

Second, NFCFs are often characterized by more frequent changes in operations, equipment and processes, which are necessitated by treatment or production campaigns, new product development, research and development, and continuous improvement.

Third, the treatment processes in most NFCFs use large quantities of hazardous chemicals, which can be toxic, corrosive and/or combustible.

Fourth, the major steps in NFCFs consist of chemical processing of fissile materials, which may result in the inadvertent release of hazardous chemicals and/or radioactive substances, if not properly managed.

Fifth, the range of hazards in some NFCFs can include inadvertent criticality events, and these events can occur in different locations and in association with different operations.

Finally, in NFCFs a significantly greater reliance is placed on the operator, not only to run a facility during its normal operation, but also to respond to anticipated operational occurrences and accident conditions [15].

Whereas the reactor core of an NPP presents a very large inventory of radioactive material and coolant at high temperature and pressure¹³and within a relatively small volume, the current generation of NFCFs operate at near ambient pressure and temperature and with comparatively low inventories at each stage of the overall process. Accidents in NFCFs may have relatively low consequences when compared against nuclear power plants. Exceptions to this are facilities used for the large scale interim storage of liquid fission products separated from spent fuel and, where applicable, facilities for separating and storing plutonium.

In some cases in an NFCF, there are rather longer timescales involved in the development of accidents and less stringent process shutdown requirements are necessary to maintain the facility in a safe state, as compared to an NPP. Nevertheless, the INPRO area of NFCF safety applies the principles of the DID concept and encourages the NFCF designers to enhance the independence of DID levels¹⁴ in new facilities. NFCFs also often differ from NPPs with respect to the enhanced importance of ventilation systems in maintaining their safety even under normal operation. This is because nuclear materials in these facilities are in direct contact with ventilation or off-gas systems. Various forms and types of barriers between radioactive inventories and operators may have different vulnerabilities. Fire protection and mitigation assume greater importance in an NFCF due to the presence of larger volumes of organic solutions and combustible gases. With fuel reprocessing or fuel fabrication facilities, the wide variety of processes and material states such as liquids, solutions, mixtures and powders needs to be considered in safety analysis.

From this point of view, the safety features of NFCFs are often more similar to chemical process plants than those of NPPs. In addition, radioactivity and toxic chemical releases and criticality

11 Except uranium or thorium mining and milling facilities.

12 Exceptions include the molten salt reactors to be developed.

13 Pressure is high in water and gas cooled reactors, but not in liquid metal or molten salt reactors.

14 Taking into account the graded approach.

issues warrant more attention in NFCFs than in NPPs¹⁵. Further comparisons of the relevant features of an NPP, a chemical process plant and an NFCF are presented in Table 2.

TABLE 2. TYPICAL DIFFERENCES BETWEEN NPPs, CHEMICAL PROCESS PLANTS AND NFCFs (MODIFIED FROM REF [16])

Feature NPP Chemical Process Plant NFCF

Type of

As outlined above, from a safety point of view, NFCFs are characterized by a variety of physical and chemical treatments applied to a wide range of radioactive materials in the form of liquids, gases and solids. Accordingly, it is necessary to incorporate a correspondingly wide range of specific safety measures in these activities. Radiation protection requirements for the personnel are more demanding, especially in view of the many human interventions required for the operation and maintenance of an NFCF. The safety issues encountered in various NFCFs have been discussed in Refs [14, 15]. A comprehensive description of the safety issues of fuel cycle facilities is provided in Ref [17].

For most existing NFCFs, the emphasis is on the control of operations using administrative and operator controls to ensure safety as well as engineered safety features, as opposed to the emphasis on engineered safety features used in reactors. There is also more emphasis on criticality prevention in view of the greater mobility (distribution and transfer) of fissile materials. Because of the intimate human contact with nuclear materials in the process, which may include (open) handling and transfer of nuclear materials in routine processing, special attention is warranted to ensure worker safety. Potential intakes of radioactive materials require control to prevent and minimize contamination and thus ensure adherence to specified

15 The objective of improving radiological safety remains unchanged. It should nevertheless be noted that the consequences of conventional toxic chemical releases, chemical fires and explosions usually include direct or indirect radiological impacts. The public perception of the risk of radiological hazards is much higher than that for chemical or industrial hazards and the design needs to ensure that all risks are appropriately addressed.

16 For most types and design of NFCFs containment/confinement is provided by a combination of a ‘static barrier’ and a complementary ‘dynamic’ (e.g. provided by a ventilation system) barrier that, together, provide effective containment/

confinement.

17 Except for molten salt reactors.

operational dose limits. In addition, releases of radioactive materials into the facilities and through monitored and unmonitored pathways can result in significant exposures.

The number of physical barriers in an NFCF that are necessary to protect the workers, the environment and the public depends on the potential internal and external hazards, and the consequences of failures; therefore the barriers are different in number and strength for different kinds of NFCFs (the graded approach). For example, in mining, the focus is on preventing contamination of ground or surface water with releases from uranium mining tails. Toxic chemicals and uranium by-products are the potential hazards of the conversion stage and for forms of in-situ mining. In enrichment and fuel fabrication facilities (with no recycling of separated or recovered nuclear material from spent fuel), safety is focused on preventing criticality in addition to avoiding contamination via low-level radioactive material.

It might be possible to enhance safety features in a nuclear energy system by co-location of front end (e.g. mining/ milling, conversion and enrichment, and fuel production facilities) and back end (reprocessing and waste management) facilities. This would have benefits through minimal transport, optimisation and alignment of processes, avoiding multiple handling of radioactive materials in different plants of the fuel cycle and comprehensive and integrated waste treatment and storage facilities.

Compared to safety of operating NPPs, only limited open literature is available on the experience related to safety in the operation of NFCFs. Examples of United States Nuclear Regulatory Commission regulation are provided in Refs [18-22]. Safety of and regulations for NFCFs have been discussed in IAEA meetings and conferences [14, 15]. Aspects of uranium mining have been reported extensively [23–30]. The Nuclear Energy Agency of the Organization for Economic Cooperation and Development published a comprehensive report on safety of nuclear installations in 2005 [31]. Safety guides on conversion/enrichment facilities, fuel fabrication, reprocessing and spent fuel storage facilities have also been published by the IAEA [32–36].

FIG.1. Conceptual comparison of safety characteristics between an NPP and a reprocessing facility.

It is obvious that in well-designed NFCFa, the safety related events that have a high hazard potential will have low frequency of occurrence and vice versa. For example, Fig. 1 (modified from Ref [37]) conceptually compares the relationship between potential consequences and frequency for safety related events in a nuclear power plant and a reprocessing facility.

The figure demonstrates that, compared to accidents in an NPP, an NFCF may have relatively higher consequences of accidents having higher probability of occurrence, e.g. accidental criticality. However, accidents with very high consequences have essentially lower probability

than in NPPs and can only occur in a few high inventory NFCFs, typically large reprocessing plants and associated liquid high level waste interim storage facilities [38].