25 years of RIFE-a brief history and highlights.

In 2020, the UK environmental regulators and food safety agencies, published the 25th edition of the Radioactivity in Food and the Environment (RIFE) report. This marks a quarter of a century since the landmark RIFE report was first published by the Ministry of Agriculture, Fisheries and Food in 1996, which represented the first joint monitoring and assessment report for the United Kingdom. This paper provides a summary of the RIFE report, how it has evolved and presents some case studies from over the 25 year period.

A statistical approach to investigating enhancement of polonium-210 in the Eastern Irish Sea arising from discharges from a former phosphate processing plant.

Since the cessation of phosphoric acid production (in 1992) and subsequent closure and decommissioning (2004) of the Rhodia Consumer Specialties Limited plant in Whitehaven, the concentration levels of polonium-210 ((210)Po) in local marine materials have declined towards a level more typical of natural background. However, enhanced concentrations of (210)Po and lead-210 ((210)Pb), due to this historic industrial activity (plant discharges and ingrowth of (210)Po from (210)Pb), have been observed in fish and shellfish samples collected from this area over the last 20 years. The results of this monitoring, and assessments of the dose from these radionuclides, to high-rate aquatic food consumers are published annually in the Radioactivity in Food and the Environment (RIFE) report series. The RIFE assessment uses a simple approach to determine whether and by how much activity is enhanced above the normal background. As a potential tool to improve the assessment of enhanced concentrations of (210)Po in routine dose assessments, a formal statistical test, where the null hypothesis is that the Whitehaven area is contaminated with (210)Po, was applied to sample data. This statistical, modified "green", test has been used in assessments of chemicals by the OSPAR commission. It involves comparison of the reported environmental concentrations of (210)Po in a given aquatic species against its corresponding Background Assessment Concentration (BAC), which is based upon environmental samples collected from regions assumed to be not enhanced by industrial sources of (210)Po, over the period for which regular monitoring data are available (1990-2010). Unlike RIFE, these BAC values take account of the variability of the natural background level. As an example, for 2010 data, crab, lobster, mussels and winkles passed the modified "green" test (i.e. the null hypothesis is rejected) and as such are deemed not to be enhanced. Since the cessation of phosphoric acid production in 1992, the modified "green" test pass rate for crustaceans is ∼53% and ∼64% for molluscs. Results of dose calculations are made (i) using the RIFE approach and (ii) with the application of the modified "green" test, where samples passing the modified "green" test are assumed to have background levels and hence zero enhancement of (210)Po. Applying the modified "green" test reduces the dose on average by 44% over the period of this study (1990-2010).

An estimate of the inventory of technetium-99 in the sub-tidal sediments of the Irish Sea.

Published results from earlier studies have provided indications that measurable quantities of technetium-99 ((99)Tc) have accumulated in the sub-tidal sediments of the Irish Sea. This is due to the enhanced discharges from the Sellafield nuclear reprocessing plant in Cumbria, UK (between 1994 and 2004). Depth distributions of (99)Tc concentrations in sub-tidal sediments have been determined from a limited number of Irish Sea sites, following the collection of deep sediment cores (up to 2 m in depth), sampled in two research cruise surveys in 2005 and 2006. Vertical concentration profiles of (99)Tc from a range of substrates in the Irish Sea are presented here and these have been used to produce an estimate of the total inventory of (99)Tc residing in the sub-tidal sediments of the Irish Sea. Significant variation was observed between (99)Tc concentrations in the sediment samples, as well as in the shape of individual depth profiles. As anticipated, concentrations tended to be greater on fine-grained (muddy) substrates and showed a general decrease with distance from Sellafield. Vertical concentration profiles of (137)Cs, and (137)Cs data from published work, have also been considered to evaluate the use of the relatively few (99)Tc core data (upon which to determine the (99)Tc inventory). The inventories of (99)Tc and (137)Cs are estimated to have been of the order of 30 and 455 terabecquerels (TBq), respectively, or ∼2% of the total cumulative Sellafield discharge for each of the two radionuclides. The residence half-time of (137)Cs in the sub-tidal sediments of the Irish Sea is estimated to be in the order of ∼16 years. Therefore, as the Kd values for (99)Tc and (137)Cs are similar, this also provides an indicative value to predict future losses of (99)Tc from the sediment reservoir.

Dipolar interactions, molecular flexibility, and flexoelectricity in bent-core liquid crystals.

The effects of dipolar interactions and molecular flexibility on the structure and phase behavior of bent-core molecular fluids are studied using Monte Carlo computer simulations. Some calculations of flexoelectric coefficients are also reported. The rigid cores of the model molecules consist of either five or seven soft spheres arranged in a "V" shape with external bend angle gamma. With purely repulsive sphere-sphere interactions and gamma = 0 degrees (linear molecules) the seven-sphere model exhibits isotropic, uniaxial nematic, and untilted and tilted smectic phases. With gamma > or = 20 degrees the untilted smectic phases disappear, while the system with gamma > or = 40 degrees shows a direct tilted smectic-isotropic fluid transition. The addition of electrostatic interactions between transverse dipole moments on the apical spheres is generally seen to reduce the degree of molecular inclination in tilted phases, and destabilizes the nematic and untilted smectic phases of linear molecules. The effects of adding three-segment flexible tails to the ends of five-sphere bent-core molecules are examined using configurational-bias Monte Carlo simulations. Only isotropic and smectic phases are observed. On the one hand, molecular flexibility gives rise to pronounced fluctuations in the smectic-layer structure, bringing the simulated system in better correspondence with real materials; on the other hand, the smectic phase shows almost no tilt. Lastly, the flexoelectric coefficients of various nematic phases--with and without attractive sphere-sphere interactions--are presented. The results are encouraging, but a large computational effort is required to evaluate the appropriate fluctuation relations reliably.

Computer simulations of bent-core liquid crystals.

The phase behavior of model linear and bent-core molecules has been studied using isothermal-isobaric Monte Carlo computer simulations. The molecular model consists of seven Lennard-Jones spheres rigidly arranged in a "V" shape, with external bond angle, gamma. With gamma=0 degrees (linear molecules), we find isotropic, nematic, untilted smectic A, and two layered phases in which the molecules are tilted with respect to the layer normal. The latter two phases correspond to distinct branches in the equation of state, and possess different types of ordering within and between the layers; these phases are tentatively assigned as being smectic B and crystal. Apart from the possible existence of a tilted smectic B, the phase behavior of this system is broadly in line with earlier simulation studies on related linear molecular models. In the gamma=20 degrees system, isotropic, nematic, and tilted smectic- B phases are observed. Interestingly, the range of stability of the nematic phase is enhanced compared to the gamma=0 degrees system. In simulations of the gamma=40 degrees system, nematic phases are absent, and only isotropic and tilted phases are in evidence. The in-layer structure of the tilted phases shows a very clear change from smectic- B to smectic- A ordering upon increasing the temperature. In all instances of a tilted phase, the degree of molecular tilt is in the region of 30+/-5 degrees, with respect to the smectic layer normal, which corresponds closely to typical experimental observations in real bent-core liquid crystals. In our model, the tilt provides efficient packing of the spheres and favorable attractive interactions between molecules. The relevance of the present simulation model to real bent-core liquid crystals is discussed critically.