Investigation of improved methods for assessing convergence of models in MCNP using Shannon entropy

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2012."June 2012." Cataloged from PDF version of thesis.Includes bibliographical references (p. 42).Monte Carlo computationals methods are widely used in academia to analyze nuclear systems design and operation because of their high accuracy and the relative ease of use in comparison to deterministic methods. However, current Monte Carlo codes require an extensive knowledge of the physics of a problem as well as the computational methods being used in order to ensure accuracy. This investigation aims to provide better on-the-fly diagnostics for convergence using Shannon entropy and statistical checks for tally undersampling in order to reduce the burden on the code user, hopfully increasing the use and accuracy of Monte Carlo codes. These methods were tested by simulating the OECD/NEA benchmark #1 problem in MCNP. It was found that Shannon entropy does accurately predict the number of batches required for a source distribution to converge, though only when when the Shannon entropy mesh was the size of the tally mesh. The investigation of undersampling showed evidence of methods to predict undersampling on-the-fly using Shannon entropy as well as laying out where future work should lead.by Ruaridh Macdonald.S.B

Physical cryptographic verification of nuclear warheads

How does one prove a claim about a highly sensitive object such as a nuclear weapon without revealing information about the object? This paradox has challenged nuclear arms control for more than five decades. We present a mechanism in the form of an interactive proof system that can validate the structure and composition of an object, such as a nuclear warhead, to arbitrary precision without revealing either its structure or composition. We introduce a tomographic method that simultaneously resolves both the geometric and isotopic makeup of an object. We also introduce a method of protecting information using a provably secure cryptographic hash that does not rely on electronics or software. These techniques, when combined with a suitable protocol, constitute an interactive proof system that could reject hoax items and clear authentic warheads with excellent sensitivity in reasonably short measurement times. Keywords: isotopic tomography; nuclear weapons; disarmament; verificationUnited States. Department of Energy (Award DE-NA0002534

Safety and efficacy of the ChAdOx1 nCoV-19 vaccine (AZD1222) against SARS-CoV-2: an interim analysis of four randomised controlled trials in Brazil, South Africa, and the UK.

BACKGROUND: A safe and efficacious vaccine against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), if deployed with high coverage, could contribute to the control of the COVID-19 pandemic. We evaluated the safety and efficacy of the ChAdOx1 nCoV-19 vaccine in a pooled interim analysis of four trials. METHODS: This analysis includes data from four ongoing blinded, randomised, controlled trials done across the UK, Brazil, and South Africa. Participants aged 18 years and older were randomly assigned (1:1) to ChAdOx1 nCoV-19 vaccine or control (meningococcal group A, C, W, and Y conjugate vaccine or saline). Participants in the ChAdOx1 nCoV-19 group received two doses containing 5 × 1010 viral particles (standard dose; SD/SD cohort); a subset in the UK trial received a half dose as their first dose (low dose) and a standard dose as their second dose (LD/SD cohort). The primary efficacy analysis included symptomatic COVID-19 in seronegative participants with a nucleic acid amplification test-positive swab more than 14 days after a second dose of vaccine. Participants were analysed according to treatment received, with data cutoff on Nov 4, 2020. Vaccine efficacy was calculated as 1 - relative risk derived from a robust Poisson regression model adjusted for age. Studies are registered at ISRCTN89951424 and ClinicalTrials.gov, NCT04324606, NCT04400838, and NCT04444674. FINDINGS: Between April 23 and Nov 4, 2020, 23 848 participants were enrolled and 11 636 participants (7548 in the UK, 4088 in Brazil) were included in the interim primary efficacy analysis. In participants who received two standard doses, vaccine efficacy was 62·1% (95% CI 41·0-75·7; 27 [0·6%] of 4440 in the ChAdOx1 nCoV-19 group vs71 [1·6%] of 4455 in the control group) and in participants who received a low dose followed by a standard dose, efficacy was 90·0% (67·4-97·0; three [0·2%] of 1367 vs 30 [2·2%] of 1374; pinteraction=0·010). Overall vaccine efficacy across both groups was 70·4% (95·8% CI 54·8-80·6; 30 [0·5%] of 5807 vs 101 [1·7%] of 5829). From 21 days after the first dose, there were ten cases hospitalised for COVID-19, all in the control arm; two were classified as severe COVID-19, including one death. There were 74 341 person-months of safety follow-up (median 3·4 months, IQR 1·3-4·8): 175 severe adverse events occurred in 168 participants, 84 events in the ChAdOx1 nCoV-19 group and 91 in the control group. Three events were classified as possibly related to a vaccine: one in the ChAdOx1 nCoV-19 group, one in the control group, and one in a participant who remains masked to group allocation. INTERPRETATION: ChAdOx1 nCoV-19 has an acceptable safety profile and has been found to be efficacious against symptomatic COVID-19 in this interim analysis of ongoing clinical trials. FUNDING: UK Research and Innovation, National Institutes for Health Research (NIHR), Coalition for Epidemic Preparedness Innovations, Bill & Melinda Gates Foundation, Lemann Foundation, Rede D'Or, Brava and Telles Foundation, NIHR Oxford Biomedical Research Centre, Thames Valley and South Midland's NIHR Clinical Research Network, and AstraZeneca

Safety and efficacy of the ChAdOx1 nCoV-19 vaccine (AZD1222) against SARS-CoV-2: an interim analysis of four randomised controlled trials in Brazil, South Africa, and the UK

Background A safe and efficacious vaccine against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), if deployed with high coverage, could contribute to the control of the COVID-19 pandemic. We evaluated the safety and efficacy of the ChAdOx1 nCoV-19 vaccine in a pooled interim analysis of four trials. Methods This analysis includes data from four ongoing blinded, randomised, controlled trials done across the UK, Brazil, and South Africa. Participants aged 18 years and older were randomly assigned (1:1) to ChAdOx1 nCoV-19 vaccine or control (meningococcal group A, C, W, and Y conjugate vaccine or saline). Participants in the ChAdOx1 nCoV-19 group received two doses containing 5 × 1010 viral particles (standard dose; SD/SD cohort); a subset in the UK trial received a half dose as their first dose (low dose) and a standard dose as their second dose (LD/SD cohort). The primary efficacy analysis included symptomatic COVID-19 in seronegative participants with a nucleic acid amplification test-positive swab more than 14 days after a second dose of vaccine. Participants were analysed according to treatment received, with data cutoff on Nov 4, 2020. Vaccine efficacy was calculated as 1 - relative risk derived from a robust Poisson regression model adjusted for age. Studies are registered at ISRCTN89951424 and ClinicalTrials.gov, NCT04324606, NCT04400838, and NCT04444674. Findings Between April 23 and Nov 4, 2020, 23 848 participants were enrolled and 11 636 participants (7548 in the UK, 4088 in Brazil) were included in the interim primary efficacy analysis. In participants who received two standard doses, vaccine efficacy was 62·1% (95% CI 41·0–75·7; 27 [0·6%] of 4440 in the ChAdOx1 nCoV-19 group vs71 [1·6%] of 4455 in the control group) and in participants who received a low dose followed by a standard dose, efficacy was 90·0% (67·4–97·0; three [0·2%] of 1367 vs 30 [2·2%] of 1374; pinteraction=0·010). Overall vaccine efficacy across both groups was 70·4% (95·8% CI 54·8–80·6; 30 [0·5%] of 5807 vs 101 [1·7%] of 5829). From 21 days after the first dose, there were ten cases hospitalised for COVID-19, all in the control arm; two were classified as severe COVID-19, including one death. There were 74 341 person-months of safety follow-up (median 3·4 months, IQR 1·3–4·8): 175 severe adverse events occurred in 168 participants, 84 events in the ChAdOx1 nCoV-19 group and 91 in the control group. Three events were classified as possibly related to a vaccine: one in the ChAdOx1 nCoV-19 group, one in the control group, and one in a participant who remains masked to group allocation. Interpretation ChAdOx1 nCoV-19 has an acceptable safety profile and has been found to be efficacious against symptomatic COVID-19 in this interim analysis of ongoing clinical trials

TFHR for isolated locations

Thesis: S.M., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2014.Page 95 blank. Cataloged from PDF version of thesis.Includes bibliographical references (pages 67-70).In this work we describe a preliminary design for a transportable fluoride salt cooled high temperature reactor (TFHR) intended for use as a variable output heat and electricity source for off-grid locations. The goals of the project were to design an economic reactor: a) Sized for the average load of a site but able to increase output to provide peaking power. b) With safety, security and safeguard requirements met by the choice of materials and form as opposed to relying on security forces and infrastructure. Powering remote sites such as mining stations, military bases, communities or even large ships could be a significant long term market for small nuclear reactors. However, the design basis is very different. The increased cost of transporting goods to the site and maintaining a large population of specialists means a reactor must be simpler to operate and able to defend itself against attackers and proliferators without a large security force. On the other hand, the increased cost of electricity in remote places means more can be spent to meet these goals. This report discusses these issues of operating at a remote site and a general strategy for meeting the resulting design criteria. The TFHR design puts these decisions into practice. The TFHR described is a 125MWth, thermal spectrum reactor using SiC-matrix coated particle fuel which can achieve single batch discharge burnups of up to 70MWd/HMkg over an 8 year cycle. Higher burnups are possible for larger cores. The neutronics properties of SiC-matrix coated particle fuel are explored in detail and various means by which they can be incorporated into a reactor are detailed. The TFHR uses a nuclear air Brayton combined cycle (NACC) for electricity generation, adapted from an off the shelf GE aero-derivative gas turbine. The NACC incorporates a combustible fuel injection port between the high and low pressure turbines which can be used to raise the temperature of the working fluid and boost the power extracted from the system by up to 50%. This increase of electric output occurs without changing the power drawn drawn from the reactor, avoiding any transients. The ability to peak the power output removes the need for a second power system or for the reactor to be sized for the maximum power demand, which is a significant cost saving. However, using an air Brayton cycle requires a high temperature reactor. A TFHR is a better match for this purpose than a gas cooled reactor as it operates at atmospheric pressure, making it easier to meet the security goals described above.by Ruaridh R. Macdonald.S.M

A framework for comparing nuclear warhead authentication protocols

This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2019Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 173-179).Even with the end of the Cold War, nuclear arms control continues to be a cornerstone of strategic stability and international non-proliferation efforts. New treaties are necessary to build upon, or at least maintain, the status-quo, and will rely upon verification technologies and protocols to ensure all sides are dismantling their warheads as promised. The nuclear weapon states refuse to participate in any process which might reveal the design of their warheads to an adversary or would-be proliferator. This makes warhead authentication, the critical verification step where the object to be dismantled is confirmed to be an authentic warhead, especially challenging. Despite several decades of research, there is no agreed means of describing or assessing warhead authentication protocols. This has hindered protocol development, and made it more difficult for the policy and technical communities to communicate what is important and feasible.This thesis presents a framework for describing warhead authentication protocols and quantifying their performance. The framework draws on methods used to assess digital authentication protocols, as well as information theoretic analysis of privacy. A model is developed for describing authentication protocols; showing how authentication questions, physical properties, and measurable data relate to one another. This allows the objectives and assumptions of a protocol to be made explicit, helping to ensure that protocols are compared fairly. It was found that the protocols in the literature have made use of very different assumptions, and that has influenced their choices of measurement technology and concepts of operation. Having established how to describe protocols, the thesis investigates how best to quantify the completeness (type I error rate), soundness (type II error rate), and information privacy of a protocol.While the absolute soundness cannot be calculated without knowledge of all possible hoaxes, a conditional soundness can be estimated using a minimax approach. A new measure of information privacy is presented, based on a change in the KL divergence between an inspector's beliefs and the actual warhead design, when the inspector starts from an incorrect prior.by Ruaridh Reid Macdonald.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Nuclear Science and Engineerin