3 September 2002. Thanks to R.

The Unique Signal Concept for Detonation Safety in Nuclear Weapons
S.D. Spray and J. A. Cooper
Sandia Report SAND91-1269
Sandia National Laboratories
June 1993
73 pages
Unlimited Release

http://nunce.org/sand91-1269.zip (Zipped PDF, 3.5MB)


The purpose of a unique signal (UQS) in a nuclear weapon system is to provide an unambiguous communication of intent to detonate from the UQS information input source device to a stronglink safety device in the weapon in a manner that is highly unlikely to be duplicated or simulated in normal environments and in a broad range of ill-defined abnormal environments. This report presents safety considerations for the design and implementation of UQSs in the context of the overall safety system.

Mathematical Aspects of Unique Signal Assessment
J. Arlin Cooper
Sandia Report SAND2002-1306
Sandia National Laboratories
May 2002
78 pages
Unlimited Release

http://nunce.org/sand2002-1306.zip (Zipped PDF, 374KB)


This report is a supplement to “The Unique Signal Concept for Detonation Safety in Nuclear Weapons,” SAND91-1269, which provides a prerequisite fundamental background on the unique signal (UQS) concept. The UQS is one of the key constituents of Enhanced Nuclear Detonation Safety (ENDS), as outlined in Section 1 of that report. There have been many documents written over the past quarter of a century describing various aspects of the UQS, but none of these emphasized the mathematical approaches that help explain why the UQS is effective in resisting inadvertent pre-arming, even in abnormal environments and how UQS implementations can be quantitatively assessed. The intent of this report is to describe various pertinent mathematical methodologies (many of which have not been previously reported) without duplicating, any more than necessary, background information available in other reports. Mathematical UQS analysis is needed because of quantitative requirements associated with ENDS, and because limited comparisons of various implementation approaches can be quantified under mathematical modeling assumptions.

See also:

National Surety Training Center


NST 220 Unique Signal


Instructor: J. Arlin Cooper, Sandia National Laboratories

Course Abstract: Details on the unique signal concept including background on why the approach was chosen from among other approaches (such as pattern recognition), the principles on which the approach is based and what the use of principles accomplishes, the design details of unique signal sequence patterns, and the payoffs to safety analysts, weapon system developers, and those responsible for the delivery system.

Principles of Nuclear Weapons Security and Safety


Due to their extreme destructiveness, nuclear weapons require stringent measures to ensure that they are never detonated, either intentionally or by accident, except under properly authorized circumstances. In addition, since most nuclear weapons contain strongly radiotoxic materials (plutonium and tritium) it is important to prevent accidental release of these materials in an accident.

The first line of defense against accident is to design into the weapon an "exclusion zone" that encloses the detonation system and physically prevents electrical energy from reaching it. Access from the firing system is provided by a "strong link". This is a mechanism (a motorized switch for example) that maintains physical isolation unless it is closed by the arming system. The strong link is thus the 'draw bridge' across the exclusion zone 'moat'.

Now it is possible for an accident of some kind (a crash, fire, munition explosion, lightning strike, etc.) to destroy the integrity of the exclusion zone or the strong link and theoretically open the possibility of the detonation system being activated. To prevent this, there is one or more "weak links" is inserted into the detonation system inside the exclusion zone. These weak links will fail, rendering the weapon inoperable, when exposed to abnormal stresses (heat, acceleration forces, etc.) that are below the level that could possibly disrupt exclusion zone integrity.

Result - any accident that could circumvent the exclusion zone/strong link protections will disable the weapon by breaking the weak links first.

Design for Safety


The safety of a nuclear device depends on that device NOT working. Three basic techniques (called "positive measures") are used to prevent unintended detonation:

1.Isolation: Critical elements are kept separate by barriers.

2.Inoperability: The device is stored in an inoperable state. For example, an ignition device or arming pin may be removed while in storage.

3.Incompatibility: Detonation requires an unambiguous indication of human intent be communicated to the weapon.   Protecting the entire communcation system against all credible abnormal environments, including sabotage, is not practical. Instead, a unique signal of sufficient information complexity that it is unlikely to be generated by an abnormal environment is used.

Nuclear systems feature:

Unique signal discriminators that must:

1. Accept proper unique signals while rejecting all spurious inputs

2. Have rejection logic that is highly immune to abnormal environments

3. Provide predictably safe response to abnormal environments

4. Be analyzable and testable

Barriers that Protect unique signal sources

Removable barriers between these sources and communication channels. A diagram of the safeguards against accidental nuclear detonation is shown in the figure below.

Integrated Circuits in Nuclear Weapon Applications


Solid State UQS Discrimination Architecture

Fundamental Desired Operation of Nuclear Weapons

– They must NEVER work unintentionally, either through accidental or adversarial environments (Safety & Security)

– They must ALWAYS work when proper authority provides unambiguous intent (Reliability & Security)

– They can NEVER be tested as a final assembly (Comprehensive Test Ban Treaty)